An optimization-based model of iron-light-ammonium colimitation of nitrate uptake and phytoplankton growth

The ability to model "new" (nitrate-based) production is crucial to predicting the ocean's role in the global carbon cycle. The ability to model the distribution of high-nutrient/low-chlorophyll (HNLC) areas is particularly important in this regard. Here I draw together three elements that appear to be necessary for constructing the required model: (1) iron limitation of algal growth rates as an ultimate cause of the HNLC condition; (2) ammonium inhibition of nitrate uptake and utilization as a proximate mechanism that leads to reduced nitrate use; and (3) the dependence of both processes on algal cell size. In the model, cells are postulated to maximize their growth rates by partitioning scarce iron between nitrogen- and carbon-related demands. The: effect of iron limitation is postulated to depend on cell size through surface/volume effects on uptake efficiency; this dependence on cell size in turn affects phytoplankton community structure and community-level uptake of nitrate. The efficacy of the model is demonstrated by its ability to reproduce community-level curves of nitrate uptake versus ammonium concentration from both HNLC and non-HNLC areas. The partition formalism can be incorporated directly into ecosystem models; when implemented in an ecosystem model with multiple size classes, the model should produce HNLC versus non-HNLC conditions in appropriate locations and for appropriate reasons. In particular, the model's ability to produce iron-light and iron-light-nitrogen colimitation should be useful in understanding and predicting the HNLC condition in parts of the Southern Ocean. With suitable changes to parameter values, the postulated mechanism for iron partitioning may also prove useful in modeling iron and energy partitioning in nitrogen fixers.