Two biomass indicators (direct counts and ATP analysis) and two activity estimators (glucose uptake and dehydrogenase activity) were used to study the bacteria at 1 m water depth in Marlon Lake sediments. Direct count-biomass estimates for bacteria averaged 0.6l gC/m², were high in summer, declined rapidly in fall, then increased during the winter. Microorganisms less than approximately 30 μm diameter had a mean biomass of 1.28 gC/m² as measured by ATP analysis. Seasonal variations in this figure paralleled changes in the algal population, although algal contributions to the microbial biomass were less than 50 per cent. ATP analysis was also used to estimate the biomass of the sediment community, excluding animals greater than approximately 5 mm in length. The mean community biomass was 4.69 gC/m². Comparison of ATP data with enumeration data obtained by others, suggests that ATP is a good biomass indicator, except when cellular ATP levels are changed in reaction to biotic or abiotic environmental factors. It is proposed that, in situations such as intense grazing or rapid Increases or decreases in temperature, ATP measurements reflect not only biomass but also activity. At such times ATP-biomass data may Indicate biomass potential, or the capacity of the population to maintain its biomass under abnormally high rates of loss. Dehydrogenase activity, or respiratory potential, of the sediment bacteria was assayed using trlphenyl tetrazolium chloride. The estimate for annual rate of carbon loss as CO2 from the bacterial population (19.3 g C/m²•year), was almost identical to previously reported data obtained by measuring oxygen consumption in normal and antibiotic-treated sediment cores. Uptake of ¹⁴C-glucose was determined in mixed, diluted sediments. The maximum uptake rate (9.6 g C/m².year), natural uptake rate (5.3 S C/m².year) and the natural turnover time (0.31 h) were similar to previous data for these sediments. This suggests that, at least in terms of function, the bacterial population is quite stable from year to year. Biomass and activity of the Marion Lake sediment bacteria were found to be in phase in late spring through early fall, but activity remained low in winter despite Increases in the microbial biomass, and increased much more rapidly than biomass in early spring. The dynamics of the bacterial population are discussed in terms of these relationships. The size of the algal and bacterial populations and organic matter reservoirs, and the flux rates for carbon between these compartments are summarized. / Science, Faculty of / Zoology, Department of / Graduate