Role of Gulf Stream frontal eddies in forming phytoplankton patches on the outer southeastern shelf1

Abstract

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·liter⁻¹ surface chlorophyll isopleth enclosed an area >1,000 km² with an alongshore dimension of 130 km. Surface chlorophyll exceeded 5 µg·liter⁻¹ within the upwelled cold core of the eddy, 10–100× higher than concentrations in Gulf Stream or resident shelf surface water. Diatoms dominated the patch with the maximum observed abundance >10⁶ cells·liter⁻¹. 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.