Maerl Beds Research Articles

Controls on mesophotic carbonate facies and sediment distribution across the Maltese shelf, central Mediterranean Sea

Although ~ 20% of global carbonate production occurs on extra-tropical carbonate depositional systems, our understanding of these environments still lags behind that of tropical ones. The Maltese shelf in the central Mediterranean offers an opportunity to study in situ facies distribution and the factors controlling it in a light-dominated setting. The investigated region of the Maltese shelf visually exhibits three main depositional environments: seagrass meadows, sand flats and rhodolith and maerl beds. While visually distinctive, the grain composition of the sediments does not provide a clear differentiation of the three environments but rather a gradient. This gradient is marked by increasing grain size with water depth, a transition from green to red calcareous algae and an increase in the fraction of low magnesium calcite of total carboantes. While some of these features can be explained by changes in light availability, other factors are also in play. Baffling by seafloor vegetation and currents, storms and internal waves inducing sediment reworking appear to play important roles in governing the sediment texture and composition across the Maltese shelf. The role of seagrass meadows in regulating production and accumulation rates of carbonates appears to be of greater importance in Mediterranean C-type carbonate factories than in southern Atlantic ones and this could be an important marker to identify them in the geological record.

Real world data for real world problems: Importance of appropriate spatial resolution modelling to inform decision makers in marine management

Effective management of marine ecosystems requires informed decision-making based on accurate, comprehensive and appropriate data. Mapping habitats can form an important part of these processes so that decision makers can ensure the pressures exerted by human activities can be assessed while considering the sensitivity of the environment. Predictive models can be utilised to understand the distribution of species and habitats (e.g. biotopes) without incurring the expense of wide scale surveys. If predictive models are used in decision-making care must be taken in interpreting model results. The spatial resolution at which habitat modelling is conducted can greatly influence the decision outcomes. Whilst national resolution habitat maps serve as valuable resources for informing overarching policy making, for development level decisions fine resolution habitat information is needed. This paper explores the importance of spatial resolution modelling in marine management decision-making processes, using four spatial resolutions (50 m, 100 m, 200 m and 500 m) to model the presence of a protected habitat, maerl beds, within the Fetlar-Haroldswick Marine Protected Area in the Shetland Islands, Scotland. Outputs were compared for model performance between the resolutions and area of modelled maerl bed coverage. Simulations of real-world marine activities, explore the magnitude of overlap attributable to varied spatial resolution models, with an emphasis on the presumed ‘need’ for management. This study shows the importance of considering spatial resolution in modelling outputs and highlights the challenges associated with using models to guide decision-making, direct pressures on protected habitats, and cumulative impacts. It carries significance for maximising economic opportunity while safeguarding marine features. For real-world applications coarse resolution data may suffice for strategic, large-scale decisions, but finer resolutions are imperative for consenting or managing individual marine activities. By emphasising the need for appropriate spatial resolution modelling, these findings contribute to the development of sustainable management strategies that are appropriate to the scale of the decision. Addressing the complexities of real-world decision-making and understanding the magnitude of spatial resolution required for the marine environment are a crucial principle that can also enhance and be applied to other disciplines including, terrestrial ecology, urban planning and the assessment of potential climate change impacts. Failing to model appropriately means that real-world pressures and impacts occurring on a finer scale then the available data may have their impacts over or underestimated, hindering effective governance. Whilst we are striving to meet our national and international obligations and objectives through effective marine governance, this paper highlights the challenges of real-world decision-making where data is not yet available at a suitable resolution to address these issues.

