Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


A two-year zooplankton survey of lower Chesapeake Bay was conducted from August 1971 to August 1973. Twenty-four stations extending from the Rappahannock River mouth to the Bay entrance were sampled each month, using paired bongo nets with a 202 μm mesh. Marine Cladocera comprised >68% of the total zooplankton during two months of the survey, and were 36-40% by number three other times. The mean abundance over the whole study area of all cladoceran species combined was >62,000 m-3 in August 1971 and was >1000 m-3 on six other occasions. Of the six species of marine Cladocera found, Podon polyphemoides was most eurythermal and euryhaline, being found in abundance at temperatures of 7-26°C and salinities of 11.5-27 0/00. It was found in all months of the year with a peak occurrence in May and a smaller peak in January. Peak densities of P. polyphemoides always followed periods of rapidly changing temperatures. Podon leuckarti was found only near the Bay mouth, nearly always at densities of latitude, cold water species which requires spring conditions for its emergence, restricting its appearance to the month of May in Chesapeake Bay. Podon intermedius occurred only in the bay mouth area, at densities of 18.3-26.4°C and a minimum salinity of 20.42 0/00. Although appearing to be a warm-water organism in Chesapeake Bay, the species is found at much lower temperatures elsewhere. An inverse temperature-salinity relationship may influence the worldwide distribution of this species, with its warm water occurrences being at relatively low salinities, and vice versa. Evadne nordmanni is a high-latitude species neeaing spring conditions to develop. It does not persist throughout the summer in regions where temperatures are >21°C for two months or more. E.nordmanni occurred from late winter through June, with a peak in May. It appeared throughout the study area with highest concentrations (>100 m-3) at the bay mouth at 15-19°C and 17-26 0/00-salinity. Evadne tergestina and Penilia avirostris usually occurred at densities of >100,000 m-3 at some stations, with a mean of 30,000 m-3 over the entire area. Densities of both species were greatly reduced by September and they disappeared in October. The species reappeared in August 1972 in greatly reduced numbers, being >1000 m- 3 only at the bay mouth. Numbers were higher in August 1973 but were still an order of magnitude less than in 1971. The freshwater runoff from Tropical Storm Agnes was responsible for the great reduction in populations of Podon polyphemoides, Evadne tergestina and Penilia avirostris in the summer of 1972 as compared to 1971 and 1973. Biological influences on cladoceran populations included predation by ctenophores, which limited P. polyphemoides to the bay mouth in June, and predation by chaetognaths, a probable cause of the great reduction in E. tergestina and P. avirostris densities between August and September. Day and night sampling over the entire study area and a 24-hour station at the York River mouth showed that Evadne tergestina releases its young from the brood pouch just before dawn. An individual of E. tergestina releases its first brood of young when two days old. The parthenogenetic reproduction of Cladocera makes them among the most opportunistic of marine zooplankters and is probably partially responsible for their comparative lack of speciation in marine waters.

This dissertation is from the Joint Program Degree from the College of William & Mary and University of Virginia and awarded by the University of Virginia.



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