Climate change impacts on aquatic consumer communities

Abstract: Climate change represents a serious threat to aquatic ecosystems, with an increase in lake temperatures already observed that is expected to continue in the near future. Aside from the direct effects of warming, climate change is also partially responsible for the browning of lakes. Browning is an ongoing phenomenon related to the increased export of terrestrial dissolved organic matter into lakes. With ongoing climate changes, lakes are becoming warmer and browner. This has major impacts on the food web dynamics of these systems. Many studies have previously examined the effects of warming and browning on consumer responses, mainly through spatial surveys. However, a mechanistic understanding of how consumers in the food web will respond to simultaneously occurring warming and browning remains poorly understood.  Using two large-scale ecosystem experiments, I studied the effects of warming and browning on consumer growth, size-structure, and population responses, and the potential mechanisms that dictate the emergent responses. In general, warming led to reduced consumer biomass and size structure, whilst browning led to an increase. Specifically, warming reduced intermediate consumer biomass and fish top consumer abundance, biomass and size structure, while browning either led to little to no changes in intermediate consumers, but increased fish top consumer abundance, biomass, and production. However, these responses were determined by food-web structure, interactions, and context-dependent mechanisms. This thesis advances our understanding of the mechanisms that drive changes in consumer responses to warming and browning and provides a better understanding of how ongoing climate may affect the structure and functioning of freshwater ecosystems.