Health status and characterisation of Gibraltar’s maerl beds

Maerl beds form heterogeneous, three-dimensional habitats for numerous species, housing high levels of biodiversity and species richness across the globe. Despite the importance of maerl’s ecological and carbon-regulating role; there is still a lack of information on methods to characterise its health. Therefore, this research set out to develop a standardised health assessment framework to assess the overall health of maerl a) structural function and b) habitat function. The framework was tested in an area of Gibraltar that has historically been severely impacted from various pressures including water quality. The maerl health is in a critical status, as over 92 % of it is deemed dead, with high levels of surface erosion and unusual colourations observed, and rhodoliths were generally small in size (mean: 1.35 cm ± 0.19 cm). The habitat health was labelled as intermediate, based on the species richness values, signs of nursery function, high number of Terebellidae (n = 233), distribution and coverage, and type of sediment composition. The maerl-forming species morphologically identified as Lithophyllum cf. racemus and Lithothamnion cf. corallioides were found to be present in the study site, which are first records within Gibraltar’s waters.

Disentangling the effect of space, time, and environmental and anthropogenic drivers on coastal macrobenthic β diversity in contrasting habitats over 15 years

Coastal zones are biodiversity hotspots and deliver essential ecosystem functions and services, yet they are exposed to multiple and interacting anthropogenic and environmental constraints. The individual and cumulative effects of these constraints on benthic communities, a key component of coastal ecosystems, and their variability across space and time, remains to be thoroughly quantified to guide conservation actions. Here, we explored how the presence of biogenic habitats influences the response of benthic communities to natural and anthropogenic constraints. We investigated this effect in both intertidal and subtidal habitats exposed to different pressures. We used data collected in the North-East Atlantic over 15 years (2005–2019) as part of the REBENT monitoring program, covering 38 sites of bare sediments, intertidal seagrass beds and maerl beds. We collected a range of environmental variables and proxies of anthropogenic pressures and used variation and hierarchical partitioning with redundancy analyses to estimate their relative effect on macrobenthic communities. We used descriptors modeling spatial and temporal structures (dbMEMs) to explore the scale of their effects and potential missing predictors. The selected variables explained between 53 % and 64 % of macrobenthic β diversity depending on habitat and depth. Fishing pressures, sedimentary and hydrodynamics variables stood out as the most important predictors across all habitats while proxies of anthropogenic pressures were overall more important in intertidal habitats. In the intertidal, presence of biogenic habitat strongly modulated the amount of explained variance and the identity of the selected variable. Across both tidal levels, analysis of models' residuals further indicated that biogenic habitats might mitigate the effect of extreme environmental events. Our study provides a hierarchy of the most important drivers of benthic communities across different habitats and tidal levels, emphasizing the prominence of anthropogenic pressures on intertidal communities and the role of biogenic habitats in mitigating environmental changes.

Salmon farming alters the structure and functioning of Norwegian maerl bed communities

Abstract Effluents resulting from fish farming activities can alter the structure and functioning of benthic ecosystems. In Norway, recent technological advances within the salmon industry have facilitated the expansion of aquaculture into shallower locations, leading to additional pressures upon coastal habitats. Maerl/rhodolith beds are important bioengineers in coastal waters—supporting highly diverse associated macrofaunal communities—and are likely to be particularly at risk. This study aims to investigate the environmental changes induced by fish farms in their vicinity and their impact on the structure and functioning of maerl bed macrofaunal communities. High dissolved nutrients and organic enrichment levels were observed close to the cages, associated with lower live maerl cover. Overall, the most substantial impact of salmon farms on macrofaunal communities was observed within 100 m from the cages. Nevertheless, changes in the structure and functioning of assemblages were evidenced up to 300 m. In the vicinity of the cages, communities were characterized by the dominance of endobenthic subsurface deposit feeders and opportunistic species, notably the polychaetes Chaetozone sp., Capitella sp. and Scoloplos armiger. Conversely, sites situated farther away from the cages exhibited higher abundances of epibenthic taxa, including crawlers, grazers and species sensitive to organic enrichment. Sites unaffected by the presence of fish farms were described by high abundances of the ophiuroid Ophiura robusta, the polychaete Proclea graffi and ostracods, reflecting more favourable environmental conditions. Such changes in macrofaunal communities highlight the vulnerability of maerl beds to the impacts of aquaculture, thus emphasizing the importance of rigorous site selection when establishing new fish farming facilities.

