THE ANNUAL CYCLE IN QUANTITY AND COMPOSITION OF THE ZOOPLANKTON OF THE SARGASSO SEA OFF BERMUDA. II. THE SURFACE TO 2,000 m1

Abstract

Zooplankton samples were collected at station “S” (32° 10′ N, 64° 30′ W), 24 km SE of Bermuda in 3,200 m of water, at monthly intervals from July 1968 to June 1969, over four depth intervals from the surface to 2,000 m. In the upper 500 m highest numbers were found in October and April; between 500 and 1,000 m the highest numbers were present in July, March, and May; from 1,000 to 1,500 m the numbers were highest in August, January, and March. In the deepest waters sampled numbers were highest in July, October, March, and May. The displacement volumes for the upper 500 m showed a pronounced April maximum; the October maximum in numbers, consisting of small copepods, was not reflected in the volumes. Larger organisms were more common in the deeper waters. In going down the water column, volumes were approximately halved over each lower depth interval, but mean numbers decreased greatly between the upper waters and those of the 500–1,000‐m depth interval, indicating an increase in size of organisms between these two depth ranges. Crustacea increased in importance and in diversity below 500 m. Copepods comprised 70%. of the plankton of the upper 500 m and 85.2% in the deepest waters sampled. Pelagic ostracods were next in importance, but were relatively less numerous below 1,500 m. Highest numbers of calanoid genera and ostracod species were noted between 500 and 1,500 in. Euphausiids were most numerous between 1,500 and 2,000 m with a November maximum. Tunicates, coelenterates, chaetognaths, Foraminifera, pteropods, and various larval forms were most abundant in numbers and species in the upper waters, but occurred in small numbers below 500 m.