VARIABILITY IN ZOOPLANKTON BIOMASS DISTRIBUTION IN 



THE NORTHERN SARGASSO SEA: THE CONTRIBUTION OF 



GULF STREAM COLD CORE RINGS ^ 



Peter B. Ortner, Peter H. Wiebe, Loren Haury, and Steven Boyd^ 



t 

 ABSTRACT 



The scale and frequency of physical variability resulting from incursion of Gulf Stream cold core rings 

 into the northern Sargasso Sea makes this fauna! province more heterogeneous than previously 

 recognized. At any one time such rings may cover between 6 and IS'^ of the surface area of the northern 

 Sargasso Sea. They are more productive than the surrounding Sargasso Sea and have a zooplankton 

 biomass intermediate between the Sargasso Sea and the slope water. Cold core rings may augment by 3 

 to 7% the primary productivity and by 8 to 169c the zooplankton standing crop of the northern Sargasso 

 Sea. Compared with either the surrounding Sargasso Sea or their parent slope water, an unusually 

 large percentage of the 0-800 m biomass in rings is found at depths greater than 200 m. This 

 distribution may be related to hydrographic and biological changes associated with ring decay. 

 Because of their higher productivity, differences in vertical biomass structure, and the possibility that 

 ring food chain efficiency is lower than that of the Sargasso Sea, rings may provide a disproportionately 

 large fraction of the total supply of organic matter to the northern Sargasso deep Sea. 



A number of papers have characterized the zoo- 

 plankton biomass of the northern Sargasso Sea 

 (Menzel and Ryther 1961; Grice and Hart 1962; Be 

 etal. 1971; Deevey 1971; Deevey and Brooks 1971; 

 and others). Because of the variety of methods 

 employed in both sampling and processing, the 

 results of these studies are not readily compara- 

 ble. In general, previous authors have portrayed 

 the Sargasso Sea as a remarkably homogeneous 

 faunal province. The scale and frequency of re- 

 gional variability resulting from incursions of cold 

 core rings into the Sargasso Sea have not been 

 generally appreciated. Cold core rings are meso- 

 scale hydrological features 150 to 300 km in 

 diameter and up to several thousand meters in 

 depth. They form when southerly directed Gulf 

 Stream meanders become so accentuated as to 

 separate from the Stream and move south, enclos- 

 ing a core of cold and relatively fresh slope water 

 within a remnant of the Gulf Stream (Parker 

 1971; Fuglister 1972; Richardson 1976). It is likely 

 that in the northern Sargasso Sea, at any one time, 

 there are 10 to 15 such rings (Lai and Richardson 



'Contribution No. 3939 from the Woods Hole Oceanographic 

 Institution. This paper is part of a thesis submitted by P. B. 

 Ortner for a Ph.D. degree from the Woods Hole Oceanographic 

 Institution. 



^Woods Hole Oceanographic Institution, Woods Hole, MA 

 02543. 



Manuscript accepted September 1977. 

 FISHERY BULLETIN; VOL. 76, NO. 2, 1978. 



1977). Estimating the surface area of the northern 

 Sargasso Sea as 32.9 x 10^ km2 (Jahn 1976), cold 

 core rings may cover between 6 and 13*^ of this 

 surface. (Throughout the ensuing sections, unless 

 otherwise indicated, the terms ring, slope water, 

 and Sargasso Sea denote hydrographic, not geo- 

 graphic, entities.) 



An overview of the phytoplankton, zooplankton, 

 and midwater fish populations inhabiting cold 

 core rings has been given by Wiebe, Hulburt, Car- 

 penter, Jahn, Knapp, Boyd, Ortner, and Cox 

 (1976). The results of that study indicated that 

 mean zooplankton biomass in the upper 750-800 m 

 of rings between 3 and 10 to 12 mo of age was 

 consistently higher than that in the surrounding 

 Sargasso Sea. In these preliminary data the frac- 

 tion of biomass below 250-300 m in depth was 

 particularly large while the near surface was more 

 similar to the Sargasso Sea. We have now taken 

 vertically stratified hauls in the same ring 3 mo 

 apart. The data from these hauls confirm our ini- 

 tial interpretation. 



The objective of this paper is twofold. First, we 

 describe the zooplankton biomass distributions 

 characteristic of the northern Sargasso Sea, of a 

 cold core ring, and to a lesser extent of the slope 

 water — the source of ring water. Second, we will 

 attempt to relate the patterns observed to sys- 

 tematic variations in phytoplankton standing 



323 



