Interspecific Comparisons and the Potential Importance of Nutrient Excretion by Benthic Fishes in a Large Reservoir

—Fishes can provide an important link between benthic and pelagic habitats by removing nutrients from sediments and excreting them into the water column. The relative importance of nutrients excreted by fishes to ecosystem productivity may vary among species and with abiotic conditions. I measured excretion rates of three benthic feeding fishes, gizzard shad Dorosoma cepedianum, smallmouth buffalo Ictiobus bubalus, and river carpsucker Carpiodes carpio, to examine the potential contribution of these species to the nutrient budget of a large southern reservoir. All species showed a significant relationship between log excretion rate and log body mass but no differences among species in the slopes or intercepts. For all species combined, the slope of the relationship was less than 1, suggesting a less than proportional increase in excretion rate with size. Using mean standing crop estimates from published cove rotenone surveys in Oklahoma, the nutrient loading to Lake Texoma by these fishes was estimated to be 0.0096 kg · ha21 · d21 for phosphorus and 0.3580 kg · ha21 · d21 for nitrogen. These values exceeded external nutrient-loading rates from the watershed 12% and 43% of the time for phosphorus and nitrogen, respectively. Numerous studies in aquatic systems have shown that fishes can have important effects on ecosystem processes. Fishes can directly affect primary productivity through control of grazer populations (e.g., Carpenter et al. 1985; Power 1990) or through transport and recycling of nutrients (Hurlbert et al. 1972; Lamarra 1975; Andersson et al. 1978; Persson 1997). In particular, benthicfeeding fishes can provide a link between benthic and pelagic processes by transporting materials between the two regions. For example, they can enhance primary productivity in the water column by resuspending phytoplankton from sediments to surface waters (Breukelaar et al. 1994) or by increasing nutrient regeneration through resuspending sediments and associated microbes (Shormann and Conter 1997). Benthic fishes also have the ability to release bioavailable nutrients derived from sediments into the water column through excretion (Lamarra 1975; Brabrand et al. 1990; Schaus and Vanni 2000). Estimating the relative importance of fishes in regulating water column productivity can help managers interested in the use of these organisms to improve water quality or to enhance sport fish production (e.g., through biomanipulation; Carpenter et al. 1987; Drenner et al. 1998). Moreover, estimating the degree of * E-mail: [email protected] 1 Present address: Kansas State University, Division of Biology, Ackert Hall, Manhattan, Kansas 66506, USA. Received August 21, 2000; accepted September 13, 2001 interspecific differences in species effects can aid in modeling the effects of individual species or of a suite of species on ecosystem processes. The ability of a species to regulate ecosystem processes depends on its abundance (Power et al. 1996; Power 1997), population size structure (Schaus et al. 1997; Mehner et al. 1998), trophic level (Carpenter et al. 1992; Schindler et al. 1993), and mode of feeding (Matthews 1998; Gido 2001). In addition, abiotic conditions often mediate the magnitude of a species’ effect in an ecosystem (Brabrand et al. 1990; Power et al. 1996). For example, the effect of omnivorous fish on chlorophyll biomass in experimental tanks was shown to be greatest when nutrients were added (Drenner et al. 1998). In natural lakes and reservoirs, the effects of nutrient enrichment by fishes (e.g., Schindler et al. 1996; Vanni 1996) are thought to be important only during times when nutrient loading from the watershed (Brabrand et al. 1990) or when nutrient regeneration by plankton (Hudson et al. 1999) is relatively low. Therefore, species effects in ecosystems must be examined in the context of variable biotic and abiotic factors (Power et al. 1996; Power