Supplementary material from "Floating plastic alters nearshore communities and functions as a low-quality habitat in a freshwater lake"

Plastic pollution is accumulating in aquatic ecosystems, yet the extent to which wild animals use this novel habitat is poorly understood. Here, we investigated how floating macroplastic (>5 mm) structures alter nearshore communities in a lake in Ontario, Canada. We experimentally manipulated habitat complexity by adding macroplastics to nearshore plots (4x2 m; N = 12, n = 4 per treatment) and compared the responses to positive and negative controls (i.e. with and without floating wood) over 56 days. Macroplastics supported communities distinct from those on floating wood but generally functioned as lower-quality habitats for primary producers and consumers. Biofilm chlorophyll a concentrations were lower on plastic fragments (mean±SD; 0.01±0.01 µg/cm2) and polystyrene foam (0.02±0.03 µg/cm2) than on wood (0.3±0.1 µg/cm2). Macroinvertebrates had lower abundance (macroplastic: 52.5±14.4; wood: 638.5±667.7) and richness (macroplastic: 2.5±0.6; wood: 4.5±0.6) on plastic substrates. In contrast, fathead minnows (Pimephales promelas), a common small-bodied fish, were more abundant around plastic during their peak spawning period and successfully spawned on macroplastics. Clutch sizes were larger on macroplastics (946.2±743.2) than on wood (245.2±268.9). However, we observed negative effects on embryo development on construction foam, raising the possibility of an evolutionary trap. These findings suggest that macroplastics can simultaneously function as habitats and stressors in freshwater ecosystems.