ROLE OF ULTRAVIOLET-B RADIATION ON PHOTOCHEMICAL AND MICROBIAL OXYGEN-CONSUMPTION IN A HUMIC-RICH SHALLOW LAKE

In a humic-rich, shallow lake (Lake Neusiedl), the seasonal dynamics o f the humic and the nonhumic dissolved organic carbon (DOG) were inves tigated and the photochemical oxygen consumption rates of these DOC fr actions exposed to surface solar radiation levels were compared with t hat of the bulk DOC and bacterial respiration, Furthermore, bacterial utilization of the humic, nonhumic, and bulk DOC pre-exposed to solar radiation was compared with utilization of the different fractions of DOC held in the dark prior to inoculating natural bacterial assemblage s. The concentration of the unfractionated DOC pool ranged from simila r to 3 mmol C liter(-1) during summer to 1.3 mmol C liter(-1) in late spring. The mean contribution of humic DOC was 35.2% of bulk DOG. Unde r the full spectrum of solar radiation, photochemical oxygen consumpti on of the unfractionated DOC was 3.3 mu mol O-2 liter(-1) h(-1), 1.8 m u mol O-2 liter(-1) h(-1) of the humic DOG, and 1.7 mu mol O-2 liter(- 1) h(-1) of the nonhumic DOG. In the absence of UVB, photochemical oxy gen consumption was reduced by 35% in the unfractionated DOG, 38% in t he humic, and 27.5% in the nonhumic DOG. Under the full, spectrum of s olar radiation, the photochemical oxygen consumption normalized to DOC was more than twice as high (2.83 mu mol O-2 mmol(-1) C h(-1)) for hu mic than for nonhumic DOG. The bacterial oxygen consumption rate was s imilar to 30% of the photochemical oxygen consumption of the unfractio nated DOG. In batch culture experiments with natural bacterial assembl ages as inocula, the bacterial yield was generally higher with substra te exposed to the full spectrum of solar radiation than with substrate held in the dark prior to inoculation. Exposure of 0.8-mu m filtered water to the full spectrum of surface solar radiation for 2-3 h result ed in a decline in activity (measured by thymidine incorporation) to 4 7% of the activity measured in the dark, If UVB was excluded, bacteria l activity was 62% of that in the dark. Subsequent incubation at 5-20- cm depth under in situ radiation for another 2-3 h resulted in bacteri al activity similar to that detected in the dark incubations at the su rface, Bacteria exposed to the full range of solar radiation at the su rface and incubated subsequently in the dark exhibited significantly l ower activity than bacteria exposed to in situ solar radiation in dist inct depth layers. This result indicates that bacteria rapidly recover from previous UV stress in the absence of UVB, Based on our results, we estimate that the photooxidation-mediated residence times in the to p 5-cm layer of the water column are 90 and 45.5 d for the nonhumic an d humic fractions and 75 d for unfractionated DOG. For the entire wate r column, similar to 10% of the remineralization activity (bacterial r espiration + photochemical oxygen consumption) is due to photooxidatio n of the DOC, and the mean residence time of DOC is similar to 80 d.