Inorganic Nutrient Diffusion at the Base of the Nutricline and its Association with the Deep Chlorophyll a Maximum Layer

Even though the dynamics leading to the formation and maintenance of deep chlorophyll maxima have not been completely unveiled, there is some consensus that the community residing in these layers sits at the minimum light level that allows the complete assimilation of vertical diffusive fluxes of growth-limiting nutrients. To better understand this process, we performed concomitant high vertical resolution measurements of physical motions, nutrient concentrations, and community composition on board an oceanographic cruise in the North Pacific subtropical gyre. We found that positive rates of nutrient diffusion were localized in a narrow layer situated close to the depth of the chlorophyll maximum (and approximately in the same position for nitrate+nitrite, phosphate, and silicate). Computed volumetric rates of nutrient diffusion could locally account for the entire phytoplankton requirement of nitrogen and phosphorus based on historical measurements of primary production at a depth of 125 m. Moreover, we consistently observed a layer with negative rates of nutrient diffusion lying below the layer of positive diffusion, suggesting that high rates of mineralization took place right below the base of the nutricline and maintained steep concentration gradients. In a time span of 2 days, we observed significant variations in the isopycnal nutrient distribution suggesting the importance of advection and the presence of horizontal variability in the position of the base of the nutricline.