Amy Leventer

Temporal and spatial patterns in the Ross Sea: Phytoplankton biomass, elemental composition, productivity and growth rates

The temporal and spatial patterns of phytoplankton biomass, productivity, and particulate matter composition in the Ross Sea were assessed during cruises in January 1990 and February 1992. Biomass and primary productivity in the southern Ross Sea were greatest during mid-January, with surface chlorophyll concentrations, particulate organic carbon levels, and integrated primary productivity averaging 4.9 μg L−1, 0.54 mg L−1 and 2.63 g C m−2 d−1, respectively. Comparable mean concentrations and rates for February were 1.1 μg L−1, 0.29 mg L−1, and 0.78 g C m−2 d−1 (decreases of 76, 46, and 70%, respectively), indicative of the scale of temporal changes. A distinct south-north transition also was observed both in productivity and phytoplankton biomass, with the lowest values occurring in the northern Ross Sea. East-west gradients in phytoplankton biomass and composition occurred within the southern Ross Sea. The areal productivity of the Ross Sea ranged from 0.15 to 2.85 g C m−2 d−1 and is among the highest found in the entire Antarctic. Carbon:chlorophyll ratios were uniformly high but were highest (150) in 1990 in the diatom-dominated western Ross Sea. Surface growth rates were modest, averaging less than 0.2 day−1 during both seasons. We hypothesize that the marked seasonality in the region provides an environment in which net growth rates, although slow, are maximized through low loss rates and which allows biomass to accumulate in the surface layer. Furthermore, the temporal variations are quantitatively similar to the observed spatial variations. Therefore the dominant determinant of phytoplankton biomass and productivity at any one point on the Ross Sea continental shelf is the stage of the seasonal growth cycle.

Sediment trap diatom assemblages from the northern Antarctic Peninsula region

Abstract Quantitative floral analyses were performed on 29 samples collected during ∼24-h deployments of floating sediment traps. Traps were deployed at five sites in the northern Antarctic Peninsula region during December 1986 to March 1987. These analyses and comparison to surface sediment data provide information concerning the influence of primary production, spore formation, post-bloom mass sedimentation, advection, and resuspension on the sinking and sedimented floral assemblage. At three of the five sites, absolute diatom flux decreased by more than an order of magnitude from January to February, the result of the sinking of bloom populations and subsequent decreased levels of primary productivity. These data indicate that at least to 200m, grazing and pelletization did not obscure the primary signal. Relatively low and uniform diatom flux in Drake Passage was indicative of a deeply mixed surface layer in which peak levels of biomass were not permitted to accumulate. Conversely, uniformly high diatom fluxes at a final site may have resulted from initially high productivity followed by a resuspension event during February, perhaps the result of storm mixing that was documented by the increased relative percentage of benthic diatoms. The utility of diatoms as water mass tracers is demonstrated by the distribution of three floral assemblages, both in the sediment traps and surface sediment samples. A distinct circumpolar assemblage dominated by Nitzschia kerguelensis was observed in Drake Passage. A diatom assemblage comprised of moderate to high abundances of Chaetoceros resting spores, Nitzschia curta , and Thalassiosira antarctica was broadly distributed through Bransfield Strait and Livingston Island continental shelf. Diatom flux in Gerlache Strait was dominated by resting spores of Chaetoceros . Northeastward advection distributed these spores into Bransfield Strait where high Chaetoceros fluxes were observed at depth. Chemical data suggest the possibility that spore formation resulted from nutrient depletion. In neritic Antarctic waters, significant production and mass sinking of resting spores appears to be characteristic of the final stages of an intense phytoplankton bloom.

