Microbial Dynamics of an Epilithic Mat Community in a High Alpine Stream

Studies were conducted to examine interrelationships between the heterotrophic and phototrophic populations within an epilithic community in the outlet stream of a high alpine lake. Levels of nitrates, phosphates, and total organic compounds in the lake were consistently near the lower limits of detectability. Microscopic examination of the community by phase-contrast light microscopy and scanning electron microscopy revealed diatoms, filamentous algae, and bacteria embedded within a dense gelatinous matrix. Chlorophyll a and primary productivity measurements had peak values in early August, with subsequent declines. Bacterial heterotrophic activity, as measured by V(max), turnover rate, and relative activity, increased significantly as the phototrophic community declined. This trend in heterotrophic activity was not accompanied by an increase in total bacterial numbers as determined by epi-illuminated fluorescence microscopy. These results suggest that the phototrophic community responded to changes in, or interactions among, various chemical and physical factors throughout the study period. The catabolic activity of the sessile bacteria appeared to be positively influenced by changes in the mat environment resulting from the decline of the phototrophic populations.