Maxime Raynaud

Dominique Osso

Frédéric Marteau

Stanislas

Stanislas Nösperger

How deep does the retrofitting have to be? A cost-benefit analysis of two different regional programmes

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How deep does the retrofitting have to be? A cost-benefit analysis of two different regional programmes

Introduction

In France, the energy transition law for green growth has renewed the country's energy objectives, with the triple goal of reducing carbon emissions, final consumption and fossil fuels consumption. More specifically, with regard to buildings, France has set itself the aim of completing the energy renovation of 500,000 homes per year (Article 3), to have renovated all private residential buildings whose primary energy consumption is greater than 330 kWh of primary energy/(m²/year) by 2025 (Article 5), and to have a fully renovated building stock according to "low-consumption building" standards or similar by 2050 (Article 1) (JORF 2015).

Ambitious energy renovation is thus one of the central objectives of French energy policy. But unfortunately, the current renovation market in the diffuse sector (i.e. single-family homes (SFH)) is mainly characterised by inefficient or limited measures (in particular replacement of windows) and a relatively low average expenditure by household (€10,000 -translating a choice of step-by-step renovation). The average cost of the most efficient renovations (less of 10 % of the renovated dwellings) is €25,000, which is explained by a greater number of actions and better quality (ADEME 2016).

On the other hand, other study [START_REF] Stolyarova | Préférences et contraintes des ménages français lors du choix de la rénovation énergétique dans le logement[END_REF] has shown that households present a willingness to pay (WTP) for an additional 1% reduction in their energy bill that would be €330, indicating an attractiveness for the economic value of energy savings.

The high-performance retrofitting examined in this study is thus an important topic in terms of making it available to the mass market. The question of the absolute level of achievable performance thus arises, and the answer may depend on the aspects considered (financial profitability, environment, nonenergy impacts [NEIs]). With this in mind, this study addresses and compares the results of two regional energy efficiency programmes that drive overall performance. First, we will present these two programmes, followed by the raw data (energy saving, cost of retrofit) and NEIs (comfort, quality, green value). Finally, we will consider the marginal cost of the difference in performance between the programmes in order to question the relevance of the absolute attainable level (i.e. the performance threshold).

Regional energy efficiency programmes

In this study, we analyse two regional energy efficiency programmes that provide retrofitting incentives, with a strong emphasis on thermal insulation and air tightness. These two programmes were carried out in the same region (East of France) but in different territories. One rural territory was used for the Low-Consumption Renovation ("Réno' Basse Consommation" -RBC) programme. This territory is poorer and less attractive than the more urbanized, more dynamic and richer territory chosen for the "I Renovate Low Consumption Buildings" ("Je Rénove Bâtiment Basse Consommation" -JRBBC) programme (Table 1).

Beyond the specific assistance for each program, households could also receive financial incentives from the state (interest-free loans or tax credits).

JRBBC -"I Renovate Low Consumption Buildings"

The JRBBC programme was undertaken by the Alsace2 region and the EDF group (EDF SA and Electricité de Strasbourg, a subsidiary of EDF) and started in 2010 for the departments of Haut-Rhin and Bas-Rhin. The aim was to renovate 500 single-family houses, with the goal of controlling household energy bills and reducing CO2 emissions and energy consumption. The obligation to achieve a "low consumption" level was based on two main principles: give priority to improvements in the thermal envelope, and provide specific support for implementation in the form of project management (PM) for each operation (Mirtain-Roth, 2017). In effect, [START_REF] Nösperger | Building expertise: A system of professions approach to low-carbon refurbishment in the UK and France[END_REF] underline the importance of one intermediary in order to ensure strong cooperation between different building trades.

Financial and technical support was provided by the Region and by EDF. More specifically, the programme included the following elements [START_REF] Leroy | EDF, 500 maisons rénovées basse consommation[END_REF]Leroy and Nussbaumer, 2017a):

 An on-site visit and recommendations for works, materials and implementations combined with a regulatory thermal study (Th C E-ex) (MTES, 2008).  At least two to three measures relating to the thermal envelope. We observe in the the entire programme the following implemented measures: o Roof: 93% of the single-family houses participating in the programme underwent roof insulation work. o Walls: 93% (interior thermal insulation (ITI): 22 %; exterior thermal insulation (ETI): 78%). o Floors: 83%. o Glazing: 81%.  Installation of a single-flow or double-flow Controlled Mechanical Ventilation (CMV) system.  Change of heating system for 63% of homes (condensing boiler, water source heat pump) (Leroy et al. 2017).  Performance of a leak test (with Q4_psurf=0.8 m 3 /(m²/h) for 84% of operations.  Financial incentives from the Alsace region and EDF: 50 % of the cost of the PM capped at €3,000; up to €10,000 of assistance for insulation work (maximum of 70% of the work costs); financial support of € 1,500 for the air-tightness test.

To evaluate actual energy savings, a workable sample of 25 pre-renovation and 37 postrenovation operations were studied. Unfortunately, the homes belonging to both samples were very few (14 renovation projects). It should thus be noted that the different values obtained for the before/after sample contain discrepancies with results for the overall evaluation of the JRBBC programme thus the conclusion for the before/after sample should not be generalized for the whole JRBBC programme. Whenever possible, we will present these differences to indicate the presence or absence of sampling bias. All houses that were subject to work as part of the JRBBC programme were somewhat large (average between 150-160 m²). Those in the before/after sample, however, were slightly smaller (138 m² in average).

RBC -Low-Energy Renovation

The RBC programme took place in the departments of Meuse and Haute Marne starting in 2011 in the "Grand Est" region. The aim was to improve the attractiveness of the territory and to encourage businesses to achieve growth within the energy efficiency retrofit market. Programme participants received technical and financial support from an EDF business manager, from thermal diagnosis of the home through to completion of the project, including:

 An overall thermal diagnosis of the property and identification of the renovation work to be undertaken.  Qualified craftsmen and programme partners, sometimes in the form of clusters that allow local businesses to work together and deliver a high-quality offer.  Cumulative financial assistance from EDF from €200 to €4,300, depending on the measures.  Site handover with air-tightness test.  Support from a business manager throughout the project.

For the evaluation study, a sample of 55 homes was investigated in 2015. These single-family houses were quite old (87% built before 1975) and large in size (125 m² in average whereas the national average is 112 m² [START_REF] Arnault | Arrêté du 22 mars 2017 modifiant l'arrêté du 3 mai 2007 relatif aux caractéristiques thermiques et à la performance énergétique des bâtiments existants[END_REF]). The majority of works carried out concerned the insulation of the building:

 Insulation of walls, attics, and possibly the floor.  Installation of double-or triple-glazed windows.  Installation of a single-flow or double-flow CMV (controlled mechanical ventilation) system.  Change in the heating system for 45% of single-family houses, with the installation of either a heat pump (air-water mainly) or a condensing boiler or a biomass boiler.

