Experimental Analysis of the Direct and indirect Effects of an Omnivorous Filter-Feeding Clupeid on Plankton Community Structure

In laboratory trials, feeding rates of an omnivorous filter-feeding clupeid, Dorosoma cepedianum, increased as a function of particle size, with maximal rates on microspheres, spherical algae, and Zooplankton >40 μm; it did not efficiently feed on filamentous Anabaena flos-aquae. To examine the community level impacts of Dorosoma, we conducted four seasonal outdoor tank experiments of cross-classified design involving two or three densities of Dorosoma and two densities of the zooplanktivorous atherinid fish, Menidia beryllina. We attempted to discriminate between the direct and indirect effects of Dorosoma on phytoplankton by using Menidia to produce indirect effects on phytoplankton by suppressing Zooplankton. Experiments began in November, March, June, and September and lasted for 45–53 d. Dorosoma suppressed most Zooplankton in at least one experiment and enhanced algal standing crops in all four experiments, as indicated by increased algal chlorophyll fluorescence, turbidity, Coulter counts and microscopic algal counts, and decreased Secchi depths. Because in three out of four experiments Menidia suppressed Zooplankton biomass to a greater extent than Dorosoma without enhancing phytoplankton, we reject the hypothesis that the enhancement of phytoplankton by Dorosoma was an indirect effect of Zooplankton biomass suppression.