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Salinity-associated freshwater–seawater mixing shapes microalgal colonization on marine plastic debris in an estuarine–coastal mesocosm experiment
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Scientists found that tiny algae grow much faster on floating plastic debris in full-strength ocean water compared to brackish or estuary water, with one type of algae (a diatom) taking over almost completely in salty conditions. This matters because plastic pollution in our oceans isn't just inert trash, it's becoming a living surface that hosts specific microorganisms, which could affect how nutrients, toxins, or even harmful microbes move through the marine food chain and potentially back to the seafood we eat.
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 day) than ES (1.42 ± 0.23 μg cm day) and BCS (1.24 ± 0.25 μg cm day). 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.
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