This review explores the application of nitrogen stable isotopes measurements as a tool for monitoring nitrogenous nutrients inputs in coastal zones, focusing on oligotrophic tropical ecosystems. Pulsed nutrient enrichment leads to the proliferation of macroalgae, compromising the health of coral reefs and seagrass beds. Through the analysis of 52 studies, we compile isotopic signatures measured in macroalgae and seagrass, while proposing reference thresholds to identify nitrogen pollution sources in tropical areas. By standardizing these isotopic compositions, our review aims to provide useful benchmarks for researchers and managers of coastal ecosystems. The main pollution sources identified include wastewater, groundwater, aquaculture, and fertilizers, each characterized by distinct δ15N ranges. Among these, wastewater and sewage show the highest isotopic values (>10 ‰), while inorganic fertilizers exhibit the lowest (down to −4.5 ‰). Macroalgae, particularly Dictyota and Rhodophyta species (e.g. Laurencia), were most frequently used as bioindicators due to their wide distribution and responsiveness to nutrient variation. Thalassia testudinum emerged as the most commonly used seagrass species across studies. However, seagrasses overall were less frequently employed than macroalgae, and typically displayed less sensitively to nitrogen enrichment in their isotopic signatures. The review highlights practical aspects of isotopic monitoring, including macrophyte selection, caging techniques, sample preparation, and environmental variables affecting interpretation such as seasonality, depth, and hydrodynamic conditions. Caging, though underused in seagrass studies, proved essential in areas lacking native bioindicators. Finally, we provide recommendations for future studies to enhance consistency and standardization allowing temporal and intersites comparisons. This approach offers a cost-effective and sensitive tool for detecting nutrient inputs in vulnerable tropical ecosystems.

Seaweed strandings are recurrent in the eastern Seine Bay (English Channel) and may strongly affect nutrient dynamics at the sediment-water interface. This study investigated whether nitrogen released from decaying stranded seaweeds contributes to nitrogen availability for benthic Ulva during summer, using a combined approach of 19 months of in situ monitoring and an in vitro experiment. Variation in delta N-15 in Ulva (delta(15)NUlva) was observed, with lower values (<7 %o) in spring and higher values (>9 %o) in summer. In spring, low delta(15)NUlva values and the positive correlation between concentrations of dissolved inorganic nitrogen (DIN) in intertidal seawater and the River Orne's flow indicate a terrigenous nitrogen origin, mainly from agricultural fertilizers. In summer, concentrations of dissolved organic nitrogen (DON) and ammonium increased in both intertidal and interstitial waters. Significant positive correlations were found between delta(15)NUlva, the extent of algal wracks, and DON concentrations, the latter also increasing with the extent of wracks. These relationships suggest that, in summer, when riverine nitrogen inputs are minimal, decaying stranded seaweeds constitute an endogenous nitrogen source for benthic seaweeds. This hypothesis was supported by a controlled experiment showing an increase in delta N-15 in Ulva thalli after 9 days of culture in seawater enriched with algal leachate. Such endogenous nitrogen recycling supports the growth of opportunistic Ulva, which dominate summer strandings and negatively affect coastal water quality assessments under the European Water Framework Directive. These findings highlight the importance of accounting for seaweed strandings in strategies and management actions aimed at improving coastal water quality.

Massive Sargassum accumulations in the Atlantic form the Great Atlantic Sargassum Belt, which fuels recurrent coastal inundation events that disrupt ecosystems by reducing light availability for phytoplankton and enriching nearshore waters with nutrients released during decomposition. This study examined the short-term effects of decaying Sargassum leachate on phytoplankton productivity and growth in Guadeloupe's coral reef ecosystems using several photosynthesis measurements based on fluorescence and 13C. A range of leachate concentrations were applied, under controlled conditions, to a natural phytoplankton community. The study revealed two major effects of Sargassum leachate on phytoplankton. First, leachate at concentrations >= 1 % initially inhibited photosynthesis and growth up to Day 2, likely due to allelopathic substances such as polyphenols. This was evidenced by near-zero Fv/Fm values and disrupted electron transport in PSII. By Day 3, these inhibitory effects diminished, suggesting degradation of labile inhibitory compounds or community modification. Second, from Day 3 onward, leachate addition stimulated phytoplankton growth, as seen in increased biomass, primary production (JVIImax), and carbon fixation. This nutrient-driven response was accompanied by reduced C/Chl a ratios and improved photosynthetic efficiency (lower & Fcy;e,C values), indicating alleviation of nutrient limitations, particularly nitrogen and phosphorus. These findings underscore the dual role of Sargassum leachate: an initial suppressive impact through allelopathy and a subsequent nutrient enrichment effect driving phytoplankton blooms. Such dynamics highlight the significant and complex influence of Sargassum strandings, combining ecological stress with nutrient-driven productivity changes.

