A geographical and seasonal comparison of nitrogen uptake by phytoplankton in the Southern Ocean

Abstract

Abstract. Primary production in the Southern Ocean has been shown to be regulated by light and nutrients (such as silicate and iron) availability. However, the impact of these factors vary seasonally and differ from region to region. The seasonal cycle of primary production in this region is not fully resolved over an annual scale due to the lack of winter in situ measurements. In this study, nitrate and ammonium uptake rates were measured using 15N tracers during a winter cruise in July 2012 and a summer cruise in February/March 2013. In winter, nitrogen uptake rates were measured at 55% and 1% of the surface photosynthetically active radiation (sPAR). The summer uptake rates were measured at 4 light depths corresponding to 55, 30, 10 and 3% sPAR. The integrated nitrate uptake rates during the winter cruise ranged from 0.16–5.20 (average 1.14) mmol N m−2 d−1 while the ammonium uptake rates ranged from 0.6–32.8 (average 6.72) mmol N m−2 d−1. During the summer cruise, the mean integrated nitrate uptake rate was 0.34 mmol N m−2 d−1 with a range between 0.16–0.65 mmol N m−2 d−1. The integrated ammonium uptake rate averaged 5.61 mmol N m−2 d−1 and ranged from 1.44–11.28 mmol N m−2 d−1. The factors controlling primary production in winter and summer were investigated. During the winter cruise, it was found the different nitrogen uptake regimes were not separated by fronts. Light (in terms of day length) and ammonium concentration had the most influence on the nitrogen uptake regime. In the summer, increases in the mixed layer depth (MLD) resulted in increased nitrogen uptake rates. This suggests that the increases in the MLD could be alleviating nutrient limitations experienced by the phytoplankton at the end of summer.