Changes in maerl-associated macroalgal community dynamics as evidence of anthropogenic pressure.

Maerl-associated communities have received considerable attention due to their uniqueness, biodiversity and functional importance. Although the impacts of human activities are well documented for maerl-associated macrofauna, the spatio-temporal variations of macroalgae have comparatively been neglected, and the drivers that influence their dynamics are poorly known. We investigate the links between maerl-associated macroalgal communities, anthropogenic pressures and environmental conditions, and hypothesize that sites under human pressure would exhibit different dynamics when compared to reference sites. To better understand community variation through space and time, four subtidal maerl beds under different pressures were consistently monitored over one year in the bay of Brest, Brittany, France. Both macroalgae community monitoring and environmental data were acquired through field sampling and available models. Higher macroalgal biomass was observed within eutrophic sites, especially in summer (more than ten times higher than in the Unimpacted site), caused by free-living forms of opportunistic red macroalgae. The Dredged site also exhibited distinct macroalgal communities during summer from the Unimpacted site. Nutrient concentrations and seasonality proved to be key factors affecting the macroalgal community composition, although dredging and its effects on granulometry also had a strong influence. Over the long term, fewer than half of the species identified during historical surveys were found, indicating major temporal changes. Human pressures have strong impacts on maerl-associated macroalgal communities. Nutrient concentrations and dredging pressure appear as the main anthropogenic factors shaping maerl-associated macroalgal communities. Additionally, our results suggest historical changes in maerl-associated macroalgal communities over 25 years in response to changes in local human pressure management. This study suggests that maerl-associated macroalgal communities could be used as indicators of anthropogenically driven changes in this habitat.

Sounding out maerl sediment thickness: an integrated data approach

Maerl beds are listed as a priority marine feature in Scotland. They are noted for creating suitable benthic habitat for diverse communities of fauna and flora and in supporting a wide array of ecosystem services. Within the context of climate change, they are also recognised as a potential blue carbon habitat through sequestration of carbon in living biomass and underlying sediment. There are, however, significant data gaps on the potential of maerl carbon sequestration which impede inclusion in blue carbon policy frameworks. Key data gaps include sediment thickness, from which carbon content is extrapolated. There are additional logistical and financial barriers associated with quantification methods that aim to address these data gaps. This study investigates the use of sub-bottom profiling (SBP) to lessen financial and logistical constraints of maerl bed sediment thickness estimation and regional blue carbon quantification. SBP data were cross validated with cores, other SBP data on blue carbon sediments, and analysed with expert input. Combining SBP data with estimates of habitat health (as % cover) from drop-down video (DDV) data, and regional abiotic data, this study also elucidates links between abiotic and biotic factors in determining maerl habitat health and maerl sediment thickness through pathway analysis in structural equation modelling (SEM). SBP data were proved to be sufficiently robust for identification of maerl sediments when corroborated with core data. SBP and DDV data of maerl bed habitats in Orkney exhibited some positive correlations of sediment thickness with maerl % cover. The average maerl bed sediment thickness was 1.08 m across all ranges of habitat health. SEM analysis revealed maerl bed habitat health was strongly determined by abiotic factors. Maerl habitat health had a separate positive effect on maerl bed sediment thickness.

Quantifying maerl (rhodolith) habitat complexity along an environmental gradient at regional scale in the Northeast Atlantic

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 species-specific 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.

Effect of sea lice chemotherapeutant hydrogen peroxide on the photosynthetic characteristics and bleaching of the coralline alga Lithothamnion soriferum.

