domain: info.info-oh info sde sdv

text:
Marko Budinich

J Ér Émie Bourdon

Damien Eveillard

Impact of the species compartment definition on quantitative modeling of microbial communities

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ecent advances in genome-scale metabolic network reconstruction paved the way to the use of quantitative modelings such as FBA. However, despite the great interest of these techniques to tackle quantitative features, microbial community modeling remains unclear. Whereas studies represent a microbial community with several compartments for each microbial strains and their common pool, others advocate for the use of a single compartment that combines all reactions. Here we show that both modelings lead to different optimal quantitative solutions. This study illustrates this difference by the use of the flux module technique, that describes, in a compact way, the optimal solution space as computed by FBA-like techniques. For application, this paper computes the flux modules of a hot spring microbial community (represented by Synechococcus spp., Chloroflexus and Roseiflexus spp.) and a microbial methagenic system (Desulvovibrio vulgaris and Methanococcus maripaludis) sulfate reducing bacteria), while emphasizing the quantitative changes that occurs when one assumes either the consortium as a "single compartment" or a multiple compartment.

Introduction

Following the surge of high throughput experiments to investigate microbial ecosystems, network systems ecology techniques risen their interest by proposing a qualitative description of ecosystems. For instance, by focusing on "who is Flux Modules in Systems Ecology • Technical Report page 1 of 11 there and who is not" [START_REF] Raes | Toward molecular trait-based ecology through integration of biogeochemical, geographical and metagenomic data[END_REF], these techniques emphasize networks of microbes that co-occurred. However, the functional understanding of these communities remains often challenging when techniques solely consider qualitative community description rather than introducing (partial) quantitative knowledge.

Considering the metabolic network of a community is one way to overcome this weakness [START_REF] Zengler | A road map for the development of community systems (CoSy) biology[END_REF]. One assumes the microbial ecosystem behaviors driven by the metabolic reactions encoded by each microbial genomes. In other words, thanks to multiple metabolic enzymes, microbes interplay within the environment and promote a whole metabolic network responsible for quantitative behavior [START_REF] Karsenti | A holistic approach to marine eco-systems biology[END_REF][START_REF] Dittami | A metabolic approach to study algal-bacterial interactions in changing environments[END_REF]. Once the metabolic network identified, constraint-based modelings are standardly used to reproduce quantitative properties of the whole metabolic network at a molecular resolution (see [START_REF] Monk | Optimizing genome-scale network reconstructions[END_REF] or [START_REF] Thiele | A systems biology approach to studying the role of microbes in human health[END_REF] for review) However, whereas these modeling techniques such as FBA-like approaches became standard modeling routines for single cell systems, their applicative conditions remain to be deeper investigated when applied on microbial communities. In particular, microbial community metabolic network could be either reconstructed as a single integrated network by merging all reactions monitored by all bacterial strains (i.e. Single Cell Hypothesis -SCH), or considering natural boundaries between species by considering strain specific genes (i.e. Multiple Compartment Hypothesis -MCH) as already promoted in tissu specific networks [START_REF] Bordbar | A multi-tissue type genome-scale metabolic network for analysis of whole-body systems physiology[END_REF][START_REF] Mintz-Oron | Reconstruction of Arabidopsis metabolic network models accounting for subcellular compartmentalization and tissue-specificity[END_REF] or well-studied human gut microbial community [START_REF] Shoaie | Understanding the interactions between bacteria in the human gut through metabolic modeling[END_REF]. Because both assumptions implies a distinct experimental efforts, this study proposes an analysis of SCH and MCH consequences on microbial community metabolic models. It is worth to notice herein that such an approach is a natural extension of a previous work of [START_REF] Klitgord | The importance of compartmentalization in metabolic flux models: yeast as an ecosystem of organelles[END_REF]. In 2010, Niels Klitgord and Daniel Segrè proposed a study of the impact of compartmentalization in metabolic flux models via the consideration of organelles within the metabolic network of yeast. However, at the time of the study, no genome-scale metabolic description of microbial community was available. We propose to overcome this weakness by taking benefits from recent biotechnological progresses. We complete the previous study by exploring herein the difference of both -MCH and SCH -hypotheses on two distinct genome-scale community metabolic networks. First, we analyze a microbial mat model that uses three microbial strains: Cyanobacteria, filamentous anoxygenic phototrophs and sulfate reducing bacterias [START_REF] Taffs | In silico approaches to study mass and energy flows in microbial consortia: a syntrophic case study[END_REF], to analyze quantitative and qualitative differences in the predictions; secondly, we use Desulfovibrio vulgaris and Methanoccocus maripaludis microbial system, a classical example of syntrophic growth in anaerobic environment [START_REF] Stolyar | Metabolic modeling of a mutualistic microbial community[END_REF].

Results

For the sake of application, one first considered the phototrophic microbial community system during day light composed Synechococcus spp., called SYN, filamentous anoxygenic phototrophs related to Chloroflexus and Roseiflexus spp., called FAP, and sulfate reducing bacteria (SRB) [START_REF] Taffs | In silico approaches to study mass and energy flows in microbial consortia: a syntrophic case study[END_REF]. This community consumes CO 2 and releases O 2 by photosynthesis. As a byproduct of the rubisco activity, glycolate is produced by SYN, which will be later used as an organic substrate by FAP, along with acetate. Besides, SRB can consume organic compounds and reduce sulfate using H 2 . MCH community model describes a metabolic network for each strain as well as external metabolites such as H 2 , O 2 , NH 3 , glycogen and acetate (136 reactions) [START_REF] Taffs | In silico approaches to study mass and energy flows in microbial consortia: a syntrophic case study[END_REF]. As a modeling contribution, a community biomass function was included to represent the ecosystem growth plus one extra reaction for preserving O 2 / CO 2 ratio as used by rubisco. As reported in [START_REF] Taffs | In silico approaches to study mass and energy flows in microbial consortia: a syntrophic case study[END_REF], the so-called "Pool model" represents the SCH community model (59 reactions: 48 core and 11 exchange reactions) -see Figure 2 and Supp for details. Both MCH and SCH models reproduce previous results and are qualitatively consistant with available experiments [START_REF] Stolyar | Metabolic modeling of a mutualistic microbial community[END_REF][START_REF] Zomorrodi | OptCom: a multi-level optimization framework for the metabolic modeling and analysis of microbial communities[END_REF] (see Figure 1). Both models show similar range of biomass increase when light increase. Naively, these Flux Modules in Systems Ecology • Technical Report page 3 of 11 similar predictions may lead to over-interpret that both model are identical, which do not advocate for the use of MCH that is experimentally expensive. However, solely, these interpretations are not sufficient and might lead to misunderstandings. To overcome these shortcomings, one must indeed consider extensive simulations to investigate all solutions as provided by both SCH and MCH models, and not a unique optimal solution. Indeed, while constraintbased modelings describe sets of fluxes that go through all metabolic reactions at equilibrium, above simulations represent, among all fluxes, a unique flux combination that maximizes one given objective -herein the biomass production. More recent Flux Variability Analysis (FVA) improves this unique description by pinpointing all satisfying solutions over a wide range of environmental conditions. Promoting a systematic exploration of these solutions, Flux Module (FM) technique [START_REF] Müller | Flux modules in metabolic networks[END_REF] analyzes how, among all solutions, some reactions are systematically correlated -emphasizing subnetworks that connect a subset of substrates and products [START_REF] Kelk | Optimal flux spaces of genome-scale stoichiometric models are determined by a few subnetworks[END_REF]. These subnetworks or modules are unique and result from all potential quantitative solutions. Implicitly, different modules imply different quantitative predictions (mathematically called optimal solution space). From a biological viewpoint, its application on single cell organisms shows modules as a sensitive description of biological functions (see supplementary for E. coli modules). MCH and SCH models produce distinct modules, which clearly emphasizes fundamental differences between MCH and SCH solutions. SCH shows only one module (purple in Figure 2), containing 31 reactions (52.2% of overall reactions): 20 reactions covered by MCH modules and 11 not previously highlighted. 7 MCH module reactions do not belong to the SCH module. MCH SYN reactions are decoupled from other networks, confirming previous studies [START_REF] Taffs | In silico approaches to study mass and energy flows in microbial consortia: a syntrophic case study[END_REF] that highlights SYN as a primary producer for all possible microbial interactions. Complementary, FAP and SRB are linked via acetate and H 2 metabolisms. As additional differences, the first glycolysis phase (R1-R2) and pentose phosphate reactions (R5-R9) are connected in SCH, which is not true when each organism is considered separately. SCH module is independent from uptake reactions; whereas MHC modules depict acetate processing of FAP and SRB linked to O 2 , H 2 and CO 2 exchanges.

For the sake of generalization, a similar modeling comparison was applied on a methanogenic microbial system composed of Desulfovibrio vulgaris and Methanococcus maripaludis. D. vulgaris uses lactate fermentation and sulfur reduction to gain energy, while producing gaseous hydrogen. M. maripaludis uses hydrogen to reduce CO 2 into methane, which avoid the accumulation of H

Discussions

Despite similar quantitative simulations, this study shows significant differences between SCH and MCH. However, this communication do not advocate for either of both modeling assumptions. Biologically, SCH models have been widely employed to study metabolite exchanges between species (e.g., cocultures [START_REF] Wintermute | Emergent cooperation in microbial metabolism[END_REF][5] or species within a complex environment [START_REF] Klitgord | Ecosystems biology of microbial metabolism[END_REF]), whereas MCH models have been used to describe microbial communities, where each member seeks to maximize their own biomass [START_REF] Tzamali | A computational exploration of bacterial metabolic diversity identifying metabolic interactions and growth-efficient strain communities[END_REF]. Both assumptions are equivalent when one is interested by predicting overall quantitative behaviors of a microbial community, which is mostly explained by similar exchange reactions between SCH and MCH models.

