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Nutrient cycling by omnivorous fish in reservoirs of varying productivity.
SIGLER, KAREN1, VANNI, MICHAEL1, SUEKAWA, KIM1, KNOLL, LESLEY1, AREND, KRISTI2, STEIN, ROY2, 1 2
ABSTRACT- We examined the role of an omnivorous fish, gizzard shad (Dorosoma cepedianum), in nutrient cycling in three reservoirs of varying productivity. When zooplanktivorous, gizzard shad merely recycle nutrients already in the water column. However, when consuming detritus, gizzard shad transport nutrients from sediments into the water and excrete them in dissolved form usable by phytoplankton. Gizzard shad abundance increases with lake productivity, leading to reduced large-bodied zooplankton densities. In the most eutrophic reservoir, gizzard shad relied more on detritus, which has lower nutrient content than zooplankton, but in all lakes diet was greater than 95% detritus. Nitrogen (N) and phosphorus (P) excretion rates varied with productivity and between years. In 1999, P excretion rates were highest in the low productivity reservoir, possibly because of greater zooplankton consumption. However, in 2000 excretion rates were similar among lakes. In all lakes, N and P excretion rates exhibited negative allometry with fish wet mass. Comparison of P flux through gizzard shad with demand for P from primary production suggests that shad sustain the greatest fraction of production in the lake of intermediate productivity.
KEY WORDS: nutrient regeneration, excretion, Dorosoma cepedianum, productivity gradient