Distributions and stoichiometry of dissolved nitrogen and phosphorus in the iron fertilized region near Kerguelen (Southern Ocean)

Abstract

Abstract. During KEOPS2 (Kerguelen Ocean and Plateau Compared Study 2), we determined dissolved inorganic and organic nitrogen and phosphorus species in the naturally fertilized region of Kerguelen Island (Southern Ocean). Above 150 m, stations were clearly separated by the Polar Front (PF), with concentrations of NO3–, NO2– and PO43– overall lower north than south of the PF. Though less pronounced, a similar trend was detectable for dissolved organic nitrogen (DON) and phosphorus (DOP). At all stations offshore and above the plateau, a subsurface maximum of NH4+ was observed between 50 and 150 m. We examined nutrient stoichiometry by calculating the linear combination N* = [NO3–] − 16[PO43– ]. The majority of stations and depths revealed N* close to −3 μM, however, for surface waters north of the PF N* increased up to 6 μM. This suggests a preferential uptake of PO43– vs. NO3– by fast growing diatoms. Using the tracer TNxs = [TDN] − 16[TDP] revealed that the dissolved organic fraction significantly contributed to changes in TNxs. TNxs were negative for most stations and depths, and relatively constant in the layer 0–500 m. As for N*, the stations north of the PF had higher TNxs in the layer 0–100 m. We discuss this stoichiometric anomaly with respect to possible external sources and sinks of N and P. Additional data collected in February 2013 at two sites revealed the occurrence of a subsurface of N* located just below the pycnocline that denotes a~layer where preferential remineralization of P vs. N persists throughout the season.