Angelique Brathwaite

Nicolas Pascal

Eric Clua

When are payment for ecosystems services suitable for coral reef derived coastal protection?: A review of scientific requirements

Keywords: Payment for Ecosystem Services, Coastal Protection, Coral Reefs, Caribbean

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Payment for Ecosystem Services (PES), a tool for managing ecosystems by providing positive incentives for behavioural changes [START_REF] Bladon | Payments for ecosystem services in developing world fisheries[END_REF] has been touted as "the next best thing" for filling the conservation financing gap [START_REF] Fujita | Ecomarkets for conservation and sustainable development in the coastal zone[END_REF][START_REF] Waylen | Surveying views on Payments for Ecosystem Services: Implications for environmental management and research[END_REF]. The suitability of this scheme for marine application and in particular coastal protection, which is often rated among the most important services provided by coral reefs, is however unclear [START_REF] Moberg | Ecological goods and services of coral reef ecosystems[END_REF][START_REF] Burke | Coastal capital: economic valuation of coral reefs in Tobago and St. Lucia[END_REF][START_REF] Mehvar | Quantifying economic value of coastal ecosystem services: a review[END_REF].

PES is based on Ecosystem Service (ES) science, a relatively new field which seeks to link science, economics, conservation management and economic development [START_REF] Braat | The ecosystem services agenda:bridging the worlds of natural science and economics, conservation and development, and public and private policy[END_REF]. At its simplest, Ecosystem Services are defined as "benefits people obtain from ecosystems" (Millennium Ecosystem Assessment 2005).

Looking at ecosystems through the lens of services provided to humans, allows for their value (economic and intrinsic) to be clearly highlighted. PES then goes a step further and utilizes quantifications of these services, as a base, to devise payments between buyers and sellers of the service. This is once the agreed upon improvements to the flow of services or management of the ecosystem are provided. The link between conservation action, service flow and payment is therefore made clear [START_REF] Ingram | Evidence of payments for ecosystem services as a mechanism for supporting biodiversity conservation and rural livelihoods[END_REF].

Coastal protection in this paper refers to the ability of coral reefs to protect beaches from erosion by absorbing and dispersing significant quantities of wave energy [START_REF] Kushner | Coastal Capital: Jamaica. Coral Reefs, Beach Erosion and Impacts to Tourism in Jamaica[END_REF][START_REF] Storlazzi | Rigorously valuing the role of US coral reefs in coastal hazard risk reduction[END_REF]. This attenuation of wave energy allows for reductions in shoreline erosion, flooding, damage to coastal infrastructure and loss of life. The service can be characterised as the amount of attenuation that can be attributed to the reef or to the increase in wave energy due to reef deterioration. Coral health is declining considerably with both local and global stressors working synergistically to negatively impact the ecosystem [START_REF] De'ath | The 27-year decline of coral cover on the Great Barrier Reef and its causes[END_REF][START_REF] Jackson | Status and trends of Caribbean coral reefs[END_REF] resulting in a diminishing of the service and its value (Mumby et al., 2 2014; [START_REF] Weijerman | Managing local stressors for coral reef condition and ecosystem services delivery under climate scenarios[END_REF].This trajectory is expected to continue [START_REF] Maynard | Projections of climate conditions that increase coral disease susceptibility and pathogen abundance and virulence[END_REF] with predictions of increasing climate change induced risks to ecosystems [START_REF] Pachauri | IPCC, 2014: climate change 2014: synthesis report[END_REF].

In spite of the variety of management measures implemented, such as ecosystem based management, integrated coastal zone management, marine spatial planning and watershed management [START_REF] Mcleod | The future of resilience-based management in coral reef ecosystems[END_REF] coral reef health continues to decline and inadequate finance has been identified as a cross-cutting factor, undermining conservation action [START_REF] Bladon | Payments for ecosystem services in developing world fisheries[END_REF][START_REF] Gill | Capacity shortfalls hinder the performance of marine protected areas globally[END_REF]. Private sector financing mechanisms for coral reef conservation are scarce (Pascal et al., 2018;[START_REF] Meyers | Conservation finance: a framework[END_REF]. However PES might provide a way for conservation funds to be generated from non-public sources [START_REF] Wunder | Taking stock: A comparative analysis of payments for environmental services programs in developed and developing countries[END_REF][START_REF] Bos | Marine conservation finance: The need for and scope of an emerging field[END_REF] if coastal protection can meet PES requirements.

With no examples found of PES schemes for coastal protection in the peer reviewed literature, our paper aims to fill this knowledge gap and determine if there is adequate scientific knowledge of the provision of the ecosystem service by coral reefs, to develop a PES scheme. This is with the knowledge that social and financial structures also need to be put in place for a PES system to be implemented, and that the ecological parameters provide the foundation on which other elements (e.g., negotiations of agreements, legal structure and financing) are built.

Our focus for PES development is on Caribbean coral reefs, which are considered globally to be among the most threatened [START_REF] Gardner | Long-term region-wide declines in Caribbean corals[END_REF]. At the same time, these reefs are capable of generating huge amounts of revenue from reef associated tourism, estimated at more than USD$7.9 billion [START_REF] Spalding | Estimating Reef-Adjacent Tourism Value in the Caribbean[END_REF]. Deriving income from this sector for reef protection therefore, seems a logical course of action.

Our objectives are as follows: (i) define PES and outline the key biophysical elements required to develop a scheme; (ii) compile information on the biophysical elements required from coral reefs to provide the ecosystem service; (iii) outline the management measures on ES delivery and (iv) based on these outputs determine if the science behind both the delivery of the ES and management action is adequate for the development of a PES scheme.

Methods

We reviewed peer-reviewed journal articles using the online academic search engine SCOPUS (cut off date July 8 th , 2020). No geographical or temporal boundaries were set and key word combinations were searched within the title, abstract and keywords. Only papers and their references relevant to our objectives were assessed and additional papers were consulted as required.

Assessment# 1: Sourcing PES schemes for coastal protection. Key words -"payment for ecosystem services" and "coastal protection" and "coral reefs". We first carried out this search to gain an overall sense of what has been written about PES mechanisms for coral derived coastal protection. SCOPUS -42 articles were obtained from the search of which only 4 were directly related to developing PES specifically for coral reefs.

Assessment# 2a: Synthesising the science behind the ability of coral reefs to deliver the coastal protection ES. Key words -"coastal protection" and "coral reefs". 139 results were obtained from the search of which 104 were eliminated as coral reefs were not central to the discussion and/or the ability of reefs to provide the service was only mentioned but not further assessed. The results of the remaining 35 articles were summarised.

Assessment# 2b: The role of live coral in delivery of the service. We added the key word "live coral" to this search. 7 articles resulted, of which 2 were excluded due to lack of direct relevance (e.g., a focus on sea cages and economic valuations). A further 27 relevant papers were found in references related to reef health (e.g., maintenance of carbonate budgets). 32 articles were summarised.

3 Definition and key PES requirements PES is a market based approach, designed to provide financing for environmental management [START_REF] Waylen | Surveying views on Payments for Ecosystem Services: Implications for environmental management and research[END_REF]. The scheme is based on the principle that those who contribute to producing the service (providers) via effective conservation/management action, should be compensated, while those who benefit from the service (beneficiaries), should pay for it.

There is an ongoing debate about what is actually a PES [START_REF] Muradian | Reconciling theory and practice: An alternative conceptual framework for understanding payments for environmental services[END_REF][START_REF] Vatn | An institutional analysis of payments for environmental services[END_REF][START_REF] Moros | Pragmatic conservation: Discourses of payments for ecosystem services in Colombia[END_REF] and therefore a sliding scale of PES definitions. These range from strongly market based [START_REF] Wunder | Payments for environmental services: some nuts and bolts[END_REF]) to an overarching term for approaches that provide positive incentives for management of ecosystems [START_REF] Engel | Designing payments for environmental services in theory and practice: An overview of the issues[END_REF]. With a variety of definitions to choose from, there is also some leeway with which to fit coastal protection to PES requirements. The definition used therefore, can depend on the level of specificity obtained by service provision.

In this paper we used definitions of Wunder as both our maximum [START_REF] Wunder | Payments for environmental services: some nuts and bolts[END_REF] and minimum standards [START_REF] Wunder | Revisiting the concept of payments for environmental services[END_REF]. Wunder is acknowledged as an authority on PES, with 6 articles cited more than 8000 times between 2005 and 2020 according to Google Scholar. His 2005 definition is among the earliest and most heavily utilised [START_REF] Sommerville | A revised conceptual framework for payments for environmental services[END_REF] which he revisited in 2015. Wunder´s (2005) definition, "A voluntary transaction whereby a well-defined ES (or actions likely to secure it) is 'bought' from at least one ES provider by at least one buyer, if and only if the payment is conditional on provision," requires robust science. His 2015 definition, "Voluntary transactions (between service users and service providers) that are conditional on agreed rules of natural resource management for generating offsite services", however allows for some scientific imprecision (inherent in ecological studies) while not being so broad as to eliminate scientific accountability.

