Nutrient excretion by fish in wetland ecosystems and its potential to support algal production

We used both direct measurement and bioenergetic methods to examine nitrogen and phosphorus (P) excretion by fathead minnows in two wetlands over 2 yr. We also assessed the potential importance of nutrient flux from fish by comparing excretion estimates to empirically derived estimates of algal P demand. Population-level estimates of P excretion across the four lake-years were high relative to other published studies, peaking at 11.7 mg m23 d21 and occurring when fish densities exceeded 69 fish m23. Excretion rates peaked in late summer during maximal algal demand, and P excretion by fish exceeded algal demand in two of the four wetland-years. Detritus was a major diet component (up to 90% by mass), making fish excretion a major nutrient flux from wetland sediments to the water column. Consumer-driven effects could have strong influences on nutrient dynamics in prairie wetlands, ecosystems historically thought to be constrained largely by abiotic factors. Fish populations can have strong influences on ecological processes and the structure of aquatic ecosystems, and one common pattern is higher phytoplankton abundance and elevated water-column nutrients in the presence of dense populations of benthivorous and planktivorous fish (Hanson and Butler 1994; Vanni et al. 1997). Although this relationship is well documented, the mechanisms responsible and their relative importance are not always clear. In a general sense, there have been three (not necessarily exclusive) models proposed to explain the positive relationship between benthivorous–planktivorous fish and nutrient concentrations and algal abundance (Vanni and Layne 1997). In the first model, nutrient excretion by sediment-feeding 1 Present address: Department of Biology, University of St. Thomas, Mail OWS 390, 2115 Summit Avenue, St. Paul, Minnesota 55105. 2 Present address: Department of Ecology, Evolution and Behavior, University of Minnesota, 1987 Upper Buford Circle, St. Paul,