ABSTRACT: Seawater dilution experiments were conducted during spring and fall in the continental shelf region of the northern Gulf of Mexico. Nutrient-enhanced phytoplankton growth rates of 0.7 to 2.2 d^-1 were measured for the entire phytoplankton community; highest growth rates were associated with >8 um cells. Phytoplankton growth was nutrient limited in all May experiments, and >8 um phytoplankton, primarily diatoms, showed the strongest response to nutrient addition: their growth rates increased the most and reached the highest values. Rates of microzooplankton grazing on the entire phytoplankton community were moderate (0 to 0.7 d^-1). During a given experiment, patterns of grazing on 2 phytoplankton size fractions (<8 and >8 um) generally differed, and high rates of grazing (>1 d^-1) on both <8 and >8 um cells were sometimes observed. Across all experiments, grazing by microzooplankton averaged 30% of nutrient-enhanced phytoplankton growth. In May, when phytoplankton growth was strongly nutrient limited, grazing averaged 90% of natural (non-nutrient-enhanced) phytoplankton growth. These data indicate that microzooplankton can be a significant source of phytoplankton mortality, even in eutrophic coastal waters. The microzooplankton community, excluding cells <5 um, comprised primarily heterotrophic dinoflagellates and aloricate choreotrich ciliates. These organisms exhibited high net growth rates (mean = 0.8 d^-1) during experiments at higher irradiance levels. Ingestion of chain diatoms by the dinoflagellate Gyrodinium sp. was observed in preserved samples; such grazing pathways, in which relatively large phytoplankton cells are consumed by protozoa, may be quantitatively important in this coastal ecosystem. Due to the variety of taxa and feeding mechanisms within the microzooplankton, their grazing impact was not restricted to the smallest phytoplankton cells, indicating that size-based models of trophic structure could yield misleading predictions about patterns of energy flow in this coastal ecosystem.

KEY WORDS: Microzooplankton . Phytoplankton . Protozoa . Grazing . Growth . Coastal

Published in MEPS Vol. 130 (1996) on January 11
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1996