Nohemy Zujaila

Qui Minet

Dominique Davoult

Jacques Grall

Sophie Martin

The relative contribution of fleshy epiphytic macroalgae to the production of temperate maerl (rhodolith) beds

Keywords: maerl, fleshy epiphytic macroalgae, photosynthesis, productivity, red algal physiology, Solieria chordalis, filamentous Rhodomelaceae

Maerl beds are composed of unattached red calcareous coralline algae. When located in shallow ecosystems, these calcareous macroalgae provide substrates for the development of fleshy epiphytic macroalgae, which contribute to the productivity of maerl beds. To assess the importance of their contribution, we estimated the primary production of the main taxa of fleshy epiphytic macroalgae (Solieria chordalis or Rhodomelaceae), growing in two distinct Lithothamnion corallioides maerl beds of the Bay of Brest (Brittany, France) characterized by different depths and incident irradiances. We estimated epiphytic algal photosynthetic parameters derived from photosynthesis-irradiance curves calculated from incubations in photo-respirometry chambers at different irradiances and in the dark. A comparison with results previously obtained in L. corallioides showed that, in the two studied maerl beds, there were no differences between maerl and its fleshy epiphytic macroalgae in terms of photo-acclimation to low irradiances. However, fleshy epiphytic macroalgae had higher photosynthetic efficiencies and photosynthetic rates per unit of biomass or chlorophyll a than the maerl species. Estimations of net primary production per surface area of maerl bed indicated that fleshy epiphytic macroalgae account for 25% of maerl bed productivity.

Interactions between L. corallioides and its fleshy epiphytic macroalgae may affect their respective contributions. In the deepest maerl beds, shading by fleshy epiphytic macroalgae may have a detrimental impact on L. corallioides net primary production, whereas in the shallowest maerl beds, fleshy epiphytic macroalgae may protect maerl from photoinhibition under high irradiances.

INTRODUCTION

Macroalgae acting as ecosystem engineers form macroalgal beds, which are among the most productive habitats in coastal marine systems [START_REF] Olafsson | Sub-canopy light conditions only allow low annual net productivity of epiphytic algae on kelp Laminaria hyperborea[END_REF]. Their contribution to the biogeochemical cycles is tightly linked to the incident irradiance they receive and their metabolic rates (Hurd et al. 2014). Among macroalgal foundation species, free-living red calcareous coralline algae (known as maerl) are long-lived species that grow worldwide at different depths, from the surface under high irradiances down to 250 m under dim-light conditions [START_REF] Bahia | South Atlantic Rhodolith Beds: Latitudinal Distribution, Species Composition, Structure and Ecosystem Functions, Threats and Conservation Status[END_REF]Riosmena-Rodríguez et al. 2017). Although very slowgrowing, maerl algae can accumulate, constituting vast beds of high density and biomass, thus significantly contributing to marine coastal primary production (Martin et al. 2007, van der [START_REF] Heijden | Reviews and syntheses: Calculating the global contribution of coralline algae to total carbon burial[END_REF].

When located in shallow environments, maerl beds are home to a highly diverse flora [START_REF] Peña | The diversity of seaweeds on maerl in the NE Atlantic[END_REF]. Maerl bed flora have been assessed in tropical ecosystems (Brazil, Amado-Filho et al. 2010), in the Mediterranean (Ustica Island, Mannino et al. 2002) and in the Northeast Atlantic, from Svalbard to Portugal [START_REF] Peña | The diversity of seaweeds on maerl in the NE Atlantic[END_REF]). These studies have highlighted that maerl algae harbor higher diversity and abundance of red fleshy epiphytes than brown and green epiphytes. In the NE Atlantic, maerl beds are found from the intertidal zone down to 60 m depth (Hernández-Kantún et al. 2017). Most beds are patchily distributed and found at less than 30 m depth [START_REF] Peña | The diversity of seaweeds on maerl in the NE Atlantic[END_REF], where light conditions are also favorable for the growth of fleshy epiphytic macroalgae. NE Atlantic maerl beds harbor at least 50 fleshy epiphytic macroalgal species, mostly Rhodophyta [START_REF] Peña | The diversity of seaweeds on maerl in the NE Atlantic[END_REF]. In contrast to maerl species, fleshy epiphytic macroalgae are fast-growing, short-lived organisms. Their composition and biomass depend on environmental factors favorable to their growth, such as high temperature, light, and nutrient availability [START_REF] Qui-Minet | The Role of Local Environmental Changes on Maerl and Its Associated Non-Calcareous Epiphytic Flora in the Bay of Brest[END_REF][START_REF] Qui-Minet | Physiology of maerl algae: Comparison of inter-and intraspecies variations[END_REF]. Habitat stability and variations in maerl morphology also influence fleshy epiphytic macroalgal abundance and species composition (Pascelli et al. 2013). The hydrodynamic regime, including winter storm disturbances, affects the abundance of fleshy epiphytic macroalgae in maerl beds (Amado-Filho et al. 2010[START_REF] Qui-Minet | The Role of Local Environmental Changes on Maerl and Its Associated Non-Calcareous Epiphytic Flora in the Bay of Brest[END_REF]. It also determines maerl morphology and, in turn, influences its associated macroflora (Bosence 1976).

The contribution of fleshy epiphytic macroalgae to the primary production of maerl beds thus depends on various environmental factors. It also depends on the living biomass of epiphytic macroalgal taxa and on their capacity to capitalize on the incident irradiance they receive. In the Bay of Brest (Brittany, France), the seasonality of maerl bed flora likely plays a major role in community primary production [START_REF] Martin | Community metabolism in temperate maerl beds. I. Carbon and carbonate fluxes[END_REF]. Epiphytic macroalgal species distribution and abundance vary with season [START_REF] Grall | Biodiversité spécifique et fonctionnelle du maerl: réponses à la variabilité de l'environnement côtier[END_REF] and local environmental changes [START_REF] Grall | Biodiversité spécifique et fonctionnelle du maerl: réponses à la variabilité de l'environnement côtier[END_REF][START_REF] Qui-Minet | The Role of Local Environmental Changes on Maerl and Its Associated Non-Calcareous Epiphytic Flora in the Bay of Brest[END_REF]. Their contribution to carbon budgets in temperate ecosystems can be particularly important in summer due to their higher abundance during this season [START_REF] Grall | Biodiversité spécifique et fonctionnelle du maerl: réponses à la variabilité de l'environnement côtier[END_REF][START_REF] Guillou | Can low sea urchin densities control macro-epiphytic biomass in a north-east Atlantic maerl bed ecosystem (Bay of Brest, Brittany, France)?[END_REF][START_REF] Qui-Minet | The Role of Local Environmental Changes on Maerl and Its Associated Non-Calcareous Epiphytic Flora in the Bay of Brest[END_REF].

