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The effect of solar radiation (photosynthetically active radiation and ultraviolet radiation (UVR)) on the photosynthetic extracellular release (PER) of phytoplankton and its utilization by bacterioplankton were studied in an alpine lake (Gossenkollesee, Austria) over a seasonal cycle. For comparison, 2 Arctic lakes on the Taymir Peninsula (Siberia) were investigated as well. The contribution of PER to primary production (% net PER) ranged between 0 and 95% (mean 32%) in Gossenkollesee (May 1999 to July 2000) following a seasonal trend that was inversely related to phytoplankton biomass. PER was released only under light conditions and was positively related to the intensity of solar radiation. During the ice-covered period, the % net PER increased, while total primary production (Ptot) decreased with experimentally increasing irradiance, indicating radiation- induced stress with dark-adapted phytoplankton. During the ice-free period, the increase of % net PER with increasing irradiance was independent of photoinhibition. The fraction of PER not immedi- ately incorporated by bacteria (~60% of total PER) may be an important contribution to the generally low pool of dissolved organic carbon (DOC) in Gossenkollesee. At the Arctic site, net PER amounted to 21% of Ptot in an ice-free lake and 51% in the ice-covered Lake Nyagamya. In all lakes, the amount of PER was sufficient to meet the bacterial carbon demand. The UVR-induced inhibition of primary production in the investigated lakes was inversely related to the DOC concentration of the water column, with the lowest effect in the ice-free Arctic lake (3.5% reduction of Ptot) and the highest impact in the alpine lake (56% reduction of Ptot, 61% reduction of PER); the percentage of net PER was apparently not controlled by UVR. This suggests that the investigated alpine lake is more sensitive to changes in UVR than the Arctic lakes are.

Effect of solar radiation on photosynthetic extracellular carbon release and its microbial utilization in alpine and Arctic lakes