Interactions between warming, nutrient enrichment and detritivores on litter decomposition and associated microbial decomposers

Leaf litter decomposition constitutes an important source of energy in many aquatic environments that is controlled by the joint action of microbial decomposers such as bacteria and fungi and also animal detritivores. In view of current scenarios of global environmental change, it is predicted that rapid temperature increases could directly affect most ecosystems including freshwaters. Additionally, human activities and industrial development have impacted water quality of many streams and rivers. In freshwater systems, eutrophication is a process, whereby excessive receive of inorganic nutrients, especially N and P, that may effect on leaf litter processing. In the present study, I investigated how warming, nutrient-addition (N and P) and detritivores, interact to affect multiple parameters associated with leaf decomposition. Investigations were carried out in the laboratory in two sets. For the studies presented here leaf litter Betula pendula (Birch) and the detritivore Gammarus pulex (Amphipoda) were chosen because of their numerical importance in northern temperate ecosystem. In the first set of experiments (Chapter I), I investigated the synergistic effects of warming and nutrient-addition (N and P) on the impact of amphipods on density and community composition of leaf litter-colonizing bacteria. I found that warming significantly exhibit stronger effects on the composition of litter-associated bacterial communities, irrespective of nutrient load but amphipods mediated warming-effects on bacterial community composition by selective feeding. In addition, Short-term effects of nutrient-addition on bacterial biofilm density were stronger than warming-effects but less pronounced so at increased temperatures. Alongside, Long-term effects of nutrient-addition on bacterial density were strongest, irrespective of environmental temperature. Additionally, nutrient-addition effectively compensated for biofilm reduction upon grazing by amphipods. In second set of experiments (chapter II), I proceeded to improve understanding of leaf litter decomposition process by comprehensive experiments to investigate how warming, nutrient-addition (N and P) and detritivores, interact to affect multiple parameters associated with leaf decomposition. These parameters included microbial (bacteria and fungi) biomass and community structure, decomposition rate and detritivore growth. I found that detritivores and nutrient-addition have strong effects on leaf litter decomposition rate but relative growth of detritivores does not increase with warming and nutrient addition. Additionally, bacterial biofilm density increases by both warming and nutrient-addition, but reduced by amphipod grazing-pressure and fungal biomass also appears to be stimulated by warming and nutrient-addition but also by amphipod presence. Moreover, litter-associated fungal composition were only slightly affected by warming or nutrient-addition, but strongly responded to selective feeding by amphipods and the community composition of bacterial colonizers on birch litter was also influenced by grazing pressure of amphipods and warming. In summary, this study provides new insights into the effect of simultaneous change in temperature and nutrient-load on microbial decomposers and also helps in better understanding of selective role of detritivores on bacterial and fungal communities on litter surfaces in freshwaters.