Cascading trophic interactions in a large, shallow sub-tropical estuary, Galveston Bay, Texas

Abstract

Dilution bioassays estimate grazing impacts of zooplankton and growth rates of phytoplankton by experimentally reducing the encounter rate between predators and their prey. In this study, the dilution method was used to measure phytoplankton growth and zooplankton grazing rates across a variety of hydrographic environments in Galveston Bay. Three stations were selected to represent the range of hydrographic regimes and planktonic communities found in the estuary. Following bioassay incubations, water samples were filtered for HPLC analysis of chemosystematic photopigments to enable a reconstruction of the community composition using a factor minimization algorithm, ChemTax. The resulting zooplankton grazing rates and phytoplankton growth rates did not always conform to the expected linear response with increasing dilution. Often grazing pressure seemed to increase with increasing dilution. This shift in apparent phytoplankton growth rate was attributed primarily to an uncoupling of trophic linkages between the microzooplankton and mesozooplankton resulting in a cascade effect on phytoplankton abundance. The results of this study suggest that the impact of microzooplankton grazing on phytoplankton production is influenced by the presence and abundance of the mesozooplankton community, in Galveston Bay, Texas.