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

The pattern of biomass and abundance of microzooplankton and mesozoopl ankton were studied over an annual cycle in the Nueces Estuary, Texas. Zooplankton samples and associated hydrographic data were collected a t four locations at biweekly intervals from September 1987 through Oct ober 1988. This is a broad, shallow bay system with an average depth o f 2.4 m. The concentration of chlorophyll a in the surface waters aver aged 7.4 mug l-1 with 85% passing through a 20 mum mesh. Microzooplank ton (20-200 mum in length) were extremely abundant throughout this stu dy. Abundances of ciliates (including both aloricate ciliates and tint innids) 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 (200-200 0 mum in length) abundance averaged 6100 m-3 for samples collected dur ing the day and 10 100 m-3 for samples collected at night. Mesozooplan kton were dominated by Acartia tonsa which made up approximately 50% o f the total. Biomass estimates for microzooplankton (based on volume e stimates) were often higher than measured biomass of mesozooplankton. Given the shorter generation times and higher metabolic rate of microz ooplankton compared to mesozooplankton, microzooplankton should have a greater effect on the trophic dynamics of the Nueces Estuary than mes ozooplankton.