The influence of coastal origin eddies on oceanic plankton distributions in the eastern Gulf of Alaska

Abstract

Mesoscale anticyclonic eddies that form along the eastern continental margin of the Gulf of Alaska in winter transport large quantities of coastal water to the open ocean. The Continuous Plankton Recorder (CPR) sampled repeated transects that passed close to, or through these eddies in 2000 and 2001. Shelf species of diatoms and calanoid copepods were recorded in CPR samples within, or in close proximity to eddies and they persisted through the sampling period. Several oceanic taxa also showed higher concentrations within the eddies. Significantly reduced occurrences of shelf taxa in oceanic samples were seen in 2001, coinciding with a weaker eddy than observed in 2000. Images of ocean temperature and chlorophyll-a, prepared from satellite observations, reveal changes in offshore transport mechanisms by eddies between winter and spring, and between southern and northern eddies in this region. These effects may also explain some of the seasonal and geographical variability in the CPR samples. Between April and June of 2000 the Haida Eddy straddled the low chlorophyll-a waters of the gulf and the high-chlorophyll-a waters of the continental margin, and entrained each water type into the neighbouring region. The resulting swirling patterns were clearly observed in SeaWiFS imagery. Sitka eddies failed to penetrate into low chlorophyll-a waters during these months and were less effective in stirring high and low chlorophyll-a concentrations. Surface currents in surrounding waters, simulated by the ocean surface current simulations (OSCURS) model, were used to provide some insights into the likely fate of planktonic organisms once they leave the eddies. We show that these eddies may play a significant role in distributing plankton species.