A protocol driven by SCH might be mostly sufficient for overall predictions with non further functional investigations. Reversely, MCH driven protocols present a significant cost to decipher boundaries between species and origin of genes within a meta-genome [START_REF] Thiele | A systems biology approach to studying the role of microbes in human health[END_REF], but appear as necessary to investigate fine quantitative interactions within the community. From a methodological viewpoint, this study advocates for the use of Flux Modules to compare metabolic models. Modules represent an abstraction of all Flux Variability simulations for a given metabolic model. Indeed Flux Module technique is a natural way to resume the methodological work of [START_REF] Klitgord | The importance of compartmentalization in metabolic flux models: yeast as an ecosystem of organelles[END_REF] that proposes an extensive analysis of yeast metabolic flux estimation with and without compartmentalization. Since our study pinpoints similar conclusions to [START_REF] Klitgord | The importance of compartmentalization in metabolic flux models: yeast as an ecosystem of organelles[END_REF], both studies reinforces the need for further constraint-based modelings dedicated to multiple compartments simulations as motivated by [START_REF] Zomorrodi | OptCom: a multi-level optimization framework for the metabolic modeling and analysis of microbial communities[END_REF][START_REF] Zomorrodi | d-OptCom: Dynamic Multi-level and Multi-objective Metabolic Modeling of Microbial Communities[END_REF]. Above figure depicts modules of E. coli in both aerobic and anaerobic growth conditions. The metabolic model composed of 2381 reactions is analyzed in aerobic (left) and anaerobic conditions (right). For each conditions, one extracts 9 modules that are composed of distinct numbers of reactions (line width is proportional). Each module is associated to the pathways in which the flux module reactions are involved. Modules are in accordance to biological conditions. When challenged by an oxidative stress, most of flux modules of E. coli are conserved, except exchange reaction, respiration & electron transfert, alternate carbon metabolism, which is in accordance to physiological knowledge. To a lesser extent, nucleotide salvage pathway is impacted by oxygen growth conditions.

Metabolic modules of a methanogenic microbial system composed of Desulfovibrio vulgaris and Methanococcus maripaludis

The metabolic model of D. vulgaris contains 145 reactions [START_REF] Zomorrodi | OptCom: a multi-level optimization framework for the metabolic modeling and analysis of microbial communities[END_REF], whereas M. maripaludis model is composed of 97 reactions [START_REF] Stolyar | Metabolic modeling of a mutualistic microbial community[END_REF]. In order to link both strains within a MCH model, we duplicated exchange reactions of H 2 in order to import/export the metabolite with either the other microorganism or environment. A similar procedure was done for Formate, Acetate and CO 2 , which overall introduces 12 exchange reactions. Finally, the whole ecosystem biomass was design to fit biomass functions of D. vulgaris and M. maripaludis as already The SCH presents a disadvantage regarding the interplay between interchange fluxes; in this case, acetate and H 2 are major players of electron transfer in anaerobic systems which role is an active area of research. Besides, for these two systems, the SCH links the fluxes of pentose phosphate system which could impact interpretations in future in silico developments

2 Figure 2 :

 22 Figure 2: Description of metabolic networks related to the microbial mat community system and corresponding modules illustrations. SYN, FAP and SRB depict bacterial strains of the MCH metabolic model. SCH model represents the same metabolic system with no consideration of the compartments while conserving the naming convention of the MCH networks.For the sake of illustration exchange reactions between compartments are not shown. MCH model reveals 2 modules (26.5% of the whole set of metabolic reactions). One module contains 28 reactions (red) that span through FAP and SRB, whereas another (blue) involves 8 reactions. Reactions of the SCH module are depicted in purple.

Flux

  Modules in Systems Ecology • Technical Report page 10 of 11 published, while maintaining a respective proportion of 2:1 for both strains. SCH model of this community consist in merging both metabolic networks and removing all replicated reactions for considering one unique representative reaction. As results, SCH model is composed of 221 unique reactions, as a reduction of 243 reactions MCH model (respectively 145 and 97 reactions for D. vulgaris and M. maripaludis).

Flux

  Modules in Systems Ecology • Technical Report page 11 of 11

  SYN, FAP and SRB depict bacterial strains of the MCH metabolic model. SCH model represents the same metabolic system with no consideration of the compartments while conserving the naming convention of the MCH networks. For the sake of illustration exchange reactions between compartments are not shown. MCH model reveals 2 modules (26.5% of the whole set of metabolic reactions). One module contains 28 reactions (red) that span through FAP and SRB, whereas another (blue) involves 8 reactions. Reactions of the SCH module are depicted in purple. D. vulgaris and M. maripaludis) and the SCH model is composed of 221 unique reactions, after deletion of redundant reactions. Again MCH and SCH modules are different (see Supplementary). Both models show a unique module: 124 reactions (48.6% of MCH model) and 187 reactions (84.6% of SCH model). Reactions related to H 2 and acetate transport are not related in SCH model, whereas they are in MCH. Additionally, pentose phosphate cycle reactions of D. vulgaris and M. maripaludis are linked in SCH module but not in MCH, which might lead to misunderstand bacterial interpretations.

	Flux Modules in Systems Ecology • Technical Report	page 5 of 11

for

Acknowledgments

M.B. is supported by CNRS grant, M.B., J.B. and D.E. are supported by GRIOTE project. D.E. is supported by ANR grant SAMOSA.


Sage Jérémie
email: jeremie.sage@leesu.enpc.fr

Berthier Emmanuel

Marie-Christine Gromaire

STORMWATER MANAGEMENT CRITERIA FOR ON-SITE POLLUTION CONTROL -A COMPARATIVE ASSESSMENT OF INTERNATIONAL PRACTICES

Keywords: Guidelines, Pollution control, Review of practices, Runoff management, SUDS 1. INTRODUCTION

Over the last decade, a growing interest has been shown towards innovative stormwater management practices, breaking away from conventional "end of pipe" approaches (based on conveying water offsite to centralized detention facilities). Innovative strategies, referred to as Sustainable Urban Drainage Systems (SUDS), Low Impact Development (LID) or Green-Infrastructures, advocating for management of runoff as close to its origin as possible, have therefore gained a lot of popularity among practitioners and public authorities. However, while the need for pollution control is generally well accepted, there is no wide agreement about management criteria to be given to developers. This article hence aims to compare these criteria through literature analysis of different state or local stormwater management manuals or guidelines, investigating both their suitability for pollution control and their influence on Best Management Practices selection and design. Four categories of criteria were identified: flow rate limitations, "water quality volumes" (to be treated), volume reduction (through infiltration or evapotranspiration), and non-hydrologic criteria (such as loads reduction targets or maximum effluent concentrations).This study suggests that hydrologic criteria based on volume reduction (rather than treatment) might generally be preferable for on-site control of diffuse stormwater pollution. Nonetheless, determination of an appropriate management approach for a specific site is generally not straightforward and presents a variety of challenges for site designers seeking to satisfy local requirements in addressing stormwater quantity and quality issues. The adoption of efficient LID solution may therefore strongly depend on the guidance given to practitioners to account for these management criteria.

While urban runoff used to be merely directed to surface water bodies through combined and separated sewer networks, it is today recognized as a major source of surface water impairment. Since the early eighties, several studies indeed evidenced that urban runoff and combined sewer overflows (CSO) were responsible for surface water quality and biodiversity deterioration (US-EPA 1983;[START_REF] Marsalek | Evaluation of Pollutant Loads from Urban Nonpoint Sources marsalek[END_REF][START_REF] Saget | Base de données sur la qualité des rejets urbains de temps de pluie: distribution de la pollution rejetée, dimensions des ouvrages d'interception[END_REF][START_REF] Herricks | Stormwater Runoff and Receiving Systems: Impact, Monitoring, and Assessment[END_REF]).

Stormwater management therefore substantially evolved over the last decades. Efforts for mitigation of urban runoff and associated pollutants initially led to the adoption of "end of pipe" strategies, based on the treatment of stormwater collected by sewer networks, prior to its release to the environment [START_REF] Roy | Impediments and solutions to sustainable, watershed-scale urban stormwater management: lessons from Australia and the United States[END_REF]). These conventional approaches however proved to be insufficient due to the limited capacity of drainage systems (often overwhelmed in older cities) and because the adoption of end of pipe treatment facilities would require handling huge runoff volumes [START_REF] Mitchell | Applying integrated urban water management concepts: A review of Australian experience[END_REF]. Innovative approaches, referred to as Sustainable Urban Drainage Systems (SUDS),Green-Infrastructures or Low Impact Development (LID), advocating for management of runoff as close to its source as possible in small decentralized best management practices (e.g. rain gardens, bioswales or micro-detention facilities) to preserve natural hydrologic balance and minimize pollutant discharge, have hence gained a lot of popularity among practitioners and public authorities [START_REF] Ahiablame | Effectiveness of Low Impact Development Practices: Literature Review and Suggestions for Future Research[END_REF][START_REF] Fletcher | SUDS, LID, BMPs, WSUD and more -The evolution and application of terminology surrounding urban drainage[END_REF].

While initially focused on flood mitigation, a broader range of benefits is today expected from stormwater management and LID [START_REF] Fletcher | SUDS, LID, BMPs, WSUD and more -The evolution and application of terminology surrounding urban drainage[END_REF]. Indeed, distributed stormwater control not only reduces peak-flow and volumes so they match drainage systems capacities [START_REF] Andoh | A cost effective approach to stormwater management? Source control and distributed storage[END_REF], but also provides a cost effective solution [START_REF] Taylor | Nonstructural Urban Stormwater Quality Measures: Building a Knowledge Base to Improve Their Use[END_REF][START_REF] Qiu | Comparative Assessment of Stormwater and Nonpoint Source Pollution Best Management Practices in Suburban Watershed Management[END_REF]) for water quality improvement through management of nonpoint runoff pollution associated with frequent rain events and moderate contamination levels. Small integrated best management practices (BMPs), such as bio-filtration, are usually promoted because they have been not only shown to efficiently retain runoff (temporarily or not), but also to remove noticeable amounts of stormwater pollutants [START_REF] Hatt | Hydrologic and pollutant removal performance of stormwater biofiltration systems at the field scale[END_REF][START_REF] Gallo | Comparing the adaptability of infiltration based BMPs to various U.S. regions[END_REF]. In some cases, stormwater facilities that were not specifically designed for pollution control, but allow for temporary retention of runoff volume and infiltration, were shown to have a significant impact on pollutant loads [START_REF] Bressy | Efficiency of source control systems for reducing runoff pollutant loads: Feedback on experimental catchments within Paris conurbation[END_REF]. BMPs' efficiency for pollution control however largely lies in their hydrologic performance. Recent studies indeed indicate that pollutant load reduction mostly tallies with runoff volume reduction, although specific treatment processes (e.g. settling, adsorption…) may also retain contaminants [START_REF] Davis | Bioretention Technology: Overview of Current Practice and Future Needs[END_REF][START_REF] Trowsdale | Urban stormwater treatment using bioretention[END_REF].

