Non-redfieldian mesopelagic nutrient remineralization in the eastern North Atlantic subtropical gyre

The eastern boundary of the North Atlantic subtropical gyre (NASTG) receives organic materials produced in the nearby Canary upwelling system (CanUS), which has important implications for the metabolic balance of the region. Here we computed absolute geostrophic, Ekman and eddy diffusive transports and net budgets of oxygen, nitrate, and phosphate, with an inverse model applied to annual climatologies from the World Ocean Atlas 2013, in a large box covering the eastern boundary of the NASTG. The surface waters (<100 m) of the box exported 4.2 ± 1.1 mol O₂ m^-2 yr^-1 to the adjacent ocean and atmosphere, and imported 0.40 ± 0.17 mol N m^-2 yr^-1 and 22.2 ± 9.5 mmol P m^-2 yr^-1 of nitrate and phosphate, respectively, indicating that net autotrophy prevailed in the euphotic waters of the eastern NASTG. The central, intermediate and deep waters (100 m-bottom) imported 6.7 ± 3.3 mol O₂ m^-2 yr^-1, and exported 1.05 ± 0.35 mol N m^-2 yr^-1 and 50.2 ± 21.8 mmol P m^-2 yr^-1 of nitrate and phosphate, respectively, indicative of net remineralization. Mesopelagic remineralization rates exceeded epipelagic net community production, implying a net deficit of carbon and nutrients in the region. Remineralization occurred at high N:P molar ratios (∼ 28) in central waters, and thus, a excess nitrate (DINxs = NO₃-16 · PO₄) was produced at a rate of 2.2 ± 1.6 ×10¹¹ mol N yr^-1. This excess nitrate was exported to the gyre interior, influencing the biogeochemistry of the whole NASTG and leaving the eastern margin with a strong nitrogen deficit, relative to carbon and phosphorus. According to the available literature, the organic carbon export from the CanUS could be sufficient to account for the carbon deficit, but the source of nitrogen remains elusive.