The proliferation of sea lice (Lepeophtheirus salmonis) represents a major challenge for the salmonid aquaculture industry in Norway. Hydrogen peroxide (H2O2) is a chemotherapeutant frequently used on Norwegian farms, however, its toxicity to non-target benthic species and habitats remains poorly understood. Maerl beds are constructed by the accumulation of non-geniculate coralline algae and provide important ecological functions. Due to the rapid expansion of aquaculture in Norway and the continued use of H2O2 as an anti-sea lice treatment, it is crucial to understand the impact of H2O2 on the physiology of maerl-forming species. The effects of a 1h exposure to H2O2 on the photophysiology and bleaching of the coralline alga Lithothamnion soriferum were examined here through a controlled time-course experiment. PAM fluorimetry measurements showed that H2O2 concentrations≥200mg l-1 negatively affected photosystem II (PSII) in thalli immediately after exposure, which was observed through a significant decline in maximum photochemical efficiency (Fv/Fm) and relative electron transport rate (rETR). The negative effects on PSII induced by oxidative stress, however, appear to be reversible, and full recovery of photosynthetic characteristics was observed 48h to 28 days after exposure to 200mg H2O2 l-1 and 2000mg H2O2 l-1, respectively. At 28 days after exposure, there was evidence of two- to four-times more bleaching in thalli treated with concentrations ≥ 200mg H2O2 l-1 compared to those in the control. This indicates that despite the recovery of PSII, persistent damages can occur on the structural integrity of thalli, which may considerably increase the vulnerability of coralline algae to further exposure to H2O2 and other chemical effluents from salmonid farms.

First insights into the meiofauna community of a maerl bed in the Bay of Brest (Brittany)

Maerl beds, particularly those of Brittany, are important, structurally complex biogenic coastal habitats that form a unique ecosystem with high benthic biodiversity. Although they are relatively well studied throughout Europe, several faunal groups of maerl beds, such as those belonging to the meiofauna, have received little attention. We investigated the meiofaunal abundance, distribution and community structure, with a focus on nematode biomass and diversity, in a maerl area in the Bay of Brest, Brittany, compared with that on a sandy beach (Anse de Dinan, Brittany). Meiofauna was five times more abundant on the maerl bed than on the sandy beach, and 1.5 times more diversified (12 vs. 8 taxa, respectively). Nematode diversity was more than three times higher on the maerl bed than on the sandy beach and showed a distinctive nematode community that was absent from the sandy beach. Maerl beds create more heterogeneous microhabitats and promote a higher diversification of meiofauna and nematode communities than sandy beaches.

Structure and biodiversity of a Maltese maerl bed: New insight into the associated assemblage 24 years after the first investigation

Maerl beds are biogenic benthic habitats distributed worldwide and known to sustain high productivity and biodiversity levels. In the Mediterranean, the number of studies that have been carried out is limited, and little is known on its real distribution, mostly due to the difficulties of exploring such habitats — Due to the high transparency of Mediterranean waters, maerl can be found at depths of over 50 m making mandatory the use of benthic grabs and ROVs. The last published data from Maltese waters were taken two decades ago. In this present study, we provide new insights on this poorly known habitat, in particular regarding the north western bank, designated as a NATURA 2000 protected area, in which human activities are also carried out, such as blue fin tuna ranching. The objective of this work is to provide information regarding this delicate habitat, especially relevant for future management plans and authorities.

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

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 the 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 2 distinct Lithothamnion corallioides maerl beds in 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 2 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.

Effects of season, bottom substrate and population dynamics on fish communities in shallow subarctic northeast Atlantic waters

