Excess nitrogen as a marker of intense dinitrogen fixation in the Western Tropical South Pacific Ocean: impact on the thermocline waters of the South Pacific

Abstract. As part of the Oligotrophy to UlTra-oligotrophy PACific Experiment cruise, which took place in the Western Tropical South Pacific during the austral summer (March–April 2015), we present data on nitrate, phosphate and on particulate and dissolved organic matter. The stoichiometric nitrogen-to-phosphorus ratios of the inorganic and organic material and the tracer N^* are described. N^* allows to trace changes in the proportion of fixed nitrogen due to diazotrophy and/or denitrification. Our results showed that the Melanesian archipelago waters between 160° E and 170° W are characterized by a deficit of nitrate and phosphate in the productive layer, significant dinitrogen fixation rates and an excess of particulate organic nitrogen compared to the canonical ratio of Redfield. A positive N^* anomaly was observed in the productive layer reflecting the combined effect of phosphate uptake by diazotrophic organisms and remineralization of excess particulate organic nitrogen. The South Pacific Gyre waters between 170° W and 160° W were depleted in nitrate but rich in phosphate. Surface waters exhibited very low dinitrogen fixation rates, an absence of excess particulate organic nitrogen and a N^* signal close to zero. The higher iron availability coupled with an absence of nitrate in the suface water of the Melanesian archipelago could stimulate the diazotrophic activity, which in turn will introduce excess nitrogen, deplete the surface waters in phosphate and be the explanation for the positive N^* anomaly in the Melanesian archipelago waters. In the thermocline waters, the N^* tracer revealed its full complexity, with notably the cumulative effect of the remineralization of particulate organic nitrogen and the effects of the mixing of water masses. At the global ocean scale, calculation of N* signal from the new Global Ocean Data Analysis Project version 2 database showed a strong spatial decoupling between the thermocline waters of the Eastern Tropical South Pacific and those of the Western Tropical South Pacific. A strongly positive N^* anomaly was observed in the thermocline waters of the Western Tropical South Pacific in the Coral/Tasman Seas and in the southern part of the subtropical gyre between latitude 23° S and 32° S. A strong negative N^* signal was observed in the waters of the Eestern Tropical South Pacific between latitude 5° S and 20° S–23° S. We hypothesise that the nitrogen excess observed in the thermocline waters of the Western Tropical South Pacific is transported eastward and then northward by the circulation of the South Pacific subtropical gyre and could influence positively the thermocline waters of the South Pacific being thus at the origin of the westward increase of the strongly negative N^* signal transported by the South Equatorial Current.