Turbulent nutrient fluxes in the Iceland Basin

As part of a multidisciplinary cruise to the Iceland Basin in July–August 2007, near to the historical JGOFS Ocean Weather Station India site (∼59° N, ∼19° W), observations were made of vertical turbulent nutrient fluxes around an eddy dipole, a strong mesoscale feature consisting of a cyclonic eddy and an anti-cyclonically rotating mode-water eddy. Investigation of the spatial distribution of vertical turbulent diffusivity around the dipole shows an almost uniform horizontal distribution despite the strong horizontal gradients in water velocity and density observed. An area mean turbulent diffusivity was calculated as 0.21 (95% confidence interval: 0.17–0.26)×10−4 m2 s−1 at the base of the euphotic zone. The vertical turbulent fluxes of three major macro-nutrients into the euphotic zone were calculated as 0.13 (95% confidence interval 0.08–0.22) mmol m−2 day−1 for nitrate, 0.08 (0.05–0.12) mmol m−2 day−1 for silicate and, 8.6 (13.0–5.2 )×10−3 mmol m−2 day−1 for phosphate. The vertical turbulent flux of dissolved iron (dFe) into the euphotic zone was calculated to be 2.6 (95% confidence interval 1.3–4.3)×10−6 mmol m2 day−1. Turbulent macro-nutrient flux is estimated to contribute up to 14% of the deep winter mixing supply of silicate, nitrate and phosphate in the region. The magnitude of turbulent dFe flux is estimated to be at most 8% of the deep winter mixing supply of dFe. Deep winter mixing is hypothesised to supply an adequate amount of iron to fully utilise the deep winter mixed supply of silicate but not the deep winter mixed supply of nitrate. This suggests that while the iron supply may not limit the magnitude of the spring bloom, iron limitation may be occurring post bloom.