In order for participants to demand and make payments for a service, they should know (as clearly as possible) what is being bought and sold, as well as where and how it is delivered (Forest Trends et al., 2008;[START_REF] Fripp | Payments for Ecosystem Services (PES): A practical guide to assessing the feasibility of PES projects[END_REF].

Therefore, identifying, quantifying and assessing the service is key. This requires: (i) the selection of suitable indicators which are accepted by the scientific community to have impacts on service flow, and can be replicated via reliable methods, (ii) baselines against which success or failure will be measured and, (iii) the definition of spatial boundaries, so that it is clear where the service originates and where it is being delivered.

Ecosystem processes underlying service delivery also require identification, as they are crucial elements in designing conservation action to reduce threats.

Conditionality is considered the "conceptual core" of a PES [START_REF] Bladon | Payments for ecosystem services in developing world fisheries[END_REF] and is a key element separating it from "business-as-usual" schemes [START_REF] Wunder | When payments for environmental services will work for conservation[END_REF][START_REF] Ingram | Evidence of payments for ecosystem services as a mechanism for supporting biodiversity conservation and rural livelihoods[END_REF]. The term refers to the requirement for payments to be made only if the stipulated goals are met. These goals can be either measurements of services (via indicators) or management proxies (accepted by the scientific community to have impacts on service flow). The setting of goals is an important element that should be identified early in the process. In many cases, goals are set based on the degree of technical challenges, such as data collection and the ability to quantify the service, and costs [START_REF] Sommerville | A revised conceptual framework for payments for environmental services[END_REF]. Proving conditionality is difficult and it is often the un-met criterion of PES [START_REF] Muradian | Reconciling theory and practice: An alternative conceptual framework for understanding payments for environmental services[END_REF][START_REF] Lau | Beyond carbon: Conceptualizing payments for ecosystem services in blue forests on carbon and other marine and coastal ecosystem services[END_REF] with issues due in part to reliance on continued monitoring from an established baseline.

Additionality is identified as an advantageous but not critical parameter [START_REF] Wunder | Payments for environmental services: some nuts and bolts[END_REF][START_REF] Muradian | Reconciling theory and practice: An alternative conceptual framework for understanding payments for environmental services[END_REF].

The term refers to the measurement of an intervention´s impact, relative to no intervention being made and translates therefore to the added benefit of having a PES [START_REF] Tacconi | Redefining payments for environmental services[END_REF]. Additionality is another difficult parameter to measure, and requires not only the establishment of baselines, but also counterfactual analysis.

Such examination allows for comparison of the impact of a scenario in which there was no PES, to a PES situation, in order to prove increased benefits [START_REF] Wunder | Payments for environmental services: some nuts and bolts[END_REF].

In theory, payments are triggered by evidence of service provision or improvement to the service. However, in reality, such results-based payments are difficult to assess and compounded by time lags between intervention and results, and between monitoring and verification. An alternative is the use of a management proxy, with payments being based on evidence of changes to harmful practices or the implementation of actions proven to assist conservation [START_REF] Atmodjo | Financing marine conservation tourism: Governing entrance fees in Raja Ampat, Indonesia[END_REF]. The proxy essentially provides an escape clause, especially for situations where service provision cannot be quantified, whether though lack of data, resources or process.

All PES schemes provide incentives to those who own (or are responsible for) specific areas to maintain, restore or enhance ecosystem services [START_REF] Moros | Pragmatic conservation: Discourses of payments for ecosystem services in Colombia[END_REF]. The rationale behind development of the scheme can however determine which elements are most important, for example in those instances where PES is used to reward conservation action (environmental stewardship), additionality is not paramount [START_REF] Swallow | Compensation and rewards for environmental services in the developing world: framing pan-tropical analysis and comparison[END_REF].

Key requirements for PES schemes are summarised in Table 1. 4 Synthesis of knowledge on the ecological processes of coral reefs involved in coastal protection

Coastal Protection is a complex ES that depends on coral reefs, acting in concert with a number of other biotic and abiotic factors (Figure 1) [START_REF] Burke | Coastal capital: economic valuation of coral reefs in Tobago and St. Lucia[END_REF][START_REF] Elliff | Coral reefs as the first line of defense: Shoreline protection in face of climate change[END_REF].

Figure 1: Natural features that interact to deliver coastal protection. Shapes in orange represent coral reefs and their different ecological parameters. Shapes in blue represent waves and their physical processes.

Shapes in green represent other biophysical features that can impact service provision. Red represents natural hazards that impact all natural processes.

Coral reefs are comprised of thin veneers of live corals, growing in decadal timeframes, on top of massive depositions of their calcium carbonate skeletons [START_REF] Kuffner | A geological perspective on the degradation and conservation of western Atlantic coral reefs[END_REF]. Both living and non-living sections are important to coastal protection, as is described in Sections 4.1 to 4.3. Reefs attenuate wave energy, reducing their height, energy and velocity as they move from deeper to shallower waters [START_REF] Gourlay | Wave-generated flow on coral reefs-an analysis for two-dimensional horizontal reef-tops with steep faces[END_REF][START_REF] Lowe | Spectral wave flow attenuation within submerged canopies: Implications for wave energy dissipation[END_REF]. Morphology across entire reef profiles affects the process [START_REF] Yao | Effects of reef morphology variations on wave processes over fringing reefs[END_REF].

Fringing reefs are often responsible for coastal protection [START_REF] Van Zanten | Coastal protection by coral reefs: A framework for spatial assessment and economic valuation[END_REF] and are also most heavily affected by anthropogenic impacts [START_REF] Mumby | Towards Reef Resilience and Sustainable Livelihoods: A handbook for Caribbean coral reef managers[END_REF]. Their morphology is typically characterized by a forereef slope that terminates at a shallow reef crest and a relatively horizontal reef flat which continues to the coast (Figure 2) [START_REF] Yao | Effects of reef morphology variations on wave processes over fringing reefs[END_REF]. Variations in species, geology and hydrodynamic conditions result in high variability between different coral reefs and therefore their effectiveness at providing the service [START_REF] Quataert | The influence of coral reefs and climate change on wave-driven flooding of tropical coastlines[END_REF].

Figure 2: Cross section of a fringing reef. Image from (U.S. Geological Survey Fact Sheet 025-02). Retrieved September 16 th , 2020.

Provision of the coastal protection ecosystem service

Both anecdotal and scientific data support the fact that coral reefs protect shorelines [START_REF] Wells | In the front line: shoreline protection and other ecosystem services from mangroves and coral reefs[END_REF][START_REF] Principe | Quantifying Coral Reef Ecosystem Services[END_REF][START_REF] Ferrario | The effectiveness of coral reefs for coastal hazard risk reduction and adaptation[END_REF]. A total of 139 papers were found in the SCOPUS database on this topic, and while only 35 actually assessed the role of the reef in service provision, causality was demonstrated between coral reefs and wave attenuation. The role that coral reefs play in actually delivering the beach protection service was described in 4 of the papers assessed [START_REF] Wielgus | Coastal capital: Dominican Republic. Case studies on the economic value of coastal ecosystems in the Dominican Republic[END_REF][START_REF] Kushner | Coastal Capital: Jamaica. Coral Reefs, Beach Erosion and Impacts to Tourism in Jamaica[END_REF] [START_REF] Reguero | Coral reefs for coastal protection: A new methodological approach and engineering case study in Grenada[END_REF][START_REF] Zhao | The Status of Coral Reefs and Its Importance for Coastal Protection: A Case Study of Northeastern Hainan Island, South China Sea[END_REF], while the role of important reef processes, such as maintaining the calcium carbonate budget, was not made at all.

The ability of coral reefs to protect beaches is site specific and dependent on the factors indicated in Figure 1 among others. Enabling factors for service delivery differ from site to site, with some coral reefs having no impact on beach erosion [START_REF] Quataert | The influence of coral reefs and climate change on wave-driven flooding of tropical coastlines[END_REF]. When coral reefs do offer protection, the literature is clear that while the ecosystem is often not the sole reason for sheltered coastlines, coral reefs, particularly, fringing reefs, are major contributors to wave attenuation. In some cases, they can dissipate greater than 90% wave energy [START_REF] Kench | Wave processes on coral reef flats: implications for reef geomorphology using Australian case studies[END_REF][START_REF] Ferrario | The effectiveness of coral reefs for coastal hazard risk reduction and adaptation[END_REF].