While public authorities emphasize the need to minimize the impacts of stormwater discharge, management criteria given for on-site runoff control significantly differ from one country to another, and the relevance of these criteria is seldom questioned. This article, based on an extensive literature analysis of international practices therefore aims to compare these criteria. After a brief discussion on the importance of institutional and regulatory framework, four categories of stormwater management criteria identified from grey literature (national or regional guidelines, engineering standards, rules or local ordinances) will be presented.

The relevance of these criteria for on site pollution control will be examined, considering (1) the rationale for their definition, (2) how well they reflect the pollution control objective, and (3) discussing whether compliance to these criteria is always likely to provide the expected outcomes. Finally, this study will investigate to what extent management criteria may influence BMP selection and design. More specifically, concerns will be raised about the possible impediments to LID and innovative practices adoption.

INSTITUTIONAL FRAMEWORK

Stormwater management governance is usually described as an increasingly cross-organizational and interdisciplinary process generally involving multiple entities, that does not solely result from legal or normative framework [START_REF] Brown | Impediments to integrated urban stormwater management: The need for institutional reform[END_REF][START_REF] Roy | Impediments and solutions to sustainable, watershed-scale urban stormwater management: lessons from Australia and the United States[END_REF][START_REF] Porse | Stormwater Governance and Future Cities[END_REF]. It has thus to be acknowledged that the influence of "management and design criteria" is only one (mostly technical) aspect of a much wider issue.

At a local scale, stormwater management implementation often results from various rules or engineering standards delivered by public authorities at different institutional levels. "Criteria", as referred to in this article, thus simply consists of numerical targets or management principles given to practitioners for on-site stormwater control, which may not systematically be regulatory and can be found as prescriptions or recommendations. In the US, States have often been delegated the authority to regulate stormwater management through NPDES (National Pollutant Discharge Elimination System) permits issuance. Management criteria may hence either result from States' rules and guidelines or local codes or ordinance set by counties or municipalities (NRC 2009). Likewise, French stormwater management policies depend on various national or regional guidance documents or planning tools, master plans covering smaller territories, and eventually local rules adopted by municipalities (GRAIE 2009). Although stormwater regulation is generally complex and criteria may originate from various entities, two main institutional levels were identified here, namely local authorities and relatively centralized regional or national bodies or agencies.

Prevention of surface water pollution typically pertains to somewhat general objectives identified in most national or regional guidance documents. In most countries, national or regional agencies therefore produce management or design standards afterwards adopted by local communities (e.g. city councils) to regulate stormwater discharge to sewer systems. In the US and Canada local codes (that primarily regulate stormwater discharge) often directly refer to guidelines originating from regional agencies and must comply with federal (US) or provincial (Canada) requirements [START_REF] Bc-Mwlap | Stormwater Planning: A Guidebook for British Columbia[END_REF][START_REF] Ch2mill | Policy and Design Criteria Manual for Surface Water Management[END_REF][START_REF] Pdep | Pennsylvania Stormwater Best Management Practices Manual[END_REF][START_REF] Pwd | Stormwater Management Guidance Manual. Philadephia Water District -Office of Watersheds[END_REF].

Similarly, in Germany, Switzerland, Sweden or United Kingdom, management criteria may also be found in guidance documents produced at national level by collaborative groups including both water industry and national agencies [START_REF] Chouli | La gestion des eaux pluviales urbaines en Europe: Analyse des conditions de développement des techniques alternatives[END_REF]CIRIA 2007;DWA 2007). Direct discharges to the environment may even require, like in France or in the US, approval of federal or state agencies which is also indicative of their involvement in stormwater management policies implementation (US-EPA 2004;NRC 2009;DDT-03 2011).

Water quality management criteria or guidelines therefore often originate from centralized regional or national agencies. It should however be acknowledged that describing stormwater regulation for water quality as a strictly "top-down" process (e.g. from government and regional entities to local communities) may not always be accurate. Some pioneer communities may actually develop their own guidance and management or design criteria, going beyond national standards, like in Maryland where aquatic habitat restoration in the Anacostia river became a key driver for the adoption of innovative and more stringent local stormwater regulations (ARWRP 2010).

On the other hand, although management criteria intended to prevent flooding (e.g. quantity control criteria) or downstream erosion may be encountered in most national or regional guidelines, discharges to sewer systems are often regulated on the basis of criteria emanating from local communities (or sewer networks operators). In the perspective of pollution control, these are therefore mostly subsidiary criteria usually assumed to ensure water quality improvement through combined sewers overflow (CSO) prevention. Philadelphia Stormwater Management Guidance Manual' (PWD 2011) is for example very similar to "Pennsylvania Stormwater Best Management Practices Manual" (PDEP 2006) but additionally requires 17 l/s/ha flow rate control for discharge to city's combined sewers (PWD 2011). Similarly, "Metro Vancouver Source Control Design Guidelines" (GV-SDD 2012) are based on British Columbia's Guidebook [START_REF] Bc-Mwlap | Stormwater Planning: A Guidebook for British Columbia[END_REF], but additionally requires discharge rate to city's drainage network not to exceed 0.25 l/s/ha. There are however exceptions to this rule; in the UK, national guidelines indicate that discharges to both the environment and sewer systems should not exceed 2 l/s/ha (DEFRA 2011).

From French experience, such local "drainage systems-based" criteria may outshine regional or national water quality guidelines, either because assumed to be suitable for pollution control, or because local codes that apply to developers are not totally consistent with national standards given to drainage network operators. Indeed, although recent national or regional guidelines stress the importance of on-site pollution control [START_REF] Certu | Centre d'Etude sur les Réseaux, les Transports et les constructions publiques [Expertise center for networks[END_REF][START_REF] Aesn | Outils de bonne gestion des eaux de ruissellement en zone urbaine [Tools for sustainable stormwater management in urban areas[END_REF], flow-rate limitations often remain, in France, the only discharge criteria given to developers by local communities [START_REF] Petrucci | Do stormwater source control policies deliver the right hydrologic outcomes?[END_REF]) and treatment devices are usually only required for highly polluted urban areas (e.g. car-parks or fuel transfer stations). Similar situations could be reported elsewhere in Europe or Northern America. In the US, centralized conveyance to detention facilities (which are assumed to provide some pollutant removal) remained the preferred approach in many US communities [START_REF] Roy | Impediments and solutions to sustainable, watershed-scale urban stormwater management: lessons from Australia and the United States[END_REF]) which focus on flow-rate control rather than water quality [START_REF] Rittenhouse | Low Impact Development Strategies and Tools for NPDES Phase II Communities[END_REF]. Likewise, in Canada, Australia, Spain or Sweden, several Stormwater Master Plans or municipal guidelines were found to be mostly focused on quantity rather than quality issues [START_REF] Momparler | Los sistemas urbanos de drenaje sostenible [Sustainable Urban Drainage Sytems[END_REF][START_REF] Isle | Town of Disdbury, Storm Water Master Plan[END_REF]RCC 2011;[START_REF] References Ae | Regional Municipality of Wood Buffalo -Stormwater Management Master Plan[END_REF][START_REF] Matschoss-Falck | Lokalt omhändertagande av dagvatten i Västerås -Möjligheter till ökad användning i innerstadsområden[END_REF]. Although these documents generally mention regional water quality objectives and encourage on-site runoff and pollutant control, maximum allowable flow-rate or detention requirements (e.g. quantity control) often remains the only criteria for stormwater facilities sizing.

Fragmentation of responsibilities, as identified by [START_REF] Roy | Impediments and solutions to sustainable, watershed-scale urban stormwater management: lessons from Australia and the United States[END_REF], and more generally institutional framework can probably explain the lack of coordination or coherence between regional and local efforts for pollution control. Water quality governance has indeed traditionally consisted in a vertical approach to decision-making with local communities being "top-down recipient of State policies", hindering their involvement in the implementation of non-traditional stormwater controls [START_REF] Brown | Impediments to integrated urban stormwater management: The need for institutional reform[END_REF], and may thus contribute to the persistence of inappropriate management criteria. This resistance to change might also originate from perceived risk associated with adoption of holistic management strategies [START_REF] Olorunkiya | Risk: A Fundamental Barrier to the Implementation of Low Impact Design Infrastructure for Urban Stormwater Control[END_REF]. In France, sewer systems operators rarely give developers specific criteria to prevent pollutants from entering drainage systems because they probably remain more receptive to quantity control issues like urban flooding, rather than surface water pollution [START_REF] Martin | Urban stormwater drainage management: The development of a multicriteria decision aid approach for best management practices[END_REF][START_REF] Aires | Les techniques de contrôle des flux polluants dès la source du ruissellement. Retour d'expérience technico-économique en Île-de-France [On-site runoff and pollutant control practices. Technical and Economic lessons from the Ile-de-France regon[END_REF][START_REF] Petrucci | Do stormwater source control policies deliver the right hydrologic outcomes?[END_REF]) even if they are liable for environmental damage. More generally, the lack of institutional capacity and technical expertise are significant impediment to the adoption of innovative approaches at the local scale [START_REF] Roy | Impediments and solutions to sustainable, watershed-scale urban stormwater management: lessons from Australia and the United States[END_REF][START_REF] Porse | Stormwater Governance and Future Cities[END_REF]. One could therefore argue that these local criteria may sometimes be erroneously perceived as suitable for pollution control by practitioners, by requiring the use of stormwater BMP (this point is discussed in section 2.1).

