titleActon Lake

Acton LakeOur research uses long-term data to test hypotheses about how ecosystems respond to declining subsidies of nutrients and detritus. Most ecosystems are subsidized by inputs of nutrients, detritus and other materials that arrive from outside boundaries, but the consequences of these subsidies are variable and remain hard to predict. Some direct subsidies are well studied, e.g. effects of dissolved nutrients on plant communities. Theory predicts important food web and ecosystem effects of subsidies of detritus and nutrients, but empirical understanding of these effects is limited.

food webOur research focuses on temporal changes in nutrient and detritus (sediment) subsidies to a reservoir ecosystem, which are mediated by changes in land use practices. Specifically, nutrient and sediment subsidies to Acton Lake have been declining because of changes in agricultural practices in the lake’s watershed. These changes include a marked increase in conservation tillage, as well as reductions in fertilizer use and livestock. As a result, concentrations of phosphorus (P) and suspended sediments in Acton Lake’s inflow streams have declined markedly since 1994. The magnitude of ecosystem subsidies (nutrient and sediment loads) to the lake vary with precipitation, because this causes variation in stream discharge, but discharge-standardized loads have also declined. In Acton Lake over this period, concentrations of suspended inorganic sediment have declined and the biomass of phytoplankton has increased. The phytoplankton response is likely due to reduced severity of light limitation, because of the decline in suspended sediment. However, the increase in phytoplankton over this time suggests the importance of an alternative source of P. The phytoplankton increase in recent years may be sustained by high biomass of gizzard shad (Dorosoma cepedianum), a detritivorous fish that consumes sediment detritus and excretes dissolved nutrients into the water column, thus translocating nutrients that sustain a substantial proportion of phytoplankton production. Thus, our research examines the singular and interactive effects of three ecosystem subsidies, and how these subsidies change over time: 1) dissolved nutrient subsidy from the watershed; 2) detritus (sediment) subsidy from the watershed; and 3) subsidy of dissolved nutrients from sediment-feeding fish to the water column.

We hypothesize that the phytoplankton increase is transitory, and that phytoplankton biomass will decline as nutrient input rates continue to decline. Furthermore, we hypothesize that reductions in detritus input will induce a decline in the biomass of gizzard shad, leading to an even more precipitous decline in phytoplankton biomass. We hypothesize that phytoplankton and zooplankton species composition will change such that taxa typical of high productivity (eutrophic) habitats will decline and those typical of mesotrophic conditions will increase. We also consider the alternative hypothesis that phytoplankton will not decline because of characteristics unique to agriculturally impacted reservoirs, specifically precipitation-mediated variability in P loading, alleviation of light limitation because of reduced inorganic turbidity, and sustained high biomass of gizzard shad. Multiple response pathways are possible, and we are collecting data to carefully evaluate these alternatives.

This research is unique because it is a long-term study on interactive effects of multiple ecosystem subsidies, including nutrient and detritus subsidies from watersheds and nutrient subsidies mediated by fish. It also links subsidies across multiple ecosystems (land, streams and lake), and integrates four of the grand challenges in ecology: land use, climate, hydroecology, and biogeochemistry, while also focusing on how a dominant species mediates ecosystem processes.

tillage practices

Agricultural fields showing conventional (left) and conservation (right) tillage