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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

Tidepools are well-known structures at tidal coastal ecosystems. They mainly occur on rocky shores, while they are less abundant on sedimentary shores such as the Wadden Sea in the south-eastern North Sea due to the lack of structures, that hold back the water during low tide. However, with the introduction of the ecosystem engineering Pacific oyster Magallana gigas (former Crassostrea gigas) and its establishment on native blue mussel beds (Mytilus edulis), new biogenic structures developed, which also caused an increase of tidepools within the reefs. These tidepools offer constant submersion in the intertidal zone, which may cause changes in the occurrence of mussel and oyster reef associated species. In this study, I determined the densities of barnacles, amphipods and the common periwinkle Littorina littorea within and outside of tidepools. Furthermore, I examined the prevalence of the copepod parasites Mytilicola intestinalis and Mytilicola orientalis and the metacercarial stage of the trematode parasite Renicola roscovita in native blue mussel M. edulis occurring inside and outside of tidepools. My results revealed a lower barnacle coverage on mussels and oysters in- than outside of tidepools. An experimental removal of grazers and predators in- and outside of tidepools resulted in a strong increase of barnacle recruitment in both habitats. Densities of periwinkles were similar in- and outside of tidepools. However, the grazing and bulldozing activity of the common periwinkle L. littorea is increased in permanently flooded areas, leading to higher displacement of juvenile barnacles from hard substrate resulting in a lower barnacle coverage within tidepools. The trematode R. roscovita showed higher abundances in blue mussels collected outside of tidepools, whereas the parasite copepod Mytilicola spp. is more abundant in blue mussels within tidepools. The density of amphipods was also found to be higher inside of tidepools. The two parasites have a difference in