Farms, Fish, Phosphorus and Phytoplankton: Watersheds and Fish Regulate Midwest Reservoir Ecosystems

Aquatic ecosystems often receive subsidies of resources from outside their boundaries, which can have profound direct and indirect effects. This is especially true for reservoirs, which have relatively large watersheds and therefore receive large subsidies of nutrients, detritus, and inorganic sediments from the surrounding terrestrial landscape. Such subsidies can affect phytoplankton biomass and hence mediate trophic interactions. In addition, detritivorous fish (gizzard shad, Dorosoma cepedianum) are abundant and provide another type of subsidy to phytoplankton. These fish feed on sediment-bound, detrital nutrients and excrete some of these nutrients into the water column, making them available to phytoplankton. Gizzard shad are abundant in reservoirs of the Midwest USA, especially in highly productive lakes with a dominance of watershed agriculture. I show that watersheds and gizzard shad jointly regulate these reservoir ecosystems. Via nutrient excretion, gizzard shad modulate a large flux of phosphorus to the water column that sustains a significant proportion of primary production. P excretion by shad is especially important in productive, agriculturally-dominated lakes and during times of low external P inputs (low runoff). Stable isotope studies show that gizzard shad obtain about 35% of their energy from terrestrial detritus, averaged across lakes of contrasting land use, showing that watersheds subsidize these populations. Watersheds also provide large inputs of sediments, which can depress phytoplankton production by reducing water clarity. Analysis of a long-term data set (18 years) in hypereutrophic Acton Lake suggests that phytoplankton abundance is interactively regulated by sediment inputs and P excretion by gizzard shad; together these two variables explain 68% of the interannual variance in phytoplankton biomass. The interactive regulation of these ecosystems by watersheds and fish must be considered for effective management of water quality and fisheries.