Origin and maintenance of a high nitrate condition in the equatorial Pacific

Abstract The physical and biological causes for the equatorial nutrient anomaly were investigated using an ecosystem model embedded within an ocean general circulation model to determine the nitrate budget for the equatorial Pacific Ocean. In the 140°W region the effects of mixing on nitrate concentration were small compared to the effects of advection: upwelling and zonal transport to the east in the Equatorial Undercurrent were the major processes in the nitrate budget. At 140°W on the equator annual JNO3, the total net physical supply of nitrate to the euphotic layer, was 3.76 mmol m−2 day−1; the vertical integrated (0–120m) new production calculated from the ecosystem model was 3.36 mmol m−2 day−1 or, in carbon units, 22.26 mmol C m−2 day−1. The vertical supply of nitrate (−w∂NO3/∂z) due to the upwelling is controlled by two factors, the vertical velocity and vertical gradient of nitrate concentration. The vertical velocity reaches the maximum during climatological fall, but the vertical gradient of nitrate is weaker in the fall. Therefore, the vertical supply of nitrate is smaller than in spring. To investigate the role of physiological limitation of phytoplankton photosynthesis and specific growth rate on the maintenance of the high nutrient-low chlorophyll (HNLC) condition, a model experiment was performed that included, unchanged from previous model runs, the physical conditions and density-dependent grazing function, but greatly reduced physiological limitations by increasing α (initial slope of P-I curve) and Pmax (maximum specific growth rate) values. When this was done, vertical integrated primary production at 140°W on the equator doubled (from 83 to 166 mmol C m−2 day−1), but the zooplankton grazing on the phytoplankton also doubled (from 75 to 150 mmol C m−2 day−1). Zooplankton biomass doubled, but there was only a slight increase in phytoplankton biomass; no phytoplankton bloom formed in this model experiment. With potential physiological limitations of phytoplankton rates greatly reduced, the characteristic equatorial plume of unused surface layer nitrate still persisted; but the nitrate-rich plume was smaller in horizontal extent and the maximum concentration was reduced by half from observed concentrations. While the reduction in the extent of the nitrate-rich plume indicates that physiological limitation plays a significant role in the maintenance of the nutrient anomaly, its persistence demonstrates that physical processes and grazing also are involved.