IDENTIFICATION OF DIFFERENT CRITERIA FOR ON-SITE POLLUTION CONTROL

1. Flow-rate limitations

Peak-flow control is perhaps the most common approach to conventional stormwater management, and generally aims at preventing urban floods or combined sewer overflows during infrequent storms. In Europe and North America, allowable flow-rates are usually justified by (1) drainage network capacity, (2) preservation of downstream "pre-development runoff rate" or (3) maintenance of peak-flow rates in the receiving stream below pre-construction levels to prevent flood and stream channel erosion [START_REF] Vuathier | Limiting stormwater input of large individual connections to public sewers[END_REF][START_REF] Balascio | A survey of storm-water management water quality regulations in four Mid-Atlantic States[END_REF][START_REF] Brown | Sustainable Land Development and Restoration: Decision Consequence Analysis[END_REF]. While generally not accepted as a water quality criterion, it often remains the only numerical target given to developers for on-site stormwater management. The following interpretations may then be put forward to explain this omission of water quality criteria: there seems to be a common belief that (1) pollution control is generally unnecessary unless runoff originates from highly contaminated surfaces (e.g. trafficked roads, metal roofs, gas station…) and ( 2) that peak-flow control can be a suitable solution for water quality management.

Several local ordinances (France), guidelines (Canada) or planning documents (Denmark) were found to require specific treatment solutions for car parks, trafficked roads or storage areas in addition to flow-rate limitation (NM 2003;CAA 2010;KWL 2012), suggesting that peak-flow control (which generally does not aim at reducing pollutant discharge) would be "suitable" for other urban surfaces. Bressy et al (2011) however demonstrated that micro-pollutant concentrations in runoff could remain significant at an individual lot scale because of pollutant wash-off from building materials or atmospheric deposition. Despite low to moderate contamination levels, such surfaces thus contribute to non point source pollution of surface waters. Furthermore, even if such runoff was "clean", it should be outlined that simple peak flow control prior to discharge into sewer systems would probably not make much sense for on-site pollution control given the high cross contamination potential during transport in sewer networks [START_REF] Bressy | Towards the determination of an optimal scale for stormwater quality management: Micropollutants in a small residential catchment[END_REF].

Claiming that peak-flow control does not provide any pollutant reduction would however be inaccurate as delaying runoff requires temporary storage and usually allows for some infiltration or evapotranspiration.

Detention may additionally promote specific processes such as particle settling or adsorption of dissolved contaminants. As a consequence, many local ordinances or guidance documents today encourage the implementation of green infrastructures to provide volume or pollution control when a flow-rate criterion is adopted (LSL 2009;[START_REF] Lehoucq | Bilan et perspectives sur la gestion des eaux pluviales à la parcelle sur le territoire des Hauts-de-Seine -Points forts, réticences et axes d'amélioration [Review and prospects of stormwater source control in the Hauts-de-Seine county -st rong spots, r. Novatech 2013[END_REF][START_REF] Hcc | Interim SuDS Design Guidance for Hertfordshire[END_REF]. Similarly, the criterion itself may be envisaged as an instrument to promote more sustainable stormwater management approaches: as outlined by [START_REF] Petrucci | La diffusion du contrôle à la source des eaux pluviales urbaines[END_REF] various benefits are thus expected from the most stringent flow-rate limitations (e.g. 1 l/s/ha). Examples from France and Sweden indicate that flow-rate limitations can be intended to reduce runoff volumes entering sewer networks, as an alternative to total infiltration or evaporation (LSL 2009;[START_REF] Lehoucq | Bilan et perspectives sur la gestion des eaux pluviales à la parcelle sur le territoire des Hauts-de-Seine -Points forts, réticences et axes d'amélioration [Review and prospects of stormwater source control in the Hauts-de-Seine county -st rong spots, r. Novatech 2013[END_REF]. Similarly, flowrate limitation might be considered as relevant for pollution control (MISEN-PL 2008;LSL 2009;DDT-36 2013). Results from [START_REF] Bressy | Efficiency of source control systems for reducing runoff pollutant loads: Feedback on experimental catchments within Paris conurbation[END_REF] however evidence that, although on-site solutions designed for peakflow mitigation could achieve significant reduction of both runoff volumes and pollutant loads, their efficiency yet remained variable. Besides, the finding that flow-rate control could actually extend the duration of erosive flows cast doubt on their viability as pollution control strategy [START_REF] Emerson | Watershed-Scale Evaluation of a System of Storm Water Detention Basins[END_REF][START_REF] Tillinghast | Stormwater control measure (SCM) design standards to limit stream erosion for Piedmont North Carolina[END_REF][START_REF] Petrucci | Do stormwater source control policies deliver the right hydrologic outcomes?[END_REF]. Development and adoption of other management criteria, directly targeting pollution control or volume reduction, would therefore probably be advisable wherever stormwater management remains only based on peak flow reduction.

2. Volumes based approaches

2.1. "Water quality volume" criteria

Definition of a "water quality volume" is probably the most common approach for pollution control. This criterion is widely adopted in Northern America (US and Canada), but also in New Zealand, England (see table 1) or South Africa [START_REF] Armitage | The South African Guidelines for Sustainable Drainage Systems[END_REF]. Contrary to peak-flow control strategies, such a criterion directly aims at reducing surface water impairment through detention and treatment of a given volume. Although water quality volume definition may vary from a country to another, it generally encompasses the following objectives (as summarized in British guidelines); "Capture and treat the runoff from frequent small events and […] a proportion of the initial runoff […] from larger and rarer events" (CIRIA 2007). As detailed in table 1, this criterion is often supposed to enable capture and treatment of 80 to 90% of annual runoff volumes (MDE 2009;ARC 2010a;[START_REF] Mddep | Guide de Gestion des Eaux Pluviales: Stratégies d'Aménagement, principes de conception et pratiques de gestion optimales pour lesréseaux de drainage en mileu urbain[END_REF] and is usually expressed as a rainfall depth, either associated with a design storm (for which runoff shall be treated), or simply representing a storage volume (corresponding runoff depth is then computed from rational or "curve number" methods). Few details are however given about the rationale underlying the determination of the amount of water to be captured and volume targets value may thus differ significantly from a community to another (cf. More generally, guidance documents analysis indicates that water quality volume is often understood as a storage volume (cf. table 1). The runoff volume intercepted during a given rain events may therefore strongly depend on antecedent weather conditions [START_REF] Hatt | Hydrologic and pollutant removal performance of stormwater biofiltration systems at the field scale[END_REF], stormwater facility's design and drawdown requirements. Determining the amount of water captured in a stormwater facility over a year would thus probably require continuous modeling.

Another issue may be that while infiltration, evapotranspiration or re-use are supposedly the preferred approaches to manage the water quality volume [START_REF] Amec | Georgia Stormwater Management Manual[END_REF]MPCA 2005), treatment and release (e.g. without significant volume reduction) may also be accepted. In the US, although several states explicitly require a fraction of the water quality volume to be infiltrated, harvested or evapotranspired [START_REF] Pdep | Pennsylvania Stormwater Best Management Practices Manual[END_REF][START_REF] Mdeq | Montana Pollutant Discharge Elimination System -General Permit (MTR040000)[END_REF], others simply specify that this volume should be "managed" or "treated" on-site [START_REF] Amec | Georgia Stormwater Management Manual[END_REF]MPCA, 2005). Without volume reduction requirements, the water quality volume approach implicitly suggests that treatment of captured runoff will necessarily result in an "acceptable" pollutant load reduction, regardless of BMP type and pollutant wash-off dynamics. Indeed, in Minnesota or Michigan an 80 to 90% load reduction of total suspended solids (TSS) is expected when meeting this volume criterion [START_REF] Mdeq | Montana Pollutant Discharge Elimination System -General Permit (MTR040000)[END_REF]MPCA 2005), as long as design requirements are respected. BMPs treatment performance has however been shown to vary significantly from a facility to another (CWP 2007) and the efficiency of treatment processes (such as settling, adsorption or filtration) in fact depend on pollutant and runoff characteristics. Particle size distribution can for instance noticeably affect the pollutant removal performance of detention ponds [START_REF] Weiss | The Importance of Particle Size Distribution on the Performance of Sedimentation Practices -in Optimizing Stormwater Treatment Practices: A Handbook of Assessment and Maintenance[END_REF]). Expecting a level of performance from a BMP regardless of its design and site characteristics is thus probably inappropriate. Moreover, water quality volume criteria are often completed by drawdown requirements to provide (1) sufficient residence time for sediment to settle out and (2) sufficient capacity for the next event.

As water residence times are generally expected not to exceed 24h to 72h hours, large outflow rates may be needed to empty stormwater facilities when high rainfall volumes are captured, which may impede their efficiency for smaller rain events.

Finally, while specific "de-pollution" processes (e.g. filtration, adsorption…) in BMPs can be significant for a highly polluted runoff, stormwater, treatment may not always be relevant for on-site management on residential watersheds where concentrations in runoff often remain relatively moderate. Recent studies indicate that (1) the concentration decrease at the outlet of LID practices often depends on the concentration at the inlet [START_REF] Barrett | Performance Comparison of Structural Stormwater Best Management Practices[END_REF][START_REF] Larm | Design methods for stormwater treatm ent -Site specific parameters[END_REF] and that load reduction does not always result from this concentration decrease (associated with treatment or "de-pollution" processes) but more systematically tallies with runoff volume reduction [START_REF] Davis | Field Performance of Bioretention: Water Quality[END_REF][START_REF] Hunt | Pollutant Removal and Peak Flow Mitigation by a Bioretention Cell in Urban Charlotte, N.C[END_REF][START_REF] Trowsdale | Urban stormwater treatment using bioretention[END_REF][START_REF] Bressy | Efficiency of source control systems for reducing runoff pollutant loads: Feedback on experimental catchments within Paris conurbation[END_REF]. One could therefore argue that a diffuse pollution mitigation criterion would probably better be expressed as runoff volume reduction targets rather than a "water quality volume" (to be treated).

