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International audience; Maerl beds are ecologically important marine biogenic habitats founded on a few species of free-living coralline algae that aggregate and form highly complex rhodoliths. The high biodiversity found in these habitats have been mainly justified by the structural complexity that they provide. However, few attempts to quantify this complexity have been made. Maerl species distribution, density, rhodolith growth forms, and shapes vary with environmental conditions. Hydrodynamics and depth have been shown to drive morphology. Using speciesspecific metrics such as sphericity and branching density, as well as diameter and fractal dimension at the rhodolith level, and maerl density at the habitat level, we quantified the habitat complexity within ten maerl beds at a regional scale (along ~400 km of the coastline of Brittany in Western France). Using both long-term monitoring data and environmental models, we investigated how maerl habitat complexity varies among beds and which environmental conditions drive those differences. The effects of currents, exposure to wind-generated waves, temperature and sediment granulometry were evaluated. We confirmed variations in complexity in maerl beds at the habitat and rhodolith levels at local and regional scales, which might have ecological and conservational implications for their associated biodiversity. The analysed environmental conditions drive around a third of the variance in habitat complexity. Sediment granulometry is the main driver of maerl habitat complexity in Brittany, while the isolated effects of depth and hydrodynamics accounted for less than 5% of the variability each. Our results have important implications for paleoecology, and we suggest that maerl facies should be interpreted carefully. Our study provides a first attempt at explicitly quantifying maerl habitat complexity, and further contributes to the understanding of this fundamental ecological question. more...

Large stands of free living (calcareous) coralline algae — called maerl beds — play a major role as ecosystem engineers in coastal areas throughout the world. They are also subject to strong anthropogenic pressures at global and local scales, which threaten their survival. However, the macroalgal epiphytes growing on maerl may benefit from these pressures, developing to the detriment of maerl algae. Here, we sought to gain insight into how maerl beds and their epiphytic algae are disturbed by variations in the local environment, and how these variations affect their capacity to respond to global change. In 2015, we monitored three maerl beds located in the Bay of Brest (Brittany, France). Sites with contrasting conditions were selected, with one station lying in a zone close to the harbor (northern basin S1) and two stations (S2 and S3) located in areas away from the main urban effluents but subject to other sources of local change: higher currents at S3 (PREVIMER Ocean Forecast) and higher sedimentation rates at S2 (Ehrhold et al., 2016). We observed significant temporal variations of physico-chemical parameters, on an annual but also on a daily basis. Results showed that S2 differentiated itself from the other stations, this station experienced higher fluctuations of salinity, nutrient concentrations and carbonate system parameters and hosted the lowest (living) maerl biomass (4.38 ± 1.54 kg DW m−2). S3 observed the highest living maerl biomass (14.56 ± 1.61 kg DW m−2) and the lowest non-calcareous epiphytic macroalgal abundance (0.1–7.9 g DW m−2). S1 displayed the highest heterogeneity in terms of living maerl biomass (it varied from 0.8 to 8.6 kg DW m−2), and the highest Chl a content. However, we did not record differences in terms of physico-chemical parameters between S1 and S3. No positive relationship was observed between nutrient enrichment and macroalgal epiphyte abundance, but epiphyte abundance was higher at stations with lower maerl biomass (S1 and S2) (mean value ranged from 4.6 to 49.0 g DW m−2 at S1 and from 7.4 to 23.7 g DW m−2 at S2). Our results highlight the importance of local changes on the development, survival and capacity to adapt to global change of maerl beds. more...

