The Role of Catchment Characteristics in Phosphate Emissions to Downstream Waterbodies

Phosphate (PO₄) is often the limiting nutrient driving eutrophication and algal blooms in freshwater ecosystems. Accurate estimation of PO₄ concentrations is crucial for assessing the impacts of agriculture and urban emissions on water bodies and evaluating the effectiveness of catchment mitigation measures. However, measuring PO₄, especially at low concentrations, is technically complex, and modelling its dynamics is challenging due to the interplay between emissions, absorption, and transport processes in the natural environment. This study develops a Random Forest model to predict PO₄ concentrations in Irish water bodies, using catchment descriptors related to climate, land use, geology, topography, and anthropogenic activities. The model achieves an R² of 0.35 and an RMSE of 0.03 mg/L on an independent validation dataset, demonstrating moderate predictive accuracy. The developed model is applied to current and future climate and land use change scenarios to evaluate the influence of catchment-level mitigation measures. The findings highlights the significant roles of soil texture and permeability in influencing downstream PO₄ concentrations. Moreover, descriptors which are associated with low-intensity land use, such as peatlands and forests, were identified as having a positive effect in reducing PO₄ concentrations in water bodies. This underscoring the importance of sustainable land management practices in maintaining healthier ecosystems.