A physiological approach to parameterising variable Diatom Si:N ratios using a quota model to reproduce nutrient addition experiments in the Southern Ocean

The Si:N ratio of diatoms in the Southern Ocean (SO) is increased in response to iron limitation resulting in enhanced removal of Si from the surface waters that are entrained into the Subantarctic Mode Water (SAMW) leaving it Si deficient. Biogeochemical models usually employ direct parameterisations to represent this phenomenon empirically, however minor differences in how this process is parameterised can lead to large disparities in model results due to the large influence of SAMW on productivity in the lower latitudes. We explore the complexities of parameterisation using data from a recent cruise of the ‘Carbon Uptake and Seasonal Traits in Antarctic Remineralisation Depth’ (CUSTARD) project. This project undertook a series of factorial nutrient addition experiments including Fe, Mn and Si, along 89° W between 54° S and 59.99° S. Experimental results reinforced that Si:N ratio is dependent not only on Fe but also Si availability. To properly reproduce this data, a quota model was altered to allow phytoplankton to uptake and store luxury quantities of nutrients to then be used for growth from internal pools. This model was able to successfully reproduce quantitative patterns of nutrient limitation and the Fe-Si dependency of diatom Si:N ratios through an explicit physiological approach without the need for a direct parameterisation. Such methodology is both more authentic to the natural control of diatom stoichiometry and may avoid the potential for artificial responses created by direct parameterisations.