WOS:000419410100010; The importance of seagrass habitat for the diversity of benthic fauna has been extensively studied worldwide. Most of the information available is, however, about a diversity while little consideration has been given to 13 diversity. To fill the knowledge gaps regarding the variability of epifaunal and infaunal seagrass assemblages at large spatial and temporal scales, we scrutinized an extensive dataset covering five years of monitoring of eight intertidal Zostera marina meadows around Brittany (France). High species richness arose at the regional scale from the combination of high local diversity of the meadows and substantial among-meadows 13 diversity. Epifauna and infauna appeared as distinct self-communities as they displayed different spatial and temporal patterns and varied in their responses to local hydrological conditions. Infauna had higher total 13 diversity than epifauna due to a tighter link to the great variability of local environmental conditions in the region. Both exhibited substantial variations in species composition and community structure with variations of dominant species that were accompanied by extensive change in numerous rare species. The dominant epifaunal species were all grazers. Changes in species composition were induced mostly by species replacement and rarely by richness differences between meadows. Indeed, species richness remained within a narrow range for all seagrass beds, suggesting a potential carrying capacity for species richness of the meadows. Overall, all meadows contributed equally to the regional turnover of seagrass macrofauna, emphasizing high variability and complementarity among beds at the regional scale. The implications of this substantial within-seagrass variability for the functioning of benthic ecosystems at broad scale and for conservation purposes in habitat mosaics warrant further investigations but our results clearly advocate taking into account within-habitat variation when evaluating the diversity of benthic habitats and the potential effect of habitat loss. more...

International audience; Open sandy beaches support key ecological functions and a distinctive biodiversity, but are threatened by an increasing anthropogenic pressure. Among these threats is the occurrence of green tides of Ulva spp., expression of the eutrophication phenomenon. Our study aimed at determining the impact of green macroalgal accumulations on benthic meiofauna inhabiting two macrotidal sandy beaches of Brittany: Saint Nic (impacted) and Anse de Dinan (unimpacted). The presence of Ulva enhanced total meiofaunal abundance and nematode functional diversity when compared to a state free from macroalgal mats. The stranded Ulva supported high values of meiofaunal beta-diversity as well. This unexpected response was likely due to the highly dynamic nature of open sandy beaches that prevents O-2 depletion within sediments. Natural seasonal changes in meiofaunal biomass and composition were also evident at both beaches and changes in environmental features, apart from eutrophication alone, do drive the variability in meiofaunal biomass and nematodes diversity. more...

WOS:000604182000002; In response to ongoing global climate change, marine ecosystems in the northwest Atlantic are experiencing one of the most drastic increases in sea surface temperatures in the world. This warming can increase water column stratification and decrease surface nutrient concentrations, in turn impacting primary productivity and phytoplankton assemblages. However, the exact impacts of these changes on sources and quality of organic matter as well as its transfers to the benthic compartment remain uncertain. This survey characterized organic matter sources and quality within a highly-stratified sub-Arctic coastal system (Saint-Pierre and Miquelon) and described its transfer towards a biomass-dominant primary consumer, the sand dollar Echinarachnius parma. This study analyzed fatty acid and stable isotope (delta C-13 and delta N-15) composition of surface and bottom Particulate Organic Matter (s-POM and b-POM, respectively), Sedimentary Organic Matter (SOM) and sand dollar tissue along a near shore to offshore gradient during two contrasting seasons associated either with sharp or weak water column stratification (i.e. High vs Low Stratification Periods). Results revealed high relative abundances of polyunsaturated fatty acids (notably macro- and microalgae markers) in POM during the Low Stratification Period while the High Stratification Period was characterized by elevated relative abundance of saturated fatty acids indicating a higher organic matter degradation state. In addition, strong seasonal differences were also observed in food availability with four-fold higher concentrations in total suspended solids during Low vs High Stratification Periods. These results suggested thus multiple negative effects of stratification on pelagic-benthic coupling and POM quality. Lower nutrient repletion of surface waters during period of sharp stratification diminishes pelagic-benthic coupling by reducing food availability, POM quality and vertical transfer of organic matter. By contrast, the sediment-based diet of E. parma showed a low spatiotemporal variability reflecting the homogenous composition of the SOM. This study suggests that intensified water column stratification due to increasing sea surface temperatures may modify the pelagic-benthic coupling and future quality and composition of POM pools. more...

