Seasonal and interannual variation in nutrient fluxes from tributary inputs, consumer recycling and algal growth in a eutrophic river impoundment Joseph Shostell 1,2 and Paul A. Bukaveckas 1, * 1 University of Louisville, Department of Biology, 328 Life Sciences, Louisville, KY 40292, USA; 2 Department of Biology, Penn State University – Fayette, Uniontown, PA 15401, USA; * Author for correspondence (e-mail: pabuka01@athena.louisville.edu) Received 29 November 2002; accepted in revised form 20 February 2004 Key words: Eutrophication, Gizzard shad, Nitrogen, Nutrients, Phosphorus, Reservoirs, Zooplankton Abstract We measured tributary inputs, algal nutrient demand and excretion rates of consumers gizzard shad and zoo- plankton at a eutrophic river impoundment. During two summers with contrasting flow regimes, tributary inputs accounted for 38% 1998 and 3% 1999 of algal N demand and 95% 1998 and 17% 1999 of algal P de- mand. Gizzard shad contributions averaged 14% and 20% of algal demand for N whereas P contributions were 31% and 58% 1998, 1999; respectively. Zooplankton recycling accounted for a comparable fraction of algal P demand 47% but a larger fraction of N demand 43% because their excretia were N rich N:P = 13:1 com- pared to fish 7:1. Nutrient release by one of the consumers gizzard shad was compared with tributary loading over a nine-year period to assess inter-annual variation in their relative importance. Historical records of inflow chemistry, discharge and gizzard shad biomass showed that variation in tributary inputs was the primary deter- minant of seasonal and inter-annual variation in nutrient loading. Consumer-derived nutrients were important in late-summer and during years when tributary inputs were low. We propose a conceptual model in which primary production is regulated by external nutrient loading and consumer recycling acts to stabilize and sustain produc- tion during periods of diminished external inputs. Introduction Phytoplankton production in aquatic ecosystems is regulated in part by the supply of nutrients from ex- ternal sources ‘new’ production and from internal cycling of nutrients stored in various biotic and abi- otic compartments Dugdale and Goering 1967; Caraco et al. 1992; Smith et al. 1999. Concerns re- garding anthropogenic eutrophication stimulated the development of numerical models predicting lake chlorophyll concentrations as a function of external nutrient inputs Dillon and Rigler 1974 and continue to drive research focusing on export of N and P from catchments e.g., Soranno et al. 1996; Siver et al. 1999. The importance of physical processes in regu- lating internal nutrient supply through sediment release and hypolimnetic entrainment has long been recognized Hutchinson 1957. Recent work has em- phasized the role of zooplankton and certain fish spe- cies in recycling N and P and their potential importance in sustaining algal production Vanni 1996; Drenner et al. 1998; Schaus et al. 2002. Rates of consumer-mediated recycling reflect the abundance and species composition of consumer groups whereas rates of external nutrient loading are dependent upon the lake’s hydrogeomorphic position and intensity of land-use within its catchment. These factors may not be independent because elevated ex- Aquatic Ecology 38: 359–373, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands. 359