Wide, shallow, rugose reefs are reported as most effective at attenuating wave energy, with reef crests reducing >80% of incident wave energy [START_REF] Sheppard | Coral mortality increases wave energy reaching shores protected by reef flats: examples from the Seychelles[END_REF][START_REF] Kench | Wave processes on coral reef flats: implications for reef geomorphology using Australian case studies[END_REF][START_REF] De Alegria-Arzaburu | The role of fringing coral reefs on beach morphodynamics[END_REF]. Coral reefs attenuate waves primarily via wave breaking and bottom friction. Dissipation occurs first as waves break on the shallowest section of the reef followed by additional energy loss via friction as the bottom of the wave moves along the reef towards the shore [START_REF] Koch | Non-linearity in ecosystem services: temporal and spatial variability in coastal protection[END_REF][START_REF] Costa | Influence of reef geometry on wave attenuation on a Brazilian coral reef[END_REF]. Various reef attributes (Figure 1) impact both the amount of energy dissipated and its spatial extent, however the two primary reef related factors with major roles in wave attenuation are: (i) reef depth for wave breaking and (ii) roughness of the substrate which causes friction [START_REF] Gallop | The large-scale influence of the Great Barrier Reef matrix on wave attenuation[END_REF][START_REF] Monismith | Frictional wave dissipation on a remarkably rough reef[END_REF][START_REF] Harris | Coral reef structural complexity provides important coastal protection from waves under rising sea levels[END_REF].

4.2

The role of live coral

The role of the coral reef structure in providing coastal protection is well reported (Section 4.1) however the input of live coral and hence the impact of degrading health to ecosystem service provision has not been as extensively studied (de [START_REF] De Alegria-Arzaburu | The role of fringing coral reefs on beach morphodynamics[END_REF][START_REF] Ferrario | The effectiveness of coral reefs for coastal hazard risk reduction and adaptation[END_REF]. In this review only 7 of the relevant 139 studies spoke to the role of live corals and of these, 5 examined more closely their role in service delivery.

While few, these and other related studies, clearly indicate that healthy reefs with high abundances of scleractinian (hard) corals and structural complexity, provide greater coastal protection than degraded reefs. This is in cases of both frequent -daily erosion [START_REF] Guannel | The power of three: coral reefs, seagrasses and mangroves protect coastal regions and increase their resilience[END_REF][START_REF] Zhao | The Status of Coral Reefs and Its Importance for Coastal Protection: A Case Study of Northeastern Hainan Island, South China Sea[END_REF] and rare -storm events [START_REF] Ferrario | The effectiveness of coral reefs for coastal hazard risk reduction and adaptation[END_REF][START_REF] Van Zanten | Coastal protection by coral reefs: A framework for spatial assessment and economic valuation[END_REF]. The function of live coral for coastal protection can be encompassed within two interconnected processes: carbonate budgets and structural complexity, which are both enhanced by the presence of the framework builders Acropora spp. and Orbicella spp.

Carbonate budgets: reef growth and maintenance

Scleractinian corals are a broad taxonomic and morphological group, which form the foundation taxa of coral reefs [START_REF] Veron | Coral survey at selected sites in Arnhem Land[END_REF]. Their generation of huge amounts of calcium carbonate skeleton is crucial to the provision of coastal protection [START_REF] Guannel | The power of three: coral reefs, seagrasses and mangroves protect coastal regions and increase their resilience[END_REF]. Reef defence functions can only be maintained naturally, if vertical reef accretion allows for the shallow depths required for wave attenuation [START_REF] Waterman | Integrated Coastal Policy via Building with Nature[END_REF][START_REF] Beetham | Future reef growth can mitigate physical impacts of sea-level rise on atoll islands[END_REF][START_REF] Perry | Loss of coral reef growth capacity to track future increases in sea level[END_REF]. Within this group, specific species play dominant roles as reef builders and therefore in the delivery of coastal protection.

Corals grow by accreting calcium carbonate and at the same time erosion (biological and physical) of the skeleton produces sand. For the structure to be maintained, the carbonate budget must be positive (i.e., the rate of growth must exceed that of erosion) [START_REF] Ryan | Massive corals maintain a positive carbonate budget of a Maldivian upper reef platform despite major bleaching event[END_REF]. If the system switches to a net negative state however (as is caused by large scale coral mortality for example), net erosion can ensue, resulting in deterioration of the reef structure over time [START_REF] Perry | Caribbean-wide decline in carbonate production threatens coral reef growth[END_REF], flattening of the reef [START_REF] Alvarez-Filip | Flattening of Caribbean coral reefs: region-wide declines in architectural complexity[END_REF] and a reduction in the structure´s ability to attenuate wave energy [START_REF] Sheppard | Coral mortality increases wave energy reaching shores protected by reef flats: examples from the Seychelles[END_REF].

Hard coral cover is a predictor of carbonate production, with the abundance of historical, framework builders such Acropora spp. and Orbicella spp being especially important to the maintenance of carbonate budgets [START_REF] Perry | Loss of coral reef growth capacity to track future increases in sea level[END_REF][START_REF] Estrada-Saldívar | Functional consequences of the long-term decline of reef-building corals in the Caribbean: evidence of across-reef functional convergence[END_REF]. A significant reshaping of Caribbean coral reefs has already taken place, with a decline in reef builders and an influx of "weedy" species such as Agaricia spp. and Porites astreoides colonising their spaces. Such corals have neither fast growth rates nor the ability to generate the quantities of carbonate required for significant reef building [START_REF] Pandolfi | Ecological persistence interrupted in Caribbean coral reefs[END_REF][START_REF] Alvarez-Filip | Shifts in coralassemblage composition do not ensure persistence of reef functionality[END_REF][START_REF] Perry | Caribbean-wide decline in carbonate production threatens coral reef growth[END_REF]. Major events, such as these have been reported as primary reasons for the shifts in carbonate budgets in the Caribbean, from strongly net positive (5 kg CaCO3 m -2 y -1 ) to less so (2.6 kg CaCO3 m - 2 y -1 ) between the 1960s and 1990s [START_REF] Kennedy | Avoiding Coral Reef Functional Collapse Requires Local and Global Action[END_REF].

Maintenance of the carbonate budget is therefore essential to sustaining reef function, including coastal protection [START_REF] Lange | Carbonate budgets as indicators of functional reef "health": A critical review of data underpinning census-based methods and current knowledge gaps[END_REF] and the presence of live coral crucial. Timescales for this deterioration/erosion of reefs are not well understood (World Bank 2016) and while a figure of 6mm per year has been suggested as an approximation of the rate by which dead reefs will erode, it is also acknowledged that rates differ dramatically between reefs and even at different locations on the same reef [START_REF] Hutchings | Biological destruction of coral reefs[END_REF][START_REF] Eakin | Where have all the carbonates gone? A model comparison of calcium carbonate budgets before and after the 1982-1983 El Nino at Uva Island in the eastern Pacific[END_REF]. For example, on Australia's Great Barrier Reef, dense coral colonies in some inshore areas were lost over 50-100 years [START_REF] Hoegh-Guldberg | Coral reefs under rapid climate change and ocean acidification[END_REF], while Uva reef in Panama, was reduced to almost the same level as the surrounding sediment in approximately 15 years [START_REF] Eakin | A tale of two ENSO events: carbonate budgets and the influence of two warming disturbances and intervening variability, Uva Island, Panama[END_REF]). In the Indian Ocean, bioerosion on Chagos reef reduced the structure to rubble within 3 years [START_REF] Sheppard | Erosion vs. recovery of coral reefs after 1998 El Niño: Chagos reefs, Indian Ocean[END_REF].

A shift into a net negative phase is reported for situations where live coral cover on Caribbean reefs reaches 10% [START_REF] Perry | Caribbean-wide decline in carbonate production threatens coral reef growth[END_REF]. In a more recent study, this concept was refined further to 10% cover of structurally important species only and considered as precautionary threshold [START_REF] Darling | Social-environmental drivers inform strategic management of coral reefs in the Anthropocene[END_REF]. Nonetheless, these are targets that should be aimed for.

Structural complexity

Structural or topographical complexity of reefs refers to their three-dimensional form or layout on all spatial scales [START_REF] Zawada | A multiscale analysis of coral reef topographic complexity using lidarderived bathymetry[END_REF], and plays an important role in wave dissipation [START_REF] Lugo-Fernandez | Tide effects on wave attenuation and wave set-up on a Caribbean coral reef[END_REF]. Complexity is primarily defined by the morpho-functional characteristics (size and shape) of dominant and foundation corals [START_REF] Veron | New species described in Corals of the World[END_REF]Alvarez-Filip et al., 2011). It is measured via rugosity (on one spatial scale)

and roughness (range of spatial scales) [START_REF] Zawada | A multiscale analysis of coral reef topographic complexity using lidarderived bathymetry[END_REF] and so both terms are used.

