LTREB: Response of a reservoir ecosystem to changing subsidies of nutrients and detritus

Miami University, Oxford OH

Investigators

Abstract

This award in long-term research in environmental biology examines how long-term changes in agriculture affect streams and lakes, using an Ohio landscape as a model system. In the 1990s, this landscape experienced a pronounced increase in conservation tillage (reduced- and no-till farming), which strongly affected nutrients and sediments in streams that feed downstream Acton Lake. Also, the abundance of bottom-feeding fish in the lake increased. These fish consume sediments and excrete nutrients into the water, providing another source of nutrients to algae. In the lake, the amount of algae is controlled mostly by concentrations of sediment in the water (which block light that algae need) and the abundance of bottom-feeding fish; this project explores long-term changes in these interactions. This research is important because similar agricultural changes are occurring throughout the US, but little is known about long-term effects on streams and lakes. The investigators will mentor several undergraduate and graduate students who will conduct research projects, and offer summer workshops for high school students, targeting students from underrepresented groups. This project tests hypotheses about the long-term dynamics and consequences of ecosystem subsidies. Over the past 25 years, watershed subsidies of nitrogen, phosphorus, and detritus (sediments) to Acton Lake have changed because of an increase in conservation tillage. Furthermore, nutrient subsidies from detritivorous fish (gizzard shad), which translocate nitrogen and phosphorus from sediments to water, have varied temporally. Over the 10-12 years, stream phosphorus and sediment concentrations declined, and Acton Lake phytoplankton increased because light limitation was alleviated by decreased suspended sediments and because of increased biomass (hence nutrient excretion) of gizzard shad. However, more recently stream soluble reactive phosphorus (SRP) concentrations increased, nitrate concentrations declined, and lake phytoplankton biomass stabilized. Furthermore, phytoplankton became increasingly limited by nitrogen relative to phosphorus, coinciding with declining nitrate and increasing SRP in streams, and with greater biomass of gizzard shad, which excrete at low nitrogen:phosphorus ratio. This long-term research in environmental biology award tests the following hypotheses: 1) in streams, SRP will continue to increase while nitrate continues to decline; 2) gizzard shad excretion rates will remain consistently high; 3) phytoplankton will become increasingly nitrogen-limited; and 4) phytoplankton biomass will be stable, or decline as nitrogen becomes more limiting. In addition, it is hypothesized that long-term environmental change and short-term weather variation will modulate watershed subsidies, and thus lake ecosystem dynamics. These hypotheses will be tested using a variety of statistical and modeling analyses on long-term data. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.