ANNUAL PATTERN OF MICROZOOPLANKTON AND MESOZOOPLANKTON ABUNDANCE AND BIOMASS IN A SUBTROPICAL ESTUARY

The pattern of biomass and abundance of microzooplankton and mesozooplankton were studied over an annual cycle in the Nueces Estuary, Texas. Zooplankton samples and associated hydrographic data were collected at four locations at biweekly intervals from September 1987 through October 1988. This is a broad, shallow bay system with an average depth of 2.4 m. The concentration of chlorophyll a in the surface waters averaged 7.4 mug l-1 with 85% passing through a 20 mum mesh. Microzooplankton (20-200 mum in length) were extremely abundant throughout this study. Abundances of ciliates (including both aloricate ciliates and tintinnids) ranged from 5000 to 400 000 l-1, with a mean of 38 000 l-1 of seawater over the entire course of the study. Mesozooplankton (200-2000 mum in length) abundance averaged 6100 m-3 for samples collected during the day and 10 100 m-3 for samples collected at night. Mesozooplankton were dominated by Acartia tonsa which made up approximately 50% of the total. Biomass estimates for microzooplankton (based on volume estimates) were often higher than measured biomass of mesozooplankton. Given the shorter generation times and higher metabolic rate of microzooplankton compared to mesozooplankton, microzooplankton should have a greater effect on the trophic dynamics of the Nueces Estuary than mesozooplankton.