| Summary: | Abstract The Amundsen Sea Polynya is experiencing large increases in glacial meltwater input and hosts an extremely productive and long-lasting summer phytoplankton bloom, suggesting a crucial role for natural Fe fertilization. Early summer distributions and dynamics of the dissolved bioactive metals Fe, Mn, Zn, Cu and Ni were investigated during a three week period in 2010–2011, using GEOTRACES-compliant methods. Dissolved Fe was very low (0.06–0.12 nmol kg−1) in the upper 20 m of the central polynya, suggesting that the sub-maximal rates of in situ primary productivity reported previously for this growth phase of the bloom are attributable to insufficient Fe availability. Weeks after the sampling period, phytoplankton biomass accumulated to peak bloom conditions, implying a continuous supply of bioavailable Fe to the euphotic zone. The dominant biologically-relevant Fe source was meltwater-enriched seawater flowing from the Dotson Ice Shelf cavity and delivering Fe at 0.7 nmol kg−1 to the broader polynya. The modest Fe content of Circumpolar Deep Water (CDW; 0.3 nmol kg−1), invading through cross-shelf troughs, was strongly augmented by benthic Fe inputs, which may combine with glacial meltwater dFe in the Dotson outflow. Sea ice melting provided a modest local Fe flux, insufficient to drive large annual blooms. Dissolved Mn was strongly reduced in surface waters, but displayed a subsurface maximum likely advected through the region from shallow coastal sediments. Nutrient-type elements Zn, Cu and Ni had large to small dynamic ranges, respectively, and increasing concentrations with depth, indicating uptake and remineralization within the polynya system. Surface water drawdown ratios of metals and nutrients provided novel estimates of metal quotas (metal/P) for the dominant bloom phytoplankton, Phaeocystis antarctica. At one unique mature bloom station, Zn and Cu were scavenged to low concentrations throughout the 350 m water column, a possible result of intense removal onto sinking Phaeocystis biodetritus. The Amundsen Sea appears to be a model region for studying the biogeochemical consequences of increased glacial meltwater inputs.
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