Development and growth of microalgae are mainly sustained by two essential nutrients: nitrogen (N) and phosphorus (P). Although single-nutrient limitation has been extensively studied, the balance between N and P availability remains less explored. Scrippsiella acuminata is one of the most abundant dinoflagellates in coastal ecosystems due to its physiological plasticity, making it a key species in the understanding of acclimation to unbalanced nutrient supply. To test the acclimation of S. acuminata, semicontinuous cultures were exposed to six N:P ratios (1.6, 8, 16, 32, 90, 180). Parameters such as photosynthetic response, biovolume, carbon excretion, lipids, reactive oxygen species production, cell cycle, and alkaline phosphatase activity were analyzed. Growth, regular cell cycle progression, balanced carbon allocation carbon resource, and high photosynthesis efficiency occurred at balanced N:P ratios (16, 32). At low ratios (1.6, 8), growth was reduced but cells maintained active photochemistry, whereas high ratios (90, 180) led to an extension of the G1 phase leading to biovolume increase and a limitation of the protective capacity of non-photochemical quenching leading to reactive oxygen species accumulation. Carbon allocation followed a stoichiometric gradient where more limiting N:P ratios favored soluble extracellular polymeric substances and a pool of cellular carbohydrates production as an overflow mechanism to protect cells, whereas moderate limitation led to lipid accumulation as a metabolic reserve. These results not only highlight the plasticity of S. acuminata to acclimate to nutrient stress but also suggest that this species may be more vulnerable in P-limited environments and has a competitive advantage where N is the primary limiting factor.

BACKGROUND:Estuaries are complex ecosystems linking river and marine environments, where microorganisms play a key role in maintaining ecosystem functions. In the present study, we investigated monthly 8 sites at two depth layers and over a one-year period the bacterial and eukaryotic community dynamics along the Seine macrotidal estuary (Normandy, France). To date, the taxonomy of the microbial diversity present in this anthropized estuary remains elusive and the drivers of the microbial community structure are still unknown. RESULTS:The metabarcoding analysis of 147 samples revealed both a high bacterial and eukaryotic diversity, dominated by Proteobacteria, Bacteriodota, Actinobacteriota and Bacillariophyta, Spirotrichea, Dinophyceae, respectively. Along the estuary we only detected significant spatial patterns in the bacterial and eukaryotic community compositions for three and two months out of twelve, respectively. However, we found a clear seasonal effect on the diversity of both microbial communities driven by physical and chemical variables that were fluctuating over the year (temperature, irradiance, river flow). Biotic associations were also significant drivers of both alpha and beta diversity. Throughout the year, we identified a diverse and abundant core microbiota composed of 74 bacterial and 41 eukaryotic OTUs. These regionally abundant species include habitat generalists encompassing heterotrophs, phototrophs and consumers. Yet, many of these core OTUs remain taxonomically and functionally poorly assigned. CONCLUSIONS:This molecular survey represents a milestone in the understanding of macrotidal estuary dynamics and the Seine ecosystem, through the identification of putative markers of ecosystem functioning. It also identifies seasons and biotic associations as main drivers of the Seine estuary microbiota and reveals the importance of a core microbiota throughout the year.

