Processes influencing phytoplankton bloom development in the southern Drake Passage were studied using shipboard iron-enrichment incubations conducted across a surface chlorophyll gradient near the Antarctic Peninsula, in a region of water mass mixing. Iron incubation assays showed that Antarctic Circumpolar Current (ACC) waters were severely iron limited, while shelf waters with high ambient iron concentrations (1-2 nmol L 21 ) were iron replete, demonstrating that mixing of the two water masses is a plausible mechanism for generation of the high phytoplankton biomass observed downstream of the Antarctic Peninsula. In downstream highchlorophyll mixed waters, phytoplankton growth rates were also iron limited, although responses to iron addition were generally more moderate as compared to ACC waters. Synthesizing results from all experiments, significant correlations were found between the initial measurements of Photosystem II (PSII) parameters (F v : F m , s PSII , and p) and the subsequent responses of these waters to iron addition. These correlations indicate that PSII parameters can be used to assess the degree of iron stress experienced in these waters and likely in other regions where photoinhibition and nitrogen stress are not confounding factors.

Satellite data show that oceanic eddies generated in the Southern Antarctic Circumpolar Current Front (SACCF) are associated with increased phytoplankton biomass. Cyclonic eddies with high chlorophyll a concentration (Chl‐a) retain phytoplankton within the eddy cores and increase the light available for photosynthesis in the upper mixed layer by limiting vertical mixing and lifting of the isopycnal surfaces. Anticyclonic eddies have low Chl‐a in the core but increased Chl‐a in the periphery. Cross‐frontal mixing mediated by eddies transports nutrients (e.g., Fe and Si) to the north and contributes to the increased Chl‐a in the frontal zone. Interannual variations in the cyclonic eddy activity are positively correlated with variations in Chl‐a during the spring bloom in regions of the Antarctic Circumpolar Current around South Georgia.

Abstract. As part of the US-AMLR program in JanuaryFebruary of 2006, 99 stations in the South Shetland IslandsAntarctic Peninsula region were sampled to understand the variability in hydrographic and biological properties related to the abundance and distribution of krill in this area. Concentrations of dissolved iron (DFe) and total acidleachable iron (TaLFe) were measured in the upper 150 m at 16 of these stations (both coastal and pelagic waters) to better resolve the factors limiting primary production in this area and in downstream waters of the Scotia Sea. The concentrations of DFe and TaLFe in the upper mixed layer (UML) were relatively high in Weddell Sea Shelf Waters (∼0.6 nM and 15 nM, respectively) and low in Drake Passage waters (∼0.2 nM and 0.9 nM, respectively). In the Bransfield Strait, representing a mixture of waters from the Weddell Sea and the Antarctic Circumpolar Current (ACC), concentrations of DFe were ∼0.4 nM and of TaLFe ∼1.7 nM. The highest concentrations of DFe and TaLFe in the UML were found at shallow coastal stations close to Livingston Island (∼1.6 nM and 100 nM, respectively). The ratio of TaLFe:DFe varied with the distance to land: ∼45 at the shallow coastal stations, ∼15 in the high-salinity waters of Bransfield Strait, and ∼4 in ACC waters. Concentrations of DFe increased slightly with depth in the water column, while that of TaLFe did not show any consistent trend with depth. Our Fe data are discussed in regard to the hydrography Correspondence to: M. V. Ardelan (murato@nt.ntnu.no) and water circulation patterns in the study area, and with the hypothesis that the relatively high rates of primary production in the central regions of the Scotia Sea are partially sustained by natural iron enrichment resulting from a northeasterly flow of iron-rich coastal waters originating in the South Shetland Islands-Antarctic Peninsula region.