Animals can be important in nutrient cycling.  In some ecosystems, they supply nutrients at rates comparable to major nutrient sources, and can thus support a substantial proportion of nutrient demands of primary producers.  Gizzard shad (Dorosoma cepedianum) is a common fish in reservoirs, natural lakes and rivers of the lower midwest and southern US. It has been argued that fish are a source of nutrients because they recycle and translocate nutrients, but an alternative argument is that fish are a sink for nutrients because they sequester relatively large amounts of nutrients in their bodies.  Our study was performed to determine what happens to nutrients when gizzard shad die and decompose, within the context of whether or not these fish are a nutrient source or sink.  We conducted a field experiment in a eutrophic reservoir, in which nutrient composition of fish bodies, and consumption of carcasses by microbes and invertebrates were examined as the factors mediating decomposition of fish.  We used two different sizes of fish and allowed either microbes or microbes plus invertebrates access to fish carcasses (by using different mesh sizes). 

Results/Conclusions

We found an initial rapid decrease (% dry mass remaining) in both the fish with access to only microbial fauna and the fish with access to both microbial fauna and invertebrates.  For fish decomposed only by microbes, decomposition eventually leveled off while the mass of fish available to both microbial fauna and invertebrate decomposition continued to decrease rapidly.  We found an initial decrease in % Carbon and % Nitrogen in both sizes of shad.  The large fish size lost relatively more C than the small fish, while fish size did not seem to be a factor in %N.  Thus, after several weeks, carcasses of large fish size had a lower C:N than small fish. Our results suggest that decomposition of gizzard shad bodies contributes to the role of these fish as a source of nutrients for primary producers.