As the global population expands, marine coastal ecosystems face mounting pressures from human activities, that have led to habitat deterioration and dwindling fishery resources. In this context, Artificial Reefs (ARs) have emerged as one of the promising solutions. They are generally implemented to provide habitat, to create a protective, physical boundary, to support sustainable fisheries and to facilitate ecosystem rehabilitation. Evaluating their ecological performance is crucial to ensuring they meet their objectives. Initially, assessment relied on comparing ARs to natural reefs using mainly ecological metrics which focused on fish assemblage and dynamics. Despite there being more research and documentation on effectiveness today, assessing ARs remains challenging due to the number of environmental factors that can affect the ecological systems. Moreover, ecological studies mainly used metrics that investigated the reef fish populations or ecological metrics such as fish assemblages or trophic structure that are often overlooked in studies that primarily focus on commercial fishery dynamics. Therefore, new ways of assessing artificial reef performance and the set-up of comprehensive metrics which integrate this level of complexity are needed. In this study, we focused on the "Rade de Cherbourg" in the English Channel, employing a trophic modeling approach using Ecopath with Ecosim (EwE). The study emphasizes the importance of Ecological Network Analysis (ENA) metrics for evaluating changes in the systems’ properties—such as complexity, flow diversity, and recycling capacity— which result from AR implementation. Furthermore, we identified which metrics are suitable for assessing specific AR objectives. The proposed metrics serve as a command-and-control tool for AR site managers, enabling them to evaluate the performance of each AR objective effectively. With the anticipated increase in AR projects, especially those which compensate for human impact like the Cherbourg ARs, this research offers valuable insights and future perspectives to continuously improve the ecological performance of ARs.

To inform the performance of ecological engineering designs for artificial structures at sea, it is essential to characterise their impacts on the epibenthic communities colonising them. In this context, the present study aims to compare the community structure among natural and four different artificial hard habitats with different ages and features installed in the Bay of Cherbourg (English Channel): i) cinder blocks and ii) boulders, both installed six years prior to the study, and iii) smooth and iv) rugous concrete dykes, both installed one year prior to this study. Results showed that artificial habitats installed six years ago harboured communities with functional and taxonomic diversity characteristic of mature communities but were still different from those of natural habitat. Conversely, the two dyke habitats installed one year prior to this study presented a poorly diversified community dominated by opportunistic taxa. Furthermore, while the concrete used for the two dyke habitats presented different rugosity properties, both habitats supported similar communities, suggesting that such eco-engineering measures did not affect the settlement of early colonisers. Overall, this study highlights the need for long-term monitoring to comprehensively evaluate epibenthic colonisation of artificial structures.

Very few studies have looked at the potential biological effects of degradation products of galvanic anodes particularly on primary producers which are central to food webs in marine ecosystems. The galvanic anode cathodic protection system (GACP) is widely used to protect submerged metallic structures from corrosion. Aluminium (Al) and zinc (Zn) are the main constituents of galvanic anodes and are therefore released in the marine environment by oxidation process to form ions or oxy-hydroxides. The main objective of our study was to evaluate the effects of the metals released from an aluminium-based galvanic anode on microphytobenthos performance in term of biofilm growing through the analysis of photosynthetic parameters, the determination of chlorophyll and extracellular polymeric substances (EPS). The bioaccumulation of Al and Zn were measured in the microphytobenthic compartment collected at the surface of polyvinyl chloride (PVC) plates exposed during 13 days to seawaters enriched in different concentrations of metals released from dissolution of one anode. Determination of bioconcentration factors confirmed that the microphytobenthos has incorporated Al. A significative effect was observed on the Chl a concentration for the higher tested concentration ([Al] = 210.1 ± 60.2 µg L - 1; [Zn] = 20.2 ± 1.4 µg L - 1). The seawater exposed to the anode affected the MPB productivity (ETRIImax) with consequences on acclimatation light (Ek), absorption cross section of PSII (σPII), Fv/Fm and NPQ. Regarding the EPS production, the anode degradation presented an impact on high and low molecular weight of both carbohydrates and protein fractions of microphytobenthos suggesting that EPS play an essential role in sequestering metal contaminants to maintain the integrity of the biological membranes and the functionality of the cellular organelles. The accumulation of Al released by GACP in microphytobenthos cells could lead to physiologic problems in photosynthetic organisms.

Introduction While crucial to ensuring the production of accurate and high-quality data—and to avoid erroneous conclusions—data quality control (QC) in environmental monitoring datasets is still poorly documented. Methods With a focus on annual inter-laboratory comparison (ILC) exercises performed in the context of the French coastal monitoring SOMLIT network, we share here a pragmatic approach to QC, which allows the calculation of systematic and random errors, measurement uncertainty, and individual performance. After an overview of the different QC actions applied to fulfill requirements for quality and competence, we report equipment, accommodation, design of the ILC exercises, and statistical methodology specially adapted to small environmental networks (<20 laboratories) and multivariate datasets. Finally, the expanded uncertainty of measurement for 20 environmental variables routinely measured by SOMLIT from discrete sampling—including Essential Ocean Variables—is provided. Results, Discussion, Conclusion The examination of the temporal variations (2001–2021) in the repeatability, reproducibility, and trueness of the SOMLIT network over time confirms the essential role of ILC exercises as a tool for the continuous improvement of data quality in environmental monitoring datasets.

