The Southern Ocean is the largest of the high-nutrient, low-chlorophyll (HNLC) regions of the world ocean. Phytoplankton production fails to utilise completely the pool of inorganic nutrients in the euphotic zone, giving rise to low phytoplankton bio-mass and leaving relatively high summer nutrient concentrations. This enigma is of considerable significance for our understanding of the role of the oceans in the global carbon cycle. Various limiting factors have been considered: low light, low temperature, absence of necessary trace elements, grazing pressure and other means of biomass removal.
The dynamics of nitrogen uptake by phytoplankton are of particular importance. Classically, nitrate mixed into the surface layer during winter provides the nitrogen pool for growth in the spring bloom. Some organic material is exported to depth, whilst the remainder is recycled, providing ammonium and other reduced species as nitrogenous substrates for growth during the remainder of the season. The oxidation state of the inorganic nitrogen supply thus identifies new and recycled carbon fixation. Whilst this is convenient “shorthand” for the nitrogen nutrition of carbon export in much of the ocean, it is an inappropriate model for the Southern Ocean. Here, nitrate and ammonium use are simultaneous, and nitrate is never exhausted by the annual phytoplankton production.
We speculate that a range of environmental factors combine to make the large pool of nitrate partially inaccessible to phytoplankton. in addition to the documented effects of low iron availability and high ammonium concentrations, the low temperatures characteristic of the Southern Ocean may decrease nitrate availability because of the increased energetic overheads in its uptake and reduction. This in turn makes ammonium an important nitrogenous substrate, and its production by zooplankton and heterotrophic microorganisms is an important component of the plankton nitrogen cycle. There is some evidence that ammonium production by large grazing animals may stimulate phytoplankton growth. Microbial removal of nitrogen from sedimenting phytoplankton cells may result in local decoupling between the carbon and nitrogen cycles, allowing some reduced nitrogen to remain in the euphotic zone whilst carbon is exported to depth.