Assessment of the Triggers for Excessive Filamentous Algal Growth in Stormwater Ponds and Preventive Management

Sammanfattning

In Uppsala Municipality, several stormwater ponds (SWP) have been reported to suffer from recurring excessive filamentous algae (FA) growth, raisingconcern for the aesthetic appearance and the long-term maintenance of these structures. This thesis investigates the factors contributing to excessive FA development in four SWP compared with a reference pond with no reportedalgal issues. The study integrates field observations, communication with experts, water and algae sampling, laboratory analysis, and literature review to explore relationships between water quality, pond design, environmental triggers, and algal proliferation. Key water quality parameters, such as nutrient concentrations and pH, were analysed alongside structural features and usual maintenance of each pond. Results suggest that excessive algal growth is not attributable to a single factor but arises from complex interactions between physical design, nutrient loading, ecological dynamics, and human management. In particular, high nutrientavailability, low presence of macrophytes, and some physical features wereassociated with FA dominance. Species identification also highlighted differences in algal composition between sites, with Cladophora and other green filamentous taxa present in impacted ponds. While the findings provide valuable insights, the study was limited by a short sampling period, low frequency of sampling and difficulties in communication with some stakeholders, as well as variability in pond age, design, and catchment characteristics. These factors introduce uncertainty, particularly in understanding temporal patterns of excessive algal proliferation and maintenance actions taken for each pond. Nonetheless, the assessment offers valuable observations aimed to potentially reduce algal growth in SWP, including consideration of nutrient input, vegetation buffers, and adaptive maintenance strategies, both technical and social. The thesis underscores the importance of integrating ecological understanding into the design and maintenance of SWP. Since excessive FA growth results from the complex interactions between biogeochemical and anthropogenic processes, addressing it requires a holistic and site-specific management approach. This study contributes to the growing knowledge base needed for more resilient and sustainable urban water infrastructure.