In the other two high nutrient, low chlorophyll (HNLC) areas, studies of the size-dependent response of phytoplankton to iron addition (Price et al., 1994) and of the relation of ammonium uptake to nitrate uptake (Wheeler and Kokkinakis, 1990; Price et al., 1994) have provided valuable insight into the micronutrient contribution to the HNLC condition (Armstrong, unpub. ms.). Grazing control of phytoplankton size class and taxa (e.g., diatoms versus Phaeocystis) is also certain to be important (Banse, 1991; Frost and Franzen, 1992; Armstrong, 1994; Price et al., 1994) with individual algal size cases and taxa being controlled by predation while the overall biomass of the system is controlled by nutrients. In the Southern Ocean, the process will be complicated by the interplay of grazing by krill and salps. Deep convective mixing (Mitchell et al., 1991) and other physical processes will almost certainly be important. It is of course possible that the relative importance of these mechanisms will differ regionally in the Southern Ocean. For example, Helbling et al. (1991) showed no response to iron addition south of the Polar Front but a substantial (5-fold) response to iron addition within the ACC.
This multiplicity of possible mechanisms contributing to the HNLC condition must be critically examined to ensure a successful modeling effort. From a modeling perspective, it is therefore critical that empirical studies of these topics be coordinated with a sharp focus towards answering these fundamental questions.