ALBERT

Description: The phytoplankton community composition and productivity in waters of the Amundsen Sea and surrounding sea ice zone were characterized with respect to iron (Fe) input from melting glaciers. High Fe input from glaciers such as the Pine Island Glacier, and the Dotson and Crosson ice shelves resulted in dense phytoplankton blooms in surface waters of Pine Island Bay, Pine Island Polynya, and Amundsen Polynya. Phytoplankton biomass distribution was the opposite of the distribution of dissolved Fe (DFe), confirming the uptake of glacial DFe in surface waters by phytoplankton. Phytoplankton biomass in the polynyas ranged from 0.6 to 14 µg Chl a / L, with lower biomass at glacier sites where strong upwelling of Modified Circumpolar Deep Water from beneath glacier tongues was observed. Phytoplankton blooms in the polynyas were dominated by the haptophyte Phaeocystis antarctica, whereas the phytoplankton community in the sea ice zone was a mix of P. antarctica and diatoms, resembling the species distribution in the Ross Sea. Water column productivity based on photosynthesis versus irradiance characteristics averaged 3.00 g C /m**2/d in polynya sites, which was approximately twice as high as in the sea ice zone. The highest water column productivity was observed in the Pine Island Polynya, where both thermally and salinity stratified waters resulted in a shallow surface mixed layer with high phytoplankton biomass. In contrast, new production based on NO3 uptake was similar between different polynya sites, where a deeper UML in the weakly, thermally stratified Pine Island Bay resulted in deeper NO3 removal, thereby offsetting the lower productivity at the surface. These are the first in situ observations that confirm satellite observations of high phytoplankton biomass and productivity in the Amundsen Sea. Moreover, the high phytoplankton productivity as a result of glacial input of DFe is the first evidence that melting glaciers have the potential to increase phytoplankton productivity and thereby CO2 uptake, resulting in a small negative feedback to anthropogenic CO2 emissions.

Description: Nutrient addition experiments were performed during the austral summer in the Amundsen Sea (Southern Ocean) to investigate the availability of organically bound iron (Fe) to the phytoplankton communities, as well as assess their response to Fe amendment. Changes in autotrophic biomass, pigment concentration, maximum photochemical efficiency of photosystem II, and nutrient concentration were recorded in response to the addition of dissolved free Fe (DFe) and Fe bound to different model ligands. Analysis of pigment concentrations indicated that the autotrophic community was dominated by the prymnesiophyte Phaeocystis antarctica throughout most of the Amundsen Sea, although diatoms dominated in two experiments conducted in the marginal ice zone. Few significant differences in bulk community biomass (particulate organic carbon, nitrogen, and chlorophyll a) were observed, relative to the controls, in treatments with Fe added alone or bound to the ligand phytic acid. In contrast, when Fe was bound to the ligand desferrioxamine B (DFB), decreases in the bulk biomass indices were observed. The concentration of the diatom accessory pigment fucoxanthin showed little response to Fe additions, while the concentration of the P. antarctica-specific pigment, 19'-hexanoyloxyfucoxanthin (19'-hex), decreased when Fe was added alone or bound to the model ligands. Lastly, differences in the nitrate:phosphate (NO3- :PO4**3-) utilization ratio were observed between the Fe-amended treatments, with Fe bound to DFB resulting in the lowest NO3- :PO4**3- uptake ratios (~ 10) and the remaining Fe treatments having higher NO3- :PO4**3- uptake ratios (~ 17). The data are discussed with respect to glacial inputs of Fe in the Amundsen Sea and the bioavailability of Fe. We suggest that the previously observed high NO3- :PO4**3- utilization ratio of P. antarctica is a consequence of its production of dissolved organic matter that acts as ligands and increases the bioavailability of Fe, thereby stimulating the uptake of NO3-.

