Role of Gulf Stream Frontal Eddies in Forming Phytoplankton Patches on the Outer Southeastern Shelf

Continuous surface mapping of temperature, salinity, and chlorophyll along a 300-km segment of the Gulf Stream cyclonic front defined the spatial scales of a large diatom patch that persisted throughout a 10-day study. The patch was localized in the upwelled cold core of a Gulf Stream frontal eddy centered over the 200-m isobath off Jacksonville, Florida, in April 1979. The 2 μg· liter1 surface chlorophyll isopleth enclosed an area > 1,000 km• with an alongshore dimension of 130 km. Surface chlorophyll exceeded 5 μ.g· liter-1 within the upwelled cold core of the eddy, 10-lOOx higher than concentrations in Gulf Stream or resident shelf surface water. Diatoms dominated the patch with the maximum observed abundance > 106 cells· liter'. Several days after the initial shipboard mapping, the size, location, and strong chlorophyll gradients of the patch were confirmed with a surface chlorophyll image generated from an ocean color scanner (OCS) flown aboard a NASA U-2 aircraft. We show that the upwelling associated with eddies forming along the Gulf Stream cyclonic front results in localized zones of high near-surface production and plant biomass that lie adjacent to oligotrophic surface waters of the Gulf Stream. Seasonal phytoplankton blooms occur on the continental shelf north of Cape Hatteras, N.C. (Riley 1947, 1959; Smayda 1973, 1976; Walsh et al. 1978), but not on the outer southeastern continental shelf (Haines and Dunstan 1975; Bishop et al. 1980). Blooms in the south are shortlived, aperiodic, and are associated with episodic upwelling and intrusions of North Atlantic Central Water (NACW) onto the shelf (Dunstan and Atkinson 1976; Atkinson et al. 1978). Studies have shown that high phytoplankton biomasses are generally within nutrient-enriched subsurface layers which underlay a surface mixed layer characterized by low chlorophyll ( <0.5 μ,g · liter-1) and nutrients. 1 This work was supported by U.S. Department of Energy contracts EY-76-S-09-0936, EY-76-S-090889, Ey-76-S-05-5163, and by the National Aeronautics and Space Administration. Shiptime w as provided under NSF grant OCE76-0ll42 to G.-A. Paffenhi",fer. During the seasons (winter and spring) of high temperature contrast between surface Gulf Stream (warm) and shelf (cold) water, satellite thermal images show meanders and other disturbances of the Gulf Stream cyclonic front. A common type of disturbance looks like a "finger'' of warm water emanating from the stream which folds hack to enclose a core of colder water. From satellite images Vukovich et al. (1979) showed that off the north Florida coast these features have average lengths of 136 km, widths of 36 km, and propagate north along the front with an average speed of 30 km· d1. Earlier these features were referred to as Gulf Stream spin-off eddies (e.g. Lee 1975) hut are now called frontal eddies (Lee et al. 1981). An important point is that these frontal eddies are distinct from the larger cold-core Gulf Stream rings that detach from the stream north of Cape Hatteras and move into the Sargasso Sea. The cold core of a frontal eddy is a result of upwelling of NACW (Lee et al. 1981), not of entrainment of a resident slope