SEASONAL RELATIONSHIPS BETWEEN PLANKTONIC MICROORGANISMS AND DISSOLVED ORGANIC MATERIAL IN AN ALPINE STREAM

The relationships between the abundance and activity of planktonic, he terotrophic microorganisms and the quantity and characteristics of dis solved organic carbon (DOC) in a Rocky Mountain stream were evaluated. Peak values of glucose uptake, 2.1 nmol L-1 hr-1, and glucose concent ration, 333 nM, occurred during spring snowmelt when the water tempera ture was 4.0-degrees-C and the DOC concentration was greatest. The tur nover time of the in situ glucose pool ranged seasonally from 40-1110 hours, with a mean of 272 hr. Seasonal uptake of H-3-glucose, particul ate ATP concentrations, and direct counts of microbial biomass were in dependent of temperature, but were positively correlated with DOC conc entrations and negatively correlated with stream discharge. Heterotrop hic activity in melted snow was generally low, but patchy. In the summ er, planktonic heterotrophic activity and microbial biomass exhibited small-scale diel cycles which did not appear to be related to fluctuat ions in discharge or DOC, but could be related to the activity of bent hic invertebrates. Leaf-packs placed under the snow progressively lost weight and leachable organic material during the winter, indicating t hat the annual litterfall in the watershed may be one source of the sp ring flush of DOC. These results indicate that the availability of lab ile DOC to the stream ecosystem is the primary control on seasonal var iation in heterotrophic activity of planktonic microbial populations.