The Annual Cycle in Quantity and Composition of the Zooplankton of the Sargasso Sea Off Bermuda. I. The Upper 500 M1

A year‐round quantitative and qualitative study has been made of the zooplankton collected by oblique 0–500‐m tows with a meter net of No. 2 and No. 8 mesh nylon at semimonthly intervals from 16 March 1961 to 7 April 1962 at station “S,” 24 km southeast of Bermuda in 3,200 m of water. Highest dry weights and displacement volumes were obtained in March 1961 and April 1962, but total numbers remained fairly high for most of the year, with a January–February minimum. Seasonal cycles of all the more numerous organisms are described. In the Sargasso Sea off Bermuda the annual cycle of zooplankton abundance is related to the degree of seasonal change; when the water column is mixed down to the permanent thermocline, greater quantities of phytoplankton and zooplankton are produced. In 1961– 1962 the mean standing crop of zooplankton was 70% of that recorded for Long Island Sound and Block Island Sound and 30% that of Georges Bank. Specific diversity in plankton populations was investigated by means of MacArthur’s information‐theory model and values for the coefficient of equitability obtained for the copepods and pteropods in the Sargasso Sea and for copepods in Long Island Sound, Block Island Sound, and Texas coastal waters. A seasonal cycle in equitability of plankton populations is apparently typical of waters where there is a sufficient degree of seasonal environmental change to produce unstable conditions or to permit changes in species composition during a year. The evenness of distribution of copepod species is more dependent on environmental stability than on number of species. The equitability of the copepod populations in the Sargasso Sea appears to be inversely correlated with productivity.