The decadal variability of elemental, isotopic, and biochemical compositions of coastal Mediterranean zooplankton responds to environmental changes

English. peer reviewed

English. The functioning of Mediterranean coastal waters is deeply affected not only by global environmental changes (e.g., global warming, climate change) but also by improvements in sewage treatment as required by the European Water Framework Directive. These changes affect nutrient concentrations and cycling, and primary production. The present paper presents the time series analysis (2005–2020) of stable isotope ratios (hereafter δ13C and δ15N), elemental (C and N), and biochemical compositions (proteins, lipids, carbohydrates) within the 300–500 µm zooplankton size fraction from bimonthly monitoring samples (0–55 m) collected in the Bay of Marseille (NW-Mediterranean Sea). The zooplankton seasonality was mostly related with the dynamic of proxies of phytoplankton biomass and nutrients in the system. In spring, protein content and energy increased and δ15N of zooplankton decreased; in late spring, lipid and carbohydrate content peaked; and in late summer, δ13C increased. At the interannual scale, major changes in the zooplankton biochemical and isotopic composition have occurred since 2009. Lipid content decreased while protein and stable isotope ratios increased. These shifts were concomitant with changes in the nutrient concentrations, nutrient stoichiometry, and particulate organic matter concentrations. The changes in biochemical composition led to an increase of energy content from 2011, but the total quantity of energy available to higher trophic positions remained stable because of a reduction in biomass. The variations of the zooplankton stable isotopes and biochemical compositions can be indicative of changes in individual biochemistry (energy allocation) and/or in the zooplankton community (changes in relative abundance of lipid-rich vs protein-rich species).