Resource quantity and seasonal background alter warming effects on communities of biofilm ciliates.

The impacts of experimental warming on field-related communities of biofilm ciliates were studied in contrasting seasons (winter vs. summer), which incorporated both different species sets and environmental background conditions. The biofilms for the experiments were cultivated in river bypass systems that were exposed to increasing temperatures based on the ambient river temperature. Opposing effects of warming were observed for ciliate 'summer' and 'winter' communities. While winter warming resulted in both stimulation (abundance and biomass) of the ciliate communities and significant shifts in the community structure, summer warming induced a significant decline in the ciliate biomass, but did not affect the relative community composition. By the simultaneous manipulation of temperature and resource density in summer, it was demonstrated that negative warming effects on the ciliate quantity during summer could be compensated by increasing the availability of food. Taken together, our results indicate that the responses of ciliate communities towards warming are strongly coupled to the availability of resources, and that the strongest impacts of environmental warming should thus be expected in resource-rich environments.