Nitrogen and phosphorus excretion by detritivorous gizzard shad in a reservoir ecosystem

The detritivorous fish, gizzard shad (Dorosoma cepedianum), provides nutrients to phytoplankton in reservoirs by ingesting organic detritus associated with sediments and excreting substantial quantities of nutrients such as N and P in soluble forms that are highly available to phytoplankton, We estimated nutrient excretion by gizzard shad in a eutrophic reservoir (Acton Lake, Ohio) during April-October 1994 by measuring N and P excretion of fieldcaught fish (n = 135). Excretion rates were then extrapolated to nutrient release by the gizzard shad population using quadrat rotenone biomass estimates, electrofishing surveys, and historic seasonal trends. N and P excretion were positively correlated with fish wet mass and temperature, but mass-specific excretion declined with increasing fish mass. Lakewidc gizzard shad biomass in July 1994 was 417 kg ha -I, Our estimates of nutrient excretion by the gizzard shad population ranged from 0.487 to 0.769 pmol NH,-N liter-’ d-l and 0.022 to 0.057 pmol soluble reactive phosphorus liter -I d .I, with the highest excretion occurring during mid-summer through early fall. The low N : P ratio at which gizzard shad excrete [mean molar N : P = 16.75 (kO.89 SE)] may alter phytoplankton community composition, favoring cyanobacteria. Our results indicate that nutrient excretion by detritivorous fish can be an important source of nutrients to open waters, especially when other sources of nutrients are reduced. Fish can alter both the internal cycling of nutrients and primary producer biomass in aquatic systems through a variety of mechanisms such as incorporating nutrients into body tissue (Kitchell et al. 1975; Andersson et al. 1988; Deegan 1993), decomposition of fish tissues (Richey et al. 1975; Durbin et al. 1979; Threlkeld 1987; Parmenter and Lamarra 1991), or excretion (Lamar-r-a 1975; Shapiro and Carlson 1982; Brabrand et al. 1990). Excretion by fishes has been shown to affect phytoplankton biomass, productivity, and community structure (Reinertsen et al. 1986; Vanni and Findlay 1990; Schindler 1992; Vanni and Layne 1997). Excretion by fishes can serve to recycle nutrients within the water column or transport nutrients from the benthos to epilimnetic phytoplankton (Lamarra 1975; Shapiro and Carlson 1982; Brabrand et al. 1990; Vanni 1996; Vanni and Layne I Present address: Department of Fisheries and Wildlife, 13 Natural Resources Building, Michigan State University, East Lansing, Michigan 48824-1222. Acknowledgments We are grateful to K. Schaus, D. Wrights, A. Layne, S. Davis, D. Arnott, J. Hammond, R. Stander, J. Auch, J. Headworth, and others for field and laboratory assistance; to Joel Udstuen, Hueston Woods Park Manager; and to J. Auch, J. Evarts, J. Hammond, K. Freeman, D, Schindler, M. Pace, and one anonymous reviewer for constructive comments on an carlicr draft of this manuscript. This research was funded by NSF grant DEB 93-18452 to M.J.V.; NSF grants DEB 94-07859, DEB 91-07173, and Federal Aid in Sport Fish Restoration Project F-69-P (administered jointly by the US. Fish and Wildlife Service and the Ohio Division of Wildlife) to R.A.S.; and Electric Power Research Institute Grant 91-07 to M.T.B. and R.A.S. 1997), depending on feeding habits and movement patterns of the fishes (Shapiro and Carlson 1982; Vanni 1996; Schindler et al. 1996). By feeding on benthic organic detritus and releasing a portion of the consumed nutrients as excreta, detritivorous fish may provide an important source of nutrients to phytoplankton and may thus stimulate phytoplankton growth. This release of nutrients can be viewed as a source of new nutrients that is fundamentally different from nutrients recycled by pelagic-feeding fish or zooplankton (Dugdale and Goering 1967; Caraco et al. 1992). Unlike recycling by pelagic fish, excretion by detritivorous fish can increase the total nutrient content of lake water by releasing sediment-bound nutrients into the water column (Shapiro and Carlson 1982). ‘Thus by feeding on benthic organic detritus and releasing the nutrients in soluble forms, detritivores, (1) transport nutrients up into the water column and (2) convert these nutrients into forms that can be utilized by phytoplankton. The magnitude of nutrient release by some detritivores and the phytoplankton response observed (Lamarra 2975; Drenner et al. 1986; Brabrand et al. 1990) indicate that this source of nutrients may be important to lake phytoplankton. However, few studies have quantified nutrient (N and P or the N : P ratio) fluxes attributable to fish at the lakewide scale despite their importance to lake productivity and phytoplankton community ,composition (Schindler 1977; Tilman 1982; Smith 1983; Sterner 1990). We quantified nutrient release by gizzard shad, a detritivorous fish that often dominates the fish communities in many midwestern and southern reservoirs (Miranda 1983; Stein et al. 1995). Gizzard shad may be an important source of nutrients in reservoirs because at sizes >35 mm standard