The contribution of epiphytic red macroalgae to maerl bed productivity also depends on specific photo-physiological performance of epiphyte species (Hurd et al. 2014). Although the physiology of maerl algae has been well studied [START_REF] Martin | Primary production, respiration and calcification of the temperate free-living coralline alga Lithothamnion corallioides[END_REF], Schoenrock et al. 2018, Qui-Minet et al. 2019, Sordo et al. 2020[START_REF] Qui-Minet | Physiology of maerl algae: Comparison of inter-and intraspecies variations[END_REF], no study has assessed the physiology of red fleshy macroalgae inhabiting maerl beds. Maerl algae are known to show a high capacity for acclimation or adaptation to different depths/irradiances. For instance, in the Bay of Brest (Brittany, France), where Lithothamnion corallioides is the dominant maerl alga, summer irradiances of saturation in this species range from 100 to 200 μmol photon m -2 s -1 , depending on its location (depth and light availability) and thallus morphology [START_REF] Martin | Primary production, respiration and calcification of the temperate free-living coralline alga Lithothamnion corallioides[END_REF][START_REF] Qui-Minet | Physiology of maerl algae: Comparison of inter-and intraspecies variations[END_REF]. Maximal gross production rates in L. corallioides are observed in summer, ranging from 1 to 3 mol O2 g -1 DW h -1 . Some studies have assessed the physiology of red fleshy macroalgae, including Rhodophyta genera known to grow in temperate maerl beds of the NE Atlantic [START_REF] Dudgeon | Physiological responses to environmental variation in intertidal red algae : does thallus morphology matter?[END_REF], Johansson & Snoejis 2002[START_REF] Michler | Long-term effects of ultraviolet radiation on growth and photosynthetic performance of polar and cold-temperate macroalgae[END_REF], McCoy et al. 2019). A study on several species of red macroalgae from the Baltic Sea and the Skagerrak (Gullmar Fjord) highlighted a high species-dependent variability in maximal gross production rates, ranging from 70 to 500 mol O2 g -1 DW h -1 in summer (Johansson & Snoejis 2002). The saturating irradiances in these species are strongly related to water depth (and irradiances), with values ranging from 100 to 300 μmol photons m -2 s -1 for species typical of the upper littoral (0-5 m depth) and lower values (i.e. <100 μmol photons m -2 s -1 ) for species typical of the lower littoral (below 10 m depth).

The contribution of epiphytic algae to the productivity of vegetated marine coastal ecosystems has been mainly studied in seagrass beds [START_REF] Moncreiff | Trophic importance of epiphytic algae in subtropical seagrass beds: evidence of multiple stable isotope analyses[END_REF][START_REF] Borowitzka | Epiphytes of seagrasses[END_REF]), but remains relatively poorly understood in macroalgal habitats despite the potentially high biomass of fast-growing epiphytes that may contribute significantly to the total productivity of these ecosystems. The epiphytic algae of seagrasses are important primary producers that can indeed make up a significant proportion (up to 60%) of the total primary production of seagrass beds [START_REF] Borowitzka | Epiphytes of seagrasses[END_REF][START_REF] Berlinghof | The role of epiphytes in seagrass productivity under ocean acidification[END_REF]. In contrast, epiphytic macroalgae of the kelp species Laminaria hyperborea contribute little to the total productivity of the entire kelp forest, relative to the kelp itself [START_REF] Olafsson | Sub-canopy light conditions only allow low annual net productivity of epiphytic algae on kelp Laminaria hyperborea[END_REF]. The contribution of fleshy epiphytic macroalgae to the primary production of maerl beds has never been assessed. The lack of knowledge on the physiology of fleshy epiphytic macroalgae limits our ability to evaluate their contribution to the productivity of macroalgal ecosystems.

The main objective of this work was to estimate the relative contribution of fleshy epiphytic macroalgae to the maerl bed productivity as a function of bottom incident irradiance variability. For the first time, we determined photosynthetic parameters of temperate fleshy macroalgal maerl epiphytes and we estimated their rates of primary production and their contribution to carbon budgets in two distinct maerl beds of the Bay of Brest (two stations), characterized by different depths/incident irradiances, macroalgal biomasses and epiphytic taxa. Macroalgae physiological parameters were measured in summer, when fleshy epiphytic macroalgal biomass reaches its highest peak in the Bay of Brest [START_REF] Grall | Biodiversité spécifique et fonctionnelle du maerl: réponses à la variabilité de l'environnement côtier[END_REF][START_REF] Guillou | Can low sea urchin densities control macro-epiphytic biomass in a north-east Atlantic maerl bed ecosystem (Bay of Brest, Brittany, France)?[END_REF][START_REF] Peña | Seasonal patterns in the maerl community: case study of shallow subtidal European Atlantic beds[END_REF]. Macroalgal productivity per surface unit of maerl bed was estimated using previous data on L. corallioides and fleshy epiphytic macroalgal biomass and L. corallioides physiology [START_REF] Qui-Minet | The Role of Local Environmental Changes on Maerl and Its Associated Non-Calcareous Epiphytic Flora in the Bay of Brest[END_REF][START_REF] Qui-Minet | Physiology of maerl algae: Comparison of inter-and intraspecies variations[END_REF]. Our working hypotheses were i) physiological characteristics differ between epiphytic macroalgae according to taxon and specific environmental conditions at the two stations, and ii) fleshy epiphytic macroalgae have higher production rates than maerl (L. corallioides), and thereby significantly contribute to the productivity of maerl beds in summer.

MATERIALS AND METHODS

Study site

Epiphyte and maerl primary production rates were studied at two stations (A and B) in the Bay of Brest (Britany, France), where maerl beds cover almost one third of its bottom surface.

Mean tidal range in the bay is 4 m, and minimal and maximal tidal ranges are 0.3 and 8 m, respectively. Station A is located in the northern basin of the bay (48º21'57"N, 04º26'47''W), and Station B is located in the southern basin (48º19'58''N, 04º19'57''W). Chart datum is 2.5 m at Station A and 0.7 m at Station B.