Water quality volume requirements alone therefore do not necessarily guarantee a particular level of pollutant reduction, but they do provide at least some water quality benefits over requirements based solely on managing flow-rates. Such policies' outcome probably depends on the guidance given to developers for BMP selection and conception which may promote volume reduction over "treatment and release" strategies. Besides, it should be acknowledged that "treatment and release" presumably remain consistent for more contaminated areas.

2.2. Volume reduction strategies

"Volume reduction" or "permanent interception" implies that volumes captured in a facility will not later be discharged to sewer networks or surface waters. Although volume reduction is often specified as the best approach for stormwater management, "volume reduction criteria" do not systematically aim at providing pollution control and often address other environmental issues. In France, infiltration or "zero discharge" (total infiltration) regulations adopted by some sewer networks operators are essentially intended to prevent floods and Eventually, special attention should presumably be paid to the terms used for the definition of such management criteria. "Infiltration" may indeed either refer to temporary storage in upper soil layers prior to evapotranspiration or to water percolation down to aquifers. "Volume reduction" or "permanent interception" should therefore probably be preferred over "infiltration" since massive infiltration may not always be desired in highly pervious soils (which are more vulnerable to groundwater contamination), neither possible for low permeability substrates (which may however store non-negligible amounts of water and result in runoff volume reduction through evapotranspiration).

Non-hydrologic criteria

3.1. Concentration thresholds

Concentration thresholds in runoff or surface waters may sometimes be given for specific purposes, like direct discharge to surface waters regulation, In Europe, Environmental Quality Standards (immission standards) have been adopted under the Water Framework Directive 2000/60/EC for various contaminants. Nonetheless, assessing the impact of a stormwater management option on receiving water is generally complex and uncertain.

As a consequence, regulations are often based on "emission criteria" which are much easier to handle, and may be adapted to the ecological status of receiving waters [START_REF] Engelhard | Risk analysis and impact assessment of urban stormwater -with emphasize on the EU -Water Framework Directive[END_REF].

Maximum effluent concentrations are typically emission control criteria. However, like environmental quality standards, they usually remain mostly informative as no simple methodology can presently guarantee that a BMP will produce the expected concentrations for a given contaminant. As stated in a 2006 report to the California State Water Resources Control Board [START_REF] Currier | Storm Water Panel Recommendations to the California State Water Resources Control Board -The Feasibility of Numeric Effluent Limits Applicable to Discharges of Storm Water Associated with Municipal Industrial and Construction Activities[END_REF], while the choice of BMP could be based on effluent concentrations or pollutant removal efficiencies from the literature, such approach may not be completely satisfactory as effluent concentrations and pollutant removal efficiencies usually depend on influent concentrations [START_REF] Barrett | Performance Comparison of Structural Stormwater Best Management Practices[END_REF][START_REF] Larm | Design methods for stormwater treatm ent -Site specific parameters[END_REF] and are more generally highly variable [START_REF] Park | Evaluation of stormwater BMPs for implementing industrial stormwater permitting strategy[END_REF].

Moreover, selection of stormwater management strategies in accordance with expected effluent concentrations pushes BMP design into the background as it implicitly assumes that for a given type of BMP design is unlikely to affect their performance. In the US, compliance with water quality standards is thus simply assumed to be met through the implementation of properly designed best management practices (US-EPA 2014). Similarly, "Design effluent objectives" for copper and zinc delivered in Auckland regional council's unitary plan are supposed to be achievable with most BMP as long as design standards are respected (ARC 2013).

Halifax Both effluent and surface water concentration targets therefore remain uncommon as design criteria (or only apply to large development rather than on-site stormwater control), as a concentration can hardly be directly related to best management practices design. Furthermore, verification and enforcement of such limitations would believably be difficult provided the variability of concentrations at the outlet of a BMP [START_REF] Currier | Storm Water Panel Recommendations to the California State Water Resources Control Board -The Feasibility of Numeric Effluent Limits Applicable to Discharges of Storm Water Associated with Municipal Industrial and Construction Activities[END_REF]. Besides, definition of admissible effluent concentrations may be somewhat subjective and thresholds can noticeably differ from a community to another (see table 3), which indicates the lack of common agreement on what a "clean discharge" should be. It should additionally be outlined that concentration based criteria do not necessarily guarantee improvement of surface water quality. In the case of residential or relatively uncontaminated catchment, pollutant concentrations may remain low whereas runoff volumes are typically likely to increase as well as pollutant loads (MBWCP 2006).

In the context of on-site stormwater management, the definition of concentration thresholds for effluent or receiving water cannot totally be discarded, but these should probably remain informative and support the adoption of certain BMP solutions in the case of fairly contaminated urban runoff or sensitive receiving waters.

However, rather than assuming a given level of performance from a BMP, stormwater management facilities should be selected and designed in accordance with (1) water quality standards (themselves consistent with the ecological status of receiving waters) and ( 2) the pollutant removal processes that are likely to address these requirements [START_REF] Clark | Targeting treatment technologies to address specific stormwater pollutants and numeric discharge limits[END_REF].

3.2. Load reduction approaches

In few cases, stormwater management guidelines may directly be based on numerical targets related to minimum pollutant loads reductions, instead of hydrologic criteria such as interception volumes or flow rate control. In Northern America or New Zealand such targets can generally be found as objectives rather than design criteria (e.g. treatment of "water quality volume" expected to provide an 80 to 90% TSS load removal) and is therefore not directly used for BMP design (IDNR 2003;MPCA 2005;ARC 2010b;[START_REF] Mddep | Guide de Gestion des Eaux Pluviales: Stratégies d'Aménagement, principes de conception et pratiques de gestion optimales pour lesréseaux de drainage en mileu urbain[END_REF].

In Australia, most authorities have on the contrary implemented annual load reduction objectives as a design criterion (MBWCP 2006;[START_REF] Tcc | WSUD Technical Design Guidelines for the Coastal Dry Tropics[END_REF]. Targeted contaminants are usually total suspended solids, nitrogen, phosphorous and gross pollutants (see table 4). Such an approach hence implicitly supposes that annual load reduction for these contaminants ensures removal of all other pollutants of concern. However, when considering highly contaminated urban surfaces, it is probably questionable whether loads associated with micro-pollutants will be "acceptable" [START_REF] Strecker | Urban stormwater BMP performance : Recent findings from the international stormwater BMP database project[END_REF]), especially if treatment only targets these four contaminants.

Conversely, in the case of moderately contaminated areas, volume reduction is probably the only way to meet these load reduction objectives (as mentioned previously, BMPs are generally less likely to affect lower concentrations).

These annual performance targets might in fact simply originate from what is reasonably achievable with most conventional BMPs. In the US, the 80% TSS removal objective (supposed to be achievable when meeting volume based management criteria) specified in CZARA (Coastal Zone Act Reauthorization Amendment)

guidance is "assumed to control heavy metals, phosphorous and other pollutants" and is adopted because "analysis has shown constructed wetlands, wet ponds and infiltration basins can remove 80% of TSS" (US-EPA 1993 Table 4 Annual pollutant loads reduction targets in Australia ( 1 Capture and management of the first 10mm of runoff may alternatively be expected for site with less than 25% impervious area)

In Germany and Switzerland, while guidance documents are also based on a load reduction approach (DWA 2007; VSA 2008), an indicator system was adopted, accounting for both pollutant loads produced on urban surfaces and receiving waters' vulnerability to determine the level of treatment required. Although German standards were initially only addressing infiltration or direct discharge to the environment, they may be mentioned in local rules for treatment requirement before discharge to sewer networks (SW 2013). In DWA's guidelines (German association for water, wastewater and waste), dimensionless variables are introduced to quantify both air quality L (depending on traffic or land use) and surface contamination F (depending on cover type). A pollutant load indicator B (=L+F) or "emission value" can therefore be computed from catchment characteristics, and compared to an "admissible value" E depending on the sensitivity of receiving waters. A "reference value" D is eventually introduced so as to account for various BMP's efficiency for pollution control (no elements are however provided for the calculation of D values). If "emission value" is higher than "admissible value" (E<B), runoff treatment is required and 'emission value" may then be reduced to an admissible levels if BMP are implemented (E>B×D). This BMP selection procedure is summarized in figure2. While, somewhat equivalent to a load reduction approach, these metrics-based methods remain fundamentally different from Australian guidelines and can hardly be considered as "criterion-based". Indeed, whereas difficulties would have arisen from the use of mass unit pollution control targets (cf. 4.2.); loads calculation is here simplified and directly integrated to BMP design and selection tools. This example once more illustrates that, for a given management criterion, guidance documents given to developers might be a decisive factor in stormwater facilities design and LID implementation. Interestingly, DWA's standardized approach furthermore allows for both emission and immission control as BMP selection is based on both the vulnerability of receiving waters and catchment characteristics, which is relatively uncommon in stormwater regulations. Nonetheless, such a method may be regarded as a "black-box" since it does not provide any scientific rationale for the determination of emission or BMP efficiency values.

FROM MANAGEMENT CRITERIA TO LID IMPLEMENTATION

Several stormwater management criteria have been identified. While their relevance for pollution control has been discussed from a relatively theoretical standpoint, their relationship to low impact development implementation (through BMP selection and design) has to be investigated

1. Preferring permanent interception over capture and release

Flow-rate criteria and their relevance for on-site management strategies have been investigated by [START_REF] Petrucci | La diffusion du contrôle à la source des eaux pluviales urbaines[END_REF] who found that sizing methods often "do not guarantee […] systematic implementation of LID solutions". Indeed, flow-rate control necessarily requires temporary storage of runoff volumes; detention facilities are therefore the easiest way to comply with these criteria. In Hauts-de-Seine county (France), where stormwater management rules consist in 2 to 5 l/s/ha flow-rate limitation and sizing approach is mainly based on a storage volume calculation (CG92 2010), underground or mineral detention facilities remain very widespread [START_REF] Lehoucq | Bilan et perspectives sur la gestion des eaux pluviales à la parcelle sur le territoire des Hauts-de-Seine -Points forts, réticences et axes d'amélioration [Review and prospects of stormwater source control in the Hauts-de-Seine county -st rong spots, r. Novatech 2013[END_REF], although probably not suitable for volume and pollutant loads reduction.

