Science Serving Maryland's Coasts
Jennifer Purcell, Ph.D.
Horn Point Laboratory
Fine Scale Vertical Distribution of Zooplankton in an Estuarine System
The fine-scale vertical distribution of zooplankton in an estuarine system was analyzed by addressing the following hypotheses: 1) Physical parameters such as low dissolved oxygen (DO), salinity, and temperature have an effect on the fine-scale vertical distribution of zooplankton. 2) Given that the copepod, Acartia tonsa is relatively sensitive to hypoxia, there will be a strong relationship between fine-scale vertical distribution of mesozooplankton and DO concentrations. 3) Given that the ctenophore, Mnemiopsis leidyi is relatively tolerant of hypoxia, there will not be a strong relationship between fine-scale vertical distribution gelatinous zooplankton and DO levels. 4) The greatest M. leidyi density will be found in the same fine-scale vertical layer as their prey, mesozooplankton. To test these hypotheses sampling was completed in the Patuxent River in July 1999 and in the mainstem of Chesapeake Bay in July 2000. DO, temperature and salinity were measured at 1m intervals using a YSI meter. Zooplankton samples and underwater video footage were attained at 2 meter intervals. Laboratory studies included identification and counting of the zooplankton samples and analysis of video footage in order to determine the ctenophore abundance in each depth interval. Zooplankton distribution data relative to DO concentrations supports the first hypothesis. There was a strong oxycline in Chesapeake Bay where DO concentrations where nearly anoxic below 10m. Zooplankton were not found below 10m in these severe oxygen conditions. However, when bottom DO does not reach stressful levels (that is, in the Patuxent River), zooplankton are distributed to lower depths. The second and third hypotheses were neither supported nor refuted. This is because in order to address these hypotheses DO levels had to be stressful (1-2 mg/L), but not lethal for ctenophores and zooplankton in a large part of the water column. These physical conditions never occurred in either the Bay or the River during sampling times. Distribution of zooplankton and ctenophores supports the fourth hypothesis. Both the Bay and River stations demonstrate an overlap in zooplankton and ctenophore peaks. The results received in this project can be used to further the study of the effects that low DO levels has upon the trophic interactions taking place in an estuarine system.
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