Impact of Haida Eddies on chlorophyll distribution in the Eastern Gulf of Alaska

Abstract

Mesoscale Haida eddies influence the distribution of surface phytoplankton in the eastern Gulf of Alaska through two processes: enhanced productivity in central eddy water, and seaward advection of highly productive coastal waters in the outer rings of eddies. These two processes were observed in a sequence of monthly images over five years, for which images of SeaWiFS-derived chlorophyll distributions were overlaid by contours of mesoscale sea-surface height anomaly derived from TOPEX and ERS-2 satellite observations. Satellite measurements were supplemented with ship-based chlorophyll observations through one of the eddies. Haida eddies are deep, anticyclonic, mesoscale vortices that normally form in winter and early spring near the southwest coast of the Queen Charlotte Islands. High levels of chlorophyll observed in eddy centres indicated that they supported phytoplankton blooms in spring of their natal years, with timing of these blooms varying from year to year and exceeding in magnitude the chlorophyll concentrations of surrounding water. Elevated chlorophyll levels also were observed in eddy centres in late summer and early autumn of their natal year. Enhanced chlorophyll biomass is attributed to higher levels of macro-nutrients and higher levels of iron enclosed within eddies than in surface, deep-ocean water. By late spring and summer, when coastal water supported higher chlorophyll biomass than did oceanic offshore regions, eddies that straddled the continental margin entrained high chlorophyll coastal water into their outer rings and carried it several hundred kilometres into the Gulf of Alaska along their southern sides. On some occasions a deep-ocean eddy would entrain chlorophyll from an adjacent eddy located closer to the coast, forming a conveyor-belt transport process to inject coastal biota into the deep-sea region of the gulf. This process extended the coastal region of high-chlorophyll surface water (and therefore, phytoplankton-rich water) several hundred kilometres seaward and dominated the shelf-to-deep-ocean exchange of chlorophyll from late winter to the following autumn.