Mongin^{1, 2}, Mathieu, and David M. Nelson¹

¹College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97311, U.S.A. and ²Institut Universitaire Européen de la Mer – UMR CNRS 6539, Technopole Brest-Iroise, Place Nicolas Copernic, F-29280 Plouzané, France, Tel: 541-737-21-86, E-mail: mmongin@coas.oregonstate.edu

Simulation of the upper-ocean biochemistry in the Southern Ocean with a flexible composition phytoplankton model: Balance between iron limitation and grazing pressure

Grazing pressure and Fe limitation are believed to be the two main processes leading to HLNC conditions in the Southern Ocean. Using a numerical model, we explore the response of upper-ocean biogeochemistry to these two processes.

We used a model in which the elemental composition of the phytoplankton is flexible, and responds to changing light and nutrient conditions. This flexible-composition model was first applied in the western Sargasso Sea, where it simulated the cycling of C, N and Si. To apply a model of this kind to the Southern Ocean and other HNLC systems we derived formulations for Fe cycling and the main physiological effects of Fe limitation on phytoplankton. Two phytoplankton groups (diatoms and nonsiliceous forms) fix C in accordance with appropriate photosynthesis-irradiance relationships and take up NO₃^–, Fe and NH₄⁺ (and diatoms take up Si(OH)₄) following Michaelis-Menten kinetics. The model allows for light dependence of photosynthesis and NO₃^– uptake, and for the observed near-total light independence of NH₄⁺ uptake and Si(OH)₄ uptake. It assumes Fe uptake by phytoplankton to be independent of the ambient light intensity, as are NH₄⁺ and Si(OH)₄ uptake. The model tracks the resulting C/N and Fe/N ratios of both phytoplankton groups and Si/N ratio of diatoms, and permits uptake of C, N, Si and Fe to proceed independently of one another when those ratios are close to those of nutrient-replete phytoplankton. When the C/N or Si/N ratio of either phytoplankton group indicates that its growth is limited by N, Si or light, uptake of non-limiting elements is controlled by the content of the limiting element in accordance with the cell-quota formulation of Droop (1974).

The Fe cycle includes dissolved Fe, picoplankton Fe, diatom Fe and two detrital Fe pools. The Fe/N ratio of each phytoplankton group is tracked and used as a proxy for the cellular Fe content. When the Fe/N ratio of either phytoplankton group becomes low enough to indicate Fe limitation, the photosynthetic efficiency of the cells and their ability to use nitrate as an N source are diminished.

We applied this model at the site of the Kerfix station in the Indian sector of the Southern Ocean (where time-series data are available for model validation) using ECMWF meteorological forcing. Several simulations with different balances between grazing pressure and Fe limitation on phytoplankton growth are reported here.