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The observed increase in the atmospheric concentration of carbon dioxide due to anthropogenic emissions is predicted to lead to significant changes in climate. Recent studies highlight the importance of identifying the role of marine coastal communities in carbon exchanges. Our objective was to couple macrozoobenthos abundance data from long-term monitoring with species metabolism rates to contribute to the estimation of CO 2 fluxes from an intertidal exposed rocky shore community at a regional scale. The carbon fluxes due to respiration and calcification were calculated both during emersion and immersion, and the effect of temperature variation on carbon emissions was then predicted. Spatial and temporal natural variations of carbon fluxes were investigated and the contribution of exposed intertidal rocky shore communities to regional carbon emissions was calculated. The method was used to calculate the carbon budget allowed to account for the natural spatial variability of the community composition and carbon emissions. Mean annual calculated CO 2 emission was 14.3 mol C m − 2 yr − 2 , and the annual regional CO 2 flux was estimated at 2978 t C yr − 1 . Simulations showed that the potential feedback of a rise in temperature of 1 °C would lead to an increase of 4–7% in carbon emissions for this type of community. The results give a first quantification of intertidal exposed rocky shore carbon emissions that could be considered in evaluating further the global CO 2 budget. more...

In coastal estuarine embayments, retention of water masses due to coastal topography may result in an increased contribution of continental organic matter in food webs. However, in megatidal embayments, the effect of topography can be counterbalanced by the process of tidal mixing. Large amounts of continental organic matter are exported each year by rivers to the oceans. The fate of terrestrial organic matter in food webs of coastal areas and on neighboring coastal benthic communities was therefore evaluated, at multi-trophic levels, from primary producers to primary consumers and predators. Two coastal areas of the French Atlantic coast, differing in the contributions from their watershed, tidal range and aperture degree, were compared using carbon and nitrogen stable isotopes (delta C-13 and delta N-15) during two contrasted periods. The Bay of Vilaine receives large inputs of freshwater from the Vilaine River, displaying N-15 enriched and C-13 depleted benthic communities, emphasizing the important role played by allochtonous inputs and anthropogenic impact on terrestrial organic matter in the food web. In contrast, the Bay of Brest which is largely affected by tidal mixing, showed a lack of agreement between isotopic gradients displayed by suspended particulate organic matter (SPOM) and suspension-feeders. Discrepancy between SPOM and suspension-feeders is not surprising due to differences in isotopes integration times. We suggest further that such a discrepancy may result from water replenishment due to coastal inputs, nutrient depletion by phytoplankton production, as well as efficient selection of highly nutritive phytoplanktonic particles by primary consumers. (C) 2013 Elsevier Ltd. All rights reserved. more...

Despite a consensus about the significant damages to marine benthos and commercial fish stocks induced by mobile fishing gear, the extent and intensity of this practice have currently grown all over the world. The main problems of fisheries management are the capture and killing of juvenile and undersized fish and thus restrictions mainly concern mesh size in cod-end. However another recurrent problem and non-negligible is the by-catch of undersize commercial fish and of non-target species. Hence, regulations to reduce such by-catch have formed a part of fisheries management techniques since the early 20th century. As a consequence, successful developments and technical modification have been used to reduce capture of undersized fish and discards (i.e. mesh size, separator panels, and sorting grids) in the last decades. Technical modification concerning reduction of damage and mortality to benthic communities are less documented. Most of the tentative to replace tickler chain, panels or legs by other systems have failed, while results showed a decrease in non-target catch, and a decrease in commercial catch was observed. This paper presents fishing experiments with modified otter trawl aimed at reducing discard rates and direct mortality of benthic infauna and epifauna without affecting the level of landings (i.e. a comparison of environmental effects caused by a conventional otter trawl compared to a modified otter trawl with enlighten experimental legs). Catch composition, by-catch and short-term effects to macro- and megafauna communities of both fishing gear (conventional and modified) were investigated. Results show that no differences for commercial catch biomass or for benthic communities' structure were observed. Moreover, by-catch analysis showed no difference while significant higher damage and direct mortality were observed for target and non-target species caught by the normal otter trawl compared to those caught by the modified one. Consequently, although limited, the severity of the damage appears to be linked with the type of trawl-legs used. The modified otter trawl permits to decrease damage and mortality to captured and non-captured organism and thus to reduce impacts to exploited marine ecosystems. These results are of interest and appear attractive in the framework of marine ecosystems preservation and conservation, as well as of fisheries management. more...

