Nutrient and Energy Cycling Among Phytoplankton, Bacteria, and Zooplankton

Abstract

During the Summer of 1985 students and senior faculty who participated in the HIMB Summer studies Program utilized a variety of experimental techniques to study nutrient and energy cycling among phytoplankton, bacteria, and zooplankton. Bacterial production rates were found to vary by more than an order of magnitude between different locations in the bay and HIMB lagoon, with most of the production being due to free-living (not particle-bound) bacteria. Use of metabolic inhibitors indicated that bacteria played an important role in both the uptake and regeneration of phosphate, and that phytoplankton were by no means the only important consumers of phosphate. Fifty percent or more of phytoplankton photosynthetic rates was due to picoplankton (0.2-2 um). The productivity/biomass ratio for the phytoplankton community was consistent with recent studies which have indicated that the cells are growing rapidly with little reduction in growth rate due to nutrient limitation. A new technique for estimating zooplankton grazing rates using fluorescent rnicrospheres yielded results in excellent agreement with known grazing rates measured in a continuous culture system. Use of bacteria uniformly labeled with 14c was found to be a promising technique for estimating bacteriovore grazing rates and assimilation efficiencies. Studies of the benthic diatom Nitzschia graeffei indicated that 6-40% of the carbon assimilated by this organism was obtained via photoheterotrophy.