Effects of invasive quagga mussels on freshwater quality under climate warming

Freshwater ecosystems face unprecedented challenges from multiple co-occurring stressors, including biological invasions and climate warming. Invasive quagga mussels (Dreissena rostriformis bugensis) have become one of the most impactful invaders in the Northern Hemisphere, functioning as ecosystem engineers with the potential to fundamentally alter water quality. This thesis integrates laboratory experiments, analysis of 20 years of high frequency long-term field data (both, 10 years before and after quagga mussel invasion), and process-based ecosystem modeling, using the temperate shallow Lake Müggelsee (Germany) as a model system to comprehensively evaluate how quagga mussels affect two critical water quality parameters – trace organic compounds (TrOCs) and harmful algal blooms (HABs) – and how these interactions change under the concurrent stressor of climate warming. Quagga mussels can indirectly facilitate the removal of TrOCs. Experiments investigating the transformation of iodinated contrast media (ICM) revealed that periphyton biofilms, whose growth may be enhanced by additional substrate and nutrient excretion by mussels, significantly accelerated the degradation of iopromide (IOP). In experiments containing mussels and periphyton, IOP concentrations were reduced by up to 93% within 30 days. This reduction was positively correlated with periphyton biomass, indicating that the combination of mussels and periphyton has the potential to play a significant role in TrOC transformation at the sediment-water interface of urbanized freshwater systems. The potential of quagga mussels to control cyanobacteria was found to be highly species-specific and temperature-dependent. Laboratory filtration experiments identified Dolichospermum flos-aquae as palatable, whereas Microcystis aeruginosa, Aphanizomenon flos-aquae, and Anabaenopsis elenkinii were rejected as non-palatable by mussels. Crucially, both laboratory and field data identified a narrow thermal window where mussel filtration on cyanobacteria declines. In the laboratory, filtration rates decreased significantly above 28.9°C (CI: 27.6–30.2°C), with complete all mussels dying at 32°C. These physiological limits were confirmed in the long-term field data, where a mussel-mediated reduction in palatable cyanobacterial biovolume was only observable, when water temperatures remained below a critical temperature of 27.7°C (CI: 26.9–28.4°C). Integrating these findings by adding invasive mussels into the PCLake+ ecosystem model, substantially improved its performance for the period after invasion, reducing Root Mean Square Errors (RMSE) by 25% for chlorophyll-a and 67% for cyanobacterial biomass predictions. Simulations showed that mussel invasion reduced summer cyanochlorophyll concentrations by an average of 40% under ambient conditions. While this suppression persisted under moderate warming scenarios (RCP 2.6 and 4.5), it was entirely lost under an unmitigated climate change scenario (RCP 8.5) by 2100. In this high-warming scenario (+2.6°C epilimnion temperature), the loss of mussel filtration capacity led to a fivefold increase in cyanobacterial biomass. Model predictions indicate that strong reductions in external nutrient loads of up to 90% would be required to compensate for this functional loss and to secure water safety standards. Collectively, these findings highlight that while invasive ecosystem engineers can temporarily mitigate symptoms of eutrophication and chemical pollution, their regulatory capacity is fragile and critically confined by thermal limits. Consequently, future adaptive lake management need to include the interactive effects of invasive quagga mussels and climate change, prioritizing integrated strategies of local nutrient reduction and global climate mitigation to safeguard freshwater quality in a warming world.