The North Bay of Biscay continental shelf is a major French demersal fishery, but little was known on the trophic food web of its benthic communities. In order to determine the benthic trophic web, the objectives of this study are to describe the macro- and megafaunal benthic community structure (species richness, abundance and biomass) and to establish the trophic pathways (food sources and trophic levels) by applying carbon and nitrogen stable isotopic analysis to the main benthic and demersal species (invertebrates and fish). Two distinct benthic communities have been identified: a muddy sand community within the central part of the bay, and an outer Bay of Biscay Ditrupa sand community of higher species richness, abundance and biomass than the muddy sand community. Deposit-feeders, suspension feeders and predators, distributed in three main trophic levels, dominate both communities. Large differences in stable carbon ratio values within the primary consumers provide evidence of two different food sources: i) a pelagic food source made up of recent sedimenting particulate organic matter on which zooplankton and suprabenthos feed and ii) a benthic detrital food source supplying deposit feeders and partly benthic suspension feeders. Differences in isotopic signatures were also observed within the upper trophic levels that allowed estimation of the contribution of each food source component to the diet of the upper consumers. Finally, the use of stable isotopic composition together with the species' feeding strategy allow identification of the main differences between the trophic functioning of the two benthic communities and highlight the importance of the role of detrital pathways in the carbon cycling within the continental shelf benthic trophic web. more...

International audience; Thirty-five years after the first description of the benthic community and sediments of the North Bay of Biscay continental shelf (80-200 m depth in the "Grande Vasière" region) by Glémarec (personal communication), the sampling stations were revisited to provide a new reference on the status of the macrozoobenthic communities to help our understanding and management of fisheries that are highly developed in this area. Results showed that large modifications occurred in the communities and sediments of the central part of the "Grande Vasière", while these modifications remained moderate in the surroundings on the outer continental shelf. Revisited, macrobenthic communities differed greatly from those recorded in the 1960s, and were less numerous and more homogeneous. The dominant species which characterized the communities and sub-communities had also changed. The main factor that can explain these differences is the granulometry of the sediments which has shown large changes: a strong decrease in the mud fraction and increase in the fine sand fraction. These sedimentary changes should be linked with human activities: increase in bottom trawling effort that induces the resuspension of fine mud particles and the homogenization of sediments over large areas, and decrease in terrigenous particulate fluxes due to anthropic activities on the shoreline and in coastal waters. Effects of global climatic change on the observed evolution remain low, even if some species common in the south of the Bay of Biscay in the 1960s but rare or absent in the north, had increased in density and spread to the north. Trawling activities are probably the main force driving benthic community evolution in the North Bay of Biscay both through direct action on the fauna (inhibition and facilitation processes) and indirect action by modifying sediment characteristics. more...

International audience; In situ benthic chamber experiments were conducted in the Thau Lagoon that allowed the simultaneous determination of the benthic exchanges of trace metals (Cd, Co, Cu, Mn, Pb and U) and mercury species (iHg and MMHg). Fluxes of organotin compounds (MBT, DBT and TBT) were also investigated for the first time. The benthic incubations were performed during two campaigns at four stations that presented different macrobenthic and macrophytic species distribution and abundance (see [Thouzeau, G., Grall, J., Clavier, J., Chauvaud, L., Jean, F., Leynaert, A., Longpuirt, S., Amice, E., Amouroux, D., 2007. Spatial and temporal variability of benthic biogeochemical fluxes associated with macrophytic and macrofaunal distributions in the Thau lagoon (France). Estuarine, Coastal and Shelf Science 72 (3), 432–446.]). The results indicate that most of the flux intensity as well as the temporal and spatial variability can be explained by the combined influence of microscale and macroscale processes. Microscale changes were identified using Mn flux as a good indicator of the redox conditions at the sediment–water interface, and by extension, as an accurate proxy of benthic fluxes for most trace metals and mercury species. We also observed that the redox gradient at the interface is promoted by both microbial and macrobenthic species activity that governs O2 budgets. Macroscale processes have been investigated considering macrobenthic organisms activity (macrofauna and macroalgal cover). The density of such macroorganisms is able to explain most of the spatial and temporal variability of the benthic metal fluxes within a specific site. A tentative estimation of the flux of metals and organometals associated with deposit feeder and suspension feeder activity was found to be in the range of the flux determined within the chambers for most considered elements. Furthermore, a light/dark incubation investigating a dense macroalgal cover present at the sediment surface illustrates the role of photosynthetic activity in controlling benthic exchanges. Significant changes in benthic flux intensity and/or direction were reported for all redox sensitive elements (Cd, Co, Cu, Mn, Pb, U, and iHg). For MMHg and organotin species, other complimentary processes such as photodegradation/uptake and hydrophobic absorption/desorption need to be considered. This work demonstrates that the processes governing benthic exchanges are complex and that benthic organisms play a major role in the significant seasonal, diurnal and spatial variability of trace metals and organometals benthic fluxes in the lagoon. more...

