Modeling the nitrogen and carbon cycling in Marguerite Bay, Antarctica: Annual variations in ammonium and net community production

A quasi-two-dimensional biogeochemical model was applied to Marguerite Bay to investigate differences in the seasonal cycles of nitrogen and carbon of the plankton communities between 2000–2001 and 2001–2002. The impetus for the modeling effort came from observations made during the US SO Southern Ocean Global Ocean Ecosystem dynamics (SO GLOBEC) program field surveys in 2001 and 2002 that showed a two-fold decrease in net community production (NCP) and ammonium stocks. Simulated nutrient fields matched these observations, indicating that NCP was mainly controlled by the availability of light to phytoplankton. Greater sea-ice cover led to a shorter phytoplankton bloom in 2001–2002 than in 2000–2001. Furthermore, the simulated lower primary production led to smaller bacterial ammonification during the second year. Additionally, smaller zooplankton stocks were observed that led to less simulated excretion of ammonium during 2002. The results of the numerical simulations showed that remineralization of organic matter to ammonium by bacteria accounted for up to 45% of the ammonium production during each year.