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Response of an agriculturally impacted reservoir ecosystem to improved watershed management.
Vanni, Michael*,1, Renwick, William1, Gonzalez, Maria1, Hale, Scott2, 1 Miami University, Oxford, OH2 Inland Fisheries Research Unit, Ohio Division of Wildlife, Hebron, OH
ABSTRACT- Agricultural practices are changing in landscapes throughout the world. However, implications of these changes for downstream lakes are not well documented, particularly for reservoirs, which receive large inputs of sediments and nutrients from watersheds. We assessed effects of improved agricultural practices on a eutrophic reservoir (Acton Lake, Ohio) that resides in an agricultural watershed. During this 9-year period (1994-2002), the percentage of agricultural land implementing conservation tillage increased from about 15% to about 70%. Associated with changes in tillage practices, concentrations and export rates of suspended sediments and phosphorus (P) in streams draining into Acton Lake declined markedly (5-15% per year). These trends are consistent with known effects of conservation tillage, which tends to reduce soil erosion and P export. Despite large decreases in watershed inputs of P (the limiting nutrient), phytoplankton biomass in Acton Lake increased over this period. This increase was most likely caused by reduced light limitation, due to decreased concentrations of suspended inorganic sediment in the lake's water column. From 1994 to 2002, concentrations of inorganic suspended sediment in the lake, and attenuation of light by this sediment, have declined by about 50%. In the early years of this study, attenuation of light by sediment exceeded attenuation of light by phytoplankton self-shading, but currently phytoplankton attenuate more light than sediment. In addition, photosynthesis-irradiance curves and experiments show that Acton Lake phytoplankton are frequently light limited. Phytoplankton can maintain high biomass in the face of decreased watershed P inputs because of P translocation by sediment-feeding fish (gizzard shad). These fish consume P from sediment detritus and excrete dissolved P into the water column. We expect that in future years, phytoplankton biomass will eventually decrease as watershed P inputs continue to decline, and gizzard shad biomass decreases in response to decreased sediment inputs.
Key words: reservoir, phytoplankton, watershed, agriculture