Biomass-Dependent Diet Shifts in Omnivorous Gizzard Shad: Implications for Growth, Food Web, and Ecosystem Effects

Abstract

We examined diet patterns of omnivorous gizzard shad Dorosoma cepedianum in Acton Lake, Ohio, during 1994-1997 using a multiple stable isotope analysis to quantify the role of this species in the system. On most dates, zooplankton were relatively depleted in δ¹³C (about −30‰ to −25.5‰) compared with sediments (−25‰), permitting construction of a mixing model to determine the proportion of C derived from benthic detritus and from planktonic productivity. During periods of greater gizzard shad biomass (>35 kg/ha), gizzard shad of more than 35-mm standard length (SL) derived most of their C from sediment detritus. When gizzard shad biomass was low (<15 kg/ha), zooplankton biomass increased and all sizes of gizzard shad derived most of their C from zooplankton. Conventional gut analyses corroborated these findings. Zooplanktivorous age-0 gizzard shad grew at three or more times the rate of those that were detritivorous. Rapid age-0 growth led to high gizzard shad biomass, a decrease in large zooplankton, and a subsequent shift to detritivory. Therefore, diet quality and growth rates are strongly linked to gizzard shad biomass, and these biomass-dependent feedbacks tend to keep gizzard shad biomass high in this system during most years. Because zooplanktivorous gizzard shad recycle nutrients within the water column, whereas detritivorous gizzard shad transport nutrients from sediments to the water column, biomass-induced diet shifts modify the impact of this species on phytoplankton through both top-down and bottom-up mechanisms.