STABLE ISOTOPIC BIOGEOCHEMISTRY OF CARBON AND NITROGEN IN A PERENNIALLY ICE-COVERED ANTARCTIC LAKE

Lake Hoare (77-degrees-38'S, 162-degrees-53'E) is an amictic, oligotro phic, 34-m-deep, closed-basin lake in Taylor Valley, Antarctica. Its p erennial ice cover minimizes wind-generated currents and reduces light penetration, as well as restricts sediment deposition into the lake a nd the exchange of atmospheric gases between the water column and the atmosphere. The biological community of Lake Hoare consists solely of microorganisms - both planktonic populations and benthic microbial mat s. Lake Hoare is one of several perennially ice-covered lakes in the M cMurdo Dry Valleys that represent the end-member conditions of cold de sert and saline lakes. The dry valley lakes provide a unique opportuni ty to examine lacustrine processes that operate at all latitudes, but under an extreme set of environmental conditions. The dry valley lakes may also offer a valuable record of catchment and global changes in t he past and present. Furthermore. these lakes are modern-day equivalen ts of periglacial lakes that are likely to have been common during per iods of glacial maxima at temperate latitudes. We have analyzed the di ssolved inorganic carbon (DIC) of Lake Hoare for deltaC-13 and the org anic matter of the sediments and sediment-trap material for deltaC-13 and deltaN-15. The deltaC-13 of the DIC indicates that C-12 is differe ntially removed in the shallow, oxic portions of the lake via photosyn thesis. In the anoxic portions of the lake (27-34 m) a net addition of C-12 to the DIC pool occurs via organic matter decomposition. The dis solution of CaCO3 at depth also contributes to the DIC pool. Except ne ar the Canada Glacier where a substantial amount of allochthonous orga nic matter enters the lake, the organic carbon being deposited on the lake bottom at different sites is isotopically similar, suggesting an autochthonous source for the organic carbon. Preliminary inorganic car bon flux calculations suggest that a high percentage of the organic ca rbon fixed in the water column is remineralized as it falls through th e water column. At nearby Lake Fryxell, the substantial (relative to L ake Hoare) glacial meltstream input overprints Fryxell's shallow-water biological deltaC-13 signal with deltaC-13-depleted DIC. In contrast, Lake Hoare is not significantly affected by surface-water input and m ixing, and therefore the deltaC-13 patterns observed arise primarily f rom biological dynamics within the lake. Organic matter in Lake Hoare is depleted in N-15, which we suggest is partially the result of the a ddition of relatively light inorganic nitrogen into the lake system fr om terrestrial sources.