Resource heterogeneity and the environmental basis of productivity in flowing waters

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A very large proportion of the energy used within stream food webs is derived from terrestrial plant material and other external sources, i.e., ecosystem subsidies. There are many sources of variation in the food quality, quantity, and availability of these materials for consumer organisms within the stream community. This proposal will address the causes and consequences of such resource heterogeneity. The causes of variation include timing of availability of particular resources (e.g. leaf litter, salmon carcasses, etc.) and changes in the physical drivers of their availability, such as hydrology, geomorphology, light inputs and temperature. Many of these physical drivers affect retention of these resources, and drivers are also expected to change in timing and magnitude under climate change scenarios. The resources themselves are heterogeneous and represent a complex assortment of organic carbon and various micro-organisms (fungi, bacteria, and algae). How these various micro-organisms interact with each other and with carbon sources will influence food quality and quantity for animals but the consequences are not well known. This series of experiments will test hypotheses about the nature of the variation in these food materials as the basis of productivity of food webs of small streams and the consequences for the aquatic community. Many species within aquatic communities are considered to be obligate consumers of leaf detritus, but alternate resources for consumers in some streams include salmon carcasses and biofilms, and these are highly patchy in time and space. Hence, the contributions of other resources are short-lived, patchy, or both, but may be potentially important because of their high quality. Our experiments will manipulate the quality and quantity of resources, light inputs, physical structure, and even hydrology (using short-distance diversions) to test impacts on growth and survival of consumer invertebrates and their community. Shifts in food resources are expected as a consequence of variation in climate, changes in terrestrial vegetation, and enhanced nutrient concentrations from airborne transport, and our work will test predictions of expected impacts.