Iron transport by mesoscale Haida eddies in the Gulf of Alaska

Abstract

Abstract A two-year study of the iron distributions in anticyclonic mesoscale Haida eddies of the eastern North Pacific Ocean has shown that such eddies are a major source of iron to the high nitrate–low chlorophyll (HNLC) waters of the central Gulf of Alaska basin. These eddies, which are typically about 100 km in diameter, form off the west coast of Canada in winter and then track roughly westward into the open ocean. Therefore, they carry large quantities of iron-rich coastal waters into iron-limited waters of the oligotrophic gyre. When the eddies had first formed, their dissolved iron concentrations were nearly two orders of magnitude higher than is typically observed at Station Papa (50°N, 145°W; 3 n m versus less than 0.05 n m ), and the total iron in the eddies surface waters (10–40 m) was more than 13 n m higher than open ocean waters of the Alaska gyre (14 n m versus 0.5 n m ). While the overall iron content of the eddies decreased rapidly during the first year after they formed, the eddy we were able to track the longest still contained 1.5–2 times more iron than the surrounding waters 16 months after its formation. Therefore, although iron concentrations in the surface mixed layers of the eddies drop to levels observed outside very rapidly (within 4 months), upward transport along isopycnals and upwelling due to eddy decay, as well as vertical advective and diffusive transports typical of North Pacific waters, probably provide steady fluxes of iron into the euphotic zone from the still iron-rich eddy core waters throughout the lifetime of the eddy.