Identification of Key Water Quality Drivers in the Grunewald Chain of Lakes, Berlin

High nutrient loads, coming from urban sewage water and agricultural land use affect ecosystem functioning of surface water bodies. Urban lakes are special cases due to the anthropogenic use as storm water reservoir and the pollution caused by nutrients such as phosphorus and nitrate. Especially, the pollution by high nutrient loads causes the aquatic ecosystem to change in the trophic state to eutrophic or even worse hypertrophic, which influences the availability of oxygen and by this the occurrence of fishes, insects, plants and other aquatic beings. Moreover, the release of hydrogen sulfide due to anoxic conditions at the bottom of a lake causes odor in the surroundings of the lake. Improving the water quality of urban lakes is both a benefit for the ecosystem, and for the socio-ecological value of the waterbody. While reducing nutrient loads from inflowing water is crucial to achieve a healthy aquatic ecosystem, an in-depth understanding of transport and reaction processes in connected urban lakes is needed to guide restauration measures and water quality management. An example for connected water bodies is the Grunewald chain of lakes in Berlin, Germany, where ten lakes are connected one after the other, directly, via pumps or canals. The lakes are located at the southwest of Berlin and are surrounded by urban areas. The lakes serve as storm water reservoirs collecting rain water from urban and traffic areas. Phosphorus and other pollutants are accumulated in the lakes. To better understand the exchange between the lakes and how they affect each other, a monitoring campaign was conducted over a period of 13 months, where monthly water samples were taken at 17 sampling stations at the inlets, outlets and at the connections of the lakes. During the monitoring campaign (07/2024 - 07/2025), a heavy rain event with more than 20 mm per day was captured providing insights into nutrient transport along the chain of lakes. A feature selection algorithm (Boruta) was applied to identify the key parameters that affect the limiting nutrient in the Grunewald chain of lakes. The limiting nutrient is described by the TN:TP ratio, also known as Redfield ratio, and categorized as phosphorus limited, nitrogen limited and co-limited. In this study, an investigation about the variation of the measured parameters as well as the TN:TP ratio along the chain of lakes and the dependency of the season is conducted in addition to the Boruta feature selection. This study reveals the relevance of temperature, volume ratio, depth and phosphorus concentrations affecting the TN:TP ratio of the lakes and how water quality of the lakes are affected by each other. The study gives insights to cascading effects on nutrient accumulation along a chain of lakes, providing guidance for further management practices.