In terms of wave attenuation, live or just dead corals provide roughness, which reduces wave energy [START_REF] Harris | Coral reef structural complexity provides important coastal protection from waves under rising sea levels[END_REF][START_REF] Reguero | Coral reefs for coastal protection: A new methodological approach and engineering case study in Grenada[END_REF]. Roughness is influenced by the type and size of substrate, with sand offering little and large, branching coral creating the most friction and therefore the greatest impact on wave attenuation [START_REF] Sheppard | Coral mortality increases wave energy reaching shores protected by reef flats: examples from the Seychelles[END_REF][START_REF] Van Zanten | Coastal protection by coral reefs: A framework for spatial assessment and economic valuation[END_REF]. Reefs can be categorized according to their rugosity index, with the flattest reefs having an index of less than 1.5. Approximately 75% of Caribbean reefs fall into this category, with reefs exhibiting an index higher than 2 (complex reefs) being extremely rare.

( [START_REF] Alvarez-Filip | Flattening of Caribbean coral reefs: region-wide declines in architectural complexity[END_REF].

While structural complexity increases with coral cover and species richness, massive growth forms (e.g.,

Orbicella spp) and fast growers (e.g., Acropora spp.) are thought to contribute the most to the structure

Summary of knowledge on service provision and the role of live coral for coastal protection

The importance of living reefs and therefore reef health to service delivery is shown in Figure 3. Living reefs contribute to service delivery in the short term via wave attenuation and in the long term via the ability of the reef to grow (accrete) and therefore maintain their structures. Reef degradation affects both carbonate production and structural complexity (Alvarez-Filip et al., 2011;[START_REF] Perry | Caribbean-wide decline in carbonate production threatens coral reef growth[END_REF]. Reef requirements can therefore be summarised as healthy reefs, dominated by framework builders. These attributes are interlinked, as the same coral species, (i.e., large framework builders) are largely responsible for both [START_REF] Graham | The importance of structural complexity in coral reef ecosystems[END_REF][START_REF] Harris | Coral reef structural complexity provides important coastal protection from waves under rising sea levels[END_REF].

Figure 3: Summary of key coastal protection requirements and primary coral reef characteristics for their provision 5 Synthesis of knowledge on management for coastal protection Coral reefs are extremely complex ecosystems with high levels of genetic, species and habitat diversity, leading to a vast variety of interactions involving many different species [START_REF] Moberg | Ecological goods and services of coral reef ecosystems[END_REF][START_REF] Dikou | Ecological processes and contemporary coral reef management[END_REF]. The major stressors are well known, however understanding ecosystem responses to them can be challenging, due to this complexity [START_REF] Pandolfi | Incorporating uncertainty in predicting the future response of coral reefs to climate change[END_REF][START_REF] Steneck | Attenuating effects of ecosystem management on coral reefs[END_REF]. It is however clear that effective reef protection will require management that can mitigate the effects of at least the dominant stressors, under existing and future scenarios of global climate change [START_REF] Weijerman | Managing local stressors for coral reef condition and ecosystem services delivery under climate scenarios[END_REF][START_REF] Brandl | Coral reef ecosystem functioning: eight core processes and the role of biodiversity[END_REF]. Coral reefs have been described as the most studied marine ecosystems with scientific consensus on the range of stressors and appropriate management measures [START_REF] Mumby | Coral reef management and conservation in light of rapidly evolving ecological paradigms[END_REF]. The aforementioned authors carried out a comprehensive review of coral reef management and conservation and no attempt was made here to conduct yet another. We used this as our seminal paper, and also referred to relevant references and more current research for specific elements.

Management can be categorised as either passive or active. Passive management operates on the basis of allowing nature to heal itself with limited human interference (e.g., reducing impacts, enforcement) while active management takes the form of human intervention, such as in the growing and planting of coral [START_REF] Rinkevich | Management of coral reefs: we have gone wrong when neglecting active reef restoration[END_REF]. Knowledge of the way in which the trophic structure, biodiversity resistance (to impacts) and resilience (ability to recover from impact) of coral reefs respond to human impact, is key to determining conservation strategies [START_REF] Bellwood | Confronting the coral reef crisis[END_REF][START_REF] Côté | Rethinking ecosystem resilience in the face of climate change[END_REF]. Management action must encompass both ecological parameters and social enabling factors, working in tandem to be effective [START_REF] Gill | Capacity shortfalls hinder the performance of marine protected areas globally[END_REF], however in the paper our focus is on the ecological characteristics.

Framework Builders

Wave [START_REF] Weijerman | Managing local stressors for coral reef condition and ecosystem services delivery under climate scenarios[END_REF][START_REF] Beatty | Variable effects of local management on coral defenses against a thermally regulated bleaching pathogen[END_REF]. Dominant, local stressors on Caribbean reefs, as well as primary impacts and management strategies, are outlined in Table 2. The most significant impact is considered to be macroalgal abundance caused by anthropogenic nutrients, and unsustainable fishing [START_REF] Jackson | Status and trends of Caribbean coral reefs[END_REF][START_REF] Harborne | Multiple stressors and the functioning of coral reefs[END_REF][START_REF] Bruno | Climate change, coral loss, and the curious case of the parrotfish paradigm: Why don't marine protected areas improve reef resilience?[END_REF]. Excessive levels of macroalgae, are responsible for overgrowing and shading adult corals, inhibiting recruitment of juveniles corals and harbouring disease [START_REF] Idjadi | Recovery of the sea urchin Diadema antillarum promotes scleractinian coral growth and survivorship on shallow Jamaican reefs[END_REF][START_REF] Rasher | Chemically rich seaweeds poison corals when not controlled by herbivores[END_REF], which promote conditions for declining reef health. Key here is that a reef in a negative feedback loop, might be unable to recover even if the disturbance is removed, as processes drive the system towards macroalgal abundance [START_REF] Mumby | Towards Reef Resilience and Sustainable Livelihoods: A handbook for Caribbean coral reef managers[END_REF]. Action aimed at reducing macroalgal growth, is therefore one of the crucial factors to be controlled in conservation, and managing herbivores and improving water quality are priority conservation actions [START_REF] Mumby | Coral reef management and conservation in light of rapidly evolving ecological paradigms[END_REF][START_REF] Jackson | Status and trends of Caribbean coral reefs[END_REF][START_REF] Steneck | Managing recovery resilience in coral reefs against climate-induced bleaching and hurricanes: a 15 year case study from Bonaire, Dutch Caribbean[END_REF]. [START_REF] Maragos | Impact of coastal construction on coral reefs in the US-affiliated pacific Islands[END_REF][START_REF] Richmond | Coral reefs: present problems and future concerns resulting from anthropogenic disturbance[END_REF]), c [START_REF] Sandin | Baselines and degradation of coral reefs in the Northern Line Islands[END_REF][START_REF] Jackson | Status and trends of Caribbean coral reefs[END_REF], d [START_REF] Green | Invasive lionfish drive Atlantic coral reef fish declines[END_REF][START_REF] Galil | ICES AD HOC REPORT[END_REF]), e [START_REF] Hunte | Recruitment and population recovery of Diadema antillarum (Echinodermata; Echinoidea) in Barbados[END_REF][START_REF] Sutherland | White pox disease of the Caribbean elkhorn coral, Acropora palmata[END_REF], f [START_REF] Lewis | The Acropora inheritance: a reinterpretation of the development of fringing reefs in Barbados, West Indies[END_REF][START_REF] Hawkins | The growth of coastal tourism in the red sea: Present and future effects on coral reefs[END_REF][START_REF] Barker | Scuba diver behaviour and the management of diving impacts on coral reefs[END_REF].

It is important to note that to date, no conservation or restoration management measures have resulted in reefs recovering to pre-stress state [START_REF] Mumby | Towards Reef Resilience and Sustainable Livelihoods: A handbook for Caribbean coral reef managers[END_REF][START_REF] Mcwilliam | Deficits in functional trait diversity following recovery on coral reefs[END_REF]. However, strategies aimed at reducing some key stressors have yielded specific successes, that have aided in recovery of reefs, as is outlined in the remaining sections.

Passive management: increase herbivory

Herbivory is a critical trophic interaction on tropical coral reefs, and declines in herbivore abundance, result in a proliferation of macroalgae [START_REF] Hunte | Recruitment and population recovery of Diadema antillarum (Echinodermata; Echinoidea) in Barbados[END_REF][START_REF] Ladd | Trophic interactions in coral reef restoration: A review[END_REF]. Increasing the abundance of herbivores leads to reduced macroalgal cover via trophic cascades, or top down control [START_REF] Hughes | Phase shifts, herbivory, and the resilience of coral reefs to climate change[END_REF][START_REF] Burkepile | Herbivore species richness and feeding complementarity affect community structure and function on a coral reef[END_REF][START_REF] Steneck | Managing recovery resilience in coral reefs against climate-induced bleaching and hurricanes: a 15 year case study from Bonaire, Dutch Caribbean[END_REF][START_REF] Mcclure | Higher fish biomass inside than outside marine protected areas despite typhoon impacts in a complex reefscape[END_REF], which is expected to eventually result in increased coral cover. While this final step has not been strongly made, research by Jackson et al.

