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Hewes, CD, Reiss CS, Kahru M, Mitchell BG, Holm-Hansen O.  2008.  Control of phytoplankton biomass by dilution and mixed layer depth in the western Weddell-Scotia Confluence. Marine Ecology-Progress Series. 366:15-29.   10.3354/meps07515   AbstractWebsite

Hydrographic, nutrient and trace metal (iron, manganese, and aluminum) concentration data, collected as part of a 2-ship survey during austral summer 2004, were used to examine the influence of upwelling and horizontal mixing on phytoplankton biomass in the region of Elephant Island and South Shetland Islands, Antarctica. Temperature/salinity property analysis and changes in trace metal and nutrient concentrations show that horizontal mixing of shelf waters, not upwelling from depth, is correlated with phytoplankton biomass in the upper mixed layer (UML). The interaction between changing UML depth and nutrient and trace metal concentrations in the UML results in a unimodal distribution of phytoplankton biomass centered at intermediate surface salinities of similar to 34. Principal component (PC) analysis of hydrographic and chemical observations resolved 3 components that accounted for 99% of the variability in nutrient and trace metal concentrations. The first PC accounted for a conservative loss of nutrients through dilution across a latitudinal salinity gradient. The second and third PCs separated mixed layer depth and nutrient consumption. Although these 2 PCs accounted for just 20% of the variability in the data matrix, they accounted for 65% of the variability in mean phytoplankton biomass, and recreated the unimodal distribution of chlorophyll concentration when modeled across a salinity gradient. We propose that the distribution of phytoplankton biomass is structured by the horizontal mixing of nutrient rich waters, derived from Weddell Sea Shelf Waters, with Antarctic Surface Water that enhances stratification and shoaling of the UML.

Holm-Hansen, O, Kahru M, Hewes CD, Kawaguchi S, Kameda T, Sushin VA, Krasovski I, Priddle J, Korb R, Hewitt RP, Mitchell BG.  2004.  Temporal and spatial distribution of chlorophyll-a in surface waters of the Scotia Sea as determined by both shipboard measurements and satellite data. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 51:1323-1331.   10.1016/j.dsr2.2004.06.004   AbstractWebsite

Chlorophyll-a (Chl-a) concentrations in surface waters were measured at 137 hydrographic stations occupied by four research vessels participating in the CCAMLR 2000 Survey and the values were compared to estimates from data acquired by the SeaWiFS satellite. The Chl-a concentrations measured on board ship ranged from 0.06 to 14.6 mg m(-3), a range that includes most surface Chl-a concentrations during mid-summer in the Southern Ocean. Owing to persistent cloud cover over much of the Southern Ocean, it was necessary to acquire multi-day composites of satellite data in order to obtain reliable estimates of Chl-a at each of the hydrographic stations. The correlation between the median value for the eight-day composites and the Chl-a concentrations measured on board ship had an R-2 value of 0.82, with the satellite data under-estimating the values obtained on board ship at high Chl-a concentrations and slightly overestimating the shipboard data at Chl-a concentrations of < 0.2 mg m(-3). For Chl-a concentrations of < 1.0 mg m(-3), the ratio of the satellite estimates divided by the shipboard values was 0.89 +/- 0.45 (n = 50). As the mean Chl-a concentration in most pelagic Antarctic waters is close to 0.5 mg m(-3), satellite estimates for Chl-a concentrations in surface waters are thus close to shipboard measurements, and offer the advantage of providing synoptic maps of Chl-a distribution over extensive areas of the Southern Ocean. Satellite Chl-a images for the months preceding (December 1999) and following (February 2000) the CCAMLR 2000 Survey cruises showed that the general pattern of Chl-a concentration in the Scotia Sea and adjoining waters was similar in all three months, but that the phytoplankton biomass was generally lowest in December, reached maximal values in January, and started to decline in February. in contrast, Chl-a concentrations in Drake Passage declined progressively from early December through February. Published by Elsevier Ltd.

