Limited effects of microplastics on size-fractionated phytoplankton booming in estuarine system

Despite the extensive presence and long-term exposure risks of marine microplastics (MPs), their impact on phytoplankton at the community level is still not very clear, especially considering the various size classes of phytoplankton. To address this issue, we investigated the spatial load of MPs in Linhong Estuary and conducted in-situ experiments of algal culture with added MPs. Our investigation showed that the abundance of MPs varied from 8 n/L to 50 n/L, with an average of 21.76 ± 12.31 n/L. A high loading of MPs was spatially identified outside Linhong Estuary, and clear decreasing trend from land to sea was not observed. Further microscopic examination revealed fiber-shaped MPs dominated in all collections, reaching a proportion of 93%. The chlorophyll a concentrations were 2.69-25.50 μg/L (9.14 ± 6.59 μg/L), 0.68-3.13 μg/L (1.48 ± 0.67 μg/L) and 0.14-0.65 μg/L (0.27 ± 0.14 μg/L) for microphytoplankton (20-200 μm), nanophytoplankton (2-20 μm) and picophytoplankton (0.2-2 μm), respectively. The correlation analysis between MP abundance and chlorophyll a of phytoplankton and the in-situ experiment at an environmentally realistic level both indicated a negative relationship between MP abundance and microphytoplankton with respect to chlorophyll a content, but no adverse effects of MPs were found for nanophytoplankton and picophytoplankton with smaller sizes. Our findings indicate that exposure to realistic levels of MPs only cause a limited impact on size-specific phytoplankton communities studied, but long-term interactions between MPs and these species merit further field-based assessment in real-world scenarios beyond lab-based incubation.