Nitrogen dynamics within a wind-driven eddy

Abstract

Abstract 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 (NO 3 − ), particulate carbon (PC), particulate nitrogen (PN), their stable isotope compositions ( δ 15 N-NO 3 − , δ 13 C-PC and δ 15 N-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 NO 3 − 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 NO 3 − assimilation, we estimated that 213±59 mmol m −2 of NO 3 − was initially injected into the upper 110 m Cyclone Opal formation, implying that NO 3 − was assimilated at a rate of 3.75±0.5 mmol N m −2 d −1 . This injected NO 3 − 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 NO 3 − 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 NO 3 − 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 3 − . 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 NO 3 − was shunted into the DON pool, and 32±6% is unassimilated, then 46±16% of the injected NO 3 − remains undocumented. Alternative loss processes within the eddy include lateral exchange of injected NO 3 − 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 δ 15 N-PN, implying a rapid change in the N source. A change in NO 3 − assimilation, or a shift from NO 3 − fueled growth to assimilation of a 15 N-deplete N source, may be responsible for such observations.