Potential phytoplankton responses to iron and stratification changes in the Southern Ocean based on a flexible‐composition phytoplankton model

Abstract

Under present climatic conditions, primary production in the Southern Ocean is limited by a combination of grazing pressure, the light/mixing regime, and iron. The response of the ecosystem to a permanent increase of iron supply and/or changes in the mixing regime is analyzed with a flexible‐composition phytoplankton model that includes C, N, Si, and Fe cycling. Limitation of phytoplankton growth by light and nutrients (Si, N, and Fe) is treated through their effects on cellular elemental composition. The model is applied to the KERFIX time series in the subantarctic region. Two physical scenarios are considered, normal and reduced mixed layer depth, with four different aeolian inputs of Fe in each case ranging from 1 to 1000 times the estimated present input. These simulations suggest that Fe supply via dust and rain must be increased by more than a factor of 10 to produce significant changes. Increased Fe supply alone causes the bloom to occur later in the season (summer rather than spring), and coupled with a decrease in the mixed layer depth it produces drastic changes in the bloom intensity while preserving its present temporal development. Seasonal interaction between light and Fe limitation plays a critical role in controlling the primary and export production. If aeolian Fe input is increased by a factor of 1000, and mixed layer depth is reduced at the same time, export of carbon increases by a factor of 3. Light limitation prevents complete drawdown of nitrate, even if Fe limitation is removed and mixed layer depth reduced. This sets an upper limit on the primary production that can be achieved under the present meteorological conditions in this sector of the Southern Ocean.