(2014) recorded a significantly higher abundance of coral on Caribbean reefs with more parrotfish (Scaridae spp.). Additionally, research from both Australia [START_REF] Hughes | Phase shifts, herbivory, and the resilience of coral reefs to climate change[END_REF] and the Bahamas (Mumby and

Harborne 2010) provided some evidence of increasing numbers of herbivores supporting reef health by increasing coral recruitment. While coral reef recovery to pre-stress levels was not observed in these studies, coral recruitment is the first step in such a recovery. It is important to note though, that the dominant recruits reported in the Bahamas were Porites astreoides and Agaricia spp, which, as was shown in Section 4.2.1.

cannot provide the same ecosystem services as framework builders.

Management of herbivores is commonly dealt with within an MPAs framework (Section 5.1.4) and/or via a complete ban of primary herbivores such as parrotfish in Belize [START_REF] Cox | Evaluating Strategies for Restoring Parrotfish Populations in Belize[END_REF] and Bermuda [START_REF] O'farrell | Parrotfish sex ratios recover rapidly in Bermuda following a fishing ban[END_REF].

Passive management: reducing land based sources of marine pollution

Activities on land produce excessive flows of nutrients (primarily nitrogen and phosphorous) and sediments onto coral reefs, which have proven detrimental to their health [START_REF] Bellwood | Confronting the coral reef crisis[END_REF][START_REF] Burke | Reefs At Risk Revisited[END_REF][START_REF] Eberhard | Scientific Consensus Statement: land use impacts on the Great Barrier Reef water quality and ecosystem condition[END_REF]. Elevated nutrient levels from fertilizer and sewage (both humans and farm animals)

promote the growth of macroalgae on small scales, such as the west coast of Barbados [START_REF] Tomascik | Effects of eutrophication on reef-building corals[END_REF] and large scales, such as the entire Great Barrier Reef of Australia (De'ath and Fabricius 2010). Coastal development and riverine run off also produce elevated quantities of sedimentation, which interrupt coral processes (such as feeding), leading to mortality and/or reduced growth [START_REF] Weber | Mechanisms of damage to corals exposed to sedimentation[END_REF][START_REF] Bartley | Relating sediment impacts on coral reefs to watershed sources, processes and management: A review[END_REF]. Coral reefs with excessive sedimentation have been shown to have reduced diversity, lower coral abundance and lower accretion rates [START_REF] Rogers | Responses of coral reefs and reef organisms to sedimentation. Marine ecology progress series[END_REF]. Improving water quality around reefs is troublesome, as the sources often originate on land and are some distance away from where impact occurs on coral reefs.

However, holistic management frameworks, which incorporate actions taken from the source of pollution to impact sites have been demonstrated to improve water quality around reefs. Land management measures that reduce nutrients and sediments (e.g., agricultural practices and watershed restoration) [START_REF] Fillols | Combining weed efficacy, economics and environmental considerations for improved herbicide management in the Great Barrier Reef catchment area[END_REF] have resulted in water quality improvements around reef areas, which have been further demonstrated to improve coral cover [START_REF] Jokiel | Reef Coral Communities at Pila'a Reef: Results of the 2004 resurvey[END_REF][START_REF] Fabricius | The effects of river run-off on water clarity across the central Great Barrier Reef[END_REF][START_REF] Shelton Iii | Watershed restoration as a tool for improving coral reef resilience against climate change and other human impacts[END_REF], even if not to pre-stress levels [START_REF] Wenger | Coral reef conservation solution-scape white paper[END_REF].

Active management: reef restoration

Coral restoration's importance is emerging with the reported inability of passive management measures to restore reefs to pre-stress levels [START_REF] Rinkevich | Management of coral reefs: we have gone wrong when neglecting active reef restoration[END_REF]. This active management tool focuses on repairing reefs in order to facilitate their recovery and restore ecosystem integrity [START_REF] Basconi | Challenges in marine restoration ecology: how techniques, assessment metrics, and ecosystem valuation can lead to improved restoration success[END_REF]. It often takes the form of coral gardening which is divided into 2 activities: nursery phase for rearing coral recruits and out-planting to the reef [START_REF] Boström-Einarsson | Coral restoration in a changing world -a global synthesis of methods and techniques[END_REF]. Of specific interest to coastal protection, is that the fast growing framework builder (Acropora spp.) and massive, framework builder (Orbicella spp) are the two species primarily selected for restoration, with survival rates of >60% being reported [START_REF] Boström-Einarsson | Coral restoration-A systematic review of current methods, successes, failures and future directions[END_REF].

While there are few reports of active reef restoration resulting in long term ecological recovery [START_REF] Fox | Rebuilding coral reefs: success (and failure) 16 years after low-cost, low-tech restoration[END_REF] and concerns raised over survival and fitness of transplants and cost effectiveness of operations [START_REF] Bayraktarov | The cost and feasibility of marine coastal restoration[END_REF]; there have been many promising advances made within the last decade, such as selective breeding and assisted evolution [START_REF] Baums | Host population genetic structure and zooxanthellae diversity of two reef-building coral species along the Florida Reef Tract and wider Caribbean[END_REF][START_REF] Drury | Making biodiversity work for coral reef restoration[END_REF][START_REF] Basconi | Challenges in marine restoration ecology: how techniques, assessment metrics, and ecosystem valuation can lead to improved restoration success[END_REF] that are cause for optimism. Restoration of marine ecosystems is still in its nascent phase, however it has been predicted to be the most dominant discipline in environmental science in the 21 st century [START_REF] Hobbs | Restoration ecology: repairing the earth's ecosystems in the new millennium[END_REF], and there have been increased calls for active restoration to be added to the tools of watershed and fisheries management for coral reefs [START_REF] Rinkevich | Management of coral reefs: we have gone wrong when neglecting active reef restoration[END_REF][START_REF] Basconi | Challenges in marine restoration ecology: how techniques, assessment metrics, and ecosystem valuation can lead to improved restoration success[END_REF][START_REF] Boström-Einarsson | Coral restoration-A systematic review of current methods, successes, failures and future directions[END_REF]. Reef Restoration has therefore been included as a priority conservation action.

Management proxy: marine protected areas

Reef management within an Marine Protected Area (MPA)1 framework, is one of the most extensively used tools for conservation [START_REF] Toropova | Global ocean protection: present status and future possibilities[END_REF][START_REF] Claudet | Ecology-Ecological effects of marine protected areas: conservation, restoration, and functioning[END_REF]. MPAs work by managing human activity, within specific boundaries, with the expectation that with reduced impact, recovery will occur [START_REF] Day | Guidelines for applying the IUCN protected area management categories to marine protected areas[END_REF]. With effective management, which encompasses: adequate compliance, participatory decision making, empowerment and education of local communities [START_REF] Hughes | Rising to the challenge of sustaining coral reef resilience[END_REF] MPAs can play important roles in restoring ecosystem structure and function [START_REF] Mumby | Coral reef management and conservation in light of rapidly evolving ecological paradigms[END_REF][START_REF] Laffoley | Marine protected areas. World seas: An environmental evaluation[END_REF].

Well managed MPAs have proven effective in the reduction of many stressors. They are responsible for:

reducing unsustainable fishing [START_REF] Bellwood | Confronting the coral reef crisis[END_REF] leading to increases in the size and biomass of fish species [START_REF] Johnson | Advances in marine biology: marine managed areas[END_REF][START_REF] Sciberras | Evaluating the relative conservation value of fully and partially protected marine areas[END_REF][START_REF] Leenhardt | Exploring social-ecological dynamics of a coral reef resource system using participatory modeling and empirical data[END_REF]; improving herbivory [START_REF] Selig | A global analysis of the effectiveness of marine protected areas in preventing coral loss[END_REF][START_REF] Possingham | Optimal conservation outcomes require both restoration and protection[END_REF] and promoting coral recovery after disturbances [START_REF] Mumby | Marine reserves enhance the recovery of corals on Caribbean reefs[END_REF][START_REF] Perry | Caribbean-wide decline in carbonate production threatens coral reef growth[END_REF]. Importantly, reefs with protection for parrotfish, have also demonstrated the ability to delay loss of architectural complexity, which is a key component of coastal erosion [START_REF] Bozec | The dynamics of architectural complexity on coral reefs under climate change[END_REF]. By reducing some effects of local stressors, MPA's have also been shown to buffer global impacts (e.g., high temperatures). Further to the passive management measures outlined, active management, such as reef restoration, is recommended to be carried out within effectively managed MPAs, where efforts are most likely to result in success [START_REF] Shaver | Time to cash in on positive interactions for coral restoration[END_REF][START_REF] Basconi | Challenges in marine restoration ecology: how techniques, assessment metrics, and ecosystem valuation can lead to improved restoration success[END_REF].

