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 occan 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 degrees N 145 degrees 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. (c) 2005 Elsevier Ltd. All rights reserved.