Fish communities in the shallow waters (9–20 m depth) of outer Malangen (69o N), Northern Norway was studied to investigate effects of season, bottom substrate associations and population dynamic parameters on the fish assemblages. In four fieldtrips during one year at four locations, fish were sampled by trammel nets and bottom substrate composition was monitored by underwater video. A total of 16 species of fish were caught, including four species from Pleuronectidae and six from the Gadidae family. Cod (Gadus morhua Linnaeus, 1758) was the most numerous species followed by plaice (Pleuronectes platessa Linnaeus, 1758), lemon sole (Microstomus kitt Walbaum, 1792) and halibut (Hippoglossus hippoglossus Linnaeus, 1758). Cod dominated the catches in all months except March, in which plaice and other flatfishes were most dominant. On a community scale, fish species richness was highest in October and lowest in March. Furthermore, the species richness and diversity were generally higher at locations with greater bottom substrate evenness. Cod showed a positive association with large macroalgae coverage and were less numerous at locations dominated by sand and pebble. Plaice and common dab (Limanda limanda Linnaeus, 1758) showed positive associations with sand and pebble coverage, while lemon sole was more associated with maerl beds, cobble and low algae coverage. Only halibut younger than 8 years of age were observed, and the sampling areas are suggested to be nursery area for this species. Von Bertalanffy growth functions were estimated for cod, plaice and lemon sole, and the growth coefficient K of cod and plaice were higher than for populations from adjacent fjords or the North Sea populations. Total mortality rates of cod and plaice were lower than for areas further south, and this may contribute to high and relative stable biomasses in the shallow Malangen area.

Fossil maerl beds as coastal indicators of late Holocene palaeo-environmental evolution in the Bay of Brest (Western France)

The Bay of Brest (BB) is a mixed, tide-dominated estuarine system. The shore terraces of this bay are occupied by modern free-living (calcareous) coralline algae locally termed “maerl”, organized in bed-like morphologies (rhodolith deposits). Cores retrieved from around the bay reveal fossilized primitive maerl beds of Holocene age, interbedded in sandy-silt sedimentation. The alternation between biogenic constructions and estuarine sedimentation may provide evidence of varying environmental conditions of the late-Holocene period. This paper mainly focuses on the results of chronostratigraphic and bio-sedimentological interpretations of coring data collected in less than 15 m of water depth in an attempt to decipher the main stages of maerl colonization in the bay. In particular, this study raises several significant points allowing to draw links between centennial to millennial-scale climatic changes in marine estuary sedimentation and episodes within the development of maerl biocenoses. The paleo-bathymetry of the coastal terraces has not changed significantly over the last 5000 years. Yet, the first maerl occurrence only appeared around 2000 cal yr B.P., likely showing that the environmental conditions were not favorable for their emergence prior to that time. Pioneer maerl beds developed on coarse shell deposits inherited from the paleostorms affecting the Atlantic coasts during the colder climatic period of the Iron Age (3100–1950 cal yr B.P.). The accumulations then aggraded at various and discontinuous rates, sometimes reaching up to 2.1 m/kyr. Maerl beds temporarily disappeared in the southern part of the Bay of Brest when sedimentation rates increased throughout the bay during the Dark-Age cold period (1375–1250 cal yr B.P.), suggesting that maerl formations could not keep up with sedimentation rate exceeded a certain threshold. Muddy sedimentation conditions also dramatically changed on two occasions, with the establishment of coarse storm levels, set at the intervals 825–600 cal yr B.P. (MWP) and 113–0 cal yr B.P. But maerl deposits reseed the environment as a result of each new cold period, demonstrating the persistence of such coralline algae against drastic palaeoenvironmental changes in coastal areas.

Physiology of maerl algae: Comparison of inter- and intraspecies variations.

Free-living red coralline algae play an important role in the carbon and carbonate cycles of coastal environments. In this study, we examined the physiology of free-living coralline algae-forming maerl beds in the Bay of Brest (Brittany, France), where Lithothamnion corallioides is the dominant maerl (i.e., rhodolith) species. Phymatolithon calcareum and Lithophyllum incrustans are also present (in lower abundances) at a specific site in the bay. We aimed to assess how maerl physiology is affected by seasonality and/or local environmental variations at the inter- and intraspecific levels. Physiological measurements (respiration, photosynthetic, and calcification rates) were performed using incubation chambers in winter and summer to compare (1) the dominant maerl species at three sites and (2) three coexisting maerl species at one site. Comparison of the three coexisting maerl species suggests that L.corallioides is the best adapted to the current environmental conditions in the Bay of Brest, because this species is the most robust to dissolution in the dark in winter and has the highest calcification efficiency in the light. Comparisons of L.corallioides metabolic rates between stations showed that morphological variations within this species are the main factor affecting its photosynthetic and calcification rates. Environmental factors such as freshwater inputs also affect its calcification rates in the dark. In addition to interspecies variation in maerl physiology, there were intraspecific variations associated with direct (water physico-chemistry) or indirect (morphology) local environmental conditions. This study demonstrates the plasticity of maerl physiology in response to environmental changes, which is fundamental for maerl persistence.