In the Bay of Brest, bottom currents have a significant impact on macroalgal assemblages [START_REF] Qui-Minet | The Role of Local Environmental Changes on Maerl and Its Associated Non-Calcareous Epiphytic Flora in the Bay of Brest[END_REF]. Summer fleshy epiphytic macroalgal biomass (DW) ranges from 21 to 49 g m -2 at Station A and from 13 to 23 g m -2 at Station B, with no significant statistical differences between them [START_REF] Qui-Minet | The Role of Local Environmental Changes on Maerl and Its Associated Non-Calcareous Epiphytic Flora in the Bay of Brest[END_REF] Rhodomelaceae", the main fleshy epiphytic macroalgae at this location. At Station B, the dominant species was Solieria chordalis. Filamentous Rhodomelaceae and S. chordalis differ in their morphology. Filamentous Rhodomelaceae include taxa with multiaxial morphology in which upright filaments are laterally or dichotomously branched (polysiphonous morphology), whereas S. chordalis grows erect from a fibrous basal system and generally has two or three orders of branching (Gabrielson & Hommersand 1982). The living biomass of Lithothamnion corallioides is highly variable in maerl beds of the bay, but is not significantly different between Station A (from 850 to 8550 g m -2 ) and Station B (from 3100 to 5650 g m -2 ) (Qui-Minet et al. 2018).

Incubation procedure and physiological measurements

In the Bay of Brest, September is representative of the summer period in terms of temperature and irradiance [START_REF] Martin | Community metabolism in temperate maerl beds. I. Carbon and carbonate fluxes[END_REF][START_REF] Qui-Minet | Physiology of maerl algae: Comparison of inter-and intraspecies variations[END_REF]). Maximal metabolic rates are reached during this period in maerl beds of the Bay of Brest [START_REF] Martin | Community metabolism in temperate maerl beds. I. Carbon and carbonate fluxes[END_REF] 2021) under similar physico-chemical conditions and using the same methodology as that described below for epiphytes.

Algae were collected with a Van Veen grab (5 replicates of 0.1 m 2 per station), rinsed with filtered seawater on a 5 mm mesh sieve and then carefully cleaned to remove the sediments.

The most abundant fleshy epiphytic taxa of each station were identified and selected as filamentous Rhodomelaceae (Station A) or Solieria chordalis (Station B), both taxa being free of epiphytes.

Measurements were carried out by incubating the most abundant fleshy epiphytic algal taxon at each station. Macroalgal incubations were performed in the light and in the dark. During incubations, surface irradiance in terms of photosynthetically active radiation (PAR, μmol photons m -2 s -1 ) was recorded every minute using a LI-COR quantum sensor (LI 192 SA).

Maximum surface irradiance ranged from to 570 to 1100 μmol photons m -2 s -1 . Macroalgal net photosynthesis (NPP) was measured under different irradiances: maximum surface irradiance (100%), and reduced irradiance levels (65%, 47%, 27%, 13% and 6%), using clear chambers and opacifying neutral filters. Respiration (R) was measured in the dark using dark chambers. NPP and R rates were obtained by measuring the initial and final oxygen concentration with an oxygen probe (Oxymeter HQ40D, Hatch Lange, Ltd. portable LDO TM ) at the beginning and at the end of the incubations. For each level of irradiance and in the dark, five photo-respirometry chambers (220 mL) were filled with bottom seawater and fleshy epiphytic macroalgae (algal dry mass of 0.5-1 g DW). Five photo-respirometry chambers were only filled with bottom seawater and used as controls. Bottom seawater was collected at each station (approximately 1 m above the bottom) in a Niskin bottle. Incubations were performed on board the R/V Albert Lucas immediately after collecting the algae.

Chambers were kept in a water bath with a continuous flow of water coming from the bottom at the in situ temperature. Incubations lasted 1 hour to avoid oxygen saturation greater than 120% during light incubations and to maintain oxygen saturation above 80% at the end of dark incubations.

Sample treatment and processing

At the end of incubations, algal samples were collected and dried (60ºC, 48 h). In parallel, additional samples were rinsed with filtered autoclaved seawater to remove salts, placed in 2 mL cryotubes and frozen in liquid nitrogen. Samples were maintained at -80ºC prior to lyophilization for pigment analyses. To obtain a fine powder, samples were ground in a Tissue Lyser II (QIAGEN) bead mill in plastic tubes using 0.5 cm stainless steel beads (Brammer). Chlorophyll a of fleshy epiphytic macroalgae was analyzed using a calibrated Turner 10-AU fluorimeter, according to the [START_REF] Arar | Method[END_REF] equation: (1) where, CS.c is the corrected chlorophyll a concentration (μg/L) in the whole water sample, C.E.C. is the uncorrected chlorophyll a concentration in the water sample (μg/L), DF is the dilution factor, and extract volume is the volume (L) of extract prepared before dilution.

Data processing

To obtain actual macroalgal physiological parameters, fluxes were corrected with respect to control data. NPP and R rates were calculated as a function of algal dry weight (μmol O2 g -1 DW h -1 ) or as a function of algal chlorophyll a content (μmol O2 mg -1 Chl a h -1 ) according to the following equation:

(2) where O2 (μmol L -1 ) is the variation of dissolved oxygen concentration between the beginning and the end of the incubation, V (L) is the volume occupied by the seawater in the chamber, Alg is the dry weight (g) or Chl a content (mg) of algae in the chamber, t (h) is the incubation time. The relationships between irradiance (E, mol photons m -2 s -1 ) and NPP as a function of algal dry weight or as a function of algal Chl a content at a given irradiance, were obtained using the [START_REF] Chalker | Simulating light-saturation curves for photosynthesis and calcification by reef-building corals[END_REF] equation: (3)

Where GPPMAX is the maximal gross primary production expressed in terms of algal dry weight (in μmol O2 g -1 DW h -1 ) or algal Chl a content (μmol O2 mg -1 Chl a h -1 ). Ek (μmol photons m -2 s -1 ) is the saturating irradiance for photosynthesis and R is the respiration rate

(in μmol O2 g -1 DW h -1 or μmol O2 mg -1 Chl a h -1 ).
The compensation irradiance (Ec, μmol photons m -2 s -1 ) is the irradiance at which NPP = 0 (or GPP = R). Alpha (α), the photosynthetic efficiency ( mol O2 g -1 h -1 ( mol photons m -2 s - 1 ) -1 ) was calculated using the following equations: irradiance values. Data were obtained in terms of μmol O2. m -2 h -1 or μmol O2. m -2 day -1 and converted from μmol of O2 to mg C, using photosynthetic and respiration quotients (PQ and RQ, respectively) previously reported for L. corallioides (PQ:1.17 and RQ:0.97, [START_REF] Martin | Primary production, respiration and calcification of the temperate free-living coralline alga Lithothamnion corallioides[END_REF]) and for red fleshy macroalgae (PQ:1.20, Buesa 1980; RQ:1.00, Carvalho & Eyre 2011), and the molecular weight of C. In addition to the estimations of macroalgal NPP (mg C m -2 day -1 ), we give the 24 h NPP rates (mg C m -2 h -1 ) during three specific days with contrasting tidal ranges: low water neap tide (LWNT, "low" tidal range of 1-2 m), medium water tide (MWT, "medium" tidal range of 4 m) and high water spring tide (HWST, "high" tidal range of 6-8 m).

