This thesis examines the role of sunlight, specifically ultraviolet-B (UV-B) radiation (280-320 nanometers), on dissolved organic matter (DOM) and bacteria in lakes. Field experiments in lakes with different humic contents and geographical locations have been performed.

Photooxidation result in that part of DOM is oxidized to inorganic carbon (DIC), mainly carbon dioxide (CO2). The photooxidation rate can be of the same magnitude as respiration at lake surface on a sunny summer day. However, on a depth-integrated basis down 2 meters (m), respiration showed to be 3-12 times higher than photooxidation, as photooxidation is restricted to the surface. Simultaneously, DOM is photochemically transformed into more available forms to bacteria, thereby enhancing bacterial growth. Bacteria grew both in numbers and volumes, resulting in a several-fold increase in biomass when inoculated to sun-exposed sterile lake water.

However, bacteria are negatively affected by direct sunlight. As sunlight decreases with increasing depth, the attenuating compounds (humic acids) in the water act as a protective filter to organisms. Thus, bacterial production in a clear lake was negatively affected to 2 m depths, whereas in a humic lake negative effects were only recorded down to 0.2 m. In the investigated lakes, 10% of surface radiation values of UV-B was found at 55 centimeters (cm) in the clearest lake but only at 1.5 cm in the most humic lake. Photochemical effects on both bacteria and DOM occurred deeper than penetration of UV-B, suggesting that other wave lengths can be of importance. Even at the surface, UV-B plays a minor role in photochemical DIC-production (17%) compared to UV-A (39%) and PAR (44%). Photooxidation of DOM in lakes at different geographical places react equally to sunlight. However, within a lake, DOM exhibits seasonal variations, being most sensitive in spring and least sensitive at fall.

A balance between stimulation and suppression of bacteria in lakes exists. Bacteria are mainly inhibited during daytime, but may during night benefit from the photoproduced substances. As UV-B reaching the earth surface is likely to increase due to ozone depletion, and because CO2 is produced photochemically and indirectly by the stimulated bacterial growth, a link between ozone depletion and the ³greenhouse effect² is suggested.