Sedimentary heavy metal interactions with phytoplankton and zooplankton across the Bohai Sea

Excessive heavy metals (HMs) pose significant threats to aquatic ecosystems and are known to affect plankton communities. However, relatively little is known about the complex interactions between plankton (i.e., phytoplankton and zooplankton) and ecological risks associated with hazardous HMs in surface sediments, limiting our ability to incorporate these metals into planktonic biogeochemical cycles. Here we explored the behavior of surface sedimentary HMs (Cu, Zn, Pb, Cd, Hg, and As) and their interactions with plankton across the Bohai Sea from 2013 to 2014. During the study period, the average abundances of phytoplankton and zooplankton ranged from 0.31 × 10³ to 1.96 × 10³ cells L^-1 and 0.02 × 10³ to 0.28 × 10³ ind m^-3, respectively. The concentrations of Cu, Zn, Pb, Cd, Hg, and As in surface sediments ranged from 18.10 to 22.32, 39.85-56.15, 13.96-21.31, 0.18-1.84, 0.03-0.31, and 5.84-8.53 mg kg^-1, respectively. The sediment quality was found to potentially pose ecological risks from Cd and Hg pollution in some cases. Our analysis of the ecological risks associated with various HMs (E_HMs) revealed that phytoplankton and zooplankton abundances responded differently to these risks in the Bohai Sea. Specifically, moderate increases in ecological risks from Cu (E_Cu<8) and Zn (E_Zn<0.8) promoted phytoplankton growth, whereas elevated risks from Pb and Hg, even at low levels, inhibited their growth. For zooplankton, the optimal ecological risks from Zn were roughly 0.2, but higher E_Zn levels inhibited their growth. Zooplankton also preferred low ecological risks from Hg and As, as these metals were toxic to them even at lower concentrations, leading to a reduction in their abundance at higher levels. In contrast, zooplankton showed less sensitivity to Pb and Cd, which suggested possible adaptive responses or a greater ability to limit metal uptake at certain concentrations. These differential responses were attributed to species-specific metal sensitivities or genetic variability in tolerance to metal toxicity within plankton communities. Altogether, surface sedimentary HMs markedly influence plankton dynamics across the Bohai Sea and should be considered in future research.