The aim of the present study in the Thau lagoon was to determine spatial and temporal variations of biogeochemical fluxes at the sediment–water interface (oxygen, nutrients and DIC) associated with macrophytic and/or macrofaunal distributions during spring time, which is a period of major vegetal growth. The pattern of benthic fluxes was studied in April 2002 and May 2003 for four main benthic biota of the lagoon, with one site subject to biodeposition by oysters cultured in suspension, one site representative of the central part of the lagoon that was only under general sedimentation, one site densely covered by macroalgae and one site located within a seagrass meadow. The results emphasize the role of oyster cultures on benthic fluxes and the impact of spatial heterogeneity of the benthos (macrophytes and macrofauna) distribution at various spatial scales. Hypoxic conditions were shown to develop at the sediment–water interface in May 2003 in the central part of the lagoon. Oxygen and DIC fluxes emphasized benthic respiration overall, and spatio-temporal variability mainly due to the presence/absence of the macroalgal cover on the sediment. The absence of benthic macroinfaunal organisms in the stations under the influence of oyster biodeposition and facing a dense macroalgal cover highlighted anoxic conditions in the sediments. Major benthic macrofauna community changes were observed in the central zone between April 2002 and May 2003, emphasizing sediment organic matter enrichment. The analysis of benthic fluxes over a productive period of the annual cycle provided a basis for describing spatial and temporal variations of the dynamics of sediment–water exchanges in the lagoon. more...

The photosynthetic and repiratory metabolism of Zostera marina and maerl communities was compared, in the same area of the Bay of Brest in March–April, using benthic chambers. P–E curves for both oxygen and carbon were established for bottom irradiances between 0 and 525 μmol m−2 s−1. An exponential function was fitted to calculate daily production. Community metabolic quotients did not differ for maerl and seagrass beds. Community photosynthetic quotients were significantly higher (1.19) whereas community respiratory quotients were lower (0.70) than 1. Maerl and seagrass bed P–E curves mainly differed by the minimum saturating irradiance (Ek). Net community production was estimated to 26.8 mmol C m−2 d−1 for Z. marina meadows and 8.6 mmol C m−2 d−1 for maerl beds. The two communities can, therefore, be considered as autotrophic during the March–April period. Community respiration did not differ between Z. marina meadows and maerl beds, with an average value of 53.8 mmol C m−2 d−1 during a day. In similar environmental conditions, the production of maerl beds corresponds to approximately one third that of seagrass meadows. The maerl communities, therefore, form productive ecosystems, relevant to temperate coastal ecosystems functioning. more...

Biotic interactions within benthic megafauna were studied from quantitative dredge samples to investigate their role in the persistence of low scallop, Pecten maximus, stocks in the Bay of Brest. The distributions of 148 species of the megafauna and of four age classes of Pecten maximus were sampled. Suspension feeders dominate all megabenthic assemblages within the bay. This domination can be related to Crepidula fornicata spreading in the bay; however, no direct competition for food between scallops and the slipper limpet (introduced species) has been observed. On the other hand, Crepidula spreading leads to major changes in sediment type (silting) in the bay owing to biodeposit production, and silting would be a major factor inducing the decrease in Pecten maximus distribution. Predation would not be limiting. Within 10 years, one may predict a strong decrease in the area colonized by scallops in the bay. more...