Role of ultraviolet‐B radiation on photochemical and microbial oxygen consumption in a humic‐rich shallow lake

Abstract

In a humic‐rich, shallow lake (Lake Neusiedl), the seasonal dynamics of the humic and the nonhumic dissolved organic carbon (DOC) 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 ∼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 DOC. Under the full spectrum of solar radiation, photochemical oxygen consumption of the unfractionated DOC was 3.3 µmol O2 liter−1 h−1, 1.8 µmol O2 liter−1 h−1 of the humic DOC, and 1.7 µmol O2 liter−1 h−1 of the nonhumic DOC. In the absence of UVB, photochemical oxygen consumption was reduced by 35% in the unfractionated DOC, 38% in the humic, and 27.5% in the nonhumic DOC. Under the full spectrum of solar radiation, the photochemical oxygen consumption normalized to DOC was more than twice as high (2.83 µmol O2 mmol−1 C h−1) for humic than for nonhumic DOC. The bacterial oxygen consumption rate was ∼30% of the photochemical oxygen consumption of the unfractionated DOC. 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‐µm 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 DOC. For the entire water column, ∼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 ∼80 d.