Statistical treatment

Statistical analyses were done using R Statistical Software (version 4.1.1). The significance of the fitted curves was tested using the Fisher test. Because data did not follow a normal distribution (Shapiro-Wilk test) and/or the homogeneity of variances (Bartlett test), a nonparametric test (Kruskal-Wallis test) was used to compare fleshy epiphytic macroalgae P-E parameters between stations, bottom irradiance between stations (during the early morning, late morning, afternoon and evening) and NPP rates between taxa/stations. When necessary, tests were followed by a post-hoc Wilcoxon test (pairwise Wilcoxon-test, Wp).

RESULTS

Comparison of photosynthesis characteristics between filamentous Rhodomelaceae and

Solieria chordalis

Mean rates of respiration (R) and of maximal gross primary production (GPPMAX) rates were significantly higher in filamentous Rhodomelaceae than in S. chordalis (Fig. 1 & Table 1).

In contrast, when physiological rates were normalized to the algal chlorophyll a (Chl a) content, GPPMAX was significantly lower in filamentous Rhodomelaceae than in S. chordalis (Fig. 2 & Table 1). Irradiance of saturation (Ek) and irradiance of compensation (Ec) were significantly higher in filamentous Rhodomelaceae than in S. chordalis (Table 1).

Comparison of photosynthesis characteristics between fleshy epiphytic macroalgae and maerl within and between stations

The physiological rates expressed in terms of algal Chl a content for Lithothamnion corallioides and its fleshy epiphytic macroalgae were compared within stations A and B. At Station A, GPPMAX in L. corallioides was five times lower than in filamentous Rhodomelaceae. At Station B, it was 12-fold lower than in S. chordalis (Table 2 & Fig. 2ab). L. corallioides photosynthetic efficiency ( ) coefficients were 6-fold and 12-fold lower than those of fleshy epiphytic macroalgae at stations A and B, respectively (Table 2 & Fig. 2a-b). L. corallioides R was 3-fold and 11-fold lower than that of fleshy epiphytic macroalgae at stations A and B, respectively (Table 2 & Fig. 2a-b). Within each station, no significant differences in terms of Ek and Ec were observed between L. corallioides and its fleshy epiphytic macroalgae.

Comparison of diel cycles in incident irradiance

Mean bottom irradiance during the summer season was significantly different between the two stations (Fig. 3). Bottom irradiance was significantly lower at Station A than at Station B during the early morning (6-9:00 h), the late morning (9-12:00 h), the afternoon (12-17:00 h) and the evening (17-22:00 h) (Table 3). Median values of bottom irradiance during the early morning were 5 and 10 μmol photons m -2 s -1 at stations A and B, respectively. They

Contribution of fleshy epiphytic macroalgae to NPP at each station

Mean rates of net primary production (NPP), estimated from summer average bottom irradiances) at noon were 42 mg C m -2 h -1 in L. corallioides and 10 mg C m -2 h -1 in filamentous Rhodomelaceae at Station A, and 55 mg C m -2 h -1 in L. corallioides and 16 mg C m -2 h -1 in S. chordalis at Station B (Fig. 5). Comparison of L. corallioides and its fleshy epiphytic macroalgae NPP (mg C m -2 h -1 ) at both stations during the early morning, late morning, afternoon and evening, showed that NPP m -2 h -1 varied significantly with tidal rhythms (LWNT, MWT and HWST), being higher in L. coralloides than in its fleshy epiphytic macroalgae and at Station B than at Station A. (Table 4;Figs. 4 & 5).

Throughout the summer period, diel (24 h) NPP rates ranged from -78 to 178 mg C m -2 day - 1 in filamentous Rhodomelaceae (Station A) and from -5 to 256 mg C m -2 day -1 in S. chordalis (Station B) (Fig. 7), being significantly higher in S. chordalis (KW, H = 16.2, p-value <0.001) (Fig. 7). Diel NPP in L. corallioides ranged from -214 to 728 mg C m -2 day -1 at Station A and from 17 to 808 mg C m -2 day -1 at Station B, being significantly higher at Station B than at Station A (KW, H = 5.5, p-value <0.05). Diel NPP was significantly higher in L. corallioides than in fleshy epiphytic macroalgae at both stations (KW, H = 25.3, p-value <0.001 and H = 79.8, p-value <0.001). Diel respiration rates were 145 and 101 mg C m -2 day - 1 in filamentous Rhodomelaceae (Station A) and S. chordalis (Station B), respectively, being significantly higher in filamentous Rhodomelaceae (KW, H = 154, p-value <0.001). In comparison, diel respiration rates in L. corallioides were 470 and of 335 mg C m -2 day -1 at Stations A and B, respectively, being significantly higher at Station A (KW, H = 154, p-value <0.001). Diel respiration was significantly higher in L. corallioides than in fleshy epiphytic macroalgae at both stations (KW, H = 160.0, p-value <0.001). Diel GPP rates ranged from 67 to 323 mg C m -2 day -1 in filamentous Rhodomelaceae (Station A) and from 96 to 357 mg C m -2 day -1 in S. chordalis (Station B), being significantly higher in S. chordalis (KW, H = 16.2, p-value <0.001) (Fig. 6). Diel GPP in L. corallioides ranged from 203 to 1198 mg C m - 2 day -1 at Station A, and from 352 to 1134 mg C m -2 day -1 at Station B, being significantly higher at Station B (KW, H = 5.5, p-value <0.05) (Fig. 6). Diel GPP was significantly higher in L. corallioides than in fleshy epiphytic macroalgae at both stations (KW, H = 130.0, pvalue <0.001) (Fig. 6).

