Oxygenic primary production below the oxycline and its importance for redox dynamics

We present evidence that oxygenic primary production occurs in the virtually anoxic regions (i.e. regions where no oxygen was detected) of the eutrophic, pre-alpine Lake Rot (Switzerland). Chlorophyll-a measurements in combination with phytoplankton densities indicated the presence of oxygenic primary producers throughout the water column. While Chlorophyceae were present as the main class of oxygenic phototrophs above the oxycline, which extended from 8 down to 9.2 m, the phototrophic community in and below the oxycline was dominated by cyanobacteria. In-situ incubation experiments with (HCO3)-C-14 (-) conducted in August 2013 revealed an oxygenic primary production rate of 1.0 and 0.5 mg C m(-3) h(-1) in 9 and 10 m depth, respectively. However, measurements with optical trace oxygen sensors showed that oxygen concentrations were below the detection limit (20 nmol l(-1)) during the incubation period below 9.2 m. Potential oxygen consumption rates, which were 10-20 times higher than oxygen production rates, explain this absence of free oxygen. Our data show that oxygen production in the virtually anoxic zone corresponded to approximately 8 % of the oxygen flux driven by the concentration gradient in the oxycline. This provided an important source of electron acceptors for biogeochemical processes beyond the conventional redox boundary and in the apparently oxygen depleted zone of Lake Rot. This oxygenic primary production in the virtually anoxic zone could allow growth and activity of aerobic microorganisms adapted to low oxygen supply.