Quantifying Linkages among Gizzard Shad, Zooplankton, and Phytoplankton in Reservoirs

Food webs in northern temperate lakes frequently exhibit tightly linked interactions between adjacent trophic levels that lead to top-down effects from piscivores to phytoplankton. To determine if these interactions occur in reservoirs dominated by an omnivorous planktivore, gizzard shad Dorosoma cepedianum, we experimentally quantified crustacean zooplankton and phytoplankton responses to age-0 gizzard shad (23–90 mm total length) at four gizzard shad densities across three reservoirs, each with a different level of crustacean zooplankton productivity. Age-0 gizzard shad exerted strong top-down effects on crustacean zooplankton, eliminating it from most enclosures within 2 weeks. This decline in crustacean zooplankton did not lead to increases in phytoplankton unless Daphnia spp. were initially abundant. As detritivores, gizzard shad can remain abundant when planktonic food resources are depleted. This trophic decoupling probably allows gizzard shad to structure reservoir food webs by overconsumption of crustacean zooplankton, which in turn may compromise recruitment of sport fishes with zooplanktivorous larvae. In our view, the potential for increasing crustacean zooplankton density by controlling gizzard shad with stocked predators will be limited to reservoirs where age-0 gizzard shad density can be reduced below 10 fish/m3 and where daily zooplankton productivity exceeds 220 mg/m3.