Microzooplankton Dynamics in Lower Chesapeake Bay and Its Major Tributaries
This study identified the temporal and spatial variability of microzooplankton density and biomass at 14 stations located within Chesapeake Bay and four rivers. Microzooplankton are defined as those heterotrophic organisms which range from 20 to 200 $\mu$m in size, and in this study were mainly composed of copepod nauplii, rotifers, and ciliates. Ciliates were the most abundant microzooplankton, comprising more than 90% of the total density, with copepod nauplii representing approximately 50% of the total microzooplankton biomass (carbon content). Rotifers contributed less than 5% for both density and biomass of the total microzooplankton. Maximum ciliate abundance occurred from spring to early summer based on the cross-correlation coefficients, and was dominated by small oligotrichs and tintinnids. This development was followed by increased concentrations of rotifers and copepod nauplii that peaked in mid summer and early fall. Copepod nauplii and rotifers had an annual cycle (12 month) in density, as determined by time series analysis (ARIMA Model). However, for the smaller components (ciliates) in size, the autocorrelation function coefficients were not significant at most stations, which indicated their seasonal abundance patterns were not periodic. In terms of the spatial variation of microzooplankton density and biomass, oligohaline statons (river-estuary transitional sites) generally had the highest microzooplankton density and biomass. Pearson's correlation analysis identified several consistent relationships between microzooplankton density and the environmental variables. For instance, rotifers had a significant negative correlation with salinity, and total nitrogen and phosphorus were negatively correlated with loricated ciliates, but positively correlated with aloricated ciliates. Physical and chemical variables were much more important in explaining the geographical heterogeneity of microzooplankton density. The zooplankton community structure, including mesozooplankton, was significantly different depending on the trophic status at each site. The relative contribution of aloricated ciliates to total zooplankton biomass was much higher in the hypertrophic Elizabeth River, but loricated ciliates and mesozooplankton comprised a much lower proportion of the total zooplankton biomass when compared to the other meso- or eutrophic sites. The dominance within the zooplanliton community shifted from mesozooplankton to microzooplankton with increasing trophic state, and the microzooplankton constituted over 90% of the total zooplankton biomass in the hypertrophic Elizabeth River. In general, microzooplankton represented over 80% of the total zooplankton biomass (carbon content) and the secondary production by ciliates only (38 $\mu$gC/1/day) was about 16% the primary production in the Bay and Elizabeth River. Given the shorter generation time and faster growth rates of microzooplankton compared to the other metazoans, the microzooplankton are considered to have a greater effect on the trophic dynamics than the mesozooplankton in lower Chesapeake Bay and these tributaries.
Elizabeth River, Ciliates, Copepods, Rotifers, Ecology and Evolutionary Biology, Environmental Sciences, Marine Biology, Oceanography
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