Long-term Research in Environmental Biology: Ecosystem response to changes in subsidies of nutrients and detritus

Our research uses long-term data to test hypotheses about how ecosystems respond to changing subsidies of nutrients and detritus. Specifically, our research looks at how changes in agricultural practices alter the export of sediments and nutrients from watersheds, and how this affects a reservoir ecosystem. Our long-term research has revealed several ‘ecological surprises’ in terms of ecosystem responses to changing subsidies.

Most ecosystems are subsidized by inputs of nutrients, detritus and other materials that arrive from outside boundaries. However, the impacts of these subsidies are variable and remain hard to predict. Theory predicts important food web and ecosystem effects of subsidies of detritus and nutrients, but empirical understanding of these effects is limited. In particular, we know little about the long-term interactive effects of multiple subsidies on food webs and ecosystems.

We are studying the interactive effects of three ecosystem subsidies (Figure). Our study site, Acton Lake and its watershed, is dominated by row crop agriculture, mostly corn and soy. As is typical of such landscapes, fertilizer use is high and croplands are prone to soil erosion. Thus, Acton Lake receives large subsidies of detritus (sediments) and nutrients from its watershed. Watersheds deliver nutrients (nitrogen and phosphorus) in both dissolved and particulate forms to the lake. Dissolved inorganic forms are readily used by phytoplankton in the lake, and as a result of these large nutrient subsidies Acton Lake is highly eutrophic. Particulate nutrients delivered to the lake are less available to phytoplankton, but become available via remineralization within the lake. One important pathway of nutrient mineralization is excretion by sediment-feeding fish, especially gizzard shad (Dorosoma cepedianum). Non-larval stages of gizzard shad feed on sediment detritus and excrete dissolved inorganic nutrients into the water column, and this represents an important source of nutrients that can sustain >25% of phytoplankton production. The sediment detritus upon which gizzard shad feed is partly derived from terrestrial sources, and from phytoplankton-derived detritus.

Thus, we are investigating three ecosystem subsidies and their interactive effects: 1) Dissolved nutrient subsidies from the watershed; 2) Sediment subsidies from the watershed; 3) Dissolved nutrient subsidies via excretion by gizzard shad (Figure). Our long-term study shows that all three subsidies are important, but their importance varies over multiple time scales. Fluxes of sediments and nutrients from the watershed vary greatly over short time scales, depending on the frequency and intensity of storms that induce runoff. Nutrient subsidies from gizzard shad vary among years depending on fish biomass, and at seasonal scales depending on recruitment of young-of-year fish as well as temperature. Finally, we have observed important long-term changes in the dynamics of all three subsidies, and subsequent responses in the Acton Lake ecosystem.