The regulation of primary productivity in the Southern Ocean

The Southern Ocean is the largest of the three high-nutrient low-chlorophyll (HNLC) regions in the world ocean. In the 1980's John Martin posited and in the following two decades international scientists have demonstrated that iron plays a proximate role in limiting nutrient utilization by phytoplankton in HNLC regions. Here I show that throughout the Southern Ocean, maximum quantum yield (&phis;m) is the primary photophysiological mechanism affected by iron addition. The manifestation of iron's control on primary productivity and the mechanisms by which iron is supplied, however, are different within the six circumpolar zones that are made up by and interact with a heterogeneous array of physical fronts, retreating ice edges, and steep chemical gradients. Within the Antarctic Zone and Southern Antarctic Circumpolar Circulation (ACC) Zone, newly upwelled, iron-rich Upper Circumpolar Deep Water (UCDW) provides enough iron to support a diatom bloom that annually propagates poleward to the Southern Boundary of the ACC (SBACC). In the zone poleward of the SBACC, the absence of upwelling iron-rich UCDW prevents the bloom's progression. Further poleward, in the zone on the Antarctic continental shelf (Ross Sea), the varying strength of temporally and spatially heterogeneous iron-rich Modified UCDW (MCDW) determines the location and duration of the annual austral summer phytoplankton bloom. The ambient upwelling conditions within the Southern Ocean delimit zones that have different phytoplankton community structure and different iron dependence. Because of these differences among the zones, iron enrichment experiments conducted in the Subantarctic Zone and the region poleward of the SBACC had differing results. Poleward of the SBACC, &phis;m increased in pico-, nano-, and microplankton; and the community structure did not change. In the Subantarctic Zone, however, microplankton &phis;m increased, but that of pico- and nanoplankton did not. The phytoplankton community composition within the Subantarctic Zone changed from being picoplankton dominated to one whose size was distributed more uniformly. The differing response to iron addition underscores the importance of having a circumpolar ecosystem perspective to the Southern Ocean.