Unlike flow-rate limitation, "water quality volume" definition implies that captured volume should not only be detained but also treated, and should therefore probably promote practices that are more suitable for pollution control. These criteria are however often completed with time-to-drain requirements. Expecting a 25mm runoff volume to drain within 24 hours would for instance be similar to a 3 l/s/ha outflow rate limitation (assuming constant outflow rate). While permanent interception would generally require large seepage surfaces to promote infiltration and evapotranspiration (so as to comply with volume drawdown requirements), developers may hence be more likely to adopt the most compact stormwater practices, based on treatment rather than infiltration.

Inversely, if volume reduction policies were adopted, runoff management in small and decentralized LID practices could presumably become a relevant and cost effective solution for both runoff and pollutant control.

Indeed, green roof implementation, which is a typically distributed runoff reduction method, could probably be preferred to underground detention facilities, and become like in Philadelphia a "tool of choice" for space constrained developments (Horwitz-Bennett 2013). However, demonstrating compliance with "volume reduction" criteria may remain difficult for practitioners and development of easy-to-use methods may be needed to guarantee optimal BMP design (PBES 2008).

2. Uncertainties associated with BMP design from non-hydrologic criteria

Assessing pollutant removal efficiency of LID practices is generally difficult and simplified methods are hence often given to developers for BMP design so as to demonstrate compliance with loads reduction criteria. While such approaches do not require extensive knowledge about processes associated with pollutant removal, these may however be questionable.

New Jersey guidelines assume that TSS removal rates can be calculated from median efficiencies given for various BMPs [START_REF] Balascio | A survey of storm-water management water quality regulations in four Mid-Atlantic States[END_REF]. Corresponding pollutant removal rates are hence supposed to be achieved as long as these practices are sized for "water quality volume" treatment (NJDEP 2009). While such an approach recognizes that treatment of a given volume may not result in same TSS load reduction in every BMP, calculated rates remain approximate and arbitrary (Balascio and Lucas 2009) (as they do not consider sediment characteristics or BMP design). Similarly, in Australia and Germany, average pollutant removal efficiencies associated with LID solutions are expected under specific design conditions. In Greater Brisbane, 1.5% of drainage area for bio-filtration filter media area is for example "deemed to comply" with water quality objectives for smallest developments (SEQ-HWP 2010). In other regions, stormwater management manuals integrate "performance curves", linking BMP's size parameters (e.g. length or area) to annual TSS or nutrient removal rates [START_REF] Ntdpi | Water Sensitive Urban Design: Technical Design Guidelines[END_REF][START_REF] Tcc | WSUD Technical Design Guidelines for the Coastal Dry Tropics[END_REF]. In Germany, although an indicator-based system is used, design approach remains very similar to Australian "deemed to comply" solutions, with treatment values given for various practices and design options (DWA 2007). As mentioned previously, many elements in treatment BMP design, like filter media composition in bio-retention systems, or vegetation in sedimentation devices, may influence their pollutant removal efficiency [START_REF] Scholes | A systematic approach for the comparative assessment of stormwater pollutant removal potentials[END_REF][START_REF] Davis | Bioretention Technology: Overview of Current Practice and Future Needs[END_REF]. Direct relationship between pollutant removal and BMP's size assumption may therefore only be relevant under strict design conditions.

Alternative design approaches in Australia include use of modeling software MUSIC, for larger developments (SEQ-HWP, 2010). Nevertheless, although presumably more satisfactory, modeling may not systematically be used by practitioners and believably requires sufficient knowledge from users, as processes associated with runoff pollution remain poorly understood [START_REF] Gromaire | Operational use of urban drainage pollutant load models -Results from a French survey[END_REF].

3. A need for better guidance to promote LID practices

The example of non-hydrologic criteria indicates that the existence of simplified methods is generally a fundamental prerequisite for successful stormwater practices implementation. More generally, lack of sufficient guidelines to determine if LID practices are consistent with existing rules and standards may therefore be a significant impediment to their implementation [START_REF] Roy | Impediments and solutions to sustainable, watershed-scale urban stormwater management: lessons from Australia and the United States[END_REF].

Precise flow-rate limitations may for example be somewhat rigid, as discharge rate is in any case expected not to exceed a given value. While green roofs have been demonstrated to provide flow-rate attenuation [START_REF] Carter | Hydrologic behavior of vegetated roofs[END_REF][START_REF] Stovin | The hydrological performance of a green roof test bed under UK climatic conditions[END_REF] they are for example not very likely to comply with such rules (unless they are equipped with flow-regulators) since their effect on peak flow rate remains variable from a rain event to another. Inversely, storage facilities equipped with flow-limiting devices would probably be preferred by practitioners, as flow regulator installation is the simplest way to ensure respect of design criterion. Similarly, although most conventional stormwater facilities can easily be sized from volume based criteria, it may remain difficult to account for volume reduction in non-infiltration BMP or non-structural practices. Indeed, while facilities such as bioretention basins equipped with drains or impervious liners can nevertheless provide significant runoff volume abatement through evapotranspiration from soil surface layers [START_REF] Hatt | Hydrologic and pollutant removal performance of stormwater biofiltration systems at the field scale[END_REF][START_REF] Daly | Modelling of stormwater biofilters under random hydrologic variability: A case study of a car park at Monash University, Victoria (Australia)[END_REF] and may be relevant for volume control under restrictive soils condition, their hydrologic functioning

is not yet fully predictable.

In the US, several local authorities have therefore adopted or developed innovative methods so as to promote and account for the effect of sustainable on-site and non structural stormwater management practices (MDE 2010; [START_REF] Battiata | The Runoff Reduction Method[END_REF][START_REF] Pwd | Stormwater Management Guidance Manual. Philadephia Water District -Office of Watersheds[END_REF][START_REF] Gallo | Comparing the adaptability of infiltration based BMPs to various U.S. regions[END_REF]. In Philadelphia, roofs effective imperviousness, used for runoff volume computation, may indeed be reduced when disconnected (PWD 2011). In Seattle, where both flow rate and volume control are required, a simplified "Pre-sized approach" based on a crediting system, representing the degree to which selected "pre-sized" solutions comply with management requirements, has been adopted for small developments (CS-SPU 2009). An equivalent mitigated area can therefore be directly calculated for a various LID practices. Similarly, Pennsylvania, Georgia, Minnesota or Maryland guidelines allow for reduction of volume requirements if LID solutions are adopted [START_REF] Amec | Georgia Stormwater Management Manual[END_REF][START_REF] Pdep | Pennsylvania Stormwater Best Management Practices Manual[END_REF]MDE 2010) (cf. table 5). Table 5 Illustration of volume reduction approaches for various LID practices While such crediting systems will obviously facilitate and promote LID implementation, they often rely on several simplifying hypotheses regarding BMP efficiency for runoff and pollution control [START_REF] Battiata | The Runoff Reduction Method[END_REF].

Definition of management criteria and guidance, balancing scientific validity and ease of use, could therefore be a real challenge for regulators and environmental agencies.

CONCLUSION AND PERSPECTIVES

Four categories of criteria for on-site runoff pollution control, namely flow-rate limitations, "water quality volumes", volume reduction and non-hydrologic targets have been identified and analyzed based on a review of management practices adopted in different countries. Regarding the relevance of these stormwater management criteria for on-site pollution control, this study suggests that:

• "Treatment and release" strategies may not be relevant for moderately contaminated runoff. As a consequence, volume reduction (through infiltration, evapotranspiration or re-use) should generally be (e.g. where feasible) the preferred approach for on-site pollution control.

• Determination of an "optimal" permanent interception volume remains an open question and further investigations are required to better understand the relation between targeted volume and long-term pollutant load removal and find out whether standardized sizing approaches based on a single criterion are relevant for on-site pollution control.

• While volume reduction can be expected to assure minimum pollution control, the benefits associated with pollutant removal processes should still be considered for increased BMP efficiency. A better understanding of these "de-pollution" processes is thus needed to correctly address requirements such as maximum effluent concentrations which may arise from environmental standards or to provide acceptable pollution control for more contaminated urban surfaces.

• Volume reduction is typically an emission control strategy which does not account for the vulnerability of receiving waters. While recommendation about volume reduction targets may be provided, it is essential to acknowledge that design criteria should be adapted to site-specific requirements to meet environmental quality objectives.

Besides, Low Impact Development implementation and proper BMP design not solely depend on stormwater management criteria, but also on guidance documents provided to practitioners. The lack of technical expertise is indeed the major factor in the persistence of traditional management and design approaches at the local scale.

Diffuse pollution control is a relatively recent stormwater management objective for local communities that have traditionally focused on flood or CSO control. Demonstrating that volume reduction is compatible with existing flow-rate based requirements and provide benefits for both peak-flow and pollutant control is thus crucial for a wide implementation of LID practices. Finally, suitable design methods should be believably developed so as to deal with site specific constraints and to overcome difficulties arising from demonstration of compliance with stormwater management requirements.

  rainfall distribution). This statistical analysis proves to lead to very different values depending on rainfall event definition. As shown in Figure1, using a 15 year long rainfall record (5-min time-step) from Paris urban area with different Minimum Inter-event Times (MIT) between non-zero precipitation records resulted in water quality volumes ranging from 8mm for a 3h MIT to 21mm for a 24h MIT (cf. figure1).