To evaluate actual consumption, a workable sample of 31 pre-renovation and 46 post-renovation operations were studied. Unfortunately, again, only a few homes belong to both samples (31 renovation projects).

Methodology

Surveys of a sample of participants were carried out for both programmes. The standard questionnaire used for RBC has featured in previous studies [START_REF] Raynaud | An energy efficiency program analysis to understand the gaps between ex-ante and ex-post evaluations[END_REF], Raynaud et al. 2016), and will hence not be described here.

The calculation of the energy savings obtained for each participant is based on the difference in final energy consumption (all energies, all uses) between the years preceding and those following the renovation (ex-post evaluation). We studied the energy consumption for all uses; i.e. energy consumption reported by households via their bills. Actual energy consumptions were corrected for climate (heating degree days (HDD)) to return them to a normal climate level, assuming a linear dependence on HDD [START_REF] Suerkemper | Cost-effectiveness of energy efficiency programmes -evaluating the impacts of a regional programme in France[END_REF].

We eventually calculate the levelized cost of conserve energy (LCCE) relying on the consideration of energy savings and non energy impacts. The method is explain in a following section.

Results

In this section, we will first present the work carried out for the before/after samples associated with each programme. Next, we will study the costs of these works, as well as their energy impacts (gross energy savings) and NEIs. Finally, we will compare the results of the two programmes using a marginal approach by which to quantify the cost associated with the difference in their respective performance.

To avoid added uncertainty, however, a marginal approach was not used for the individual analysis of each programme. Indeed, considering only price and marginal savings requires a point of reference (i.e., the minimum performance of a measure accompanied by its minimum price) that we were not able to fully estimate in this instance. For example, the point of reference could be either the minimum performance required by the thermal regulation 3 (TR) in the existing building (JORF 2017) or else the lack of retrofit (initial state). However, we were not able to estimate the impact in terms of marginal price due to the significant dispersion of market prices that was unrelated to performance, as observed elsewhere [START_REF] Osso | The right price or the fair price? A quali-quantitative analysis of the formation of price for energy retrofit works in the residential sector in France[END_REF]. To count marginal energy savings -quantities that we could calculate -but with gross prices would be inconsistent. The point of reference here is therefore the absence of measures. Given that we will compare the outputs of both programmes to estimate a marginal performance, this is of little importance.

Completed work

While the two programmes pursued similar objectives, such as the significant reduction in consumption and CO2 by working mainly on the building shell, the means by which this was achieved were somewhat different. The following differences were observed between the works implemented in our before/after samples for the two programmes (Table 2,Table 3):  ETI works were carried out systematically but in smaller numbers for JRBBC, with higher insulation levels (differential of +17% in thermal resistance).  Less roof insulation for JRBBC, but with more expensive external roof insulation (sarking) measures.  Slightly more efficient glazing for JRBBC.

Energy savings

We observed average actual final energy consumptions of 216 kWh/(m²/year) before retrofit and 127 kWh/(m²/year) after works for the JRBBC sample: a reduction of 41% (gain of 89 kWh of final energy/(m²/year)).

For all the JRBBC sites surveyed (25 pre-and 37 post-renovation), average energy savings were in the order of 83 kWh/m². These values, which are similar to the previous, suggest that our small sample (14 renovation projects) made up of homes whose before and after consumptions are both known does not suffer from any bias.

We observed average actual final energy consumptions of 218 kWh/(m²/year) before measures and 155 kWh/(m²/year) after measures for the RBC sample: a reduction of 29% (gain of 63 kWh of final energy/(m²/year)).

For all the RBC sites surveyed (31 pre-and 46 post-renovation), energy savings were in the order of 72 KWh/m² (33%). This value, similar to the previous one, again suggests that our small sample is made up of homes whose before and after consumptions are both known and does not suffer from any bias.

Investment

The investments presented in this section are the gross prices excluding value added tax of the renovation, with no deductions of financial aid, corresponding to the work supplied and installed (materials, equipment and labour).

For the RBC programme, the average price of renovation for our before/after sample was €33,000, with a minimum of €6,000 and a maximum of €74,000. The majority of the renovation works cost between €25,000 and €50,000. The average renovation price was thus €290/m² (median: €270/m²).

For our JRBBC before/after sample, the average price was €56,000, with a minimum of €17,000 and a maximum of €91,000. The average surface area price was thus €415/m² (median: €400/m²). For the entire JRBBC programme, the average cost of the renovation work was €68,000 (median: €60,000) with a minimum of €13,000 and surpassing €100,000 in 14% of cases. This corresponds to an average cost of €465/m² (median: €411/m²) (Leroy, Nussbaumer 2017b). Observed costs for the before/after sub-sample were thus in the same order of magnitude as those observed for the programme as a whole.

For both programmes, there was a high dispersion in the total cost of renovations. However, the average price per square meter was higher for JRBBC. This difference can be explained in part by a larger portion of ETI being used in JRBBC than in RBC, where ITI was mostly used (see Table 3). It should also be noted that the JRBBC programme included roof insulation by sarking, which is significantly more expensive than simple interior attic insulation (see Table 3).

Also, some existing costs for the JRBBC programme were not present in RBC. In the before/after JRBBC sample, for example, the average total cost of the project management (PM) and the thermal study was €6,000. In the case of RBC, support was provided by EDF and not billed to the client. If we subtract these additional costs (i.e. PM and the thermal study), the average price for JRBBC becomes €50,000 (€370/m²) to compare with the RBC program. However, this additional cost helps to reach a better efficiency (see below).

Finally, the price difference between the two programmes can also be explained, in part, by effects linked to the difference in income of the participating households (see Table 1). An initial "purchasing power effect" shows that costs are higher when households are richer; however, this effect is very weak [START_REF] Osso | The right price or the fair price? A quali-quantitative analysis of the formation of price for energy retrofit works in the residential sector in France[END_REF]. A second effect concerns the quality or brand of products that are more accessible for wealthy households, which have no direct link to their energy efficiency but come at a higher cost. Unfortunately, we have no information on this topic.

Non-energy impacts (NEIs)

For both programmes, the household's motivation was mainly to "save energy" and "improve comfort". Other motivations may also be apparent, such as enhancements to property, aesthetics, energy performance or environmental issues.

Overall, for both JRBBC and RBC, over 90% of households were satisfied with the work carried out. Nevertheless, both programmes experienced quality problems. However, these were resolved thanks notably to on-site monitoring. Thus, the cost of PM should be included in the RBC economic balance.

In JRBBC, winter thermal comfort was the criterion that saw the greatest improvement: 79% of the respondents declared that they felt an average or poor level of comfort before the renovation work began. Upon completion, only 5% made the same statement. In the same way, no occupants declared an excellent level of comfort before the renovation works began, compared with 40% upon completion. Temperatures were measured mainly between 18° C and 21° C [START_REF] Leroy | Fascicule A : la performance énergétique globale et le confort thermique[END_REF].

