Evolution of the phytoplankton assemblage in a long-lived mesoscale eddy in the eastern Gulf of Alaska

We tracked changes in the phytoplankton species composition in an anticyclonic mesoscale eddy (Haida-2000a) in the eastern Gulf of Alaska (GOA) four times over a 20-month period as it propagated westward away from the coast and into high nitrate, low chlorophyll (HNLC) waters. Phytoplankton species diversity in the eddy was higher in late summer/early autumn (September) compared to spring (June) and it declined between the first and second years of evolution. Eddy diversity exceeded that in the surroundings in September (but not in June) in both the first and second years of evolution. Small cells dominated the phytoplankton assemblages throughout the study at all sites. The prevalence of coastal species in the eddy and its surroundings declined between the first and second years of eddy evolution. During the same period, the contribution of haptophytes and pelagophytes increased. Haida-2000a had lower abundances of diatoms compared to its surroundings after one year of evolution, possibly due to the preferential export of silica from the eddy. Pigment and phytoplankton distributions indicated that edge sites differed from the center and outside, either due to the advective entrainment of surrounding waters into eddy circulation, or as a consequence of local upwelling at the eddy margins. The data suggest that eddies may respond differently than surrounding waters to external forcing including storms and iron deposition events, thus they contribute to variability and large scale patchiness of phytoplankton populations observed in the GOA.