An experimental mesocosm study of residual and contemporary effects of an omnivorous, filter‐feeding, clupeid fish on plankton community structure 1

We conducted three changeover experiments in large outdoor tanks to evaluate the persistence of effects of an omnivorous, filter‐feeding, clupeid fish ( Dorosoma cepedianum ) on plankton community structure in different seasons. The experimental design included four prechangeover treatment combinations of Dorosoma and a zooplanktivorous atherinid fish, Menidia beryllina (neither, one, or both fishes present). At the changeover, we moved Dorosoma from half of the tanks containing Dorosoma to half of the tanks previously lacking Dorosoma. Enhancement of phytoplankton by Dorosoma in the prechangeover period was a persistent effect in the December–February and July‐September changeover experiments. In the April–June experiment, phytoplankton responded primarily to the postchangeover Dorosoma treatments and showed few residual effects. Zooplankton effects of Dorosoma were not persistent. Menidia enhanced phytoplankton in the April–June experiment only, even though it reduced crustacean zooplankton in all three changeover experiments. Seasonal differences in effects on phytoplankton by Menidia and between residual and contemporary effects of Dorosoma appear to be related to the presence of large‐bodied Daphnia in the April–June experiment.