For the RBC programme, temperatures were reported by households rather than measured. Overall, the comfort temperature increased slightly in the living rooms (+0.4° C) and more significantly in the other rooms (+1.5° C). We also observed a reduction in extreme values upon completion of the renovation works, with temperatures converging around 19-21° C.

Note that for the Grand Est region, where both programmes were located, the impact of energy performance rating on house prices (i.e. the green value) is relatively high, with a difference of 19% between classes F or G compared to class C (DINAMIC 2017). For the JRBBC programme, where thermal studies were systematically conducted, an average gain of four classes (F  B) (Leroy 2017) was observed. For RBC, 47% of households reported an increase in actual estate values (without quantification) as a significant impact of their renovation work.

If the average impact of the energy performance rating is estimated at the regional level (i.e. no detailed data at a more scaled-down level), house prices differed according to the departments in which the programmes took place (Actualitix 2012): €942/m² for Haute-Marne and €946/m² for Meuse versus €1,840/m² for Haut-Rhin and €1,910/m² for Bas-Rhin. The absolute financial impact on property values was therefore greater for JRBBC than for RBC. On the basis of only 14% (the difference between band F or G and band D), the green value would represent 45% of the renovation costs for RBC and 60% for JRBBC.

Comparison of the two programmes: the marginal cost of performance

Although the initial average surface-area energy consumption of our before/after samples for the two programmes was equivalent (216 vs. 218 kWh/(m²/year)), the observed energy savings were different (89 kWh net of final energy/(m²/year) for JRBBC and 63 kWh net of final energy/(m²/year) for RBC). These differences raise two questions:

 What is the source of these additional gains for JRBBC?  What is the marginal cost of additional energy savings?

The source of the additional gains for the JRBBC programme could be:

 The slightly higher thermal performance of the measures taken in JRBBC than in RBC, particularly for the walls (thermal resistance and ETI) and windows.  A possible higher quality of work in JRBBC via the more "technical" monitoring provided by the PM compared to that of the EDF business manager in RBC. Due to significant price dispersion and energy savings, it is not statistically valid to link the increase in energy savings with that of investments. We should hence remain cautious about any conclusions. We can thus calculate the levelized cost of conserved energy (LCCE) [START_REF] Krey | Annex II: Metrics & Methodology[END_REF] for the two programmes studied here:

𝐿𝐶𝐶𝐸 = 𝐶𝑅𝐹 * ∆𝐼-∆𝐺𝑉𝐿-𝑊𝑇𝑃 𝑐 ∆𝐸
eq. 1

Where: CRF: capital recovery factor = a(1+a) n /((1+a) n -1) with a=discount rate and n=number of years, I: investment, E: energy savings, GVL: depreciated and annualized green value, WTPc: willingness to pay for comfort [START_REF] Nösperger | A proposal to go beyond the rebound effect: how to evaluate the financial value of comfort after retrofitting?[END_REF], it depends on the energy prices and on their evolution, we consider a general energy prices escalation rate of 2%).

and estimate the marginal LCCE based on the differences in price and energy savings for the two programmes. As a first step, without taking into account the term associated with the green value, and ignoring the cost of the PM for JRBBC, the marginal LCCE4 is then €0.154/kWh -a value lower than the average gross costs of energy savings GCES (GCES =CRF*I/E) in both programmes of between €0.20-0.22/kWh. If we include the price5 of the PM in the investment costs for both programmes, the marginal cost increases slightly (€0.164/kWh) but remains under the average gross energy savings costs.

However, if we integrate the depreciated and annualized minimum green value (GVL, with GV =14 %) [START_REF] Osso | Regional efficiency programme valuating energy and multiple benefits: a balance between bill and comfort and far beyond[END_REF] as a deduction from investments in the LCCE calculation (eq. 1), this gives us a marginal LCCE of €0.096/kWh. The integration of the green value therefore significantly reduces the LCCE (albeit the impact of green value is greatly diminished by its depreciation and discounting) to a level below the average gross energy savings costs in the two programmes. These results can be understood as the desire to invest with a view to increasing energy savings and reaching a better energy performance band, resulting in a home with a greater green value. From the perspective of households, however, the green value can be considered more as an "option value" on the future and hypothetical sale of their property6 than a tangible financial gain.

Finally, the impact of the WTP for comfort further reduces the LCCE either slightly or more significantly depending on the assumptions retained (i.e. low or high WTPc as observed in [START_REF] Nösperger | A proposal to go beyond the rebound effect: how to evaluate the financial value of comfort after retrofitting?[END_REF].

Conclusion & policy implications

This study compared two ambitious and similar regional energy efficiency programmes located in the same region but in different territories. Both programmes emphasized the reduction of energy needs via the quasi-systematic treatment of the walls of the building and improvements to ventilation (installation of a CMV and permeability test).

The energy savings obtained were consistent with higher average prices for more savings (-29% and -41% energy savings for a price of €290/m² and €415/m² for RBC and JRBBC, respectively).

Beyond a slightly better performance for JRBBC, these differences in energy savings can be explained by the different techniques employed (ITI vs. ETI) and closer technical support (dedicated PM).

The LCCE was slightly higher for JRBBC due to the cost of the PM. When we integrate the green value, however, this figure is lower than that of RBC. This seems to indicate that while the PM allows for additional energy savings via higher quality implementation, their presence increases the average price of kWh avoided. However, the financial value of the PM is retained when the green value is taken into account. Comprehensive financial assistance for on-site support -and not only for equipment -could be an interesting approach, in particular to improve the acceptability of this additional service, which rarely features in the current renovation market.

The marginal gain of energy savings is highly sensitive to assumptions (integrated PM or not). It is thus difficult to draw a conclusion as to the economic relevance of the additional gains in energy savings without taking other parameters into account (NEIs).

Taking the green value, which is different in absolute value according to the territories, into account in the LCCE, even if this reduces the cost of kWh saved, further reduces the marginal cost of the additional savings. However albeit this is largely minimized in its strict economic calculation and does not necessarily reflect the feelings about property value that households seem to allocate it at the time of the renovations being carried out. A different approach could improve clients' attitudes to this green value.

Unfortunately, due to the somewhat limited data in our ex-post evaluation studies, it remains difficult to draw definitive conclusions. However, this paves the way for future reflection.

Figure 1 :

 1 Figure 1: gain (kWh/m²) as a function of price (€/m²).

  ∆𝑉 𝐹 /𝐺→𝐷 : the difference in the sale price associated with the transition from energy performance band F or G to energy performance band D.

