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 field‐caught 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. Lakewide gizzard shad biomass in July 1994 was 417 kg ha −1 . Our estimates of nutrient excretion by the gizzard shad population ranged from 0.487 to 0.769 µmol NH 3 ‐N liter −1 d −1 and 0.022 to 0.057 µmol soluble reactive phosphorus liter −1 d −1 , 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 (±0.89 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.