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 of the humic and the nonhumic dissolved organic carbon (DOG) were investigated and the photochemical oxygen consumption rates of these DOC fractions exposed to surface solar radiation levels were compared with that 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 assemblages. The concentration of the unfractionated DOC pool ranged from similar 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. Under the full spectrum of solar radiation, photochemical oxygen consumption of the unfractionated DOC was 3.3 mu mol O-2 liter(-1) h(-1), 1.8 mu 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 oxygen consumption was reduced by 35% in the unfractionated DOG, 38% in the humic, and 27.5% in the nonhumic DOG. Under the full, spectrum of solar 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 humic than for nonhumic DOG. The bacterial oxygen consumption rate was similar to 30% of the photochemical oxygen consumption of the unfractionated DOG. In batch culture experiments with natural bacterial assemblages as inocula, the bacterial yield was generally higher with substrate 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 resulted in a decline in activity (measured by thymidine incorporation) to 47% of the activity measured in the dark, If UVB was excluded, bacterial 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 bacterial activity similar to that detected in the dark incubations at the surface, Bacteria exposed to the full range of solar radiation at the surface and incubated subsequently in the dark exhibited significantly lower activity than bacteria exposed to in situ solar radiation in distinct 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 top 5-cm layer of the water column are 90 and 45.5 d for the nonhumic and humic fractions and 75 d for unfractionated DOG. For the entire water column, similar to 10% of the remineralization activity (bacterial respiration + photochemical oxygen consumption) is due to photooxidation of the DOC, and the mean residence time of DOC is similar to 80 d.