Watersheds deliver ‘subsidies’ of water, carbon, nutrients, pollutants, and other materials to downstream ecosystems. We study how these subsidies affect food webs and ecosystem processes in lakes. In particular, we study reservoirs, which are human-made lakes with large watersheds. Of particular interest is the long term response of a reservoir ecosystem (Acton Lake) to changes in agricultural practices, research carried out with Miami colleagues María González, Bill Renwick and Bart Grudzinski and others. Our long term research, funded since 2003 by the NSF Long-Term Research in Environmental Biology (LTREB) program, shows that subsidies of sediments, nitrogen, and phosphorus are all changing due to shifts in watershed agricultural practices (Renwick et al. 2018). Furthermore, nutrient and sediment subsidies interact with each other, and with an internal subsidy provided by detritivorous fish (Kelly et al. 2018). Although phosphorus subsidies to the lake decreased over approximately the first decade of our study, phytoplankton in the lake increased because of increased light penetration due to declines in sediment inputs. In addition, biomass of detritivorous fish (gizzard shad, Dorosoma cepedianum) increased; these fish act as nutrient pumps by feeding on sediment detritus and excreting nutrients into the water (Williamson et al. 2018). The combination of increased light and increased nutrient flux through fish fueled the increase in phytoplankton, despite the decrease in phosphorus subsidies from the watershed (Kelly et al. 2018). Our ongoing research seeks to understand how these interactions will play out as we approach three decades of change.
Animal mediated nutrient cycling
The conventional view of nutrient cycling is that animals are relatively unimportant relative to microbes. Yet, many examples exist showing that animals are important. Our research strives to understand when and where animals are important in nutrient cycling in aquatic ecosystems, and encompasses research that spans ecological levels of organization from individuals (e.g., Vanni et al. 2002; Vanni & McIntyre 2016) to the ecosystem (e.g., Vanni et al. 2006; Kelly et al. 2018; Williamson et al. 2018). We study animal-mediated nutrient cycling using several approaches, including ecosystem nutrient budgets, field and lab experiments, and literature syntheses (Vanni 2002; Vanni & McIntyre 2016; Atkinson et al. 2017; Sitters et al. 2017). Our research is part of a broader effort to place the role of animal-mediated nutrient cycling in context.