DISCUSSION

Epiphytic macroalgal physiology

Filamentous Rhodomelaceae and Solieria chordalis are ubiquitous, cosmopolitan and opportunistic taxa inhabiting cool temperate shallow coastal areas [START_REF] Bunker | Seaweeds of Britain and Ireland: Second Edition[END_REF]).

Filamentous Rhodomelaceae are common in brackish estuaries, in both intertidal and subtidal areas [START_REF] Baweja | Chapter 3 -Biology of Seaweeds[END_REF]. S. chordalis generally grows in wave-sheltered habitats, from Ireland to the south of Morocco and the Canary Islands (Guiry & Guiry 2020). Despite knowledge on the ecology of these epiphytic macroalgae, few studies have been carried out on their photosynthesis capacities and their contribution to the primary production or to the C cycle in the coastal ecosystems where they are present remains unknown. Physiological characteristics of algae depend on their local environment. In the Bay of Brest, although light intensity varies considerably at stations A and B, incident bottom irradiance is significantly lower at Station A (deeper) than at Station B (shallower) [START_REF] Qui-Minet | The Role of Local Environmental Changes on Maerl and Its Associated Non-Calcareous Epiphytic Flora in the Bay of Brest[END_REF]. This difference in environmental conditions suggests that species may show potential photoacclimation/adaptation to their local environment. The characteristics of photosynthetic parameters of filamentous Rhodomelaceae and S. chordalis are typical of shade-growing macroalgae and in agreement with the low incident irradiance they receive at both Station A (<100 μmol photons m -2 s -1 ) and Station B (<150 μmol photons m -2 s -1 ) during most of the day due to the turbidity in the Bay of Brest (light extinction coefficients of 0.4 at both stations, Qui-Minet et al. 2018). Accordingly, both taxa displayed low values of compensation irradiance (Ec) and saturation irradiance (Ek) and high photosynthetic efficiency (α), as previously observed in other red macroalgae acclimated to low irradiances [START_REF] Dawes | Response of the tropical seaweed Gracilaria cornea to temperature, salinity and irradiance[END_REF][START_REF] Johansson | Macroalgal photosynthetic responses to light in relation to thallus morphology and depth zonation[END_REF]. However, despite the distinct depths/incident irradiances at the two stations, lower values of Ec and Ek, and higher values of α and maximal gross primary production (GPPMAX) were observed at the shallower location (Station B) in S. chordalis than at the deeper location (Station A) in filamentous Rhodomelaceae, suggesting greater photo-acclimation to low irradiances in S. chordalis than in filamentous Rhodomelaceae. This difference in performance is in agreement with the very shallow locations where filamentous Rhodomelaceae are found [START_REF] Baweja | Chapter 3 -Biology of Seaweeds[END_REF], suggesting that this taxon is better adapted/acclimated to higher irradiances. To survive under a constantly changing light environment and to optimize photosynthesis activity, red macroalgae have evolved several long-and short-term adaptation strategies, such as thallus anatomical changes at the individual level, morphological or organizational changes at the cellular level (long-term adaptation), or functional changes at the molecular level (short-term adaptation) (Talarico & Maranzana 2000). No significant differences were observed between stations in terms of seawater temperature, salinity, pH, or nutrients during the incubations. Nevertheless, due to its shallowness and proximity to freshwater run-off, Station B is characterized by higher variability in physicochemical parameters (particularly in temperature, salinity, nutrients, and carbonate system parameters) than Station A [START_REF] Qui-Minet | The Role of Local Environmental Changes on Maerl and Its Associated Non-Calcareous Epiphytic Flora in the Bay of Brest[END_REF]. Hence, differences in physiological parameters, Ec and Ek, between filamentous Rhodomelaceae and S. chordalis may be a result of long-term adaptation or short-term acclimation to the station-specific environmental conditions (Talarico & Maranzana 2000) and/or interspecific physiological differences.

Morphological differences between epiphytic species are also likely to affect their physiology. Differences in macroalgal morphology translate into differences in the surface area available for dissolved inorganic carbon and nutrient uptake, which result in differences in photosynthetic rates (Wallentinus 1984[START_REF] Dudgeon | Physiological responses to environmental variation in intertidal red algae : does thallus morphology matter?[END_REF][START_REF] Hurd | Effects of seawater flow velocity on inorganic nitrogen uptake by morphologically distinct forms of Macrocystis integrifolia from sheltered and exposed sites[END_REF], Johansson & Snoejis 2002, Roleda & Hurd 2019). Consequently, the polysiphonous structure of filamentous Rhodomelaceae taxa provides them with a higher surface-to-volume ratio and thus higher photosynthesis capacities. Similarly, when comparing photosynthesis capacities of several species from Rhodophyta, Johansson & Snoejis (2002) also observed that species previously classified as Polysiphonia spp. (filamentous Rhodomelaceae) are among those with the highest photosynthetic performances: Polysiphonia brodiaea (m. Leptosiphonia brodiei) and Polysiphonia fucoides (m. Vertebrata fucoides) (350 and 460 mol O2 g -1 h -1 , respectively).

Epiphytic macroalgal physiology vs. maerl physiology

Red macroalgal taxa dominate the fleshy epiphytic macroalgal flora in the maerl beds of the Bay of Brest [START_REF] Qui-Minet | The Role of Local Environmental Changes on Maerl and Its Associated Non-Calcareous Epiphytic Flora in the Bay of Brest[END_REF] and in other temperate maerl ecosystems [START_REF] Peña | The diversity of seaweeds on maerl in the NE Atlantic[END_REF]. Red algae possess several physiological mechanisms to acclimate to different light intensities (Talarico & Maranzana 2000). In this study, fleshy epiphytic macroalgae also demonstrated a capacity to adapt to deeper and low irradiance environments, with low values of Ec (22 and 15 μmol photons m -2 s -1 at stations A and B, respectively) and Ek (171 and 106 μmol photons m -2 s -1 , at stations A and B, respectively). However, due to its calcareous structure, Lithothamnion corallioides has lower metabolic rates (per g) than its fleshy epiphytic macroalgae counterparts (GPP rates more than 50-fold higher). Even when photosynthetic rates were normalized to algal Chl a content, epiphytic macroalgae had higher rates of GPP and photosynthetic efficiency than L. corallioides. Due to their opportunistic nature, these epiphytic species perform better in shaded environments (Burnham et al. 2022).

