Salinity-associated freshwater-seawater mixing shapes microalgal colonization on marine plastic debris in an estuarine-coastal mesocosm experiment.

Marine plastic debris (MPDs) provide artificial substrates for microalgal attachment, but how water-column phytoplankton and plastic-associated periphyton respond to environmental conditions associated with freshwater-seawater mixing remains poorly understood. We conducted a 30-day mesocosm experiment by mixing freshwater (salinity 0) and seawater (salinity 35) to establish three treatments: estuarine salinity (ES; ∼15), brackish coastal salinity (BCS; ∼30), and coastal salinity (CS; ∼35). Polypropylene attachment plates were used to examine periphyton development. In the water column, ES showed a late Chl. a maximum on day 20, BCS peaked earlier on day 10, and CS maintained low biomass with relatively stable Fv/Fm until day 20. The ES bloom did not persist despite high nitrate+nitrite concentrations, indicating possible constraints from low phosphate availability, nutrient imbalance, and low-salinity stress. On the plastic plates, Chl. a accumulated most rapidly in CS, with significantly different accumulation rates among treatments (ANOVA, F = 403.06, p < 0.001); CS showed a much higher rate (6.16 ± 0.02 μg cm-2 day-1) than ES (1.42 ± 0.23 μg cm-2 day-1) and BCS (1.24 ± 0.25 μg cm-2 day-1). Periphyton communities were dominated by diatoms, especially Navicula sp., which accounted for more than 94% of the CS periphyton community during the late phase. These results indicate that water-column phytoplankton and plastic-associated periphyton responded differently to salinity-associated environmental conditions. Overall, our findings highlight the capacity of MPDs to support distinct microalgal communities under contrasting estuarine-coastal conditions.