Seasonal dynamics and environmental drivers of particulate algal toxins in coastal planktonic food webs.

Particulate algal toxins are emerging particle-associated pollutants whose seasonal dynamics across planktonic food webs remain poorly understood in marginal seas. Using four seasonal surveys in the Yellow-Bohai Seas, we quantified diarrhetic shellfish poisoning (DSP) and paralytic shellfish poisoning (PSP) toxins in phytoplankton- and zooplankton-sized fractions. Phytoplankton-sized DSP (DSPp) was widespread (40-75% detection), whereas PSP toxins (24-48%) exhibited pronounced seasonal shifts in toxin composition. Zooplankton-sized DSP (DSPz) was predominantly observed in summer, showing the strongest apparent trophic coupling between plankton size fractions during this season. Two-stage generalized additive models (GAMs) suggested distinct environmental and biological associations with toxin occurrence and concentration. Toxin occurrence showed associations with temperature, salinity, nutrients, diatom biomass, and toxin-producing phytoplankton, with relationships varying among toxin groups and seasons. DSPp concentrations showed the strongest association with temperature among the evaluated predictors, whereas DSPz concentrations were more closely associated with particulate DSP availability, with DSPp showing the highest relative contribution among the predictors in the annual DSPz GAM. These findings suggest that the relative distribution of particulate toxins between plankton size fractions is more closely associated with particulate toxin availability than with the occurrence of toxic algae, supporting integrated monitoring strategies for improving ecosystem risk assessment in coastal waters.