Phytoplankton contribute to the regulation of the Earth's climate. In this study, we investigated the factors which affect the amount of nitrogen that phytoplankton in the Southern Ocean are able to use for their growth. We collected data during a winter cruise and a summer cruise. We found that in winter, day light and the amount of ammonium (a nitrogenous nutrient) were the two most important factors. In summer, it seems that the influx of nutrients (such as iron) plays a major role.