Holm-Hansen, O, Kahru M, Hewes CD.  2005.  Deep chlorophyll a maxima (DCMs) in pelagic Antarctic waters. II. Relation to bathymetric features and dissolved iron concentrations. Marine Ecology-Progress Series. 297:71-81.   10.3354/meps297071   AbstractWebsite

A deep chlorophyll a maximum (DCM) at depths between 60 and 90 m in waters south of the Antarctic Polar Front (APF) occurs only in pelagic waters where the chlorophyll a concentrations in the upper mixed layer (UML) are very low (generally < 0.2 mg m(-3)). Dissolved Fe concentrations in these waters with DCMs are also very low (generally < 0.2 nM) and are probably a limiting factor for phytoplankton growth and biomass. DCMs occur in the upper portion of the temperature minimum layer (TML), which is the winter residue of the Antarctic Surface Water (AASW). The higher phytoplankton biomass at these depths is thought to result from higher Fe concentrations in the winter remnant of the AASW as compared to that found in the overlying UML. A survey of the literature indicates that DCMs are located predominately over the deep ocean basins where enrichment of surface waters with Fe from either coastal sediments or from upwelling processes would be minimal. DCMs are not found in coastal waters or in pelagic regions where complex bottom topography causes upwelling of deep water with sufficiently high Fe concentrations to enhance surface chlorophyll a concentrations. Such enrichment of surface waters overlying or downstream of topographical seamounts or ridges that rise to within a few thousand meters of the surface usually results in elevated phytoplankton biomass in the UML and no DCM due to decreased solar irradiance in the TML. The effect of such enrichment of Fe in surface pelagic waters that results from upwelling processes is most pronounced in the Scotia Sea, in the Polar Frontal region downstream of South Georgia, over the Southwest Indian Ridge, over the Kerguelen Plateau, and over the Pacific Antarctic and Southeast Indian Ridges.

Holm-Hansen, O, Naganobu M, Kawaguchi S, Kameda T, Krasovski I, Tchernyshkov P, Priddle J, Korb R, Brandon M, Demer D, Hewitt RP, Kahru M, Hewes CD.  2004.  Factors influencing the distribution, biomass, and productivity of phytoplankton in the Scotia Sea and adjoining waters. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 51:1333-1350.   10.1016/j.dsr2.2004.06.015   AbstractWebsite

During January and February 2000 four research vessels, from Russia, the UK, Japan, and the United States, conducted an oceanographic survey with 137 hydrographic stations within the Scotia Sea and adjoining waters as part of a survey sponsored by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) to estimate the biomass and distribution of Antarctic krill in the Scotia Sea. Chlorophyll-a (Chl-a) measurements showed great variability in phytoplankton biomass within the Scotia Sea, with some areas having among the lowest Chl-a concentrations found in Antarctic waters (<0.1 mg m(-3) in surface waters) while other areas were among the richest with > 10 mg m(-3). This paper describes the distribution and concentration of Chl-a in the upper 100 m of the water column and relates the Chl-a profiles at individual stations to profiles of upper water-column stability, to the depth of the upper mixed layer, and to the mixing of different water masses. The 58 stations with the lowest Chl-a values in surface waters also had low values for integrated Chl-a (33.9 +/- 19.5 mg m(-2)) and a Chl-a maximum at depths of between 70 and 90 m, in contrast to all other stations where deep Chl-a maxima did not occur. The T/S diagrams at many of these stations were indicative of Antarctic Circumpolar Current (ACC) waters. The central Scotia Sea and areas to the west and north of South Georgia had significantly higher integrated Chl-a values (98.1+/-46.0 mg m(-2), n = 57), in addition to five stations with very high Chl-a values (mean of 359+/-270 mg m(-2)). The mean rate of integrated primary production, which was estimated using the Chl-a data and the mean incident solar radiation measured from previous cruises as well as from satellite data, was estimated to be 994 mg carbon m(-2) day(-1). The temperature profiles at these stations suggested that considerable interleaving and mixing of water types had occurred, which was also evident in the T/S diagrams, which indicated mixing of ACC waters with coastal waters originating from Bransfield Strait or the Weddell Sea. There was no significant correlation between integrated Chl-a values and the profiles of upper water column stability or the depth of the upper mixed layer. The spatial variability in phytoplankton biomass within the Scotia Sea is discussed in relation to the hypothesis that low iron concentrations are the major factor controlling phytoplankton biomass in these pelagic Antarctic waters and that concentrations of iron available for phytoplankton uptake are strongly influenced by fronts and the mixing of different water masses. Published by Elsevier Ltd.