Seawater mercury species and phytoplankton biomass drive bioaccumulation in a coastal plankton food web

Plankton plays a key role in the bioconcentration and biomagnification of mercury (Hg) in marine food webs. Few studies have investigated the driving factors in coastal plankton food webs. Total-mercury (tHg) and total methylated-mercury (MeHg) were measured in seawater, size-fractioned phytoplankton (0.7 to 60 μm), zooplankton (60 to >2000 μm) and mysids, sampled monthly between February 2020 and November 2021 in the Bay of Marseille (NW Mediterranean Sea, France). Total-Hg and MeHg concentrations varied widely in seawater (193-903, 4-39 pg L⁻¹), phytoplankton (37-5690, 0.02-15 ng g⁻¹DW) and zooplankton (5-478, 2-36 ng g⁻¹DW). Temporal variability suggests that, under high biomass conditions, high MeHg uptake by phytoplankton may deplete seawater concentrations, limiting passive bioaccumulation in zooplankton. Conversely, low biomass was consistent with low MeHg uptake in phytoplankton, most likely favoring passive bioaccumulation in zooplankton. Nutrient limitation was related to higher MeHg concentrations and higher δ¹⁵N values in zooplankton. MeHg content in zooplankton was correlated with the particulate organic carbon (POC). Inorganic-Hg (iHg) and MeHg exhibited different dynamics in a plankton food web consisting of three trophic levels, i.e., pico-nanoplankton → herbivorous copepods → carnivorous mysids: iHg was biodiluted (decreased) while MeHg was biomagnified (increased) from the first trophic level. Plankton appears to be a good proxy for the bioaccumulation and biomagnification of MeHg, rapidly reacting to variations in MeHg seawater concentrations and trophodynamics. Marine areas exposed to higher Hg contamination and oligotrophic conditions thus promote the bioaccumulation and trophic transfer of Hg.