Topical Area: Upper Trophic Levels and Zooplankton

M. Elizabeth Clarke, Marine Biology and Fisheries, University of Miami/RSMAS, Miami, FL and Stephen M. Bollens, Department of Biology, San Francisco State University, San Francisco, CA

Dramatic changes in the biota of the Florida Bay have occurred over at least the past decade. Several environmental parameters have been postulated to be the causal factors in these changes, among them increasing salinity and eutrophication. In fact, the exact mechanisms by which changes in salinity and nutrients affect the biota are not presently well known, nor are the linkages between primary producers and upper trophic levels. Intuitively one might expect salinity and/or nutrient effects on primary producers in Florida Bay to ramify through the food web, leading to changes in higher trophic levels. These effects may be manifested directly via changes in survival and species composition of zooplankton and ichthyoplankton, or indirectly and more subtly via changes in fecundity, food habits, or growth of planktonic species.

Our chief objective is to experimentally determine the effects of salinity and nutrient changes on the plankton dynamics of Florida Bay. Specifically, we are testing for effects on species composition and abundance of phytoplankton, protozoan, and metazoan (including larval fish) components of the plankton. Additionally, we are testing for effects on the feeding and growth of selected species, i.e.; larval spotted sea trout and the copepods Acartia tonsa and Parvocalanus (Paracalanus) crassirostris.

To identify the causal mechanisms underlying these changes in Florida Bay, and thus be able to predict future ecosystem response requires the application of field experiments. Therefore, we used well-controlled and heavily replicated experimental mesocosms to investigate the importance of salinity and nutrients in controlling the plankton dynamics in Florida Bay. Experiments were conducted in January and March 1998. We tested the effects of salinity (25, ambient, 41 ppt) and nutrients (ambient, +N, +P) on the composition and abundance of phytoplankton, protozoan and metazoan zooplankton.

A raft was constructed in protected waters adjacent to the Keys Marine Laboratory, Long Key, Florida and a series of 2300 liter polyethylene enclosures were suspended from it. Enclosures were cylinders (1.0 m diameter x 4.0 m deep) constructed of 1.5 m wide panels of polyethylene heat-sealed together and providing water-tight seals. Plankton were stocked in the bags by pumping subsurface seawater and associated microplankton into the enclosures and making net hauls for mesozooplankton (this method has worked well for us in our previous studies). Larval spotted sea trout from the fish hatchery at the University of Miami also were stocked into bags.

Salinities were manipulated by adding RO water or saline solution (Instant Ocean) to ambient subsurface seawater. This additional water was pumped slowly and gently into each enclosure over a period of many hours, thereby minimizing the acute stress to the plankton. Salinity was monitored daily over the course of each experiment. Nutrients were manipulated by adding either nitrate or phosphate to each enclosure in the nutrient enrichment treatments.

Each enclosure was sampled daily for zooplankton and chlorophyll as well various abiotic parameters (salinity, nutrients, temperature) over the course of each experiment. At the end of each experiment all fish larvae were removed from the enclosures by repeated towing of a 1/2 meter diameter, 110 um mesh net. Larvae and zooplankton were reserved for analysis of condition and growth.

Sampling was completed in mid March and we are now beginning a detailed analysis of species composition, condition and growth of the micro and macrozooplankton in the mesocosms. Preliminary analyses indicate that the highest survival of larval fish occurred in treatments with highest salinities. We also found that protozoans comprised a large component of the fish diets. The dominant macrozooplankton were Oithona spp. Parvocalanus (Paracalanus) crassirostris, Acartia tonsa and Eutrepina acutifrons. During the first five days of the salinity manipulation experiment, relative abundances of Parvocalanus increased and Oithona decreased in all treatments, whereas relative abundance of Longipedia increased in only the low salinity treatment.