Whole genome genotyping reveals discrete genetic diversity in north-east Atlantic maerl beds.

Maerl beds are vital habitats for a diverse array of marine species across trophic levels, but they are increasingly threatened by human activities and climate change. Furthermore, little is known about the genetic diversity of maerl‐forming species and the population structure of maerl beds, both of which are important for understanding the ability of these species to adapt to changing environments and for informing marine reserve planning. In this study, we used a whole genome genotyping approach to explore the population genomics of Phymatolithon calcareum, a maerl‐forming red algal species, whose geographical distribution spans the north‐east Atlantic, from Norway to Portugal. Our results, using 14,150 genome‐wide SNPs (single nucleotide polymorphisms), showed that P. calcareum maerl beds across the north‐east Atlantic are generally structured geographically, a pattern likely explained by low dispersal potential and limited connectivity between regions. Additionally, we found that P. calcareum from the Fal Estuary, south‐west England, is genetically distinct from all other P. calcareum sampled, even from The Manacles, a site located only 13 km away. Further analysis revealed that this finding is not the result of introgression from two closely related species, Phymatolithon purpureum or Lithothamnion corallioides. Instead, this unique diversity may have been shaped over time by geographical isolation of the Fal Estuary maerl bed and a lack of gene flow with other P. calcareum populations. The genomic data presented in this study suggest that P. calcareum genetic diversity has accumulated over large temporal and spatial scales, the preservation of which will be important for maximizing the resilience of this species to changes in climate and the environment. Moreover, our findings underline the importance of managing the conservation of maerl beds across western Europe as distinct units, at a site‐by‐site level.

Comparison of benthic oxygen exchange measured by aquatic Eddy Covariance and Benthic Chambers in two contrasting coastal biotopes (Bay of Brest, France)

To the best of our knowledge, the understanding of benthic metabolism of coastal sedimentary areas is still limited due to the complexity of determining their true in situ dynamics over large spatial and temporal scales. Multidisciplinary methodological approaches are then necessary to increase our comprehension of factors controlling benthic processes and fluxes. An aquatic Eddy Covariance (EC) system and Benthic Chambers (BC) were simultaneously deployed during the winter of 2013 in the Bay of Brest within a Maerl bed and a bare mudflat to quantify and compare O2 exchange at the sediment–water interface. Environmental abiotic parameters (i.e., light, temperature, salinity, current velocity and water depth) were additionally monitored to better understand the mechanisms driving benthic O2 exchange. At both sites, EC measurements showed short-term variations (i.e. 15 min) in benthic O2 fluxes according to environmental conditions. At the Maerl station, EC fluxes ranged from -21.0 mmol m−2 d−1 to 71.3 mmol m−2 d−1 and averaged 22.0 ± 32.7 mmol m−2 d−1 (mean ± SD), whilst at the bare muddy station, EC fluxes ranged from -43.1 mmol m−2 d−1 to 12.1 mmol m−2 d−1 and averaged -15.9 ± 14.0 mmol m−2 d−1 (mean ± SD) during the total deployment. Eddy Covariance and Benthic Chambers measurements showed similar patterns of temporal O2 flux changes at both sites. However, at the Maerl station, BC may have underestimated community respiration. This may be due to the relative large size of the EC footprint (compared to BC), which takes into account the mesoscale spatial heterogeneity (e.g. may have included contributions from bare sediment patches). Also, we hypothesize that the influence of bioturbation induced by large-sized mobile benthic fauna on sediment oxygen consumption was not fully captured by BC compared to EC. Overall, the results of the present study highlight the importance of taking into account specific methodology limitations with respect to sediment spatial macro-heterogeneity and short-term variations of environmental parameters to accurately assess benthic O2 exchange in the various benthic ecosystems of the coastal zone.