Biogenic sedimentation in McMurdo Sound, Antarctica

Surface sediments from eastern McMurdo Sound and fjord basins of the Victoria Land Coast are enriched in organic carbon and opal. At depths below 600 m, opal contents commonly exceed 30%; organic carbon contents average 1.5% and are as high as 3.5%. Opal and organic carbon are supplied by local production within and below sea ice and by advection from open water areas of the southwestern Ross Sea. The distribution of organic carbon and opal at the sea floor is consistent with cyclonic (clockwise) water circulation in McMurdo Sound. Advective transport from the Ross Sea supplies biogenic sediment to eastern and northern McMurdo Sound; the southwestern shelf is bathed by waters derived in part from beneath the Ross Ice Shelf which transport very little allochthonous carbon. The supply of biogenic debris in southwestern McMurdo Sound is further curtailed by sea-ice conditions, e.g., more prevalent multi-year sea ice which reduces photosynthesis, and the absence of summer basal melting and ice breakout which restricts the flux of sea ice and open-water production to the sea floor. Consistent with this hypothesis, organic carbon fluxes measured via sediment trapping beneath fast ice are one to two orders of magnitude higher in eastern versus western McMurdo Sound. Pronounced cross-sound gradients in shallow water benthic biomass and species diversity have previously been attributed to east-west variations in productivity. Our surface sediment data suggest that similar or even more dramatic contrasts in benthic community structure may exist in the deeper water areas of McMurdo Sound. Pervasive resuspension is documented by pronounced near-bottom flux increases measured by sediment trapping. Resuspension events, which may be more common during the austral winter, act to winnow and transport biogenic debris and fine terrigenous sediment from shallow regions (< 400 m) to protected shelf basins. Despite oxidative loss within the water column, the accumulation rate of organic carbon in the deep basins of McMurdo Sound averages 45 mg C m−2 day−1, more than an order of magnitude higher than the world average organic carbon flux to continental margins and equivalent to accumulation rates observed in many anoxic settings. Preservation efficiency of opal during transport and deposition appears anomalously high and the most opal-rich sediments of McMurdo Sound are depleted in organic carbon relative to mid- and low-latitude siliceous deposits. Despite this observation, if similar patterns of biogenic sedimentation exist on many other parts of the shelf, the Antarctic continental margin is an important sink for sedimentary organic carbon, as has been previously suggested for the silica system.

Diatom evidence for late Holocene climatic events in Granite Harbor, Antarctica

Downcore diatom data from a deep basin in Granite Harbor, Antarctica, provide new, high-resolution information concerning changes in oceanographic and depositional processes that have occurred during approximately the last 1250 years. The influence of global-scale climatic change, during the time of the Medieval Warm Period and possibly the Little Ice Age, is revealed by long-term changes in floral assemblages, as recorded by changes in the relative concentration of Nitzschia cylindrus. The diatom data suggest that Granite Harbor experienced less annual ice and pack ice cover during both these periods, possibly because of higher temperatures during the Medieval Warm Period and stronger offshore winds during the Little ice Age. The sedimentary record reveals several distinct, short-lived events, in which nearly monospecific blooms of Corethron criophilum and Chaetoceros spp. occurred, possibly the result of strong stabilization of the upper water column and were most likely a response to regional, rather than global conditions. Although short-lived, these events may play a significant role in the delivery of organic carbon and biogenic silica to the seafloor.

Diatom flux in McMurdo Sound, Antarctica

Abstract Quantitative floral analyses have been performed on sea ice, sediment trap, and surface sediment samples collected from six sites in McMurdo Sound, Antarctica. Although diatom abundances in the sea ice reach 108–109 cells m−2, maximum diatom fluxes between October–December, 1984, as measured in the sediment trap samples, are only 105–107 individuals m−2 day−1. The cumulative flux of diatoms for this three month period is approximately 107–109 individuals m−2, possibly an order of magnitude less than the sea ice abundances, implying that diatom and opal flux must significantly increase when the sea ice melts and releases those particles previously trapped in the ice. Five species dominate the sea ice assemblage -Amphiprora sp.,Pleurosigma sp.,Nitzschia stellata, Pinnularia quadratarea, andNitzschia curta. These species are also common in the water column, along withThalassiosira spp., a genus rarely found within the sea ice. Within the upper 250 m of the water column, at a site in Granite Harbor, diatom flux decreases between 47–79% from 34 to 220 m. Opal flux, however, decreases by only 13–40% over this same interval, indicating that dissolution of thinly silicified diatom frustules has occurred. At mid-water depths, increases in absolute diatom flux of two species in particular,Nitzschia curta andThalassiosira spp., may indicate their transport from other areas. At all sites, the greatest increase in diatom flux occurs just above the sea floor. Resuspension of diatom tests and/or lateral advection creates a near-bottom nepheloid layer in which significant modification of the diatom assemblage occurs. A combination of preferential dissolution of those forms which dominate the sea ice and dilution of the assemblage with resuspended and/or advected diatoms representative of many year averages of the flora may be responsible for the production of a sediment assemblage primarily made up ofThalassiosira spp.,Nitzschia curta, and other robust forms ofNitzschia.

Cruise reveals history of Holocene Larsen Ice Shelf

In May 2000 the U.S. Antarctic Research Vessel Nathaniel B. Palmer braved extensive ice and the frigid temperatures of the Northwest Weddell Sea to penetrate the coastal leads along the Nordenskold Coast (Figures 1–3). The scientific objective of this international endeavor was to understand the natural variability of the Larsen Ice Shelf, the largest of several ice shelves along the northern extremities of the Antarctic Peninsula to have undergone catastrophic decay over recent years (Figure 2) [Vaughan and Doake, 1996; Skvarca et al., 1999]. The study sought to determine the history of the Larsen Ice Shelf beyond the limits of our historical observations. Such knowledge is needed in order to assess the potentially unique nature of the current disintegration pattern and to determine the role of regional versus global warming scenarios on the future of the break-up trend.