Sources of nitrogen for primary production in warm - core rings 79 - E and 8 1 - D 1

Profound changes in concentrations of phytoplankton biomass and rates of primary production in warm-core Gulf Stream rings can occur, apparently in response to nutrient enrichment, within a few months after a ring separates from the Gulf Stream. In Ring 8 1 -D at 3 months of age the dependence on NO,was 2%3 times as high as in the northern Sargasso Sea and in Ring 79-E shortly after formation. The N03gradient at the base of the mixed layer in the new Ring 79-E was 0.09 pmol kg-* m-l, whereas that for the older Ring 8 1 -D was 0.30. The increase in proximity ofNO,to the mixed layer may be a general phenomenon associated with the aging process of warm-core rings. The increase in this gradient enhances the potential for episodic bursts of production in warm-core rings in response to turbulent mixing at the base of the mixed layer. Following an interaction with the Gulf Stream and a gale force storm, the mixed layer of Ring 8 1 -D cooled and its NO,content increased. In response to increased availability of NO,-, both the rate of phytoplankton utilization of N03and the rate of primary production increased. Integrated rates for N03uptake in the mixed layer increased sevenfold (from 28 to 195 pmol m-2 h-1 1. Meanders of the Gulf Stream regularly contribute to the formation of coldand warm-core eddies, or rings, in the northwest Atlantic Ocean (Robinson 1983). Cold-core rings occur in the Sargasso Sea and contain central masses that are of Slope Water origin, whereas warm-core rings occur in the Slope Water and contain central masses that are of Sargasso Sea origin. Both are surrounded by a high velocity region (HVR) which consists of remnants of the Gulf Stream. In old cold-core rings the surface thermal contrast between the ring and the surrounding water can be lost as the surface waters of the ring warm. However, for warmcore rings strong thermal contrast usually persists until the ring is reabsorbed by the Gulf Stream, some months after formation. The ease with which warm-core rings can be tracked with satellite infrared imagery during their migration southwest through the Slope Water region is a feature that facilitates the use of these mesoscale eddies in the study of plankton nutrition (Joyce and Wiebe 1983). For cold-core rings the evolution of several biological properties, particularly zooplankton abundance, has been well documented in recent years (Ring Group 198 1). The pattern that emerges is for young rings to have features that resemble the Slope Water, while old rings more closely resemble the Sargasso Sea. It can by hypothesized that the observed transition to a more oligotrophic condition with time in the core of a cold-core ring is related to the availability of nutrients for primary production. If this is correct, then, for reasons that are not well understood, the degree of nutrient limitation must increase with time until the central region of the aging cold-core ring resembles the typical conditions of the western or northern Sargasso Sea. In general this progressive change in biological properties has been presumed to occur relatively smoothly, since cold-core rings of intermediate age have been observed to be intermediate in character. This generalization holds for the biomass and species composition of phytoplankton and zooplankton, and for primary productivity (Wiebe et al. 1976; Ring Group 198 1). Before the Warm-Core Ring Program there were no comparable data for transitions in biological properties and processes within warm-core rings. There was some I This work was supported by NSF grants OCE 78indication in remotely sensed data from the 26011 and OCE 80-22990. Coastal Zone Color Scanner (CZCS) that