Figure 1

 1 Figure 1 Calculated volume targets depending on rain event definition (MIT = Minimum Intra-event Time) for a 15 year rainfall record from Paris urban area using Auckland Regional Council procedure (ARC 2010a)

Figure 2

 2 Figure 2 BMP selection process for pollution control according to German standards DWA-M-153 (DWA 2007)

"

  Minimizing soil compaction" ∆V = A × 6.4 / 1000 Where: ∆V = net volume reduction (m 3 ), A = Area of minimal soil compaction (m²) Maryland (MDE 2009) "Grass swales"" P E = 254 × SA / DA Where: P E = equivalent rainfall volume managed in the swale (mm), A = drainage area, SA = swale surface area Georgia (AMEC and CWP 2001) "Rooftop disconnection" ∆V = 30.5 × C × (A-DIA) Where: ∆V = net volume reduction (m 3 ), C = runoff coefficient, A = site area (m²), DIA = disconnected impervious area(m²)

Table 1

 1 table1). Illustration of "water quality volume" criteria for various communities -Volume targets are expressed as rainfall depth unless specified.The water quality volume is generally established from the analysis of long-term rainfall records (although design storm approaches may as well be adopted). Many authorities such as Auckland Regional Council or Iowa Department of Natural Ressources(IDNR 2003; ARC 2010a;[START_REF] Mddep | Guide de Gestion des Eaux Pluviales: Stratégies d'Aménagement, principes de conception et pratiques de gestion optimales pour lesréseaux de drainage en mileu urbain[END_REF] thus indicate that water quality volume can be computed by identifying a rainfall depth that include up to 80 or 90% of monitored events. In this case, captured volume is however not necessarily equal to 80 to 90% of annual runoff volumes (as depending on

	Country/Community	Volume	Details
		targets
	US
	Georgia (AMEC and CWP 2001) (regulatory)	31 mm	Storage volume = corresponding runoff depth
	Maryland (MDE 2009) (regulatory)	23 to 25 mm	Storage volume = corresponding runoff depth
	New-Jersey (NJDEP 2009) (regulatory)	32 mm	Design storm approach
	Canada
	Québec (MDDEP 2012) (non-regulatory)	25 mm	Design storm approach
	Alberta (AEP 1999) (non-regulatory)	25 mm	Storage volume = corresponding runoff depth
	England
	National guidelines (CIRIA 2007) (non-regulatory)	10 to 15 mm	Storage volume (stormwater ponds only)

Table 2

 2 temporal variability of concentrations in runoff). Furthermore, volume reduction criteria are either accepted as daily performance targets or management objectives for a design storm (cf. table 2); determination of corresponding storage volume is hence not straightforward and proper BMP design presumably requires providing sufficient guidance to practitioners to ensure that volume reduction objectives are met. Illustration of "volume reduction" criteria for various communities -Volume targets are expressed as rainfall depth unless specified

	evaporated volumes are entirely mitigated while additional pollution control may be obtained from treatment
	processes like filtration, adsorption or sedimentation. As for water quality volume criteria, the amount of water
	to be captured nevertheless significantly differs from a community to another (cf. table 2), and definition of
	performance targets, when justified, only relies on simple statistical rainfall analysis, similar to that described in
	3.2.1 (BC-MWLAP 2002) or estimation of predevelopment infiltration volumes. Indeed, although British-
	Columbia guidelines indicate that such analysis is sufficient for setting performance targets (and continuous

CSO (HBCA 2010; SyAGE 2013)

, although national and regional agencies' guidance documents indicate that infiltration should generally be preferred for on-site pollution control

[START_REF] Certu | Procédures d'autorisation et de déclaration des projets d'aménagement au titre du Code de l'Environnement. Rubrique 2.1.5.0 -Rejets d'eaux pluviales -Repères à destination des services instructeurs de la police de l'eau et des milieux aquatiques[END_REF][START_REF] Driee | Doctrine relative à l'instruction des dossiers de rejets d'eaux pluviales dans le cadre de la police de l'eau (rubrique 2.1.5.0.) et de la police des ICPE[END_REF]

. Likewise, some states in the US require a fraction of the water quality volume to be infiltrated to maintain pre-development groundwater recharge and to preserve water table elevation, but this strategy is not systematically associated with pollution control

[START_REF] Vanr | [END_REF] NJDEP 2009)

. Conversely, in Oregon, New-York State or British Columbia in Canada runoff volume reduction pertains to both stormwater quantity and quality management.

Chilliwack policy and design criteria manual states that "reducing volume at the source -where the rain falls -is the key to protecting […] water quality" (CH2MILL 2002). Likewise, it can be found in Portland Stormwater Management Manual that "infiltrating stormwater on site […] is a multi-objective strategy that provides a number of benefits including […] pollution reduction […]"

(PBES 2008)

.

"Volume reduction" strategies probably provide less variable pollution control and could presumably be preferred over treatment criteria (e.g. "water quality volume"), as pollutant loads corresponding to infiltrated or modeling is thus not always needed) (BC-MWLAP 2002), definition of an optimal "volume reduction" criterion for pollution control is arguably complex, as pollutant loads and runoff volume abatements may not be equal (because of

  ). Similarly, Australian TSS, TN or TP reduction targets are often similar to most BMP's median efficiencies reported by Center for Watershed Protection (CWP 2007).

	City of Logan (LCC 2013) (regulatory)	80%	55%	45%	90%
	City of Melbourne (CM 2006) (regulatory)	80%	45%	45%	70%
	Authority	TSS	Phosphorous	Nitrogen	Gross pollutant
	Parramatta Catchment (URSA 2003) (non-regulatory)	50 to 80%	45%	45%	70%
	South East Queensland 1 (MBWCP 2006) (non-regulatory)	80%	60%	45%	90%
	State of Victoria (VSC 2006) (regulatory)	80%	45%	45%	70%
	City of Townsville (TCC 2011) (regulatory)	80%	60%	45%	90%

ACKNOWLEDGEMENTS

This research was carried out under the OPUR research program. We acknowledge OPUR partners and the French Ministry of Ecology, Sustainable Development and Energy for financial support.


Nikos Bozionelos
email: nbozionelos@audencia.com

Sumona Mukhuty
email: s.mukhuty@hull.ac.uk

Why CEOs Resign: Poor Performance or Better Opportunities?

Research Questions

Why do CEOs resign? This is an important question given the impact CEOs have economically and in society [START_REF] Hambrick | Toward more accurate contextualisation of the CEO effect on firm performance[END_REF]. A common view is that CEOs resign because shareholders and the board want them to leave because of lower-thanexpected performance. The implication here is that power resides with shareholders and the board, with CEOs sitting around passively waiting for their fates to be decided. After all, CEOs are compensated extremely well for delivering results. So it seems only fair that their positions are precarious and in the hands of shareholders and board members. Yet in many cases, the reasons CEOs resign are murky and appear to have little to do with performance.

Consequently, in a recent paper, Yun Liu from the University of California at Riverside Director Gender and Firm Acquisition Behavior 2 challenged the dominant notion that CEO resignations are shareholder and board driven.

Instead, Liu attempted to shed more light on why CEOs resign by adopting their perspective. Specifically, Liu argues that CEOs often resign not because they are forced out, but because they find better deals elsewhere. Liu based his premise on employment contracting theory, particularly its idea of "participation constraint." Basically, the notion is that someone will stick with a current employment contract as long as he or she believes they are not worse off relative to outside opportunities. When, and if, someone perceives that better prospects exist elsewhere, then the current contract may be abandoned. In the case of CEOs, this simply means that CEOs may leave because they can land better deals in other firms or institutions.

But since executive and director positions are rarely advertised and few and far between to begin with, how do CEOs obtain information about such opportunities elsewhere? Liu believes that access to such information comes from CEOs own personal networks or "connectedness." We know that connectedness generally plays a role in finding employment and moving up the corporate ladder (Bozionelos, in press). However, Liu considers that connectedness is particularly important for CEOs because most information on executive openings is not publicly available. Furthermore, being an executive and overtly looking for employment opportunities involves some risk (e.g., one's motives and loyalties may be questioned). Finally, unlike most other types of hiring, there is very little margin for error when it comes to CEOs-an outstanding fit between candidate and position is imperative.

Consequently, connectedness may be crucial both for executives on the lookout for new opportunities and for firms seeking to fill executive positions.

In a nutshell, Liu developed and tested the idea that CEOs with richer networks are more likely to leave their firm for an opportunity elsewhere.

Study Design & Method

Director Gender and Firm Acquisition Behavior 3 Liu used the Board Ex database to extract detailed information on more than 7,500 CEOs from nearly 5,000 publicly held firms over an 18-year period (1991-2009). During this timeframe, nearly 3,000 CEO resignations took place in the firms studied.

Liu also used the database to construct the personal networks or connectedness of these CEOs. Subsequently, Liu linked these data with data from other databases, such as Compustat and CRSP, which provide financial information on the firms involved. On average, each CEO had ties with 12 other executives in each given year. Liu measured connectedness with an index that reflects the richness or variety of information CEOs have access to. The more varied the information a CEO can access via his or her ties with directors outside and inside the firm, the greater their connectedness. Liu investigated the relationships between CEOs' connectedness and their mobility up to two years after their resignations from their current position. In his calculations, Liu took into account characteristics of both firms and of CEOs themselves (such as education and personality). As a result, he was able to isolate the influence of connectedness on mobility and produce generalizable conclusions.

Liu identified three types of destinations for CEOs who resigned. About two thirds of these CEOs took positions internal to the firm, usually chairman of the board or executive director. About 20% took full-time positions outside the firm--normally CEO positions in other firms. Finally, almost a third of the sample took part-time positions outside the firm, normally non-executive directorships in other firms. There was also a respectable proportion of CEOs (about 14%) who took no new position after resigning. An interesting finding was that those aged 60 and above were more likely to hold a new position two years after resigning than those below 60! This illustrates that CEOs who leave at "normal" retirement age (that is 60 and above) remain very active in their work lives, perhaps more active that their much younger counterparts.

Director Gender and Firm Acquisition Behavior 4

Key Findings

Liu's findings confirmed that the greater the CEO connectedness the greater the probability of CEO resignation. Indeed, for every one standard deviation increase in connectedness the chance of CEO resignation increased roughly 10%. This was in line with Liu's idea that CEO resignations are often driven by their connectedness. Interestingly, connectedness with other executives inside the firm reduced the probability of CEO resignation. So on the one hand, richer links with executives in other firms seem to increase the likelihood that CEOs will leave while stronger connections within the firm seem to make it less likely. That said, the results also showed that a substantial proportion of CEO resignations were indeed linked to poor firm performance, which is in line with common perceptions about why CEOs leave.

