Long-term (50 years) trends of oxygen and drivers of anoxia in a managed small temperate lake

In this study, we used a long-term data series of 50 years to identify which factors drive lake deoxygenation. Our results suggest the existence of two feedback loops operating simultaneously in lakes, which are triggered by anoxic conditions and later on facilitate the occurrence of further anoxia. The first feedback consists of anoxic conditions that enhance sediment phosphorus release and increases availability of this element in the water column. This boosts algal growth, eventually resulting in more biomass available for aerobic mineralisation and further reducing oxygen concentrations. The coefficient estimates of previous models to detect this feedback loop were inflated due to the large variance across lakes. To avoid this, this study aimed to identify this feedback loop by analysing the parameters from a single lake with a much longer time series. Long-term trends of all parameters contributing to this loop were analysed using linear regressions and breakpoint analyses. Furthermore, the correlations between these parameters were tested. All of these links making up the loop were confirmed by the results of this study. Additionally, external phosphorus sources to the lake were analysed and showed a continuously decrease in concentration, further supporting the idea that the observed increase in this element comes from the internal lake load. Further, we found the existence of a second feedback loop operating simultaneously with the first one. Chlorophyll-a, as proxy of algal biomass, and oxygen concentrations significantly correlated with concentrations of reactants of anaerobic mineralisation pathways, namely sulphate, nitrate and ammonium. The results suggest that the rate of anaerobic mineralisation increased over the last 30 years, releasing more reduced substances into the water column. These substances diffuse towards oxygenised water layers, following a concentration gradient and are oxidised when in contact with dissolved oxygen. Direct measurements
Masterarbeit Universität Innsbruck 2024