Drivers of Carbon and Oxygen sources and sinks in standing water bodies: A case study of the Rappbode Reservoir, Germany

Global warming and land use changes exert environmental strains on terrestrial aquatic ecosystems. This has consequences on metabolic balances of water bodies that primarily depend on dissolved oxygen (DO) and dissolved inorganic carbon (DIC) because these phases are involved in photosynthesis and respiration. DO concentrations are usually inversely proportional to temperature. With global warming this implies that future O2 fluxes from the atmosphere to open water systems might be lower. Global warming also enhances phytoplankton growth and can favour the growth of harmful cyanobacteria. For instance, in the last two decades the cyanobacterium Planktothrix rubescens prospered likely due to rising temperatures and changes of land use with altered nutrient inputs. Blooms of this species preferentially occur in the metalimnion of lentic water bodies and are well-known for the release of hepatotoxins. They have also been linked to the so-called Metalimnetic Oxygen Minimim (MOM), a phenomenon characterized by an abrupt decrease in DO. This thesis reports metabolic equilibria as well as causes of MOM in the Rappbode Reservoir, Germany´s largest drinking water reservoir. Amongst others, new tools were concentration patterns and stable isotopes of DO (δ18ODO) and DIC (δ13CDIC) as well as stable isotopes and concentrations of dissolved and particulate organic carbon (DOC and POC, respectively). The MOM zone showed the lowest DO concentrations (down to 0.1 mmol L-1) coupled with the highest δ18ODO (up to +36.5 ‰) and the lowest δ13CDIC (down to -15.1 ‰) values. It was also marked by the highest respiration to photosynthesis ratios (R/P up to 60) as determined from DO concentrations and its isotopes. Moreover, P. rubescens was also studied in a laboratory experiment with a non-axenic culture. This allowed the identification of a fractionation factor (αR) of 0.980 for respiration of a community dominated by P. rubescens. This finding can help to quantify oxygen depletion linked to this cyanobacterium via Rayleigh fractionation. Another part of this thesis was able to show that autochthonous POC is the main contributor to DIC pool through mineralisation in both the metalimnion and the hypolimnion of Rappbode reservoir. This PhD work could quantify for the first time the MOM as a layer of intense respiration via δ18ODO and δ13CDIC measurements. Another new aspect is that metabolic balances for the entire water column of the Rappbode Reservoir could be established with stable isotope systematics. With this, the fragility of the Rappbode System to global warming with links to atmospheric O2 inputs and mineralisation rates of POC into DIC could be described. Moreover, this work provided new isotope insights into the metabolism of the harmful algae P. rubescens that also occurs in other lakes and reservoirs worldwide. In summary, this thesis offers a basis for future studies on the ecology of standing water bodies.