Table 1 :

 1 characteristics of the territories of the RBC (Meuse and Haute-Marne) and JRBBC (Haut-Rhin and Bas-Rhin) regional programmes (source: INSEE, 2014).

	Meuse	Haute-Marne	Bas-Rhin	Haut-Rhin
	(55)	(52)	(67)	(68)

Table 2 :

 2 Measures implemented during the renovation and thermal performance (average value) associated with samples before/after the JRBBC and RBC programmes

	Programme	Roof/ high-floor	Wall (ITI, ETI)	Floor	Glazing (double, triple)	CMV	Space heating system
		% works	R (m²K/W)	% works	R (m²K/W)	% works	R (m²K/W)	% works	Uw (W/m²K)	% works	% works
	JRBBC	78	7.28	57	4.45	57	3.13	71	1.25	93	57
	RBC	91	7.15	93	3.8	63	3.41	74	1.39	91	45

Table 3 :

 3 Details of measures implemented during the associated thermal renovation and performance for samples before/after the JRBBC and RBC programmes

	Programme	External roof	insulation	Roof Renovated attics	Uninhabitable	attics	Window (double-glazing) % works Uw (W/m²K)	Window (triple-glazing) % works Uw (W/m²K)	Single-flow CMV % works	Double-flow CMV % works
	JRBBC	46	18	36	93	1.28	7	1.0	85	15
	RBC	0		18	82	88	1.4	12	1.1	84	16

3 Which only applies in case of retrofit. 2018 International Energy Policy & Programme Evaluation Conference -Vienna, Austria

2018 International Energy Policy & Programme Evaluation Conference -Vienna, Austria

Table 4 :

 4 GCES and LCCE estimate (€/kWh) between programmes, with the assumption of a lifetime (n) of 30 years and a discount rate (a) of 2.5 % including or not the JRBBC PM cost. High WTPC = €/m²0.18, low WTPc = €/m²1.44 (assessed from[START_REF] Nösperger | A proposal to go beyond the rebound effect: how to evaluate the financial value of comfort after retrofitting?[END_REF].

							w/ JRBBC	w/ JRBBC	w/ JRBBC
	programme		w/o JRBBC PM	w/ JRBBC PM	PM & green value	PM, green value & high WTPc	PM, green value & low WTPc
		€/m² kWh/m² €/kWh €/m² kWh/m² €/kWh	€/kWh	€/kWh	€/kWh
	RBC -GCES	290	63	0.220 329	63	0.249	0.222	0.191	0.218
	JRBBC -GCES 370	89	0.201 415	89	0.225	0.186	0.164	0.183
	LCCE	80	26	0.154 125	26	0.164	0.096	0.019	0.087

CU = 1 st adults =1 ;

nd adults = 0.5 ; child = 0.3…

Now "region Grand Est" (Alsace -Champagne Ardenne -Lorraine).

Calculation based on the differences in price and energy savings between the RBC and JRBBC programmes.

In as much as no data concerning PM cost for the RBC programmes (business manager) we used the JRBBC PM cost adjusted to the income level of the region.

The approach used in the tertiary sector whereby the green value is considered as a risk premium on the cash flow of the invested capital is more difficult to apply for owner-occupier households, unless one considers an imputed rent (fictitious rent).

Acknowledgements

Sabine Mirtain-Roth, JRBBC project manager, Patrick Leseck, RBC project manager and Franck Lejuez, energy efficiency project manager on Haute-Marne et Meuse territories.

Disclaimer

Any opinions, findings, and conclusions or recommendations expressed in this material are those the authors and do not necessarily reflect the views of their affiliations.


Adrien Jacotot
email: adrien.jacotot@protonmail.com

Cyril Marchand

Michel Allenbach

Tidal variability of CO 2 and CH 4 emissions from the water column within a Rhizophora mangrove forest (New Caledonia)

Keywords:

Introduction

Mangroves are considered as major ecosystems in the carbon cycle along tropical and subtropical coastlines, being among the most efficient blue carbon sinks [START_REF] Kauffman | Ecosystem carbon stocks of Micronesian mangrove forests[END_REF]. Due to their high primary productivity, estimated at 218 ± 72 TgC year -1 [START_REF] Bouillon | Organic matter exchange and cycling in mangrove ecosystems: recent insights from stable isotope studies[END_REF], and their long-term carbon storage capacities [START_REF] Donato | Mangroves among the most carbon-rich forests in the tropics[END_REF], mangroves have been recognized as having a key role in climate change mitigation [START_REF] Howard | Clarifying the role of coastal and marine systems in climate mitigation[END_REF][START_REF] Mcleod | A blueprint for blue carbon: toward an improved understanding of the role of vegetated coastal habitats in sequestering CO 2[END_REF]. However, part of mangroves primary productivity is mineralized in their soils, producing greenhouse gas (GHG) that can be subsequently emitted towards the atmosphere [START_REF] Chen | Rich soil carbon and nitrogen but low atmospheric greenhouse gas fluxes from North Sulawesi mangrove swamps in Indonesia[END_REF]. Among GHG, methane may be of major concern due to its global warming potential, 34 times higher than CO 2 using a 100 year time frame and climate carbon feedback [START_REF] Stocker | Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change[END_REF]. In anoxic mangrove soils, when electron acceptors such free oxygen, metal oxides, nitrates, and sulfates have been exhausted, methanogenesis can occur. Recently, it was demonstrated that sulfate reducing and methanogens microorganisms can coexist in mangrove soils and it was thus suggested that methane emissions have been underestimated [START_REF] Chauhan | Factors influencing spatiotemporal variation of methane and nitrous oxide emission from a tropical mangrove of eastern coast of India[END_REF][START_REF] Lyimo | Diversity of methanogenic archaea in a mangrove sediment and isolation of a new Methanococcoides strain[END_REF].

