Microplastics and warming: dual threats to planktonic protist communities in freshwater ecosystems

Climate change and microplastic (MP) pollution are major threats to freshwater ecosystems. To assess warming and microplastics effects, we conducted a four-day microcosm experiment exposing the aquatic communities to three MP concentrations (No MP, low [-MP] and high [+ MP]) and three temperatures (26 °C, 28 °C and 30 °C), individually and in combination. We analyzed taxonomic richness, abundance, and community composition of planktonic ciliates. A total of 32 morphospecies of ciliates were recorded, with Hymenostomata dominating all treatments. In treatments with -MP concentrations, richness increased only on ambient temperature, but this effect was weaker at higher temperatures. A negative effect of MPs on abundance was evident only at ambient temperature (26 °C), while at high temperatures (28 °C and 30 °C), the opposite was observed, with an increase in abundance. Community composition shifted with MP presence, especially at lower temperatures. However, the composition seems to become more homogeneous as the temperature increases, possibly favoring generalist species. In summary, our results suggested that MPs and temperature have antagonistic effects, possibly saturating the ciliate response, with temperature masking the MPs’ isolated effects. Our findings underscore the combined impact of MPs and warming on freshwater biodiversity and ecosystem function, stressing the importance of considering multiple stressors when evaluating ecological risks.