Reduced physiological performance in a free-living coralline alga induced by salmon faeces deposition

Maerl beds are formed by the accumulation of free-living coralline algae and have considerable ecological significance due to the high diversity of associated fauna and flora. The rapid expansion of the Atlantic salmon Salmo salar aquaculture industry in Norway may have major impacts on surrounding maerl beds through the release of effluents, including fish faeces. This study is the first to test the effects of salmon faeces and inorganic sediment deposition on the photosynthesis, respiration, calcification and pigment content of the coralline alga Lithothamnion soriferum. In a 6 wk laboratory experiment, inorganic sediment and salmon faeces deposition significantly reduced the amount of light reaching the surface of coralline algae. No impact of inorganic sediment deposition was detected on L. soriferum physiology, while salmon faeces deposition increased respiration rate and reduced net primary production and calcification. The accumulation of salmon faeces stimulates proliferation of bacteria, with adverse consequences on L. soriferum physiology due to the potential release of toxic compounds. Burial by salmon faeces deposition also affects the physiology of coralline algae due to the flocculation of sticky faeces particles, which may limit nutrient and gas exchanges in the vicinity of thalli. Carbon dioxide accumulation in the vicinity of L. soriferum may lead to a decline in pH and alter the calcification process in cell walls. In natural maerl beds, the negative effect of faeces deposition may be exacerbated by longer-term exposure and the presence of other chemicals released by fish farms.

Impact of fishing gears and fishing intensities on maerl beds: An experimental approach

• Maerl beds are very diverse yet very vulnerable habitats, especially to dredging. • Bivalve dredging leads to profound maerl alteration and to diversity loss. • The fishing pressure and the gear specificities cause long-term impacts (>1 year). • At low fishing pressure, the type of gear is conditioning the level of destruction. • Fishing pressures and gears specificities must be considered in management policies.

Using stable isotope analysis to determine the effects of ocean acidification and warming on trophic interactions in a maerl bed community

AbstractOcean acidification and warming are likely to affect the structure and functioning of marine benthic communities. This study experimentally examined the effects of ocean acidification and warming on trophic interactions within a maerl bed community by using stable carbon and nitrogen isotope analysis. Two three‐month experiments were conducted in winter and summer seasons with four different combinations of pCO2 (ambient and elevated pCO2) and temperature (ambient and +3°C). Experimental assemblages were created in tanks held in the laboratory and were composed of calcareous (Lithothamnion corallioides) and fleshy algae (Rhodymenia ardissonei, Solieria chordalis, and Ulva sp.), gastropods (Gibbula magus and Jujubinus exasperatus), and sea urchins (Psammechinus miliaris). Our results showed higher seaweed availability for grazers in summer than winter. Therefore, grazers were able to adapt their diet seasonally. Increased pCO2 and temperature did not modify the trophic structure in winter, while shifts in the contribution of seaweed were found in summer. Combined acidification and warming increased the contribution of biofilm in gastropods diet in summer conditions. Psammechinus miliaris mostly consumed L. corallioides under ambient conditions, while the alga S. chordalis became the dominant food source under high pCO2 in summer. Predicted changes in pCO2 and temperature had complex effects on assemblage trophic structure. Direct effects of acidification and warming on seaweed metabolism may modify their abundance and biomass, affecting their availability for grazers. Climate change may also modify seaweeds' nutritive value and their palatability for grazers. The grazers we investigated were able to change their diet in response to changes in algal assemblages, an advantage given that warming and acidification alter the composition of algal communities.