Understanding dynamics of agricultural runoff to Oslofjord: A causal loop modelling approach.

Agricultural nutrient runoff remains the primary driver of eutrophication in the Oslofjord, despite decades of mitigation efforts that have failed to reverse ecological decline. Agriculture accounts for approximately 39% of the total nitrogen load entering the fjord, yet regulatory interventions have not translated compliance into measurable ecological improvement. This persistent challenge is partly attributable to interventions that focus on isolated practices rather than the interconnected social, economic, and soil-system processes underlying nutrient losses. To address this gap, this study employs qualitative causal loop modelling (CLM), applied here for the first time to the Oslofjord catchment, to integrate insights from agronomy, soil science, agricultural economics, and Norwegian policy documents into a unified causal loop diagram (CLD) of the agricultural nutrient runoff subsystem. The model illustrates how farm profitability pressures, soil degradation pathways, and long-term incentives for farm enlargement and specialisation collectively reinforce nutrient runoff. The analysis demonstrates that well-intentioned regulations, such as restrictions on autumn tillage, may inadvertently intensify runoff by inducing soil compaction or promoting yield-maximising behaviours. Simultaneously, the model identifies balancing mechanisms that reduce the nutrient runoff. The analysis identifies four feedback loops, two reinforcing and two balancing, with farm profitability emerging as the dominant structural leverage point. By exposing hidden feedback and unintended consequences, the CLD serves as both an analytical and communication tool, demonstrating that effective nutrient management in the Oslofjord requires co-designed economic and environmental policies rather than the sequential layering of compliance instruments on unaddressed structural conditions.