In this respect, the higher surface-to-volume ratio of the thin and filamentous thalli of epiphytic algae maximizes light harvesting [START_REF] Hurd | Review: Water motion, marine macroalgal physiology, and production[END_REF].

Due to their seasonal and patchy presence, fleshy epiphytic macroalgae contribute to the temporal and spatial heterogeneity of the physico-chemical environment by increasing the seawater diel variations in pH, pO2 and pCO2 [START_REF] Semesi | Seagrass photosynthesis controls rates of calcification and photosynthesis of calcareous macroalgae in a tropical seagrass meadow[END_REF], Cornwell et al. 2013, Short et al. 2015[START_REF] Guy-Haim | Epiphytes provide micro-scale refuge from ocean acidification[END_REF]. According to our estimations for the summer season, during sunny days when low tide occurs at noon, bottom irradiances can reach 300 μmol photons m -2 s -1 at Station A and >500 μmol photons m -2 s -1 at Station B. Nevertheless, despite its shallowness, the Bay of Brest experiences high turbidity [START_REF] Qui-Minet | The Role of Local Environmental Changes on Maerl and Its Associated Non-Calcareous Epiphytic Flora in the Bay of Brest[END_REF]) and during the summer season high irradiances are not reached on a daily basis. According to our estimations, maerl beds experience dim irradiances during the early morning and the evening, as well as during rainy/cloudy days. Consequently, fleshy epiphytic macroalgae may overshadow L. corallioides, thereby significantly reducing its NPP. According to our estimations, this effect may be detrimental for L. corallioides physiology in the early morning and in the evening. In this context, the overall impact of fleshy epiphytic macroalgae on L. corallioides physiology and survival varies among stations is a function of the patchy distribution of fleshy epiphytic macroalgae, bottom irradiance and macroalgal physiological parameters. Large areas of maerl beds are not covered by fleshy epiphytic macroalgae and, when present, the impact of fleshy epiphytic macroalgae on the incident irradiance received by maerl algae depends on their morphology, density and biomass. Furthermore, under dim irradiances, L. corallioides and fleshy epiphytic macroalgae have similar values of Ec ( 22and 19 μmol photons m -2 s -1 at Station A, and 15 and 12 μmol photons m -2 s -1 at Station B, respectively), and compete for light. In our study, incident irradiance differed significantly between stations A and B. At Station A, incident irradiance remained lower than Ek values measured in L. corallioides (171 μmol photons m -2 s -1 , Qui-Minet et al. 2021) and filamentous Rhodomelaceae (185 μmol photons m -2 s -1 ), because during most part of the day, the highest irradiance value remained lower than Ek, reaching100 μmol photons m -2 s -1 . Therefore, if fleshy epiphytic macroalgae overshadow L. corallioides, they may have a negative impact on maerl primary production and subsequently its growth and survival. At Station B, macroalgae experience irradiances higher than Ek (> 150 and up to 600 μmol photons m -2 s -1 ) allowing them to reach photosynthetic saturation. Therefore, at least during the afternoon, when irradiances are maximal, fleshy epiphytic macroalgae may here protect L. corallioides from photoinhibition and from any potential photodamage. Interactions with other biotic and abiotic parameters may diminish or enhance these predicted effects. For instance, reduced turbidity due to water renewal from tides and bottom currents or a reduction in fleshy epiphytic macroalgae due to herbivorous grazing can maintain a favorable physicochemical environment for maerl under low irradiances. Nevertheless, our study highlights the ambivalent relationship between maerl and its fleshy epiphytic macroalgae inhabiting highly turbid and shallow temperate ecosystems such as the Bay of Brest.

Heterogeneity of primary production in the Bay of Brest

The comparison of macroalgal primary production (NPP) between the two maerl beds studied here demonstrated that maerl bed primary production has heterogeneous spatio-temporal patterns. In the Bay of Brest, variability in depth among stations translated into higher bottom irradiances at Station B, and therefore higher algal NPP for both L. corallioides and its fleshy epiphytic macroalgae. In this context, due to the lower irradiance required by algae to reach photosynthetic saturation at Station B (i.e. Ek of 100 mol photon m -2 s -1 for both L. corallioides and S. chordalis), maximal productivity can be reached, at least around noon, but also during the morning and afternoon, depending on tidal height. However, at Station A, maximal macroalgal productivity may only be reached when low tide occurs around noon during high water spring tides. Due to the higher spatial heterogeneity of living L. corallioides biomass in the maerl bed at Station A (1 to 9 kg DW m -2 ) than at Station B (3-6 kg m -2 ) (Qui-Minet et al. 2018), maerl NPP showed higher variability at this location. The contribution of fleshy epiphytic macroalgae to the NPP also varies with bottom irradiance. Filamentous Rhodomelaceae have a significantly higher GPPMAX than S chordalis, but their productivity is limited by incident light. When bottom light intensity is below 50 μmol photons m -2 s -1 , filamentous Rhodomelaceae and S. chordalis have similar NPP per m 2 . On the other hand, above 400 μmol photons m -2 s -1 , filamentous Rhodomelaceae can exceed the value of S. chordalis NPP by two-fold. Nevertheless, according to our estimations for summer 2015, maximal bottom irradiance at Station A was 300 μmol photons m -2 s -1 (around noon and under a HWST) and thus, L. corallioides and its fleshy epiphytic macroalgal NPP values were higher at Station B.

The impact of fleshy epiphytic macroalgae on maerl bed primary production depends on their biomass and taxon composition, fleshy epiphytic algal biomass being similar between S. chordalis (Station B) and filamentous Rhodomelaceae (Station A) [START_REF] Qui-Minet | The Role of Local Environmental Changes on Maerl and Its Associated Non-Calcareous Epiphytic Flora in the Bay of Brest[END_REF]).

The differences in NPP between these epiphytic algae were mainly correlated with bottom irradiance and their physiological rates. Despite significantly higher biomass of L. corallioides relative to epiphyte biomass on maerl beds (fleshy epiphytic biomass was less than 50 g DW m -2 at both stations, whereas maerl biomasses ranged from 0.85 to 8.5 kg DW m -2 at Station A and from 3.1 to 5.6 kg m -2 at Station B, Qui-Minet et al. 2018), fleshy epiphytic macroalgae had higher photosynthetic rates, and accounted for one fourth of maerl bed NPP (22 ± 13% and 24 ± 0.02% of NPP at stations A and B, respectively), independently of tidal height and incident surface irradiance. Species interactions were not considered here, but may affect these macroalgal productivity estimations. The relative contribution of L. corallioides to the estimated NPP can be effectively overestimated due fleshy epiphytic macroalgae shading the host L. corallioides. Consequently, fleshy epiphytic macroalgae may actually have a relatively higher contribution to maerl bed productivity than estimated here.

