DIATOMS IN SEDIMENTS OF PERENNIALLY ICE-COVERED LAKE-HOARE, AND IMPLICATIONS FOR INTERPRETING LAKE HISTORY IN THE MCMURDO DRY VALLEYS OF ANTARCTICA

Diatom assemblages in surficial sediments, sediment cores, sediment tr aps, and inflowing streams of perennially ice-covered Lake Hoare, Sout h Victorialand, Antarctica were examined to determine the distribution of diatom taxa, and to ascertain if diatom species composition has ch anged over time. Lake Hoare is a closed-basin lake with an area of 1.8 km(2), maximum depth of 34 m, and mean depth of 14 m, although lake l evel has been rising at a rate of 0.09 m yr(-1) in recent decades. The lake has an unusual regime of sediment deposition: coarse grained sed iments accumulate on the ice surface and are deposited episodically on the lake bottom. Benthic microbial mats are covered in situ by the co arse episodic deposits, and the new surfaces are recolonized. Ice cove r prevents wind-induced mixing, creating the unique depositional envir onment in which sediment cores record the history of a particular site , rather than a lake-wide integration. Shallow-water (<1 m) diatom ass emblages (Stauroneis anceps, Navicula molesta, Diadesmis contenta var. parallela, Navicula peraustralis) were distinct from mid-depth (4-16 m) assemblages(Diadesmis contenta, Luticola muticopsis fo. reducta, St auroneis anceps, Diadesmis contenta var. parallela, Luticola murrayi) and deep-water (26-31 m) assemblages (Luticola murrayi, Luticola mutic opsis fo. reducta, Navicula molesta). Analysis of a sediment core (30 cm long, from 11 m water depth) from Lake Hoare revealed two abrupt ch anges in diatom assemblages. The upper section of the sediment core co ntained the greatest biomass of benthic microbial mat, as well as the greatest total abundance and diversity of diatoms. Relative abundances of diatoms in this section are similar to the surficial samples from mid-depths. An intermediate zone contained less organic material and l ower densities of diatoms. The bottom section of core contained the le ast amount of microbial mat and organic material, and the lowest densi ty of diatoms. The dominant process influencing species composition an d abundance of diatom assemblages in the benthic microbial mars is epi sodic deposition of coarse sediment from the ice surface.