Primary production in and below the oxycline of Lake Rot
The thesis investigates oxygenic and anoxygenic primary production in and below the oxycline of Lake Rot. Starting point were periodically observed traces of oxygen in the hypolimnion of Lake Rot as well as the hypothesis that methane oxidation in the hypolimnion of Lake Rot may be at least partly controlled by aerobic methane oxidation, even though no oxygen is measured in the deeper parts of the hypolimnion. Assessments were based on four main aspects: (1) the variability of the penetration depth of oxygen, (2) the distribution of phytoplankton and sulphur bacteria, (3) the oxygenic and anoxygenic primary production and (4) the possibility of aerobic methane oxidation fuelled by photosythetically produced oxygen.
Methods applied include in-situ profiling, laboratory measurements and in-situ incubations. Physico-chemical conditions of the whole water column were assed with a custom-built profiler for ion selective analysis (PIA) recording temperature, conductivity, turbidity, hydrogen sulphide and oxygen concentrations. In addition, photosynthetically available radiation (PAR) was measured with a PAR probe. By comparing different recordings, the variability of the penetration depth of oxygen (1) was assessed under different light conditions. To measure the distribution of phytoplankton and sulphur bacteria (2) chlorophyll a (Chl a) measurements, phytoplankton classification and cell counts by light microscopy and flow cytometry, were conducted. Primary production (3) was assessed with in-situ 14C incubations. Thereby, aerobic and anaerobic primary were distinguished by comparing incubation samples with and without the addition of DCMU ((3-(3,4-dichlorophenyl)-1,1-dimethylurea)), herbicide blocking aerobic photosynthesis. In addition, dark incubations with Lake samples from the highest and the lowest analysed depth were conducted.
Results are based on two field campaigns conducted in June and August 2013. Indications were found, that the penetration depth of oxygen below the oxycline (1) varies with surface irradiance. Accordingly, the variability of the oxic-‐anoxic boundary layer could be caused by oxygenic primary production below the oxycline. The phytoplankton distribution (2) indicated that the potential for aerobic primary production was given through the whole water column. The hypolimnion was dominated by different cyanobacteria as well as a smaller number of other photoautotrophic phytoplankton genera during both field campaigns. In addition, the number of sulfide bacteria was observed to increase from the first detection of sulfide towards the sediment. Both, oxygenic and anoxygenic primary production (3) were observed to operate below the oxycline at H2S concentrations up to 70 μM and total sulfide concentrations up to 140 μM. While anoxygenic primary production increased with increasing sulfide concentrations, oxygenic primary production decreased with decreasing irradiation. While the oxycline was observed to vary between a depth of 8.6 and 9.8 m, oxygenic primary production was measured down to a depth of 12 m. In conclusion, indications for aerobic methane oxidation below the oxycline found in other studies as well as the observation of oxygenic primary production in the anoxic hypolimnion of Lake Rot found in this study suggest, that aerobic methane oxidation in an below the oxycline of Lake Rot could be fuelled by oxygen produced by oxygenic primary production (4).