MPAs are however not "silver bullets" and even when well-managed, have not always resulted in increases in coral cover [START_REF] Cox | Establishment of marine protected areas alone does not restore coral reef communities in Belize[END_REF]. Most MPAs are susceptible to impacts that originate outside of their boundaries such as anthropogenic pollutants (e.g., sewage, heavy metals), invasive species and disease [START_REF] Hughes | Rising to the challenge of sustaining coral reef resilience[END_REF]. [START_REF] Mumby | Coral reef management and conservation in light of rapidly evolving ecological paradigms[END_REF] in an extensive review of the causes and consequences of reef decline, documented the status of knowledge of the ability of MPAs to meet management goals. They found clear successes in terms of increasing fish populations within the protected spaces, however the expected results of increased coral recruitment and hard coral abundance were less widely documented. Modelling has shown that with an objective of increasing coral cover (as is required for coastal protection) reduction of land based sources of pollution is more effective than on site MPA activities [START_REF] Weijerman | Managing local stressors for coral reef condition and ecosystem services delivery under climate scenarios[END_REF]. MPA action therefore would have to encompass land based sources of pollution and be part of a broader programme of Watershed Management within an Integrated Coastal Zone Management (ICZM)2 [START_REF] Belfiore | Incorporating marine protected areas into integrated coastal and ocean management: Principles and guidelines[END_REF][START_REF] Cicin-Sain | Linking marine protected areas to integrated coastal and ocean management: a review of theory and practice[END_REF]. This is demonstrated on the Great Barrier Reef where an important policy and management focus for marine park managers is on nutrients and pesticides from agricultural practices outside of their boundaries [START_REF] Kroon | Towards protecting the Great Barrier Reef from land-based pollution[END_REF].

Increasing coral reef health resilience within an MPA framework that includes watershed management can therefore can be taken as a proxy; a means of ensuring the likelihood of the continuance of the ES by reducing threats to coral reefs (World Bank 2016;[START_REF] Mcleod | The future of resilience-based management in coral reef ecosystems[END_REF].

Summary of knowledge on management for coastal protection

Management for coastal protection, equates to providing a hard coral reef, dominated by framework builders.

Priority measures as well as the primary impact and management frameworks are outlined in Figure 4. To date PES has not been extensively considered for coral reef coastal protection. Only 4 peer reviewed articles were found, in which PES for coastal protection was considered, and in none of these was a PES scheme actually developed [START_REF] Lau | Beyond carbon: Conceptualizing payments for ecosystem services in blue forests on carbon and other marine and coastal ecosystem services[END_REF][START_REF] Castaño-Isaza | Valuing beaches to develop payment for ecosystem services schemes in Colombia's Seaflower marine protected area[END_REF][START_REF] Pascal | Economic valuation of coral reef ecosystem service of coastal protection: A pragmatic approach[END_REF][START_REF] Elliff | Coral reefs as the first line of defense: Shoreline protection in face of climate change[END_REF]. One report was identified in which a PES scheme for coastal protection in San Andres, Colombia was considered [START_REF] Lau | San Andres Island, Colombia: A Case Study of Stacking and Bundling in the Marine and Coastal Context. Prepared for the USAID Bundling and Stacking for Maximizing Social, Ecological, and Economic Benefits: A Framing Paper for Discussion at the "Bundling and Stacking Workshop[END_REF], however this never came to fruition due to the departure of one of the key local stakeholders (T.

Agardy 2019, pers. comm.). The ability of coral reefs to meet each PES requirement is described and displayed via the use of tables 5-7.

Analyses were based on outputs of the syntheses of knowledge (Sections 4 and 5). Measurement Indicators were categorised as: N -Not Documented (No assessed papers documented the output); E -Expected result, but confirmed by less than 5 studies and C -Confirmed. The Management Proxy status (Section 6.3) was based on knowledge of the ability of the identified system to produce the desired output.

ES identification, quantification and spatial boundaries

Coastal Protection can be defined and measured by different indicators, which include: the biophysical processes responsible for providing the service, the service itself and the social benefits derived by humans [START_REF] Guerry | Natural capital and ecosystem services informing decisions: From promise to practice[END_REF]. Principie et al ( 2012) quantified at least 57 different indicators that can be clearly calculated and replicated (as is required for PES) and a sub set of those deemed most relevant is shown in Table 3. Indicators of measurement vary in their direct relevance to human well-being [START_REF] Wainger | Valuing ecosystem services. Ecosystem-based Management for the Oceans[END_REF], with biophysical processes requiring some means of translation into either ecosystem service or socioeconomic measurements, for the benefits to humans to be made clear. [START_REF] Storlazzi | Rigorously valuing the role of US coral reefs in coastal hazard risk reduction[END_REF] for example used wave processes and reef health indicators (biophysical processes), to determine the protection offered by coral reefs against flooding (ecosystem service measurement). This was then calculated in terms of avoided damages (socioeconomic measurement). Here, we consider only the biophysical elements -processes and the service as outlined in our objectives.

The measurements of indicators for the biophysical processes (e.g., wave heights and hard coral abundance) and the service (beach erosion) are well established in literature [START_REF] Principe | Quantifying Coral Reef Ecosystem Services[END_REF]. The height of waves in back reefs for example is indicative of wave energy which will impact the shoreline. The measure is widely used in engineering studies which examine the impact of reefs on shorelines [START_REF] Ferrario | The effectiveness of coral reefs for coastal hazard risk reduction and adaptation[END_REF][START_REF] Quataert | The influence of coral reefs and climate change on wave-driven flooding of tropical coastlines[END_REF][START_REF] Guannel | The power of three: coral reefs, seagrasses and mangroves protect coastal regions and increase their resilience[END_REF][START_REF] Storlazzi | Rigorously valuing the role of US coral reefs in coastal hazard risk reduction[END_REF].

Coral reef health is most often indicated by the variable of hard coral cover [START_REF] Obura | Coral Reef Monitoring, Reef Assessment Technologies, and Ecosystem-Based Management[END_REF]. This metric, as well as hard coral species diversity, fish abundance and fish diversity are standard parameters in many established monitoring programmes such as the Global Coral Reef Monitoring Programme (GCRMP) [START_REF] Hill | Methods for Ecological Monitoring of Coral Reefs[END_REF]; the Atlantic and Gulf Rapid Reef Assessment (AGRRA) [START_REF] Lang | Atlantic and Gulf Rapid Reef Assessment (AGRRA) Protocols Version 5.4[END_REF] and Reef Check (Reef Check Instruction Manual 2006). Rugosity, the more commonly utilised metric for structural complexity in coral reefs, is less often included in monitoring programmes but there are clear methods established for quantification, ranging from low tech "chain and tape" to remote sensing [START_REF] Figueira | Accuracy and precision of habitat structural complexity metrics derived from underwater photogrammetry[END_REF]. This parameter, as well as the previously mentioned hard coral and fish metrics, are identified as key indicators in determining reef health [START_REF] Flower | Interpreting coral reef monitoring data: A guide for improved management decisions[END_REF] and as important in the determination of coastal protection [START_REF] Beck | The global flood protection savings provided by coral reefs[END_REF]. The carbonate budget is not a commonly measured metric, but could be used as a proxy measurement for maintenance and reef function is carbonate budgets [START_REF] Lange | Carbonate budgets as indicators of functional reef "health": A critical review of data underpinning census-based methods and current knowledge gaps[END_REF].

Defining boundaries is not simple for coastal protection, due in part to the diffuse and interconnected nature of water [START_REF] Bladon | Payments for ecosystem services in developing world fisheries[END_REF]. Additionally, benefits are provided and impacts can originate some distance away from the coral reef, which increases this complexity. However, while no standard methods exist for determining where the service is provided, (i. tech, at different levels of specificity and range from complex simulation models [START_REF] Reguero | Coral reefs for coastal protection: A new methodological approach and engineering case study in Grenada[END_REF][START_REF] Storlazzi | Rigorously valuing the role of US coral reefs in coastal hazard risk reduction[END_REF] to a simple reliance on the distance from the reef to the shore [START_REF] Burke | Coastal capital: economic valuation of coral reefs in Tobago and St. Lucia[END_REF][START_REF] Silver | Advancing Coastal Risk Reduction Science and Implementation by Accounting for Climate, Ecosystems, and People[END_REF].

A summary of the ability of coastal protection to meet the first three PES requirements -ES identification, quantification and spatial boundaries is provided in Table 4.

Management measures linked to specific outcomes and MPA proxy

Restoring coral reefs will not occur with a single management tool. Rather, a combination of marine and terrestrial management actions, as is possible within an ICZM 3 framework, and coupled with a reduction in carbon dioxide emissions [START_REF] Weijerman | Managing local stressors for coral reef condition and ecosystem services delivery under climate scenarios[END_REF]. Priority management measures aimed at delivering increased hard coral cover, were identified as, improving herbivory, water quality and reef restoration (Section 5). However, the ability of these measures to increase and sustain hard coral cover populations, while expected, have not been convincingly demonstrated in the literature (Table 5).

