Planktivory and planktivore biomass effects on zooplankton, phytoplankton, and the trophic cascade. Limnology and Oceanography

Five outdoor tank experiments were conducted to evaluate how season, fish biomass, and ac-cidental mortality of introduced fish contributed to enhancement ofphytoplankton.growth by the zooplanktivorous atherinid fish, Menidia beryllina. Menidia enhanced phytoplankton in all seasons. Three experiments examined effects of fish density. A4enidia gained weight when stocked in the experimental tanks at low densities (<1.5 g dry wt m ‘3), suppressed crustacean zooplankton, and enhanced blue-green filaments. Water transparency and other phytoplankton were unaffected. Menidia lost weight. and occasionally died when stocked at higher densities (> 1.5 g dry wt m-3). Under these conditions, Menidia suppressed crustacean zooplankton and reduced water transpar-ency, and enhanced rotifers, algal chlorophyll fluorescence, and seston particles. Two additional experiments of factorial design used live A4enidia and dead Menidia as separate, cross-classified treatment factors, Live fish effects were limited to the suppression of crustacean zooplankton and enhancement of blue-green filaments, while dead fish reduced water transparency or enhanced other phytoplankton. The importance of lost fish biomass (either weight loss or mortality of introduced fish) to the enhancement of phytoplankton by planktivorous fish in these experiment