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In a broader sense, fish may influence ecosystem structure and nutrient cycling in aquatic ecosystems. Feeding and egestion activities of fish can elevate water-column nutrient concentrations, increasing planktonic algal biomass and reducing water transparency (Breukelaar et al. 1994, Vanni and Layne 1997). Shallow, eutrophic lakes reportedly exist in two alternative "states", either clear-water, macrophyte dominated, or turbid-water, phytoplankton dominated (Scheffer et al. 1993, Hanson and Butler 1994). In shallow lakes, increased turbidity associated with fish may induce shifts toward, or sustain, turbid-water, phytoplankton-dominated conditions. Abundance of macrophytes and associated periphyton and aquatic invertebrates decrease with such shifts, resulting in functionally different ecosystems (Bouffard and Hanson 1997).

It is possible that some fish species have similar influences on wetlands throughout the PPR. Fathead minnows (Pimephales promelas (Rafinseque)) are native to many prairie wetlands (Peterka 1989) and are capable of explosive population growth (Payer and Scalet 1978). Here we compare ecosystem structure and sizes of nutrient pools in two wetlands, one fishless and the other supporting a population of fathead minnows. We predict the wetland with fathead minnows will emulate the turbid-water state, the fishless site the clear-water state, and hypothesize that the size of nutrient pools will differ dramatically between these ecosystems. The turbid-water wetland will be characterized by a simple, pelagic-type food web, and fathead minnows, phytoplankton, and seston will be the major nutrient pools. In contrast, the clear-water wetland will have a complex, littoral-type food web, and macrophytes, periphyton, and aquatic invertebrates will be the largest nutrient pools.