Edge enrichment in an ocean eddy

Abstract

In October 1981, relatively high concentrations of surface phytoplankton (chlorophyll a~1.5 µg1^-1 relative to a background of ~0.4 µg 1^-1) were observed in a cyclonic crescent of cool water along the south-eastern margin of a warm-core eddy in the south-westem Tasman Sea. Associated with this phytoplankton peak were relatively high concentrations of surface nitrate (~50µg I^-1), low concentrations of oxygen (<90% satn), and a population of a copepod (Calanoides carinatus) often associated with upwellings. The salinity, oxygen and nitrate distributions suggest that an upwelling-downwelling circulation cell existed at the interface between (anticyclonic) eddy and (cyclonic) crescent. Biological enrichment was greater at the site of the crescent than where the margin of the eddy was comparatively straight. The hypothesis is advanced that such enrichment may be the basis for the association that exists between schools of southern bluefin tuna and sharp surface temperature fronts.