The high-nutrient, low-chlorophyll regime of the ocean: limits on biomass and nitrate before and after iron enrichment

In high-nutrient, low-chlorophyll (HNLC) regions of the ocean, phytoplankton biomass remains low despite an abundance of major nutrients. Platt et al. [Proc. Roy. Soc. A 459 (2003) 1063] constructed a simple two-component (chlorophyll and nitrate) model of HNLC regions and used it to determine analytically the upper bound on chlorophyll and the lower bound on nitrate in terms of the bio-optical and physical properties of the system. Subsequently, Platt et al. [Mar. Ecol. Prog. Ser. 254 (2003) 3] showed that the response of HNLC regions to iron addition could be captured through the effect of iron on the parameters of the growth term in the model. Here, we extend this approach to derive a procedure for less-conservative bounds on chlorophyll and nitrate. We also examine the consequences of replacing the linear loss term in the original model with a quadratic loss term. The application of the model is illustrated using parameters typical of the eastern Equatorial Pacific in its unperturbed state and also after enrichment with iron. The results are consistent with observations made during an experimental manipulation of the region by addition of iron (IronEx I). Our work emphasises the value of simple mathematical models as tools to address complex issues in biological oceanography. The method has generality as well as simplicity: it could be applied by non-modellers to investigate other problems in other regions, and to facilitate this we make our computer programs freely available on the Ecological Modelling Website at http://www.elsevier.com/locate/ecolmodel.