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Since the beginning of the industrial revolution, atmospheric carbon dioxide (CO2) concentrations have risen steadily and have induced a decrease of the averaged surface ocean pH by 0.1 units, corresponding to an increase in ocean acidity of about 30 %. In addition to ocean warming, ocean acidification poses a tremendous challenge to some marine organisms, especially calcifiers. The need for long-term oceanic observations of pH and temperature is a key element to assess the vulnerability of marine communities and ecosystems to these pressures. Nearshore productive environments, where a large majority of shellfish farming activities are conducted, are known to present pH levels as well as amplitudes of daily and seasonal variations that are much larger than those observed in the open ocean. Yet, to date, there are very few coastal observation sites where these parameters are measured simultaneously and at high frequency. To bridge this gap, an observation network was initiated in 2021 in the framework of the CocoriCO2 project. Six sites were selected along the French Atlantic and Mediterranean coastlines based on their importance in terms of shellfish production and the presence of high- and low-frequency monitoring activities. At each site, autonomous pH sensors were deployed, both inside and outside shellfish production areas, next to high-frequency CTD (conductivity–temperature–depth) probes operated through two operating monitoring networks. pH sensors were set to an acquisition rate of 15 min, and discrete seawater samples were collected biweekly in order to control the quality of pH data (laboratory spectrophotometric measurements) as well as to measure total alkalinity and dissolved inorganic carbon concentrations for full characterization of the carbonate system. While this network has been up and running for more than 2 years, the acquired dataset has already revealed important differences in terms of pH variations between monitored sites related to the influence of diverse processes (freshwater inputs, tides, temperature, biological processes). Data are available at https://doi.org/10.17882/96982 (Petton et al., 2023a).

To gain insight into the impact of bottom-up changes in the plankton community on planktivorous fish in the context of the decline of small pelagic fisheries in the Northwestern Mediterranean Sea, we have conducted an extensive year-long study. The investigation combined biochemical analyses (proteins, carbohydrates, and lipids) with C and N stable isotope analyses (SIA) to simultaneously study phytoplankton, zooplankton, and eight planktivorous fish species (Engraulis encrasicolus, Sardina pilchardus, Sardinella aurita, Sprattus sprattus, Cepola macrophthalma, Chromis chromis, Boops boops, and Spicara maena). This study is the first to analyze both stable isotope and biochemical compositions in coastal particulate organic matter (POM) size classes (0.7-2.7 μm, 2.7-20 μm, and 20-200 μm), zooplankton size classes (200-300 μm, 300-500 μm, 500-1000 μm, 1000-2000 μm, and >2000 μm), and taxonomic groups. We demonstrated that: (1) POM stable isotope compositions varied based on its spatial origin, the taxonomic composition of its biota, and its biochemical content; (2) δ 15 N values increased with zooplankton size classes and groups, indicating different trophic levels; (3) Phytoplankton exhibited a lipid-rich composition (∼55%), while zooplankton and fish muscles were protein-rich (∼61% and ∼66%, respectively). Bayesian stable isotope mixing models revealed that, on average: (1) POM from oceanic waters contributed the most to the POM in the bay (>51%), with a dominance of pico-POM (∼43%); (2) The 200-1000 μm zooplankton primarily consumed nano-POM, the 1000-2000 μm zooplankton mostly consumed micro-POM (∼64%), and the >2000 μm zooplankton also mostly consumed micro-POM; (3) Mesozooplankton (200-2000 μm) constituted the main portion (∼42%) of the diet for planktivorous fish species, while macrozooplankton organisms (>2000 μm) were the primary food resource (∼43%) for both B. boops and S. sprattus. Our study underscores the complexity of the pelagic food web and highlights the bottom-up transfer of organic matter from the smallest phytoplankton size fractions to planktivorous fish., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Along with their important diversity, coastal ecosystems receive various amounts of nutrients, principally arising from the continent and from the related human activities (mainly industrial and agricultural activities). During the 20th century, nutrients loads have increased following the increase of both the global population and need of services. Alongside, climate change including temperature increase or atmospheric circulation change has occurred. These processes, Ecosystem state changes are hard to monitor and predict. To study the long-term changes of nutrients concentrations in coastal ecosystems, eleven French coastal ecosystems were studied over 20 years as they encompass large climatic and land pressures, representative of temperate ecosystems, over a rather small geographical area. Both univariate (time series decomposition) and multivariate (relationships between ecosystems and drivers) statistical analyses were used to determine ecosystem trajectories as well as typologies of ecosystem trajectories. It appeared that most of the French coastal ecosystems exhibited trajectories towards a decrease in nutrients concentrations. Differences in trajectories mainly depended on continental and human influences, as well as on climatic regimes. One single ecosystem exhibited very different trajectories, the Arcachon Bay with an increase in nutrients concentrations. Ecosystem trajectories based on ordination techniques were proven to be useful tools to monitor ecosystem changes. This study highlighted the importance of local environments and the need to couple uni- and multi-ecosystem studies. Although the studied ecosystems were influenced by both local and large-scale climate, by anthropogenic activities loads, and that their trajectories were mostly similar based on their continental influence, non-negligible variations resulted from their internal functioning., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)