Conditionality and additionality

In PES schemes, payments should be based on either service flows or conservation action. If the outcomes identified by the conditions are unmet, then payments are not triggered. This further implies that the effect of conservation action must be observed in the short term and during the life of the PES.

In order for buyers to ensure that the environmental actions they have paid for have: (i) the desired impact (conditionality) and (ii) a beneficial impact greater than what is occurring at present (additionality), there needs to be some form of verification of the sellers´ actions and impacts. Therefore, baseline conditions,

measured by specific indicators that can be replicated are required [START_REF] Lau | Beyond carbon: Conceptualizing payments for ecosystem services in blue forests on carbon and other marine and coastal ecosystem services[END_REF]) and these were identified in Section 6.1. Proving both conditionality and additionality will depend on the indicators chosen and how well these are monitored. The difficulty here appears to be more related to the practical issues (such as expense) involved in establishing baselines and monitoring over time [START_REF] Bladon | Payments for ecosystem services in developing world fisheries[END_REF], rather than the ability to measure. As the ecological requirements for additionality are the same for those of conditionality, only the latter will be considered in the assessment.

3 ICZM is a resource management system that includes a participatory and holistic approach to addressing the complex coastal management issues ibid., [START_REF] Saffache | Integrated Coastal Zone Management in small islands: A comparative outline of some islands of the Lesser Antilles[END_REF]. Integrated Coastal Zone Management in small islands: A comparative outline of some islands of the Lesser Antilles. Revista de Gestão Costeira Integrada-Journal of Integrated Coastal Zone Management 10

(3), 255-279.

Table 6 lays out some indicators of coastal protection (identified in Section 6.1). For each indicator, examples of conditions and an indication of the ability of priority management actions (Section 5) to deliver these outcomes, are identified. Outputs are based on conclusions/evidence drawn from Sections 4 and 5. For example, the condition for 1a is that wave heights do not exceed baseline measurements. There are no indications in the literature that increasing herbivory (H) will result in this effect, hence not documented (N) is the output. For measurements of service flow, the only condition that can be fully met by the management measures outlined is 2e -Parrotfish abundance is equal to or greater than the baseline. Actions aimed at improving herbivory both inside and outside of MPAs have been demonstrated to increase parrotfish abundance. For Conditions 2a and 2b, management actions have been documented to increase hard coral cover and structural builders, however these studies are few and increases have not been recorded to pre-stress levels (or to 10%). Neither physical nor service indicators measurements can meet the conditionality requirement, as none of the aforementioned management actions will have a foreseeable impact on wave heights or beach width in the short term. Therefore, these measures are not suitable for PES.

Conditionality via environmental action/proxy

Using MPAs as a management proxy of environmental action is the strongest means of fulfilling conditionality. A system of monitoring the effectiveness of MPAs such as the Management Effectiveness

Tracking Tool (METT) can be used. This tracking tool allows one to monitor progress in improving management effectiveness via a system of scoring [START_REF] Stolton | Reporting Progress at Protected Area Sites[END_REF]. Priority management actions and stipulations such as being linked to broader Watershed or ICZM programmes, can be appended.

Other PES considerations

In addition to meeting the aforementioned requirements, the suitability of PES will be driven by the specific situation. Knowledge on whether the specified beach is eroding or stable is important. For an eroding beach, even if the situation is due to coral decline, no existing management measures can ensure that corals grow to attain the adequate depth and rugosity required to attenuate waves and ensure stable beaches in the short term. In such cases, most hoteliers opt for the more established grey infrastructure such as breakwaters [START_REF] Silver | Advancing Coastal Risk Reduction Science and Implementation by Accounting for Climate, Ecosystems, and People[END_REF]. For a stable beach, however, where a decline in reef health can cause a concomitant decline in service provision [START_REF] Kushner | Coastal Capital: Jamaica. Coral Reefs, Beach Erosion and Impacts to Tourism in Jamaica[END_REF][START_REF] Monismith | Frictional wave dissipation on a remarkably rough reef[END_REF], payments can be made for better management, aimed at maintaining or improving services from an existing healthy reef [START_REF] Beck | The global flood protection savings provided by coral reefs[END_REF][START_REF] Iyer | Finance Tools for Coral Reef Conservation: A Guide[END_REF]. Causality has been demonstrated in the literature between a toolbox of conservation action (as is carried out within MPA frameworks) and some improved reef health parameters and between reef health and greater beach protection. This is therefore a more feasible option. A PES scheme, with a goal of rewarding such environmental stewardship allows for potential PES schemes to be developed.

Social parameters must also be taken into consideration and while not being the focus of this paper, knowledge of potential beneficiaries (buyers) and providers (sellers) of the ES are required to showcase an example of a PES scheme. A variety of stakeholders would benefit from coastal protection, however, for most Caribbean Islands with the importance of tourism and its strong relationship to coastal protection, the most obvious beneficiaries are coastal hoteliers, who desire beach presence as a selling point for their businesses [START_REF] Uyarra | Island-specific preferences of tourists for environmental features: implications of climate change for tourism-dependent states[END_REF]Pascal et al., 2018), as well as governments. Sellers of the service for the MPA Proxy, will most likely be MPA Managers, who, while not owning the marine space, are designated by governments to manage them.

A PES could be carried out with a hotelier as the buyer and MPA managers as sellers of a range of environmental actions (both ecological and social) that would enhance beach stability (Figure 5). Payments could be conditional on the measurement of indicators, such as achieving 10% hard coral cover, or delivery of an MPA workplan, or METT scores. The goal must be clear; forestalling potential loss in revenue from beach erosion, by investing in environmental stewardship of reefs that actively protect beaches. PES is a multifaceted approach, with specific ecological requirements, which can be met by the coastal protection ecosystem service.

An analysis of literature, indicates that the scientific knowledge behind the ability of coral reefs to provide coastal protection, is adequate for the development of a PES scheme, under certain conditions. The service is complex, with different factors, both reef and non-reef related, contributing to reducing beach erosion. Not all fringing reefs protect beaches, but some, within specific contexts, are largely responsible for wave attenuation and therefore can play major roles in reducing beach erosion. Studies measuring the impact of coral reef parameters on waves are well reported. However, those demonstrating causality between coral health parameters (e.g., coral cover, rugosity, fish abundance) and the service itself (e.g., decreased erosion) are rarer, possibly due to the complexity of coral reefs and coastal processes [START_REF] Reguero | Coral reefs for coastal protection: A new methodological approach and engineering case study in Grenada[END_REF].

A range of methods exist in the literature to identify, quantify and define geographical boundaries of coastal protection delivery, as required for a PES. Our limited ability to prove causality between management action and ecosystem service delivery causes some difficulty, though not insurmountable, in fulfilling the conditionality requirement. In some very specific cases, such as increasing the number of herbivores, management measures have been shown to be successful for coral reef health and a PES can be designed around this. However, since increasing herbivore abundance has not been strongly demonstrated to have impacts on beach erosion, it is highly unlikely that a PES would be successful. Instead, a PES scheme can be developed with payments triggered by specified conservation action or management proxy.

MPAs cannot solve all issues, however, once effectively managed, they provide a framework within which a range of management measures can be effectively carried out. This toolbox of measures has been demonstrated to improve coral reef health and increase hard coral cover under some conditions. Such interventions therefore remain our best tool at improving overall reef health [START_REF] Roberts | Marine reserves can mitigate and promote adaptation to climate change[END_REF][START_REF] Sala | Assessing real progress towards effective ocean protection[END_REF]. MPAs, linked to a broader programme of watershed management and/or under an ICZM programme, are expected to significantly strengthen health outcomes (Cicin-Sain and Belfiore 2005), and therefore present a sensible management proxy for PES.

It is clear that even with passive conservation measures promoting reef health, some sort of active management would be required to attain the reef depth and rugosity parameters required for reefs that have lost their ability to attenuate wave energy in the short term. In order to achieve optimal outcomes, a combination of active (e.g., reef gardening) and passive (e.g., habitat protection) actions should be utilised [START_REF] Possingham | Optimal conservation outcomes require both restoration and protection[END_REF].

We suggest utilising PES in cases where the beach is stable with a goal of ensuring continued and even improved stability. Consideration of gray-green solutions such as reef augmentation, a concept that integrates nature into the building process [START_REF] Waterman | Integrated Coastal Policy via Building with Nature[END_REF]) might be a means of creating short term improvements specifically for coastal protection, while allowing for the longer term benefits of overall reef protection. The potential of using PES for such innovative concepts should be explored further, but it is outside of the scope of this paper. Future research should concentrate on the most effective means of reef augmentation to promote service flow.