In mangrove soils, organic matter (OM) decay processes depend on numerous factors, including waterlogging (Kristensen et al., 2008a). In fact, mangroves are subjected to tidal cycles that influence the redox characteristics of their soils, modifying the rate of GHG production. In addition, tides can also influence GHG emissions towards the atmosphere since gas diffusion differs when the soil is immerged or unsaturated. Recently, many studies focused on CO 2 or CH 4 emissions at the soil-air interface at low tide [START_REF] Bulmer | Sediment properties and CO2 efflux from intact and cleared temperate mangrove forests[END_REF][START_REF] Chanda | Measuring daytime CO2 fluxes from the inter-tidal mangrove soils of Indian Sundarbans[END_REF][START_REF] Chauhan | Factors influencing spatiotemporal variation of methane and nitrous oxide emission from a tropical mangrove of eastern coast of India[END_REF][START_REF] Chen | Soil greenhouse gas emissions reduce the contribution of mangrove plants to the atmospheric cooling effect[END_REF][START_REF] Chen | Summer fluxes of atmospheric greenhouse gases N2O, CH4 and CO2 from mangrove soil in South China[END_REF][START_REF] Leopold | Influence of mangrove zonation on CO 2 fluxes at the sediment-air interface (New Caledonia)[END_REF][START_REF] Leopold | Temporal variability of CO 2 fluxes at the sediment-air interface in mangroves (New Caledonia)[END_REF][START_REF] Livesley | Temperate mangrove and salt marsh sediments are a small methane and nitrous oxide source but important carbon store[END_REF][START_REF] Wang | Temporal and spatial variations of greenhouse gas fluxes from a tidal mangrove wetland in Southeast China[END_REF], or at the water-air interface in adjacent tidal creeks [START_REF] Borges | Atmospheric CO 2 flux from mangrove surrounding waters[END_REF][START_REF] Bouillon | Inorganic and organic carbon biogeochemistry in the Gautami Godavari estuary (Andhra Pradesh, India) during pre-monsoon: the local impact of extensive mangrove forests[END_REF][START_REF] Call | Spatial and temporal variability of carbon dioxide and methane fluxes over semi-diurnal and spring-neap-spring timescales in a mangrove creek[END_REF][START_REF] Maher | Methane and carbon dioxide dynamics in a subtropical estuary over a diel cycle: insights from automated in situ radioactive and stable isotope measurements[END_REF]. However, to our knowledge, no study focused on water to atmosphere GHG emissions when the forest is inundated, which may frequently occur, depending on the tidal range and the position of the forest in the intertidal zone.

Stable isotope of carbon (δ 13 C) is a widely used tool for studying carbon dynamic in natural environments [START_REF] Gonneea | Tracing organic matter sources and carbon burial in mangrove sediments over the past 160 years[END_REF][START_REF] Graham | A Study of Carbon and Nitrogen Stable Isotope and Elemental Ratios as Potential Indicators of Source and Fate of Organic Matter in Sediments of the Forth Estuary[END_REF][START_REF] Lu | Identifying organic matter sources using isotopic ratios in a watershed impacted by intensive agricultural activities in Northeast China[END_REF][START_REF] Yamamuro | Chemical tracers of sediment organic matter origins in two coastal lagoons[END_REF], including the partitioning of the different sources [START_REF] Midwood | Challenges in measuring the δ 13 C of the soil surface CO 2 efflux[END_REF][START_REF] Millard | Quantifying the contribution of soil organic matter turnover to forest soil respiration, using natural abundance δ 13 C[END_REF][START_REF] Paterson | Through the eye of the needle: a review of isotope approaches to quantify microbial processes mediating soil carbon balance: Tansley review[END_REF]. In mangrove ecosystems, mangrove litter, benthic microalgae, phytoplankton and seagrass detritus are the main OM sources (Kristensen et al., 2008a), and the latter may be distinguishable thanks to their specific δ 13 C values. Marine OM has typical value of δ 13 C ranging from -30 to -16‰, whereas mangroves plants are C3 photosynthetic pathways, and therefore produce OM that have a δ 13 C value ranging from -32 to -21‰ [START_REF] Lamb | A review of coastal palaeoclimate and relative sea-level reconstructions using δ 13 C and C/N ratios in organic material[END_REF]. In addition, due to a low carbon fractionation during respiration processes [START_REF] Lin | Carbon Isotopic Fractionation Does Not Occur during Dark Respiration in C3 and C4 Plants[END_REF][START_REF] Maher | Methane and carbon dioxide dynamics in a subtropical estuary over a diel cycle: insights from automated in situ radioactive and stable isotope measurements[END_REF], the δ 13 C of the CO 2 produced is close to the δ 13 C value of its source. Recent development of advanced technologies such as cavity ring-down spectroscopy (CRDS) allows high resolution in situ measurements of δ 13 CO 2 . Therefore, these new analytical means could help to identify the origins of the CO 2 emitted from mangrove ecosystems.

Within this context, the present study aimed to (i) quantify the CO 2 and CH 4 emissions at the water-air interface within a Rhizophora spp. mangrove forest, (ii) evaluate the variability of these emissions along tidal cycles, and (iii) identify the origin of the CO 2 fluxes measured. To reach our goals, we measured CO 2 and CH 4 fluxes, as well as δ 13 CO 2 values, in the field, using a dark floating chamber connected to a cavity ring-down spectrometer analyzer (CRDS), along different tidal cycles from neap to spring tides.

Material and methods

Study site

The present study was conducted in the mangrove of Ouemo (22°16′50″S, 166°28′16″E), in New Caledonia, a French overseas archipelago located in the South Pacific (21°21′S, 165°27′E). The studied mangrove was dominated by three Rhizophora mangrove species: R. stylosa, R. samoensis and R. selala. Climate in the region is semi-arid and strongly influenced by the inter-tropical convergence zone and by the El Niño Southern Oscillation (ENSO). Average air temperature varied between 20.5 and 26.6 °C, with a mean annual precipitation of 1070 mm (data from meteofrance.com). The tidal regime is semidiurnal, with a tidal range ranging from 1.10 to 1.70 m.

Gas fluxes measurements

CO 2 , δ 13 CO 2 and CH 4 measurements were performed using a dark custom-built floating chamber (466 cm 2 ; 5050 cm 3 ) connected to a G2131-i CRDS analyzer (Picarro Inc., Santa Clara, CA, USA) that measures gas concentrations at a frequency of 1 Hz. Guaranteed precision of the analyzer are 200 ppb + 0.05% of reading, 50 ppb + 0.05% of reading and 0.1‰, for CO 2 and CH 4 , and δ 13 CO 2 , respectively. Accuracy of the CRDS analyzer was periodically checked using certified N 2 (0 ppm CO 2 and CH 4 ), CO 2 (503 ppm) and CH 4 (100 ppm) gas standard samples (Calgaz, Air Liquide, USA).

We are aware that floating chambers may induce a bias in flux measurements during windy conditions or with high current velocity that induce artificial turbulences and, as a result, increase fluxes [START_REF] Kremer | Technical note: conditions for using the floating chamber method to estimate air-water gas exchange[END_REF][START_REF] Yang | Progress in the studies on the greenhouse gas emissions from reservoirs[END_REF]. However, in low turbulence environment, like inside the mangrove forest where the wind is almost null, and the water flow is slow, the floating chamber technique can be a powerful method [START_REF] Lorke | Technical Note: Drifting Versus Anchored Flux Chambers for Measuring Greenhouse Gas Emissions from Running Waters[END_REF][START_REF] Vachon | The relationship between near-surface turbulence and gas transfer velocity in freshwater systems and its implications for floating chamber measurements of gas exchange[END_REF]. Thus, we feel confident about the validity of our measurements. In addition, floating chambers have the capability to capture ebullition events that may account for a large proportion of the gas transferred to the atmosphere, particularly CH 4 [START_REF] Chuang | Methane fluxes from tropical coastal lagoons surrounded by mangroves, Yucatán, Mexico: methane fluxes from coastal lagoons[END_REF].