Nevertheless, the shading effect of fleshy epiphytic macroalgae may also protect L. corallioides from photo-inhibition under high irradiances, as occurs at Station B when bottom irradiance is greater than the irradiance of photosynthetic saturation of L. corallioides and S. chordalis (> 100 μmol photons m -2 s -1 ) at Station B.

Due to the highly variable irradiance and tidal rhythms, significant daily changes in macroalgal diel (24 h) NPP can be observed at each station through the summer season. When the tide is high, summer days with low light intensity can lead to a negative macroalgal NPP (R > GPP). This situation is more accentuated at the deepest station (Station A) and correlated with the lower bottom irradiances at this location, which rarely reach saturating irradiances for L. corallioides and filamentous Rhodomelaceae. Previous studies have defined maerl beds in the Bay of Brest as heterotrophic communities, and maximal community respiration measured during summer has been estimated at 2 g C m -2 day -1 [START_REF] Martin | Community metabolism in temperate maerl beds. I. Carbon and carbonate fluxes[END_REF], including fleshy epiphytic macroalgae, the epiphytic microbiome (bacteria and microflora), the benthic fauna and the microphytobenthos associated with the sediment [START_REF] Martin | Community metabolism in temperate maerl beds. I. Carbon and carbonate fluxes[END_REF]. Although this value may vary among maerl beds, according to our data, macroalgal (maerl and epiphytes) contributions to the maerl bed community respiration are 29-31% at Station A (21-24% for L. corallioides and 7% for filamentous Rhodomelaceae) and 22% at Station B (17% for L. corallioides and 5% for S. chordalis). On the other hand, mean community GPP in the Bay of Brest in summer has been estimated to be 1.3 g C m -2 day -1 [START_REF] Martin | Community metabolism in temperate maerl beds. I. Carbon and carbonate fluxes[END_REF]).

According to our estimations, summer algal GPP represents 21% (Station A) and 35% (Station B) of this value during low irradiance days, but would exceed this value by 15% during high irradiance days (1.5 g C m -2 day -1 at both stations A and B).

CONCLUSIONS

The impact of fleshy epiphytic macroalgae on maerl physiology is equivocal and it depends on bottom incident irradiance, and therefore on the location of maerl beds and their specific water depths and turbidity. Under low irradiances, fleshy epiphytic macroalgae will compete with maerl for light, and under high irradiances they will protect maerl. The overall impact of fleshy epiphytic macroalgae on maerl physiology not only depends on their abundance, but on taxon composition and morphology (i.e. shading effect). In addition to the interaction between coralline algae and their epiphytes, future research should also address other biotic interactions such as grazing or interactions with local abiotic conditions, such as hydrodynamic regimes, because they may preserve maerl algae from any potential negative impact of their epiphytes.

At the ecosystem scale, bottom incident irradiance in coastal ecosystems is affected by anthropogenic activities, including nutrients and organic matter, that may increase the seawater turbidity and sedimentation rates. Our results highlight the importance of monitoring the incident irradiance received by macroalgal species acting as ecosystems engineers, because they play fundamental roles in biodiversity preservation and biogeochemical cycles. Assessing maerl bed health and applying ecosystem management policies to define threshold values of physico-chemical parameters (incident irradiance, turbidity, nutrients release, etc.) thus requires better knowledge on the physiology of maerl algae as well as of their fleshy epiphytes. This is the first study to compare the physiological parameters of Lithothamnion corallioides and its fleshy macroalgal epiphytes. This is also the first assessment of the contribution (~ 25%) of fleshy epiphytic macroalgae to maerl bed C budgets. Although physiological rates of maerl are significantly lower than those of fleshy epiphytic macroalgae, due to their high living biomass in the Bay of Brest, their contribution to the C cycles is very high (~ 75% of the total NPP). A more accurate estimation of the fleshy epiphytic macroalgae contribution to maerl bed productivity requires studying their physiology under different environmental conditions, such as temperature, irradiance or nutrient concentrations, throughout their period of occurrence. Finally, this study is also among the first to assess the contribution of fleshy epiphytic macroalgae to the primary production of coastal ecosystems dominated by foundation macroalgal species.

  each station, values of bottom irradiance were calculated using chart datum, tidal height and light extinction coefficients measured at each station during the summer season (0.38 for Station A and 0.43 for Station B). Bottom incident irradiance was calculated every 20 min from surface irradiance data (SOMLIT-MAREL), tidal ranges (www.maree.info) and light extinction coefficients previously obtained for summer season at each station (Qui-Minet et al. 2018). Surface irradiance (PAR) was monitored at high frequency (every 20 min) by the autonomous COAST-HF-MAREL-Iroise buoy (MAREL-Iroise/SOMLIT-Brest scientific platform) located in the Bay of Brest. Summer is here defined as the period from 16 June to 15 September 2015. Values of NPP (mg C m -2 ) per h (or per day) were individually estimated for L. corallioides, filamentous Rhodomelaceae and S. chordalis using photosynthesisirradiance (P-E) parameters presented here (epiphytes) or previously published (maerl, Qui-Minet et al. 2021), algal biomass data per m 2 (Qui-Minet et al. 2018) and calculated bottom

  were below the Ec values for L. corallioides (16 and 13 μmol photons m -2 s -1 at stations A and B, respectively), filamentous Rhodomelaceae (19 μmol photons m -2 s -1 , Station A) and S. chordalis (13 μmol photons m -2 s -1 , Station B). During the late morning, median values of bottom irradiance remained above Ec and below Ek for the macroalgae taxa here studied.During the afternoon, median values of bottom irradiance were below Ek for L. corallioides and filamentous Rhodomelaceae at Station A, whereas at station B median values of bottom irradiance were equal to Ek for L. corallioides (106 μmol photons m -2 s -1 ) at Station B. During the evening, median values of bottom irradiance at Station A (11 μmol photons m -2 s -1 ) were below Ec for L. corallioides and filamentous Rhodomelaceae, but this was not the case at Station B, where median values of bottom irradiance (18 μmol photons m -2 s -1 ) remained above Ec for L. corallioides and S. chordalis. In addition to the variability of bottom irradiance among stations, due to surface irradiance and height of tide variability, bottom irradiance also varied significantly among days(Table 3; Fig. 3; Suppl. Figs. 1 & 2).

