Nutrient utilization ratios in the Polar Frontal Zone in the Australian sector of the Southern Ocean: a model

To investigate the non-Redfield N/P depletion ratio in the Polar Frontal Zone (PFZ) of the Southern Ocean, we simulated the seasonal nitrate, phosphate, and silicate cycle in the upper ocean with a biophysical model. Total phytoplankton biomass was prescribed from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) estimates, and we included two phytoplankton types, diatoms and nondiatoms. We set the nondiatoms N/P uptake ratio to 16 while the diatoms N/P and N/Si ratios were determined by fitting the observed seasonal nitrate, phosphate, and silicate cycle in the mixed layer. The best model fit to the observations required an annual N/P utilization ratio of 13.2, but this low N/P ratio still overestimated the nitrate utilization during the summer. We considered three mechanisms for improving the simulated nitrate cycle: (1) seasonal variation in the N/P ratio of the horizontal nutrient supply to the PFZ, (2) different remineralization length scales for particulate organic nitrogen (PON) and particulate organic phosphorus (POP), and (3) seasonal accumulation and decomposition of labile dissolved and suspended organic matter (OM). Model simulations showed that the seasonal variability in the N/P ratio of horizontal supply failed to reduce the simulated excess nitrate utilization in summer. Preferential recycling of PON compared to POP below the mixed layer degrades the simulation and cannot produce results that satisfy both the observed seasonal nitrate and phosphate cycle in the mixed layer. The most realistic model simulation was obtained with preferential recycling of POP over PON, but this mechanism alone was incapable of satisfying the summer nitrate and phosphate data. With the inclusion of an OM pool in our model we were able to reproduce the observed seasonal mixed layer nitrate and phosphate cycles. Satisfactory results can be achieved through various combinations of the N/P ratio of OM and the lifetime of the OM. Seasonal observations of dissolved and suspended organic phosphorus, nitrogen and carbon are needed to confirm their role. The important conclusion of our model study is that in the PFZ the annual nutrient utilization ratio of nitrate to phosphate is considerably less than the classical Redfield value of 16.