Measurements were performed every three weeks, from December 2016 to September 2017, at a single sampling station. Measurements were done during sunny days, with the slack high tide around noon, and included different tidal regimes. During neap tides, the tidal range varied from 1.10 to 1.25 m, which corresponded to a water column thickness above the mangrove sediment of 15 to 30 cm at the maximum of the high tide. During spring tides, the tidal range varied from 1.25 to 1.55 m, and, therefore, the water column thickness at the maximum of the high tide ranged from 30 cm to 60 cm. Measurements were performed all along the high tide, from the beginning of the flow to the end of the ebb. For each incubation, an integrating period of 3 to 6 min was chosen, depending on the linearity of the signal. Therefore, 19 to 40 measurements were performed during each campaign, resulting in a total of 284 flux measurements.

In addition, before each incubation, water level was measured thanks to a water gauge, and air temperature was recorded thanks to a handheld Skymate SM-19 thermometer.

Flux calculation

Water to atmosphere fluxes of CO 2 and CH 4 were integrated as a function of time, and calculated using the following formula:

F (CO2,CH4) = (d(CO 2 ,CH 4 )/dt) * V/(R * S * T) * 3.6
where F is the water to atmosphere fluxes of CO 2 or CH 4 (mmolC m -2 h -1 ); d(CO 2 , CH 4 ) / dt is the variation in CO 2 or CH 4 as a function of time (ppm s -1 ); V is the total volume of the system (m 3 ); R is the ideal gas constant of 8.205746 10 -5 (atm m 3 K -1 mol -1 ); T is the absolute air temperature (K); and S is the area of the bottom of the incubation chamber (m 2 ).

δ 13 CO 2

To measure the isotopic value of the CO 2 (δ 13 CO 2 ) released from the water column, we used a Keeling plot approach [START_REF] Keeling | The concentration and isotopic abundances of atmospheric carbon dioxide in rural areas[END_REF][START_REF] Keeling | The concentration and isotopic abundances of carbon dioxide in rural and marine air[END_REF][START_REF] Pataki | The application and interpretation of Keeling plots in terrestrial carbon cycle research[END_REF]. By plotting the δ 13 CO 2 value CO 2 as a function of the inverse of the CO 2 concentration (δ 13 CO 2 = f(1/CO 2 )) during each incubation, the intercept of a linear regression with the y-axis is equivalent to the δ 13 CO 2 value of the flux.

In addition, three supplementary incubations were realized at high tide outside the mangrove forest to evaluate the δ 13 CO 2 value of the CO 2 produced within the water column in the lagoon.

Statistical analyses

Student's t-tests were used to test the significant differences (p b 0.05) in the emissions between flow and ebb tides as well as between spring and neap tides. All statistical analyses were performed under R software version 3.3.1 (R Development Core Team, 2008).

Results and discussion

CO 2 and CH 4 emissions from the water column to the atmosphere varied from 0.13 to 18.41 mmolC m -2 h -1 and from 0.18 to 172.07 μmolC m -2 h -1 , respectively (Fig. 1). Thus, when converted into CO 2equivalent and considering its 100-year global warming potential [START_REF] Stocker | Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change[END_REF], methane represented 18% of the total emissions measured (CO 2 + CH 4 ). Gas transfer to the atmosphere may occur by upward molecular diffusion (Kristensen et al., 2008b) but also through gas bubble ebullition, mainly for methane due to its lower solubility [START_REF] Barnes | Tidal dynamics and rainfall control N2O and CH4 emissions from a pristine mangrove creek[END_REF][START_REF] Komiya | Contribution of ebullition to methane and carbon dioxide emission from water between plant rows in a tropical rice paddy field[END_REF][START_REF] Stamp | The importance of ebullition as a mechanism of methane (CH4) loss to the atmosphere in a northern peatland[END_REF]. However, in the present study, considering the linear evolution of CO 2 and CH 4 concentrations with time during each flux measurement, we suggest that only the diffusive fluxes were measured. As a consequence, the emissions reported herein may have been under-estimated, particularly methane.

To our knowledge, this study was the first one interested in measuring GHG emissions at the water-air interface within a mangrove forest. Therefore, we decided to compare our results with fluxes from other interfaces (i.e. from mangrove soils and creek waters to the atmosphere). CO 2 emissions were in the same order of magnitude and even higher than from mangrove soils at low tide or from mangrove tidal creeks (Table 1). It was demonstrated that soil surface biofilm may limit GHG emissions from mangrove soils, notably by acting as a physical barrier but also by using the CO 2 for primary production through photosynthesis [START_REF] Bulmer | Sediment properties and CO2 efflux from intact and cleared temperate mangrove forests[END_REF][START_REF] Leopold | Temporal variability of CO 2 fluxes at the sediment-air interface in mangroves (New Caledonia)[END_REF][START_REF] Leopold | Influence of mangrove zonation on CO 2 fluxes at the sediment-air interface (New Caledonia)[END_REF]. As a result, some authors reported negative fluxes at the soil surface (e.g. [START_REF] Chen | Rich soil carbon and nitrogen but low atmospheric greenhouse gas fluxes from North Sulawesi mangrove swamps in Indonesia[END_REF][START_REF] Leopold | Temporal variability of CO 2 fluxes at the sediment-air interface in mangroves (New Caledonia)[END_REF], which was never the case from the water column in the studied Rhizophora forest. Closed canopy and elevated water turbidity probably limited the development of microalgae that might have used the CO 2 produced in the soil. In addition, low tide fluxes measurements are strictly representative of the surface that is covered by the incubation chamber, and, usually, roots and crab burrows are excluded to avoid overestimations of the fluxes. At the opposite, water-air fluxes integrate the entire soil surface that is flooded by the water, and, therefore, roots and burrows are an important factor to take into account as they represent preferential diffusion pathways for the gases to escape from the sediment. The density of Rhizophora roots and crab burrows could not be measured in our study site, however, the density of crab burrows was reported for other mangrove in New Caledonia and was evaluated to 83 burrows m -2 for the fiddler crab Uca spp. [START_REF] Nielsen | Impact of fiddler crabs (Uca spp.) on rates and pathways of benthic mineralization in deposited mangrove shrimp pond waste[END_REF], and from 8.9 to 29.1 crabs ha -1 for the mud crab Scylla serrata [START_REF] Dumas | Mud crab ecology encourages sitespecific approaches to fishery management[END_REF].

