An investigation of sediments from the littoral (shallow water) and profundal (deep water) zones of Blelham Tarn, a shallow eutrophic lake, showed marked differences in the microbial decomposition processes. These differences were due largely to differences in the degree of oxygenation, supply of electron acceptors, and mean summer temperature at the two sites. The changes in the hypolimnion (the deep water zone formed on thermal stratification, which may be treated essentially as a closed system) could be used to calculate profundal rates of aerobic respiration, NO and SO reduction, and methanogenesis, relative to the accumulation of CO Laboratory measurements demonstrated that NH accumulation, SO reduction and methanogenesis were more intense in the profundal than in the littoral zone. Anaerobic processes that occurred in the littoral sediments did so at greater depths than in the profundal sediments. The release of CH and N bubbles also provided estimates of the importance of these processes at the two sites. At both sites aerobic respiration was the most important component (about 50%) of carbon mineralization; SO reduction was the least important, accounting for only a small percentage of carbon turnover. Pathways of NO reduction and methanogenesis accounted for approximately equal proportions (varying between 15 and 25%) of the carbon mineralized. When the results were adjusted to account for the relative areas of the profundal and littoral zones, the former was the more important site of methanogenesis and SO reduction, whereas aerobic respiration and NO reduction were greater in the littoral zone. The major end-product of NO reduction was NH in the profundal and N in the littoral zone. The higher and continued levels of nitrification, which recycled the NH in the littoral sediments, were thought to contribute to this.


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