It should be noted that average coral cover on Caribbean reefs (reported at the last regional census in 2012) was 14.3% with coral cover declining at 75% of the 88 sites [START_REF] Jackson | Status and trends of Caribbean coral reefs[END_REF]. If the trajectory continues, as is expected with the impeding threats of global climate change, the ability of reefs in this region to attenuate wave energy and maintain themselves will be severely diminished. Coral health will therefore become even more important.

PES could play a stronger role in coral reef conservation for this ES, especially if this service was bundled with other more clearly defined and quantified services, such as aesthetics and fish biomass.

(

  Alvarez-Filip et al., 2011;[START_REF] Kuffner | A geological perspective on the degradation and conservation of western Atlantic coral reefs[END_REF] and hence to wave dissipation. A significant decline in coral cover and hence structural complexity results in crumbling of the reef and a change from a more varied topographical surface to a flatter surface, with less ability to reduce wave heights[START_REF] Alvarez-Filip | Flattening of Caribbean coral reefs: region-wide declines in architectural complexity[END_REF][START_REF] Osorio-Cano | Ecosystem management tools to study natural habitats as wave damping structures and coastal protection mechanisms[END_REF].

Figure 4 :

 4 Figure 4: Summary of primary impact, priority management measures and management frameworks to deliver positive outcomes for coastal protection

Figure 5 :

 5 Figure 5: Potential PES Scheme between buyers (coastal hoteliers) and sellers (MPA managers) of coastal protection

Table 1 :

 1 Summary table of key PES requirements

	Key PES Elements	Status
	ES Identification	Required
	ES Quantification	Required only in absence of proxy
	ES Spatial Boundaries	Required
	Proven Management Measures/ Proxy	Required
	Conditionality	Required
	Additionality	Desired

  al.,

	2011;			Hazard
				events
		Structural complexity	Velocity	Energy	Coastal vegetation	geography Coastal
	Reef health	Coral reefs	Attenuate	Waves	plus	features Other natural	Calm Provide	Coastal protection
							conditions
	Reef height,
	depth, shape
		Carbonate budget +ve	Period	Height	Coastal elevation	Sediment & grain size
								6

  Coral reefs are impacted by a range of local and global stressors. Local management has however been reported as providing a buffering effect on coral degradation under climate change scenarios[START_REF] Hughes | Review: Climate Change, Human Impacts, and the Resilience of Coral Reefs[END_REF]

	Coastal Protection	Occurs when	Function of	Reef Characteristics
	Requirements
				High % Fast growing
		breaking	Reef Depth	hard corals e.g.
				Acropora spp
	Wave
	attenuation
		Bottom friction	Reef Roughness	Hard coral dominated
				High % branching or
	Coral growth	+ve Carbonate		massive species e.g.
		budgets		Acropora & Orbicella
				spp.

Table 2 :

 2 Prominent documented local stressors, impacts and passive management action on Caribbean coral reefs.

	Reef Threat	Primary Impact	Management Action
	Eutrophication a
	Agriculture	Macroalgae	Watershed Management
	Sewage	Macroalgae	Treatment Plants, Watershed Management
	Hurricanes b
	Industry	Outright Mortality	Policy -Water Quality Standards
	Coastal Construction	Outright Mortality	Policy -Integrated Coastal Zone Management
	Unsustainable Fishing c	Macroalgae	MPAs -no take zones
		Outright mortality
	Invasive Species d
	Lionfish	Trophic Pathway disruption	Culling
	Harmful Orgs. Ballast Water	Disease	Policy -Ballast Water Treatment Protocols
	Disease e
	of Keystone Species	Macroalgae
			Removal of diseased colonies
	of Corals	Outright Mortality	Barriers to disease progression
			Application of chemicals to kill microbes
	Physical Damage f
	Anchors		Permanent Moorings
	Divers	Outright Mortality	ELE*
	Hurricanes		Rapid Repair
	*ELE -Education, Legislation and Enforcement are cross cutting management actions and are only indicated when they are the only
	action identified.

a

(Weil and Rogers 2011; Mumby et al., 2014) b

  management proxy can be used to satisfy conditionality. Additionality is desired and is discussed with conditionality, as its basic requirements are the same.

	possible, a
	Required Reef	Primary local	Priority Management	Management
	Characteristics	impact	Measures	Framework
			Improve herbivory	MPA
	Hard coral
	reef corals & fast growers on dominated by large	Macroalgae	Improve water quality	Watershed Management
	reef crest
			Coral reef restoration	MPA
	The ability of coral reefs to meet each PES requirement (Section 3) is discussed in this section. Requirements
	are as follows: the ES must be identified, spatial boundaries defined, management measures linked to specific
	outcomes known and conditionality satisfied. It is important to note that if quantification of the service is not

Table 3 :

 3 Examples of ecosystem measures and indicators used to quantify coastal protection

	Measures	Indicators	Citation
		Physical	Harris et al 2018, Yao et al 2019
		Wave energy, height, velocity
	Biophysical Processes	Biological
		Reef rugosity	Monnismith et 2015, Lowe et al 2005
		Fish abundance	Wainger and Boyd 2009
	Ecosystem Service	Beach Erosion	Reguero et al 2018, Kushner et al 2011
		Coastal Inundation	Beck et al 2018, Ferrario et al 2014
	Socioeconomic	Avoided damages	Storlazzi et al 2019, vanZanten et al 2014
		Property values	Burke et al 2008, Pascal et al 2016
		Breakwater replacement costs	Ferrario et al 2014, Beck et al 2019

  e., what reefs protect what beaches/properties/lives) (van Zanten et al., 2014), the literature is replete with examples of methods used. They include both high and low

Table 4 :

 4 Summary of the ability of coastal protection to meet PES requirements -ES identification,

	quantification and geographical boundaries
	PES Requirement	Possible Coastal Protection Parameters	Measurement	Status Explanatory Notes
					Back reefs wave heights are important indicators of
		Wave height	m	C	wave energy
					Health of reef, related to its ability to attenuate wave
		Reef state -1. Hard coral abundance	% cover	C	energy and to accrete
	ES Identification &				Indicator of the rate at which the reef produces and
	Quantification	Reef state -2. Carbonate budgets	kg CaCO3m -2 yr -1	C	accumulates CaCO3
					Keystone species on Caribbean reefs and indicators of
		Reef state -3. Parrotfish density	#/m 2	C	overall health
		Beach erosion	m/yr	C	Sand lost from the beach due to wave energy
	Geographical boundary	Area of beach protected	m	C	Index methods, numerical and physical models can indicate spatial boundaries
	Key: N -Not Documented, E -Expected result but confirmed by less than 5 studies and C -Confirmed.

Table 5 :

 5 Table of management frameworks for priority management action and indications of success for

	improving coral health
	PES Requirement	Management Framework	Status	Explanatory Notes
		Improve herbivory	MPA	E	Increased numbers of parrotfish is documented in MPAs
					and linked in a few cases to minimal increases in hard
					coral
	Management measures
	required for increased
	abundance hard corals & framework builders	Reef restoration	MPA	E	Methods for gardening Acropora spp and Orbicella spp have proven successful in increasing hard coral
					abundance. Issues remain with post-transplantation
					reproductive ability and survivability.
		Improve water quality	Watershed	E	Reducing terrestrial nutrients is documented in few
					cases to increase hard coral abundance
	Key: N -Not Documented, E -Expected result but confirmed by less than 5 studies and C -Confirmed.

Table 6 :

 6 Coastal protection indicators, possible conditions and an assessment of the ability of priority management actions to be able to meet each condition.

	Indicators		Possible Conditions	H	Management Actions WQ AR	MPA	Explanatory Notes
								No documented relationship
		1. Physical	(a) Wave heights do not exceed baseline under normal conditions.	N	N	N	N	between passive management actions and condition. Active Restoration is expected to ultimately result in reduced wave heights, but has not been
								demonstrated.
			(a) Hard coral cover is equal to or exceeds 10%.	E	E	E	E	Few cases documented where management actions result in increases in hard coral cover and not to 10%.
								No cases documented where
	Measurements	2. Biological	(b) Framework builder cover is equal to or exceeds 10%.	N	N	E	N	passive management actions result in increases in framework builders and not to 10%. Active Restoration has been documented to increase abundance of Acropora spp. (framework builders and fast growers). However, not to 10% and additional concerns re survival and fitness in the long term.
								Expected that improving conditions
			(c) Carbonate					for reef health will result in positive
			budget is	N	N	N	N	carbonate budgets, but not
			positive.					documented for management
								actions.
			(d)Rugosity index is equal to or higher than 2.	N	N	N	N	Expected that improving conditions will result in higher rugosity index, but not documented for management actions.

An all-encompassing term that includes reserves and multiuse zones

ICZM is a resource management system following an integrative, holistic approach and an interactive planning process in addressing the complex management issues in the coastal area[START_REF] Thia-Eng | Essential elements of integrated coastal zone management[END_REF]. Essential elements of integrated coastal zone management. Ocean & Coastal Management 21 (1-3), 81-108.