However, regarding CH 4 , some studies reported much higher values from mangrove soils but lower from tidal creeks (Table 1). When CH 4 diffuses from anoxic mangrove soils into oxic lagoon water, it may be subject to aerobic oxidation by methane-oxidizing bacteria or to photo-oxidation [START_REF] Morana | Methanotrophy within the water column of a large meromictic tropical lake (Lake Kivu, East Africa)[END_REF][START_REF] Smith | Exchange of greenhouse gases between soil and atmosphere: interactions of soil physical factors and biological processes[END_REF], therefore limiting emissions at the water-air interface both in mangrove forests and in tidal creeks. We thus suggest that further measurements of GHG emissions must be performed in mangroves at high tide with different physiographic conditions. We also suggest including these results in future mangrove carbon budgets to make them more accurate, this ecosystem being flooded part of the time.

Our results demonstrate that CO 2 and CH 4 fluxes were highly variable with tides. As the thickness of the water column above the mangrove soil increased, CO 2 and CH 4 emissions decreased (Fig. 2a, b, c andd). The δ 13 CO 2 value of the CO 2 fluxes measured at the lowest tidal level was -26.88‰ (Fig. 2e), which was close to the δ 13 C values of Rhizophora roots, ranging from -32 to -25‰ [START_REF] Weiss | Soil organic carbon stocks in estuarine and marine mangrove ecosystems are driven by nutrient colimitation of P and N[END_REF], and organic matter, ranging from -30 to -20‰ (Bouillon et al., 2003a;[START_REF] Saintilan | Allochthonous and autochthonous contributions to carbon accumulation and carbon store in southeastern Australian coastal wetlands[END_REF][START_REF] Weiss | Soil organic carbon stocks in estuarine and marine mangrove ecosystems are driven by nutrient colimitation of P and N[END_REF]. Oppositely, the δ 13 CO 2 value measured at the highest tidal level that was -8.6‰ (Fig. 2e andf), which was similar to the values of δ 13 CO 2 measured in the lagoon outside the mangrove. Consequently, we suggest that (i) the variability of the CO 2 fluxes measured resulted from a mixing between these two sources, i.e. the CO 2 produced within the soil and the one produced within the water column, and that (ii) emissions decreases with the increasing water column thickness resulted from the dilution of pore waters, enriched in CO 2 , by lagoon waters. We also suggest that few biogeochemical processes affected CO 2 dynamic within the water column due to (i) its low thickness that implied a low residence time of the gas, and (ii) limited photosynthetic processes resulting from dense canopy and water turbidity, as explained earlier.

Regarding CH 4 , water mixing and aerobic oxidation, within the water column, as it was observed in other studies [START_REF] Abril | Enhanced methane oxidation in an estuarine turbidity maximum[END_REF][START_REF] Bouillon | Importance of intertidal sediment processes and porewater exchange on the water column biogeochemistry in a pristine mangrove creek (Ras Dege, Tanzania)[END_REF][START_REF] Dutta | Reviews and syntheses: methane biogeochemistry in Sundarbans mangrove ecosystem, NE coast of India; a box modeling approach[END_REF] may partly explain the results obtained. However, measurements of both δ 13 CH 4 and dissolved oxygen are needed to comfort these hypotheses.

In addition, significant higher emissions were measured during the ebb than during the flow (two-samples t-test, CO 2 : t (205.66) = 5.11, p b 0.001, CH 4 : t (146.3) = 7.96, p b 0.001), but also during spring tides than during neap tides (CO 2 , t (180.56) = -4.60, p b 0.001, and CH 4 , t (134.1) = -5.32, p b 0.001) (Fig. 2a, b, c andd). We suggest that these differences may result from a combination of different factors including: residence times of the water in the soil, volume of lagoon water entering the mangrove, area of submerged mangrove, and electron acceptor renewal.

During neap tides, CO 2 emissions reached up to 9.59 mmolC m -2 h -1 at the beginning of the rising tide, and up to 14.42 mmolC m -2 h -1 at the end of the ebb tide (Fig. 2a andb). Meanwhile, as water thickness decreased during ebb, the δ 13 CO 2 values showed clearly an increased contribution of the CO 2 originated from mangrove soils, with more depleted values at the end of the ebb tides (Fig. 2e andf). Several forcing factors may enhance the exchanges of gaseous compounds between the soil and the overlying water, including: (i) flushing of the upper centimeters Table 1 Various dark CO 2 (mmolC m -2 h -1 ) and CH 4 (μmolC m -2 h -1 ) emissions from mangrove forests during emersion periods, and from mangrove creeks and estuaries.

