Ecological responses of phytoplankton community from the Río de la Plata estuary to polyester fiber exposure

Fibers are the most frequently reported type of microplastic in aquatic environments. Therefore, this work aimed to evaluate the impact of polyester fibers on the phytoplankton community from the Río de la Plata estuary using a 14-day ex situ assay under laboratory conditions. Treatments consisted of three fiber concentrations and a control, based on environmentally reported values. Community responses were assessed through phytoplankton density, taxonomic composition, specific growth rate, chlorophyll-a and pheophytin concentrations, and extracellular polymeric substances (EPS). Total phytoplankton density significantly increased over time in all groups, but no significant differences were observed among treatments or between treatments and control. However, chlorophytes dominated the community, and Kirchneriella and Scenedesmus showed significantly higher abundances under medium and high fiber concentrations, with positive correlations to fiber levels. Chlorophyll-a increased significantly in low and medium treatments, while EPS concentrations slightly decreased in all exposed groups, with no statistical differences. Although overall abundance was unaffected, genus-specific responses indicate that microplastics can subtly modify phytoplankton community composition. These findings provide novel evidence of early and taxon-specific phytoplankton responses to fibers, suggesting that such changes may have broader ecological implications for primary production and ecosystem functioning in estuaries, which remain less studied than other aquatic environments.