Surface and groundwater nutrient loading to the lowermost lake in a multi-lake network

Lakes may exist within chains or networks where surface water and groundwater connectivity facilitate the transport of nutrients, complicating efforts to manage eutrophication in intermediate and lowermost lakes. Despite the recognised role of this connectivity, there is limited understanding of how these pathways influence nutrient loading and retention in these lakes, particularly where complex hydrogeology enables groundwater flow across topographic catchment boundaries. In this study, we investigated the pathways of nitrogen (N) and phosphorus (P) to Lake Tarawera—a moderate-sized, deep, monomictic lake set within complex volcanic geology in the Bay of Plenty region, New Zealand—which is the lowermost lake in an eight-lake network within a partly agricultural landscape, potentially increasing its risk of eutrophication. We constructed 7-year water and nutrient budgets from a detailed dataset of discharge and nutrient concentrations. Gross nutrient loads were 119.6 t N yr −1 and 13.2 t P yr −1 , while nutrient retention was 81.0% for N and 80.8% for P. Groundwater was the primary pathway of N (46%) and P (49%), reflecting the highly fractured local volcanic geology with substantial subsurface connectivity to upstream lakes. The approach effectively quantified nutrient loads associated with different pathways (including groundwater), characterised inter-annual and intra-annual variability, and identified key uncertainties, underscoring the importance of management approaches that consider the broader hydrologically connected landscape. The nutrient budgeting approach used in this study provides a robust method for quantifying pathway fluxes that could be applied to similarly hydrogeologically complex and connected multi-lake networks.