Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


The community structure and vertical distribution patterns of mesozooplankton in the lower York River, Virginia were examined during August, 1978. Samples were collected in the upper and lower water strata before, during and after destratification of the water column due to tidal mixing. In the stratified pre-mixed water column, distinct zooplankton communities were identified above and below the pycnocline. Pronounced diurnal vertical migrations of Acartia tonsa (all stages) and Pseudodiaptomus coronatus were documented. During the well-mixed hydorgraphic regime, the previously identified communities were not as well defined. There was a greater homogeneity of species assemblages and less pronounced differentiation of the vertical distributions for the dominant species during all light conditions. Following restratification, the species assemblages and diurnal vertical distributions closely resembled those identified in the well-mixed sampling period rather than the communities and patterns observed from the initial stratified environment. The homogeneous nature of the zooplankton communities in the well-mixed environment is attributed to the absence of water column stratification. The inability of the zooplankton to reestablish vertically distinct assemblages following restratification of the water column is proposed to be a result of low oxygen concentrations.



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