Macronutrient dynamics in an anticyclonic mesoscale eddy in the Gulf of Alaska

Each winter, long-lived anticyclonic eddies are spawned off the southern tip of the Queen Charlotte Islands off the British Columbia coast. An anticyclonic eddy that formed in winter 2000 (Haida-2000) was sampled six times over 20 months as it traveled from the coast into High Nitrate-Low Chlorophyll waters of the Subarctic northeast Pacific. Repeated shipboard observations coupled with satellite radar altimetry and ocean colour suggest that Haida-2000 underwent a phytoplankton bloom early in life while still in coastal waters. This bloom caused a near depletion in eddy surface nutrients (nitrate, phosphate, silicic acid). While nitrate concentrations were restored to initial levels during winter 2001, the silicic acid inventory within Haida-2000 remained lower than initial observations. Below the euphotic zone, deep nutrient concentrations were altered by eddy decay, interactions with bathymetric features, and by the coalescence of a second, younger eddy that restored coastal characteristics within the core of Haida-2000. Estimates of new production (3–3.5 mmol NO3− m−2 d−1) derived from seasonal changes in nitrate inventories fell between values previously reported for coastal and mid-gyre environments for both years studied. In contrast, removal of silicic acid was twice as high (7.0 mmol Si(OH)4 m−2 d−1) as nitrate during the first year, but less than half as high in Year 2 (1.3 mmol Si(OH)4 m−2 d−1). Changes in the timing of nutrient drawdown accompanied the shift from high to low Si(OH)4:NO3− drawdown ratios, with the maximum changing from spring to autumn, similar to long-term observations at Ocean Station P (50°N, 145°W). Relative to the local environment, the eddy evolved from a nutrient-rich to a nutrient-poor body of water, indicating that the path of these anticyclonic eddies determines their role in nutrient supply and distribution in the Gulf of Alaska.