Linking Landscapes and Food Webs: Effects of Omnivorous Fish and Watersheds on Reservoir Ecosystems

E increasingly recognize the need to understand how food webs and landscapes are linked (Polis et al. 2004). Fluxes of organisms, detritus, nutrients, and other materials across landscapes can strongly affect recipient food webs. For example, many studies show that subsidies of plant-available nutrients stimulate primary production and cause changes in species composition of plants as well as organisms at higher trophic levels (Carpenter et al. 1998). Inputs of detritus or organisms to a food web are less studied but may have complex effects that depend on the trophic position at which these subsidies enter a food web. For example, inputs of detritus can increase the abundance of detritivores in recipient food webs, which can thus increase their effects on other species through various direct and indirect pathways (Moore et al. 2004, Polis et al. 2004). Although most ecosystems are subsidized, the consequences of subsidies remain unknown for most subsidies and for most food webs. Freshwater ecosystems are highly subsidized because their downhill position relative to their terrestrial watersheds facilitates movement of materials toward them. Inputs of dissolved nutrients and detritus to aquatic ecosystems can regulate primary productivity, food web interactions, and energy flow (e.g., Carpenter et al. 1998,Wallace et al. 1999, Pace et al. 2004). Reservoirs (lakes formed through impoundment of rivers by dam construction) are the dominant lake type in North America at latitudes below 42° north, and are highly subsidized ecosystems (Thornton 1990). Because they have relatively large watersheds, reservoirs receive substantial inputs of nutrients and detritus from streams that drain terrestrial landscapes. Nutrient and detritus subsidies are particularly large to reservoirs in agricultural landscapes, where many eastern US reservoirs reside, because of the use of fertilizers and high rates of soil erosion (Carpenter et al. 1998). There is evidence that detritus subsidies are important in these reservoir ecosystems. For example, reservoir fish productivity often cannot be supported entirely by in-lake primary production but rather must be subsidized largely by inputs of detritus (Adams et al. 1983).