Next, Liu tested whether resignations were related to the nature of external opportunities available (full-time or part-time) via connectedness. His results showed that CEO connectedness increased the probability of resignation for a full-time position elsewhere (such as becoming CEO in another firm) only for CEOs below 60 years of age. Similarly, a decline in firm performance was associated with increased probability of CEO resignation, but again mostly for "young" CEOs. These findings basically mean that "young" CEOs are more likely to use their connections in order to find more alluring CEO positions in other firms. On the other hand, the reverse pattern was found for part-time positions. In short, "old" CEOs were more likely to use their connections to become non-executive directors in other firms. Such positions include good financial packages and prestige, but are less time demanding-perhaps ideal for CEOs at retirement age.

So the results clearly suggested that connectedness may help identify alternative opportunities for CEOs, prompting them to resign to take CEO positions in other firms or to retire and take a non-executive position somewhere else. Naturally the next issue that Liu Director Gender and Firm Acquisition Behavior 5 examined was which explanationoutside opportunities or poor performance -weighs heavier in CEO resignations. Importantly, his analysis showed that connectedness increased the probability of resignation to seek a CEO or non-executive position somewhere else more than poor performance did.

In the final step, Liu investigated whether connectedness was related to probability of resignation under particular conditions of firm performance. As it turned out, while CEO connectedness was related to the probability of CEO resignation under all performance conditions (poor, medium and strong), the relationship was especially strong when firms were performing poorly.

According to Liu this may mean that well-performing CEOs have less need to rely on their connections to move to other firms because their records will "speak for themselves."

On the other hand, poor performers must counterbalance their poor outcomes with additional information if they are to move on--and their networking serves as a conduit for transmitting such information. Furthermore, Liu pointed out that high performing CEOs may find it best to continue with their current firms because those firms are likely to reward them with even better conditions and pay. On the other hand, poorly performing CEOs may have limited futures within the firm and may also experience internal pressure to resign. This may leave them with few alternatives other than any outside opportunity that they learn about through their networks.

Conclusions & Implications

CEO turnover is an important corporate event. Liu explored a new dimension in CEO resignation by taking the perspective of CEOs themselves and considering them as actors that shape their own future rather than as passive objects who simply react to board demands and decisions. His findings suggested that options outside their current firm influence CEOs resignations beyond their performance. This means that in many cases CEOs leave not Director Gender and Firm Acquisition Behavior 6 because they are fired but because their networks with other executives help them uncover better opportunities outside their firms.

Overall, well-connected CEOs appear more likely to leave their firms than poorly connected ones. In addition, poor performing CEOs seem especially keen to use their networks to find external jobs, probably because of limited internal opportunities and pressure to leave. And while well-performing CEOs with richer networks may also leave, they will likely have more to gain by staying put.


Nikos Bozionelos
email: nbozionelos@audencia.com

Marianne Afanassieva
email: m.afanassieva@hull.ac.uk

Does Having More Women on the Board Impact Firms' Acquisition Behavior?

Research Questions

Is there a difference between how women and men behave as board directors when they are deciding whether to acquire other firms? And do boards with larger representations of women act differently when they consider buying other firms and deciding how much to pay?

Women and men may differ in ways that are manifested in certain behavioural differences. For example, at the risk of generalizing, men may tend to be somewhat more assertive that women in certain situations. That said, identifying clear gender differences in economic behaviour has proved elusive. For instance, some research indicates that women directors are less prone to financial risk-taking and more reliable in terms of their obligations than their male colleagues. On the other hand, there has been no convincing evidence to date illustrating actual behaviour differences between men and women serving as board members.

Board behaviour is important to understand since they are associated with key firm outcomes, such as the decision to acquire another firm and figure out how much to pay in the Director Gender and Firm Acquisition Behavior 2 2 process. Indeed, a better understanding of mergers and acquisitions (M&A) would give us a better grasp of their economic impact and help us tease apart why so many fail or do not add shareholder value. Moreover, the extent to which the presence of women on the board of directors influences major outcomes for firms may be critical, especially since women are slowly but surely increasing their presence on corporate boards.

One reason why good insights about the impact of board gender composition are lacking is that short-term outcomes or day-to-day activities really do not reveal the true impact of the board. Instead, it may be better to focus on major decisions with long-term consequences that permit clearer evaluations of the board's behaviour and decision-making.

Of course, bidding to acquire other firms is an example of such "major decisions." Indeed, acquisitions involve board-level discussions where individual directors play significant roles. This is exactly what Maurice Levi and Kai Li from the University of British Columbia along with Feng Zhang from the University of Utah wanted to study. They used M&A information to discover whether female board representation is associated with differences in firms' acquisitions behaviour. In particular, Levi and his colleagues investigated whether the fraction of female directors on a board is related to a firm's propensity to bid for other firms as well as the premiums it offers to pay. M&A accounted for nearly US$4 trillion of economic activity worldwide in 2014, meaning that these board outcomes are of prime importance.

Levi and his colleagues expected that larger proportion of female directors on a firm's board would be associated with lower firm acquisitiveness (i.e., a weaker propensity to acquire other firms). They saw this expectation in line with other research suggesting that women are sometimes less overconfident that men in business settings. Overconfidence, which can play a major role in bidding behaviour, has two dimensions: 1) certainty about the precision of one's views about the future; and 2) the belief that future outcomes will be favourable. Women generally score lower than men on both dimensions-they tend to be less certain of the future and less optimistic about future outcomes. According to Levi and his colleagues, this should result in women viewing the same acquisition as less attractive than would their male counterparts. In turn, this would render boards with more women, everything else being equal, less likely to initiate acquisition bids. Levi and his colleagues also hypothesized that the larger the representation of women on a bidding board, the lower the premium of the bid. This expectation was also based on the potential gender gap in overconfidence. In short, women directors would tend to attach lower value to firms targeted for acquisition than would their male counterparts.

Study Design & Method

To answer their questions, Levi and his colleagues collected data on board composition and bidding behaviour from all 1500 firms in the Standard & Poor's index over a 12 year period (1997 to 2009). On average, nearly 10% of the directors on these boards were women.

Other studies reported a lower figure, suggesting that female board representation has been slowly rising. Overall, from 1997-2009 the firms studied made nearly 20,000 acquisition bids, with 5,301 resulting in actual deals. Levi and his colleagues took into account firm and board size, along with various aspects of firm's performance and financial situation, such as sales growth, return on assets, and the degree of leverage. This means that their conclusions were generalizable to firms of all sizes and financial circumstances.

Key Findings

The results confirmed that boards with higher percentage of women members tended to make fewer acquisition bids. In particular, every 10% increase in female board membership (which roughly corresponds to one additional woman director) was associated with a 7.5% decrease in the number of bids. Levi and his colleagues performed additional analyses to account for two other factors that could have contaminated their results: 1) firm maturity (i.e., older, more mature firms may be more cautious and less likely to initiate acquisitions); and 2) CEO aggressiveness or the so called "testosterone" effect (i.e., younger male CEOs who may be itching to "make their mark" and more prone to initiate acquisitions). However, neither of these factors altered the findings in any way. Firm maturity and CEO aggressiveness notwithstanding, having greater female board representation still meant acquisition behaviour was less likely.

Moreover, the results showed that the higher the proportion of female board members, the smaller the premium that was offered in bids that were made. The bid premium represents the additional amount (in percentage terms) -or "how much more"relative to the actual value of the target firm the bidding firm offers. To illustrate, a bid premium of 10% means that the acquiring firm has paid 10% more than what was the actual value of the target firm.

The calculations of Levi and his colleagues indicated that for every 10% increase in the proportion of female directors (or roughly for each additional female director on the board) the bid premium dropped by 15.4%, which represents a sizeable amount in financial terms. It is worth noting that in performing these calculations, not only were the characteristics of the bidding firms taken into account, but also the characteristics of the target firms. This means that these findings are generalizable pretty much across all types of organizations.

Finally, one analysis indicated that greater gender diversity on the board was linked with longer time to reach a resolution about a bid. This implies that women's presence on a board leads to fewer but better quality decisions. It supplements the explanation that women tend to suffer from less overconfidence and that greater representation of women on a board is therefore likely to result in more prudent board behaviour.

Conclusions & Implications

There is a slow but steady increase when it comes to women on corporate boards. This increase is likely to accelerate due to legislative requirementsalready in place in certain countries such as Norway (requiring at least 40% female corporate board membership). The work of Levi and his colleagues suggests that presence of more women is associated with substantial changes in a board's acquisition behaviour. Overall it seems that boards with more women take a more prudent approach-they are less likely to target other firms for acquisition in the first place, and once they do, are less likely to overpay. In essence, women seem to be less motivated by sheer empire building and, as a consequence, having greater female board representation means the board is less likely to make decisions that destroy shareholder value.

Levi and his colleagues conclude that behavioural differences that occur when gender diversity increases on boards may be attributed to two factors that are not necessarily mutually exclusive. First, women are less overconfident, meaning that they are less likely to hold an over-optimistic view of the future and their capabilities to forecast and influence it.

Indeed, overconfidence at the top is associated with lower quality acquisition decisions [START_REF] Huang | Gender and corporate finance: Are male executives more overconfident relative to female executives[END_REF][START_REF] Malmendier | Who makes acquisitions? CEO overconfidence and the market's reaction[END_REF]. Second, diverse boards may be more thorough and methodical in their approach and evaluation when are considering an acquisition.

Regardless, Levi and his colleagues offer us a strong set of arguments for increasing gender diversity on corporate boards, including better acquisition decisions and better consequent shareholder value. And these are arguments that go beyond discussions of social Director Gender and Firm Acquisition Behavior 6 6 responsibility, equality and social justice. Indeed, they are arguments that corporate stakeholders and their bottom-line interests should find attractive if not compelling.