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In a general sense, there have been three, not necessarily exclusive, models proposed to explain the positive relationship between planktivorous fish and nutrient concentrations and algal abundance (Vanni and Layne 1997). In the first model, algal abundance increases because fish predation reduces the abundance of large-bodied zooplankton, which are effective grazers on phytoplankton (Carpenter et al. 1985). This model explains higher algal abundance, but not necessarily the higher nutrient concentrations. In a second model, fish predation changes the zooplankton community structure, which modifies zooplankton nutrient cycling and stimulates phytoplankton growth (Sterner 1990, Sterner et al. 1992). In the final model, nutrient excretion by the fish increases nutrient levels and facilitates higher algal abundance (Braband et al. 1990, Schindler et al. 1993). The last two are considered "bottom-up" models and can potentially explain both higher nutrient concentrations and phytoplankton abundance. In this last model, fish consumption and subsequent excretion results in "internal loading" of nutrients that stimulates algal growth. However, the degree to which fish populations influence nutrient cycling is variable (Schindler and Eby 1997), as is the specific role of fish, with some populations serving as potential sinks (DeVries and Stein 1992, Kraft 1992) and others as sources of nutrients (Carpenter et al. 1992b, Pérez-Fuentetaja et al. 1996). Thus, the role of fish in nutrient cycling is an area of active research in aquatic ecology.

Though much work has been done on influences of fish on nutrient cycling in lakes, the role of fish in wetland ecosystems is virtually unknown. Fathead minnows (Pimephales promelas) occur in many wetlands in the Prairie Pothole Region (Peterka 1989), and are often the dominant fish species. The degree to which a fish population influences nutrient cycling is probably species and ecosystem dependant, and is influenced by population density, body size, growth rates, and type of prey (Kraft 1992, Schindler and Eby 1997). The characteristics of fathead minnows and wetland ecosystems suggest that these fish may play a large role in nutrient cycling and dynamics in these ecosystems.

Fathead minnows are opportunistic feeders, but Daphnia spp. and macroinvertebrates are often important prey items (Held and Peterka 1974, Price et al. 1990, Duffy 1998). These prey have relatively high concentrations of phosphorus, thus minnow populations have the potential to excrete large amounts of phosphorus due to stoichiometric relationships between predator and prey (Schindler and Eby 1997). Additionally, fish feeding on benthic and littoral macroinvertebrates (such as fathead minnows) can have large impacts on nutrient cycling by transferring nutrients from benthic and littoral areas to the water column (Braband et al. 1990, Carpenter et al. 1992b). Finally, young-of-the-year (YOY) fish appear to have the highest potential to impact nutrient cycling due to metabolic allometry (Post 1990, Kraft 1992). Fathead minnow populations are often dominated by YOY fish; Payer and Scalet (1978) reported that 99% of annual production by a fathead minnow population in a South Dakota wetland was contributed by YOY fish.

Fathead minnows appear to have a number of impacts on wetland ecosystems, generally these impacts are similar to those of other planktivorous fish in lake ecosystems. Hanson and Zimmer (1998) compared 10 wetlands with fathead minnows to 10 fishless sites and found that wetlands with fathead minnows had significantly higher concentrations of total phosphorus and phytoplankton in the water column, and lower abundance of large-bodied zooplankton. However, the mechanisms responsible for this relationship are unclear. Reduced grazing pressure associated with lower abundance of large-bodied zooplankton may facilitate the higher algal abundance. However, increased algal abundance may also be due to increased availability of phosphorus. Higher levels of phosphorus suggests different nutrient cycles and pathways in wetlands with fathead minnows; these differences may be due to the altered zooplankton community, or they may be a direct result of fish consumption and excretion. To date, influences of fathead minnows on nutrient cycling in wetlands has not been examined.

Understanding the role of fish in nutrient cycling in wetlands is critical given the high productivity of these ecosystems. Primary production is limited by the availability of phosphorus in many lentic ecosystems (Schindler 1977) and altered cycling of this nutrient by fish may have ecosystem-scale impacts. High consumption rates may reduce the amount of phosphorus stored in invertebrate and periphyton pools, and subsequent excretion into the water column may stimulate phytoplankton growth. The net result may be a shift in primary production from macrophyton and periphyton to phytoplankton. Such a shift would have a dramatic impact on water quality and potentially decrease the value of wetland habitats for other wetland-dependant species (Bouffard and Hanson 1997).

Here we examine influences of fathead minnows on nutrient cycling in a wetland included in a larger 20-wetland study assessing impacts of fathead minnows on wetland ecosystems. Specifically, we estimated nutrient consumption by fish and effects of fish consumption on 1) the amounts of nutrients sequestered from various prey-nutrient pools, 2) nutrients incorporated into the fish pool, 3) nutrients released via fish mortality and decomposition, and 4) nutrients excreted directly to the water column by fish. In essence, we are assessing internal nutrient loading by fish, whereby fish translocate nitrogen and phosphorus from invertebrate and detrital pools to the water column pool. We test the hypothesis that high population densities and metabolic rates of these fish can result in high levels of nutrient consumption from invertebrate nutrient pools, and high levels of excretion into the water-column pool. Thus, fathead minnows act as a nutrient source, increasing water-column levels of nutrients and stimulating phytoplankton growth. This, coupled with decreased grazing pressure, results in higher algal abundance and a functionally different ecosystem compared to a fishless analog.