Plankton Size Structure Impacts PAH Bioaccumulation and Fate in Marine Pelagic Food Webs

Plankton constitutes the primary entry point of contaminants into pelagic food webs, yet the role of plankton size structure in governing hydrocarbon fate remains poorly understood due to methodological limitations. Here, polycyclic aromatic (PAHs) and aliphatic (AHs) hydrocarbons were investigated across seven plankton size fractions (0.7 μm to >500 μm) collected in coastal waters of the northwestern Mediterranean Sea using a locally designed online sequential filtration system. Biochemical, isotopic, and molecular marker analyses were combined to characterize plankton composition, hydrocarbon sources, and PAH bioaccumulation patterns across size fractions. Molecular markers confirmed the transition from phytoplankton-dominated signatures in the smallest fractions to increasing zooplanktonic contributions in larger fractions. The 0.7–2.7 and 2.7–20 μm fractions, dominated by pico- and nanoplankton, accounted for the highest particulate organic carbon and hydrocarbon concentrations (3.0–3.4 μg g–1 dw for Σ26-PAHs) while zooplankton fractions (>60 μm) displayed lower values (0.09–0.69 μg g–1 dw). The passive uptake of low-molecular-weight (LMW) PAHs dominated across all size fractions but decreased with increasing plankton size. In contrast, high-molecular-weight (HMW) PAHs showed preferential accumulation in nanoplankton. These results demonstrate that plankton size structure and biomass dependent processes strongly influence PAH bioaccumulation and trophic transfer in marine pelagic food webs.