Planktivory and planktivore biomass effects on zooplankton, phytoplankton, and the trophic cascade1

Abstract

Five outdoor tank experiments were conducted to evaluate how season, fish biomass, and accidental mortality of introduced fish contributed to enhancement of phytoplankton growth by the zooplanktivorous atherinid fish, Menidia beryllina. Menidia enhanced phytoplankton in all seasons. Three experiments examined effects of fish density. Menidia 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 transparency, and enhanced rotifers, algal chlorophyll fluorescence, and seston particles. Two additional experiments of factorial design used live Menidia 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 experiments suggests that zooplankton control of phytoplankton by grazing is not as important as implied by the trophic cascade hypothesis.