Description: Dissolved iron (DFe) and total dissolvable Fe (TDFe) were measured in January-February 2009 in Pine Island Bay, as well as in the Pine Island and Amundsen polynyas (Amundsen Sea, Southern Ocean). Iron (Fe) has been shown to be a limiting nutrient for phytoplankton growth, even in the productive continental shelves surrounding the Antarctic continent. However, the polynyas of the Amundsen Sea harbor the highest concentrations of phytoplankton anywhere in Antarctica. Here we present data showing the likely sources of Fe that enable such a productive and long lasting phytoplankton bloom. Circumpolar Deep Water (CDW) flows over the bottom of the shelf into the Pine Island Bay where DFe and TDFe were observed to increase from 0.2 to 0.4 nM DFe and from 0.3-4.0 to 7-14 nM TDFe, respectively. At the southern end of Pine Island Bay, the CDW upwelled under the Pine Island Glacier, bringing nutrients (including Fe) to the surface and melting the base of the glacier. Concentrations of DFe in waters near the Pine Island Glacier and the more westward lying Crosson, Dotson, and Getz Ice Shelves varied between 0.40 and 1.31 nM, depending on the relative magnitude of upwelling, turbulent mixing, and melting. These values represent maximum concentrations since associated ligands (which increase the solubility of Fe in seawater) were saturated with Fe (Thuroczy et al., 2012, doi:10.1016/j.dsr2.2012.03.009). The TDFe concentrations were very high compared to what previously has been measured in the Southern Ocean, varying between 3 and 106 nM. In the Pine Island Polynya, macronutrients and DFe were consumed by the phytoplankton bloom and concentrations were very low. We calculate that atmospheric dust contributed 〈 1% of the Fe necessary to sustain the phytoplankton bloom, while vertical turbulent eddy diffusion from the sediment, sea ice melt, and upwelling contributed 1.0-3.8%, 0.7-2.9%, and 0.4-1.7%, respectively. The largest source was Fe input from the PIG, which could satisfy the total Fe demand by the phytoplankton bloom by lateral advection of Fe over a range of 150 km from the glacier. The role of TDFe as a phytoplankton nutrient remains unclear, perhaps representing an important indirect Fe source via dissolution and complexation by dissolved organic ligands (Gerringa et al., 2000, doi:10.1016/S0304-4203(99)00092-4; Borer et al., 2005, doi:10.1016/j.marchem.2004.08.006).

Description: Here we present concentrations of chlorophyll a, phaeopigments, particulate organic carbon and nitrogen from water samples collected at discrete depths with a CTD-rosette during the European Iron Fertilization Experiment (EIFEX). The experiment was carried out from February 11 to March 20, 2004 in the 60-km diameter, rotating core of an eddy, formed by a meander of the Antarctic Polar Front (centred at around 49°10' S and 2°10' E). Samples were taken within the eddy inside and outside the fertilized patch, and in a few cases outside the eddy.Chlorophyll concentrations were determined by fluorometry using a Turner Design Model 10-AU digital fluorometer. Sampling, measurements and calibration of the fluorometer was carried out following the JGOFS protocol procedure (Knap et al, 1996). Results of the fluorometer calibration diverged by 5% between beginning and end of the cruise. Chlorophyll a content was calculated using average parameter values from the two calibrations. Measurement uncertainty was estimated from triplicate water samples taken from depths ranging between 10 and 100 m depth and averaged 5% of measured values. Samples for particulate organic carbon and nitrogen (POC and PON) were filtered onto precombusted Whatman GF/F filters and processed following recommendations by Lorrain et al. (2003). Samples were measured independently on three different analysers: a CN2500 CHN Analyser (Thermo Finnigan MAT) coupled to a Delta+ mass spectrometer (Thermo Finnigan MAT) via Conflo II interface (Thermo Finnigan MAT), a Carlo-Erba NA-1500 Series II elemental analyzer coupled to a Finnegan Delta+ mass spectrometer and a Euro EA Elemental Analyser. Differences due to methods were within the range of measurement variability (below 2%). The particulate organic phosphorus (POP) content was determined colorimetrically using the method from Hansen and Koroleff (1999; measurement variability 4%). Biogenic silica (BSi) was measured following the wet alkaline digestion method according to Müller and Schneider (1993; measurement variability 2%).