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

Abstract

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 (Gossenköllesee, 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 Gossenköllesee (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 immediately incorporated by bacteria (∼60 % of total PER) may be an important contribution to the generally low pool of dissolved organic carbon (DOC) in Gossenköllesee. 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 P tot) 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. © Inter-Research 2007.