This study aimed to assess the influence of nutrient enrichment on the development of microalgal biofilm on concrete and PVC cubes. Three mesocosms were utilized to create a nutrient gradient over a period of 28 days. Various parameters including biomass, photosynthetic activity, microtopography, and extracellular polymeric substances (EPS) were measured. Imaging PAM techniques were employed to obtain surface-wide data. Results revealed that nutrient availability had no significant impact on Chl a biomass and the maximum quantum efficiency of PSII (Fv/Fm). The photosynthetic capacity and efficiency were minimally affected by nutrient availability. Interestingly, the relationship between microphytobenthic (MPB) biomass and photosynthesis and surface rugosity exhibited distinct patterns. Negative reliefs showed a strong correlation with Fv/Fm, while no clear pattern emerged for biomass on rough concrete structures. Overall, our findings demonstrate that under conditions of heightened eutrophication, biofilm photosynthesis thrives in the fissures and crevasses of colonized structures regardless of nutrient levels. This investigation provides valuable insights into the interplay between nutrient availability and surface rugosity.

Twenty-six species of the cosmopolitan genus Pseudo-nitzschia can produce domoic acid (DA), a neurotoxin responsible for amnesic shellfish poisoning (ASP). To improve knowledge on this issue, we studied the physio-logical conditions favorable to DA production and accumulation by three Pseudo-nitzschia species from French coastal waters: P. australis, P. pungens, and P. fraudulenta. They were grown in batch cultures under silicate limitation to characterize their physiological traits and calculate their DA production rates. Three strains were studied per species to consider intraspecific diversity and better characterize interspecific differences. DA pro-duction was not influenced by growth or silicate limitation in any of the three species. In contrast, silicate limitation in the stationary phase led to DA accumulation by inhibiting cell division, while DA production was still active. The maximum cellular DA (cDA) production rate was 2.95 pg cell-1 d-1 for P. australis, 0.07 pg cell-1 d-1 for P. pungens, and 0.03 pg cell-1 d-1 for P. fraudulenta. The physiological conditions favorable to cDA production and accumulation by P. australis and P. pungens differed. The three species presented similar growth rates, but P. australis had higher photosynthetic capacities that could partly explain its higher DA production potential. The cDA production and the net dissolved DA (dDA) production in the culture medium were favored by different growth conditions. The cDA production to net dDA production ratio was species specific, with P. pungens excreting proportionally more of its produced DA. These laboratory results on cultures imply that cDA production and net dDA production can occur during all phases of P. australis, P. pungens, and P. fraudulenta blooms. The interactions between the species composition of the bloom, the species-specific capacity for DA production, and the effect of silicate limitation - among other factors - on DA cell quotas drive the toxigenicity of Pseudo-nitzschia blooms.

The Seine estuary is a typical model of a system in which phosphorus (P) inputs have been considerably reduced to reduce past eutrophication, with a parallel decrease in phytoplankton biomass. However, reducing P alone while concentrations of nitrate (N) remains high led to a dystrophic nutrient balance in the estuary (high N/P and N/Si). To identify the drivers of primary production and phytoplankton communities in the highly anthropised Seine estuary, sampling was performed along a 110-km stretch over a period of 3 years. Photosynthetic parameters were measured with a single turnover active fluorimeter and the phytoplankton community was assessed using a fluoroprobe and flow cytometry. The results revealed an annual primary production of 33 g C.m −2 .y −1 which was largely controlled by light availability (turbidity) but also by nutrients in late spring and summer period. Massive blooms, not seen since 2002, were observed in a specific area of the estuary. None of the nutrients measured explained the particular location of the blooms but phytoplankton production and productivity indicators were higher in this specific zone excluding a biomass accumulation phenomenon. The local effect of tide slowdown increasing water residence time in this exact part of the estuary could explain the bloom area. The question thus arises: does phosphate play a role as a limiting nutrient? The diatom bloom led to total depletion of Si, but no clear limitation of P was observed. Nevertheless, the decrease in Si appears to have played a key role by triggering a succession from diatoms (microphytoplankton) to chlorophytes (nanophytoplankton) and hence affecting phytoplankton composition. The N/P balance appeared to be more important than the absolute concentration of P to explain the phytoplankton dynamics and the biomass decrease observed the last decades.