FIGURESFigure 1 .

 1 FIGURES

Figure 2 .

 2 Figure 2. Relationship between net primary production per Chl a content (in O2 flux per mg

Figure 3 .

 3 Figure 3. Estimated 24-h bottom irradiance at Station A and Station B considering average

Figure 4 .

 4 Figure 4. Estimated mean net primary production over 24 h in filamentous Rhodomelaceae at Station A and Solieria chordalis at Station B, from average summer bottom incident irradiance and during specific summer days with low water neap tide (LWNT, 1 September 2015), medium water tide (MWT, 20 June 2015), and high water spring tide (HWSP, 25 June

Figure 5 .

 5 Figure 5. Estimated mean net primary production over 24 h considering average bottom irradiance during summer season in Lithothamnion corallioides at stations A and B, from average summer bottom incident irradiance and during specific summer days with low water neap tide (LWNT, 1 September 2015), medium water tide (MWT, 20 June 2015), and high water spring tide (HWSP, 25 June 2015). The shaded area represents the standard deviation as a function of the heterogeneity of L. corallioides biomass at stations A and B, respectively (n = 5).

Figure 6 .Figure 1 .

 61 Figure 6. Estimated diel (24-h) net primary production in epiphytes (filamentous Rhodomelaceae and Solieria chordalis) and Lithothamnion corallioides at stations A and B during the summer season. The shaded area represents the standard deviation as a function of the heterogeneity of macroalgal biomass at stations A and B, respectively (n = 5). APPENDICES/SUPPLEMENTARY FIGURES Figure 1. Estimated daily bottom irradiance at station A during the summer season.Figure 2. Estimated daily bottom irradiance at station B during the summer season.

Figure 2 .

 2 Estimated daily bottom irradiance at station B during the summer season.

  .

	Physiological measurements were carried out on 17 September 2015 under the following
	physico-chemical conditions: temperature, 17.8 ± 0.2°C [Station A] and 16.3 ± 0.0°C
	[Station B]; salinity, 35.1 ± 0.0 [A] and 35.0 ± 0.0 [B], and pH 7.92 ± 0.03 [A] and 8.03 ±
	0.02 [B]. L. corallioides physiological measurements were taken on 16-18 September 2015
	(see Qui-Minet et al. (

Table 4 .

 4 Summary of Kruskal-Wallis tests used to compare the rates of net primary production (in mg C m -2 h -1 ) per m -2 h -1 in filamentous Rhodomelaceae (Station A), S. chordalis (Station B) and L. corallioides (stations A and B) under high water spring tide (HWST), mean water tide (MWT) and low water neap tide (LWNT), during the early morning, late morning, afternoon and evening (n> 10). Bold values indicate statistical significance at p < 0.05. *** p <0.001; ** p <0.01; * p <0.05.

			H	p-value	H	p-value	H		p-value	H	p-value
	Station A		71.5	<0.001*** 144.1 <0.001*** 229.5	<0.001*** 80.1	<0.001***
	Station B		80.8	<0.001*** 167.4 <0.001*** 222.4	<0.001*** 66.7	<0.001***
								C (mg m -2 h -1 )
		Early morning	Late morning	Afternoon	Evening
		H	p-value	H	p-value	F	p-value	F	p-value
	Filamentous	11.3	<0.01**	15.8	<0.001***	22.5	<0.001***	19.3	<0.001***
	Rhodomelace
	ae
	Tides
		HWST > LWNT	HWST < MWT &	HWST & MWT >	LWNT & MWT >
		MWT = LWNT		LWNT		LWNT	HWST
	S. chordalis:	8.8	<0.05*	11.5	<0.01**	28.2	<0.001***	17.1	<0.001***
	Tides
		Inconclusive	Inconclusive		HWST> MWT>	LWNT & MWT>
								LWNT	HWST
	L.	11.3	<0.01**	15.8	<0.001***	22.5	<0.001***	19.3	<0.001***
	corallioides
	Station A:
	Tides
		HWST > LWNT,	HWST> MWT,			MWT & LWNT>
		HWST > MWT	HWST > LWNT,	HWST & MWT>	HWST
		MC = LC		HWST = MWT	LWNT
	L.	12.4	<0.01**	17.1	<0.001***	23.2	<0.001***	19.7	<0.001***
	corallioides
	Station B:
	Tides
		HWST = MWT	LWNT > HWST &	HWST & MWT >	LWNT & MWT >
		LWNT & MWT >		MWT		LWNT	HWST
			HWST

ACKNOWLEDGEMENTS

We thank the crew of the R/V Albert Lucas, and in particular Franck and Daniel, for their invaluable help with sampling. This work benefited from a grant from the Consejo Nacional de Ciencia y Tecnología (CONACyT) and from support from French National Research through the investment expenditure program IDEALG ANR-10-BTBR.

Semesi IS, Beer S, Bjork M (2009) Seagrass photosynthesis controls rates of calcification and photosynthesis of calcareous macroalgae in a tropical seagrass meadow. Mar Ecol Progr Ser 382: 41-47 Schoenrock KM, Bacquet M, Pearce M, Rea BR, Schofield JE, Lea J, Mair D, Kamenos N (2018) Influences of salinity on the physiology and distribution of the Arctic coralline algae, Lithothamnion glaciale (Corallinales, Rhodophyta). J Phycol 54: 690-702 Short J, Kendrick GA, Falter J, McCulloch MT (2014) ( mol photons m -2 s -1 ) -1 ) 0.6 ± 0.3 1.9 ± 0.7 8.2 <0.05* Table 2. Summary of Kruskal-Wallis tests used to compare the physiological parameters between non-calcareous epiphytic macroalgae and Lithothamnion corallioides at stations A and B (n = 5). GPPMAX and R are the rates of maximal gross primary production and respiration, respectively, expressed per unit of Chl a ( mol O2 mg -1 Chl a h -1 ), Ek is the irradiance of saturation ( mol photons m -2 s -1 ), Ec is the irradiance of compensation ( mol photon m -2 s -1 ), and ( mol O2 mg -1 Chl a O2 mg -1 Chl a h -1 ( mol photons m -2 s -1 )