A coupled ocean-ecosystem model of the Ross Sea: 2. Iron regulation of phytoplankton taxonomic variability and primary production

[1] A model of the Ross Sea pelagic ecosystem has been developed by incorporating biological and nutrient dynamics into an ocean general circulation model. Surface heat and salt fluxes are calculated in the model using sea ice concentrations that are specified daily using the 20 year climatology derived from passive microwave satellite data. Surface spectral irradiance is calculated and propagated through both the water column and the sea ice. The biological state variables include two phytoplankton groups (diatoms and Phaeocystis antarctica), two nutrients (NO3 and Fe), detritus, and zooplankton. Nutrients are supplied to the sea surface from upwelling of nutrient-rich deep waters, and in the case of Fe, from melting sea ice and snow which have accumulated aeolian-derived Fe during the preceding autumn and winter. Phytoplankton growth rate is computed as a function of light or nutrient limitation. The model is able to accurately simulate the temporal evolution of the two well-described phytoplankton blooms that dominate the southwestern Ross Sea: the diatom bloom in the shallow mixed layer of the western continental shelf (including Terra Nova Bay) and the P. antarctica bloom in the more deeply mixed Ross Sea polynya region. The simulated temporal progression of both phytoplankton blooms as well as their magnitude of primary production are in good agreement with time series estimates made using satellite ocean color data as validated by in situ observations. Model results suggest that Fe availability controls annual primary production in the Ross Sea, with only about two thirds of the available macronutrients in surface waters being consumed during the course of the bloom, but that Fe has little role in determining phytoplankton community composition. Rather, the taxonomic composition of the phytoplankton blooms is determined by the differential responses of shade-adapted P. antarctica and high light-adapted diatoms to the different mixing (and hence light) regimes within the Ross Sea. The model also predicts the development of a previously undescribed phytoplankton bloom along the eastern continental shelf, just north of the Ross Ice Shelf. The physical environment of this third phytoplankton bloom differs from that of the western shelf and the Ross Sea polynya region in that it receives a larger input of sea ice-derived Fe and experiences complete exhaustion of surface NO3. Consequently, annual production on the eastern continental shelf is higher than anywhere else in the Ross Sea and supports an unusually large grazer population.