Effects of warming and nutrient enrichment on how grazing pressure affects leaf litter-colonizing bacteria.

In view of current scenarios of global environmental change, we investigated the effects of warming and nutrient addition (N and P) on the impact of detritivores on density and community composition of leaf litter-colonizing bacteria in a freshwater environment. Within 10 d, detritivorous amphipods () reduced bacterial numbers at 10°C and to a lesser degree at 15°C. However, the detritivore-induced decrease in bacterial numbers was compensated for by nutrient addition. After 31 d of incubation, amphipods reduced bacterial numbers only at 15°C, and nutrient addition did not counteract detritivore effects. Similarly, changes in bacterial numbers in response to nutrient addition were more pronounced at low temperature within 10 d, whereas nutrient effects were stronger at high temperature after 31 d of incubation. Thus, warming without detritivores did not affect bacterial numbers under low-nutrient conditions (10 d). When detritivores were present, warming increased bacterial density significantly just under high-nutrient conditions ( < 0.05). After 31 d of incubation, warming did not affect bacterial density in detritivore-free controls in both nutrient conditions and did not affect bacterial density in both nutrient conditions in the presence of detritivores. Warming exhibited a significant effect on the composition of litter-associated bacterial communities irrespective of nutrient load ( < 0.05), whereas nutrients had less consistent effects. We conclude that warming, and to a lesser degree nutrient enrichment, may have influence on grazer-induced changes in bacterial biofilm composition, whereas nutrient enrichment may mostly interfere with those processes that depend on bacterial density.