Biogeochemical evolution of the carbon dioxide system in the waters of long-lived mesoscale eddies in the Northeast Pacific Ocean

Abstract

Abstract An anticyclonic eddy, named Haida-2000, formed off the southern Queen Charlotte Islands early in the year 2000. The eddy subsequently tracked northwest, transporting fresh, warm, and nutrient-rich water into the Northeast Pacific Ocean. Based on total dissolved inorganic carbon (C T ), total alkalinity (A T ), and nutrients from six seasonal cruises between February 2000 and September 2001, we found that the oceanic carbonate system of the surface waters in the eddy changed significantly. The greatest loss of C T from the surface waters of Haida-2000 occurred during the first year, between February and June 2000. Based on a concomitant loss in nitrate (NO 3 ), the large C T loss we observed was likely due to biological production of organic carbon and probably resulted in a large drawdown of atmospheric CO 2 , as supported by the observed fugacity of carbon dioxide ( f CO 2 ) in the surface waters. During fall, C T , f CO 2 , and NO 3 values increased, probably due to oxidation of organic matter and vertical entrainment of C T and nutrient-rich water from below with fall mixing. As a result, the surface waters of the eddy were highly oversaturated in CO 2 relative to the atmosphere. Another Haida eddy (Haida-2001) had a significantly smaller nitrate drawdown than Haida-2000 during its first spring, but the C T losses were similar in the two eddies. In early summer (mid-June) of the second year, the surface and sub-surface waters in Haida-2000 gained C T , A T , and NO 3 , partly caused by a merging with another 2001 Haida eddy, showing that exchange with surrounding waters took place and was significant at times. The high nitrate/low chlorophyll (HNLC) surface waters at Ocean Station Papa (OSP), showed less f CO 2 undersaturation during summer and less total nitrate and C T loss than Haida-2000, indicating that both eddies were larger sinks for carbon than HNLC waters. The waters surrounding the eddies generally had concentrations of C T , A T , and NO 3 that were similar to or slightly higher than the waters of both Haida eddies throughout the study. Also, the sub-surface waters in these eddies showed no significant change in total carbon content, suggesting that at these depths eddies are recycling, not exporting systems.