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 10 8–10 9 cells m −2, maximum diatom fluxes between October–December, 1984, as measured in the sediment trap samples, are only 10 5–10 7 individuals m −2 day −1. The cumulative flux of diatoms for this three month period is approximately 10 7–10 9 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, and Nitzschia curta. These species are also common in the water column, along with Thalassiosira 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 and Thalassiosira 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 of Thalassiosira spp., Nitzschia curta, and other robust forms of Nitzschia.