Decomposition Processes in the Profundal Region of Blelham Tarn and the Lund Tubes

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Abstract
During 1977 one of the experimental enclosures (Lund tubes) in Blelham Tarn was fertilized at weekly intervals with nitrate, phosphate, silicate and Fe to encourage phytoplankton growth. A 2nd tube was untreated. Decomposition processes in the profundal region, including O2 and nitrate uptake, CO2, CH4, NH3 and sulfide accumulation, and loss of CH4 and N2 as bubbles, were studied for the duration of thermal stratification. C and N input as sedimenting particulate material was also measured. Sediment input was greater, decomposition more rapid and anaerobic respiration proportionately more important in the fertilized tube than in the unfertilized control. The CO2 which accumulated in the profundal zone came from aerobic respiration (42%), denitrification (17%) and sulfate reduction (2%); methanogenesis was equivalent to 25%, and presumably fermentations contributed to the remainer, although these processes were not measured. The relative importance of each process was similar in the tarn and the tubes. If aerobic respiration were to continue at a similar rate during isothermal conditions, the profundal decomposition processes would account for about 75% of the organic C sedimenting during the summer months.

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