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We investigated the bacterial response to sunlight in 5 oligotrophic lakes of different humic content [dissolved organic carbon (DOC) 3.9 to 19 mg I-', water color 0 to 140 mg platinum (Pt) 1-'1 in southern Sweden. Lake water including free-living, heterotrophic bacteria was exposed in situ in quartz tubes at 5 depths (0 m to Secchi depth) from sunrise to sunset during July. In sunlight exposed surface water. 3H-leucine uptake, measured immediately after retrieval of the tubes at dusk, was inhibited in all lakes by 23 to 85 % (most severely in clear water) compared to dark controls. Inhibition was detectable at 2 m in the clearest lake while in the most humic lake there was no effect below 0.2 m. Depth integrated loss of bacterial production in light tubes compared to dark ones decreased with increasing DOC content, ranging from 23% (clear lake) to 1-4 % (humic lakes). To study the indirect effects of solar light on bacteria, due to phototransformation of dissolved organic matter (DOM), 0.2 pm filtered lake water was exposed from sunrise to sunset at 4 depths (0, 0.2, 0.65 and 2 m) and then inoculated with bacteria, which were allowed to grow in darkness. At stationary phase the abundance and cell volume increased by 23 to 99% and 20 to 123 %, respectively, related to dark exposed controls, resulting in increases in biomass between 83 and 175%. The depth integrated light mediated increase in bacterial carrying capacity (23 to 34%) showed no relation to DOC content. Direct inhibition of growth by sunlight, and lndirect stimulation of growth from increased availability of DOM, were detectable to depths greater than detectable UV-B penetration, indicating that wavelengths >320 nm affect bacterial growth. We suggest that during exposure to daylight, bacteria are mainly negatively affected by UV light, especially in clear waters, but a simultaneous stimulation due to phototransformation of recalcitrant DOM into more available forms occurs. Time spent by cells and DOM in light exposed zones during mlx~ng determines the relative importance of these counteracting processes.

Effects of sunlight on bacterial growth in lakes of different humic content