Phytoplankton community-driven regulation of microplastic sinking via marine gel formation and aggregation: mesocosm...

Microplastic (MP) aggregation and vertical transport in marine environments are increasingly recognized as biologically mediated processes that influence the environmental fate of MPs. Because nutrient enrichment shapes phytoplankton blooms and community composition, it may affect MP fate in coastal waters. In this study, a 32-day summer mesocosm experiment (∼1 ton scale) was conducted under five treatments: control (C) and four nutrient-enriched conditions (N25, N50, N100, N200), to investigate MP aggregation and sinking behavior. Water temperature ranged from 24.8 to 30.2 °C, while salinity remained relatively stable (30.7-32.6). Phytoplankton responses varied among treatments, and increased chlorophyll a (Chl. a) was accompanied by the formation and sedimentation of marine gel-like particles. The control maintained low Chl. a (maximum 5.54 μg L⁻¹), whereas nutrient-enriched treatments showed higher biomass, peaking in N50 (34.15 μg L⁻¹), followed by N25 (28.12 μg L⁻¹) and N100 (18.32 μg L⁻¹). In contrast, N200 exhibited delayed growth with a lower peak (12.83 μg L⁻¹). MP sinking ratios differed markedly and were not directly proportional to biomass, remaining below 4.9% in the control, increasing to 27.6% (N25) and 34.1% (N100), but remaining lower in N50 (11.6%) and N200 (7.3%). Redundancy analysis indicated that MP sinking was primarily governed by phytoplankton community composition rather than total biomass. Diatom-dominated conditions enhanced marine gel production, promoting MP aggregation and sinking, whereas dinoflagellate dominance resulted in prolonged cell survival and reduced gel formation, delaying MP export. These findings demonstrate that MP vertical transport in eutrophic coastal systems is regulated by bloom dynamics, species composition, and aggregation processes linked to cell mortality and exudate production, providing a mechanistic basis for improving MP transport models and supporting future assessments of MP exposure and ecological risk in nutrient-enriched coastal waters.