The photoautotrophic, unicellular N2-fixer, Cyanothece, is a model organism that has been widely used to study photosynthesis regulation, the structure of photosystems, and the temporal segregation of carbon (C) and nitrogen (N) fixation in light and dark phases of the diel cycle. Here, we present a simple quantitative model and experimental data that together, suggest external dissolved inorganic carbon (DIC) concentration as a major limiting factor for Cyanothece growth, due to its high C-storage requirement. Using experimental data from a parallel laboratory study as a basis, we show that after the onset of the light period, DIC was rapidly consumed by photosynthesis, leading to a sharp drop in the rate of photosynthesis and C accumulation. In N2-fixing cultures, high rates of photosynthesis in the morning enabled rapid conversion of DIC to intracellular C storage, hastening DIC consumption to levels that limited further uptake. The N2-fixing condition allows only a small fraction of fixed C for cellular growth since a large fraction was reserved in storage to fuel night-time N2 fixation. Our model provides a framework for resolving DIC limitation in aquatic ecosystem simulations, where DIC as a growth-limiting factor has rarely been considered, and importantly emphasizes the effect of intracellular C allocation on growth rate that varies depending on the growth environment.

In this study, the diversity and structures assemblages of benthic communities present on artificial reefs (AR) immersed for 5 years were monitoring during a full year in 2020. The comparison of two different sites Bernières in the Bay of Seine and Cherbourg in the central part of the English Channel brings innovative results on the efficiency of such structures. Benthic fauna and macroalgae communities were studied; several biotic indices like Shannon-Wiener diversity index were calculated. Benthic fauna was classified according to their trophic group and the biomass was estimated. Our results pointed out strong differences for several indicators between sites and seasons. Benthic fauna was more abundant in the Bay of Seine and more diversified than in the Bay of Cherbourg. Primary producers’ diversity and biomass were higher in the Bay of Cherbourg and dominated by Rhodophyceae species. Primary production results showed that the Bay of Cherbourg was a more productive system than the Bay of Seine. This study highlighted the efficiency of such structures to create habitats and promote biomass and diversity of associated living communities. In comparable conditions, different systems were highlighted: a “primary producer reef” and a “primary consumer reef”.

Colonisation of artificial structures by primary producers is an important determinant for eco-engineering projects. In this context, interactions between the colonisation by microphytobenthic biofilm and macroalgae were explored on 48 samples of marine infrastructures (MI) immersed for one year in the English Channel. Marine infrastructures samples with smooth and rough surface were compared to evaluate the influence of surface micro-scale rugosity. Microphytobenthos biomass (MPB), macroalgal diversity and photosynthetic parameters of both were assessed during colonisation. No significant differences were found as a function of the surface rugosity of MI samples, which was unexpected, but can be explained by biogenic rugosity provided by barnacles. Marine infrastructures were largely colonised by a red encrusting alga, Phymatolithon purpureum, which showed poor photosynthetic capacity compared to the microphytobenthos present next to it. Colonisation by monospecific encrusting algae tended to reduce the primary productivity of hard substrate.

The current development of human activities at sea (e.g. land reclamation, maritime activity and marine renewable energy) is leading to a significant increase in the number of infrastructures installed in marine settings. These artificial structures provide new hard-bottom habitats for many marine organisms and can thus modify the structure and functioning of coastal ecosystems. In order to better evaluate the nature of these modifications as well as the potential benefits and/or impacts generated, it becomes essential to develop assessment methods that can be applied to a wide variety of study sites from harbours to coastal offshore environments. In this context, our study aims to review the different methods and indicators available which are used to measure the modifications of biodiversity and ecological functioning generated by such structures. Among the methods reviewed, we highlight some that were developed specifically for artificial structures, and others intended for various primary uses but which have been successfully transposed to artificial structures. Nevertheless, we also point out the lack of reliable methods concerning some biological ecosystem components impacted by artificial structures. In this context, we require the adaptation or creation of brand-new indicators to achieve a better characterisation of the ecological impacts generated by these structures. Overall, this study highlights a very high number of existing methods, which provide stakeholders with useful tools to study the impacts of artificial structures, and identifies the need to develop integrative indicators to enhance the deployment of new artificial structures.