Nitrogen dynamics within a wind-driven eddy

Wind-driven cyclonic eddies are hypothesized to relieve nutrient stress and enhance primary production by the upward displacement of nutrient-rich deep waters into the euphotic zone. In this study, we measured nitrate (NO3-), particulate carbon (PC), particulate nitrogen (PN), their stable isotope compositions (delta N-15-NO3-, delta C-13-PC and delta N-15-PN, respectively), and dissolved organic nitrogen (DON) within Cyclone Opal, a mature wind-driven eddy generated in the lee of the Hawaiian Islands. Sampling occurred in March 2005 as part of the multi-disciplinary E-Flux study, approximately 4-6 weeks after eddy formation. Integrated NO3- concentrations above 110 m were 4.8 times greater inside the eddy (85.8 +/- 6.4 mmol N m(-2)) compared to the surrounding water column (17.8 +/- 7.8 mmol N m(-2)). Using N-isotope derived estimates of NO3- assimilation, we estimated that 213 +/- 59 mmol m(-2) of NO3- was initially injected into the upper 110 m Cyclone Opal formation, implying that NO3- was assimilated at a rate of 3.75 +/- 0.5 mmol N m(-2) d(-1). This injected NO3- supported 68 +/- 19% and 66 +/- 9% of the phytoplankton N demand and export production, respectively. N isotope data suggest that 32 +/- 6% of the initial NO3- remained unassimilated. Self-shading, inefficiency in the transfer of N from dissolved to particulate export, or depletion of a specific nutrient other than N may have led to a lack of complete NO3- assimilation. Using a salt budget approach, we estimate that dissolved organic nitrogen (DON) concentrations increased from eddy formation (3.8 +/- 0.4 mmol N m(-2)) to the time of sampling (4.0 +/- 0.09 mmol N m(-2)), implying that DON accumulated at rate of 0.83 +/- 1.3 mmol N m(-2) d(-1), and accounted for 22 +/- 15% of the injected NO. Interestingly, no significant increase in suspended PN and PC, or export production was observed inside Cyclone Opal relative to the surrounding water column. A simple N budget shows that if 22 +/- 15% of the injected NO3- was shunted into the DON pool, and 32 +/- 6% is unassimilated, then 46 +/- 16% of the injected NO3- remains undocumented. Alternative loss processes within the eddy include lateral exchange of injected NO3- along isopycnal surfaces, remineralization of PN at depth, as well as microzooplankton grazing. A 9-day time series within Cyclone Opal revealed a temporal depletion in delta N-15-PN, implying a rapid change in the N source. A change in NO3- assimilation, or a shift from NO3- fueled growth to assimilation of a N-15-deplete N source, may be responsible for such observations. (C) 2008 Elsevier Ltd. All rights reserved.