Location

Climate CO 2 (mmolC m -2 h -1 ) C H 4 (μmolC m of soil driven by bottom currents inducing pressure gradients at the soil surface, concept of "skin circulation" introduced by [START_REF] Billerbeck | Surficial and deep pore water circulation governs spatial and temporal scales of nutrient recycling in intertidal sand flat sediment[END_REF]; (ii) macrofaunal burrowing activities during inundation, causing an increase in soil permeability [START_REF] Santos | The driving forces of porewater and groundwater flow in permeable coastal sediments: a review[END_REF]; (iii) flushing of crab burrows, the later significantly enhancing the effective surface area of mangrove soils (up to 7-fold), therefore increasing the diffusive fluxes towards the water column [START_REF] Stieglitz | Passive irrigation and functional morphology of crustacean burrows in a tropical mangrove swamp[END_REF][START_REF] Stieglitz | The mangrove pump: the tidal flushing of animal burrows in a tropical mangrove forest determined from radionuclide budgets[END_REF][START_REF] Heron | The tidal flushing of multiple-loop animal burrows[END_REF]. We suggest that the increased residence time of the water during inundation resulted in enhanced exchanges across the soil-water interface due to the different processes listed above, inducing its enrichment in gaseous compounds, and explaining thus the higher emissions measured during the ebb than during the flow. Additional processes may be involved for CH 4 since the difference between the beginning of the flow and the end of the ebb was much higher than for CO 2 (Fig. 2); CH 4 concentrations was b21 μmolC m -2 h -1 during the flow and reached N170 μmolC m -2 h -1 during the ebb when the water levels were minimum (Fig. 2c andd). During low tide, atmospheric air can penetrate deeper in the soil, limiting its anoxic character and thus CH 4 production (Kristensen et al., 2008a). However, although its production can occur deeper in the soil, CH 4 may be oxidized during its transit to the atmosphere within the soil, resulting in the low emissions measured at the beginning of the flow. Conversely, at the end of the ebb, most of electron acceptors may have been exhausted due to the installation of an anoxic environment during the high tide, and thus, methanogenesis may occur, resulting in higher CH 4 emissions. During spring tides, emissions intensity differed from neap tides. If fluxes were not significantly higher during the flow, they were during the ebb, with CO 2 and CH 4 emissions reaching up to 18.41 mmolC m -2 h -1 and 172.07 μmolC m -2 h -1 , respectively (Fig. 1 and Fig. 2a, b, c andd). One possible explanation is a higher renewable of the electron acceptors pool within the soil during spring tide, which enhanced the microbial activity and consequently greenhouse gas production. Then, at low tide, the water level within the soil may be lower during spring tide, resulting in higher contribution of the soil deep layers to CH 4 emissions, as gas diffusion is higher in air than in water. Additionally, higher water level during spring tides may have flooded the upper intertidal zones within the mangrove forest, increasing therefore the exchanges between the surface pore waters and the overlying water column. The flushing of surface pore waters and burrows of this upper intertidal zone, due to changes in hydrostatic pressure along the topographic gradient during ebb, may be involved in the enhancement of the emissions, analogically to the concept of "mangrove tidal pumping", which has been described to significantly enrich tidal creeks in nutrients, gaseous and dissolved compounds [START_REF] Call | Spatial and temporal variability of carbon dioxide and methane fluxes over semi-diurnal and spring-neap-spring timescales in a mangrove creek[END_REF][START_REF] Dittmar | Molecular evidence for lignin degradation in sulfate-reducing mangrove sediments (Amazonia, Brazil)[END_REF][START_REF] Maher | Groundwaterderived dissolved inorganic and organic carbon exports from a mangrove tidal creek: the missing mangrove carbon sink?[END_REF][START_REF] Maher | Methane and carbon dioxide dynamics in a subtropical estuary over a diel cycle: insights from automated in situ radioactive and stable isotope measurements[END_REF]. In addition, the more depleted δ 13 CO 2 values during ebb of spring tides, in comparison to ebb of neap tides (Fig. 2f), and except at the lowest water column thickness, support the hypothesis of a higher contribution of CO 2 produced within mangrove soils to the water column. Surprisingly, the lowest δ 13 CO 2 values were measured at the beginning of the flow (Fig. 2e), which were close to the δ 13 C values of mangrove soil organic matter or mangrove roots as explain earlier in this study, suggesting than mangrove pore waters strongly dominated the water column composition at this period of the tide, which is clearly different from neap tides (Fig. 2e). This result may be related to the tidal wave spread when entering the mangrove, inducing higher pore water advection during the spring tides notably through crab burrows. However, although higher mean CO 2 and CH 4 emissions were observed for the flow during spring tides, the maximal value reached by the emissions was not higher than during neap tides (Fig. 2a). One possible explanation is that CO 2 and CH 4 did not accumulate within the soil at low tide due to the more efficient diffusion of GHG in air than in water, and thus were rapidly emitted to the atmosphere. When the tide increased, the deep soil pore waters, depleted in δ 13 CO 2 , were transported to the surface, resulting in low δ 13 CO 2 values emitted at the water surface. This study thus highlighted that tides (flow/ebb, water column thickness, neap/spring) is a key factor controlling CO 2 and CH 4 emissions from the water column during mangrove forests immersion periods. In a future research effort, the influence of other environmental parameters such as seasonal temperature variations should be studied. Although the sampling period in this study encompassed the two main seasons in New Caledonia, they were not entirely covered, and considering the high tidal variability, our data set did not allow us to conclude on a possible seasonal effect.

Conclusion

This preliminary study demonstrates that tide characteristics (flow/ ebb, water column thickness, neap/spring) drove CO 2 and CH 4 emissions at the water-air interface within the studied Rhizophora spp. mangrove forest. Simultaneous measurements of CO 2 , CH 4 and δ 13 CO 2 values highlighted that:

-the water column above the forest floor was a mix between soil pore waters, enriched in gaseous compounds originated from OM decomposition and root respiration, and incoming lagoon water, poor in CO 2 and CH 4 and characterized by enriched δ 13 C values, resulting in lower fluxes when the water column thickness increased, -the progressive enrichment of the water column in CO 2 and CH 4 by diffusive fluxes, as its residence time over the forest floor increased, induced higher fluxes during the ebb tide than during the flow, -CO 2 and CH 4 emissions were higher during spring tides than during neap tides possibly due to (i) higher renewable of the electron acceptor pool within the soil that enhanced microorganism's activity, and (ii) flooding of the upper intertidal zones that induced a higher exchange surface between the soil and the water column.

These results also showed that CO 2 and CH 4 emissions were not negligible and therefore, we suggest that these fluxes should be integrated in future carbon budget to make them more accurate. Further studies should now examine the variability of these emissions across mangroves that differ by their physiographic conditions to assess their spatial variability.

Fig. 1 .

 1 Fig. 1. Mean CO 2 (mmolC m -2 h -1 ) and CH 4 (μmolC m -2 h -1 ) (+SD) emissions during the complete tidal cycles (n = 284), the flow tides (n = 141) and the ebb tides (n = 143) in the mangrove of Ouemo for the period December 2016 to September 2017. Different letters indicate significant differences (two-samples t-test, CO 2 : t (205.66) = 5.11, p b 0.001, CH 4 : t (146.3) = 7.96, p b 0.001).

Fig. 2 .

 2 Fig. 2. Emissions of CO 2 and CH 4 fluxes, and δ 13 CO 2 values (‰) as a function of the water level above the sediment: (a, c and e) during flow tides, and (e, d and f) during the ebb tides.

  -2 h -1 ) Reference

	Mangrove waters
	New Caledonia	Semi-arid	0.13 to 18.41	0.18 to 172.07	This study
	Mangrove soils
	New Zealand	Temperate	7.02 ± 1.91	-	Bulmer et al. (2015)
	New Caledonia	Semi-arid	-0.71 to 2.41	-	Leopold et al. (2015)
	China	Subtropical	6.92 to 20.56	34.24 to 5168.60	Chen et al. (2010)
	China	Subtropical	-0.19 to 4.62	21.56 to 1919.68	Wang et al. (2016)
	Indonesia	Tropical	-1.34 to 3.88	-6.05 to 13.14	Chen et al. (2014)
	Tanzania	Tropical	1.50 to 3.67	0 to 3.65	Kristensen et al. (2008b)
	Mangrove creeks and estuaries
	Brazil	Semi-arid	0.62 ± 0.30	0.0000093	Nóbrega et al. (2016)
	Australia	Subtropical	0.38 to 26.21	0.54 to 26.38	Call et al. (2015)
	Florida	Subtropical	-	-2.60 to 9.61	Cabezas et al. (2017)
	Australia	Subtropical	0.79 to 2.92	0.29 to 2.13	Maher et al. (2015)
	India	Tropical	-	0.08 to 5.61	Biswas et al. (2007)
	India	Tropical	-	0.37	Dutta et al. (2015)

Acknowledgments

This work was funded by the Province Sud of New Caledonia, the City of Mont Dore, KNS Koniambo Nickel SAS, Vale NC, and the GOPS. We thank the Air Liquide Foundation for funding the CRDS analyzer. We are grateful to Inès Gayral for her support in field work.

Declaration of interest

Authors have no conflict of interest to declare.