Macro‐scale exhaustion of surface phosphate by dinitrogen fixation in the western North Pacific

Abstract

In the subtropical oceans, nutrient concentrations are frequently below the detection limits of standard analytical methods. We applied a highly sensitive method to the surface water of the western and central Pacific between 42°N and 40°S and between 141°E and 158°W except in the equatorial zone, and detected overall depletion of nitrate + nitrite and an excess of SRP. However, a remarkable exception was found: an almost complete exhaustion of SRP (<10 nM) existed at a horizontal scale of >2000 km in the western subtropical North Pacific in both summer and winter. The SRP exhaustion was a consequence of an elevated dinitrogen fixation, which occurred in areas with high dust deposition from the Asian continent that likely enhanced SRP consumption. A coupling among nutrient dynamics, dinitrogen fixation and dust deposition produces the extremely low P availability spanning a large area, which appears to be unique to the western North Pacific.