Can Septic-System Management Practices Outweigh Climatic and Anthropogenic Drivers of Phosphorus-induced Lake Eutrophication?

Excess phosphorus loading is a major driver of lake eutrophication, with significant consequences for aquatic ecosystems and human health. The contribution of septic systems to lake phosphorus concentrations has recently become an increasing concern. However, the combined effects of evolving septic-system conditions and climate change remain poorly understood. Here, we assess lake phosphorus dynamics using an integrated septic system-lake phosphorus model that simulates septic contributions as a function of system age, operational performance, and soil characteristics. For future conditions, climate scenarios are based on an ensemble of climate projections under Shared Socioeconomic Pathways (SSPs) 2-4.5 and 5-8.5, while anthropogenic scenarios include variations in household population density, occupancy rates, and septic-system compliance. Results for a lake in Quebec, Canada, indicate that climate change alone increases the likelihood of a transition from oligotrophic toward meso-eutrophic conditions under status quo human activities. Anthropogenic pressures exert a stronger influence: increased population density, followed by higher occupancy rates and reduced septic-system performance, substantially accelerates lake degradation. The implementation of source-separation technology significantly improves water quality, although its impacts differ slightly between the two SSPs. Under current anthropogenic conditions, septic-system upgrades can maintain a healthy lake state, while under increased human pressure, they can delay the onset of eutrophic conditions. Proactive management of watershed development and septic-system performance is therefore essential to prevent long-term degradation of lake water quality under changing conditions.