Abstract. Dissolved organic carbon (DOC) in Lake Fryxell, 10 streams flowing into the lake, and the moat surrounding the lake was studied to determine the influence of sources and biogeochemical processes on its distribution and chemical nature. Lake Fryxell is an amictic, permanently ice-covered lake in the McMurdo Dry Valleys which contains benthic and planktonic microbial populations, but receives essentially no input of organic material from the ahumic soils of the watershed. Biological activity in the water column does not appear to influence the DOC depth profile, which is similar to the profiles for conservative inorganic constituents. DOC values for the streams varied with biomass in the stream channe1,and ranged from 0.2 to 9.7 mg CIL. Fulvic acids in the streams were a lower percentage of the total DOC than in the lake. These samples contain recent carbon and appear to be simpler mixtures of compounds than the lake samples, indicating that they have undergone less humification. The fulvic acids from just above the sediments of the lake have a high sulfur content and are highly aliphatic. The main transformations occurring as these fractions diffuse upward in the water column are 1) loss of sulfur groups through the oxycline and 2) decrease in ali hatic carbon and increase in the heterogeneity of aliphatic moieties. The fraction of modem ' C content of the lake fulvic acids range from a minimum of 0.68 (approximately 3000 years old) at 15m depth to 0.997 (recent material) just under the ice. The major processes controlling the DOC in the lake appear to be: 1) The transport of organic matter by the inflow streams resulting in the addition of recent organic material to the moat and upper waters of the lake; 2) The diffusion of organic matter composed of relict organic material and organic carbon resulting from the degradation of algae and bacteria from the bottom waters or sediments of the lake into overlying glacial melt water, 3) The addition of recent organic matter to the bottom waters of the lake from the moat.

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Abstract. The relationships between the abundance and activity of planktonic, heterotrophic microorganisms and the quantity and characteristics of dissolved organic carbon (DOC) in a Rocky Mountain stream were evaluated. Peak values of glucose uptake, 2.1 nmol L-' hr -', and glucose concentration, 333 nM, occurred during spring snowmelt when the water temperature was 4.0 C and the DOC concentration was greatest. The turnover time of the in situ glucose pool ranged seasonally from 40-1110 hours, with a mean of 272 hr. Seasonal uptake of 3 H-glucose, particulate ATP concentrations, and direct counts of microbial biomass were independent of temperature, but were positively correlated with DOC concentrations and negatively correlated with stream discharge. Heterotrophic activity in melted snow was generally low, but patchy. In the summer, planktonic heterotrophic activity and microbial biomass exhibited small-scale diel cycles which did not appear to be related to fluctuations in discharge or DOC, but could be related to the activity of benthic invertebrates. Leaf-packs placed under the snow progressively lost weight and leachable organic material during the winter, indicating that the annual litterfall in the watershed may be one source of the spring flush of DOC. These results indicate that the availability of labile DOC to the stream ecosystem is the primary control on seasonal variation in heterotrophic activity of planktonic microbial populations.

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