Nutrient uptake over Koko Seamount, North Pacific

Abstract

Water temperature and salinity profiles were measured on April 9–10, 2019 above the summit of Koko Seamount (35^o15’ N 171^o35’ E, with minimal depth of 260 m) and at the slope, with water sampling to measure nutrients concentration, namely dissolved inorganic phosphorus (DIP) and nitrite and nitrate summed as dissolved inorganic nitrogen (DIN). Both DIP and DIN decreased from depth to sea surface proportionally to salinity increasing, with some deviations from this inverse relationship, which were interpreted as the nutrient uptake for the synthesis of new organic matter. These deviations were mostly minor, apparently due to weak density stratification that prevented active phytoplankton growth, but increased up to 0.21 μmol/L for phosphorus and 4.8 μmol/L for nitrogen at 100 m depth above the southern slope and to 0.13 μmol/L and 1.7 μmol/L, respectively, at the summit of seamount. In the first case, the uptake was presumably caused by photosynthesis in the upward flow that transferred nutrient-rich water into the photic layer and simultaneously retained phytoplankton cells in this layer. In the second case, the nutrients were consumed by phytoplankton which were sunk to the lower part of the thick mixed layer but still had sufficient light for photosynthesis at the top of the seamount. The arrangement and characteristics of these productive areas do not contradict to the Taylor column hypothesis that a stream induces an anticyclonic eddy over a seamount with downward flow in the centre and compensating upward flows at the periphery, although such water structure is not observed in the fields of temperature and salinity, possibly because of strong vertical mixing. The N:P ratio in utilized nutrients was inclined to greater nitrogen uptake in the first case and greater phosphorus uptake in the second case, relative to the Redfield ratio. As a result, the DIN:DIP ratio in water decreased above the southern slope and increased above the eastern slope located near the summit, downstream of the Kuroshio Extension. These results approve the thesis on heightened bioproductivity of seamounts and explain one of its possible mechanism.