On the Importance of Lateral Nutrient Transport: A Shift in the New Production Paradigm for the Subtropical Ocean Gyres

A widely used assumption for estimating the strength of the organic carbon export flux is that one-dimensional vertical processes dominate the supply and loss of nutrients to the euphotic zone. However, for the North Atlantic subtropical gyre, lateral inputs of nutrients by convergent Ekman transport have been suggested to be important. Here we use a biogeochemical ocean circulation model constrained by dissolved organic matter concentration measurements to show that on annual timescales, lateral transport of organic and inorganic nitrogen (N) and phosphorus (P) from the gyre margins and its subsequent biological utilization supplies a flux that can exceed vertical nutrient supply across all subtropical gyre systems. Lateral nutrient transport supplies 25-40% of the N and 40-70% of the P necessary to balance new and export production in these regions. We also find that the proportion of the nutrient supply sustained by lateral transport in the gyres is strongly correlated with ecosystem stoichiometry, exhibiting high N:P and C:P within the gyres receiving the most nutrients laterally. Our results suggest that the predicted future decline in biological carbon export due to decreasing vertical inputs of nutrients to surface waters might be partly offset by a concomitant increase in ecosystem carbon to nutrient ratios accompanying a shift to an increased importance of lateral nutrient inputs in the subtropical gyres.