Loads of Confusion: Rethinking the total phosphorus loading paradigm to highlight the importance of dissolved phosphorus inputs to lakes

The paradigm that watershed total phosphorus (TP) loading controls lake eutrophication has become the foundation of nutrient management worldwide, resulting in billions of dollars of investments into tributary TP reductions annually. The convention of expressing nutrient inputs as loads, together with the focus on total phosphorus rather than 15 specific fractions of the phosphorus pool, has obscured important relationships between tributary and lake phosphorus concentrations. Using unique datasets from two important North American lakes we demonstrate that inflow DP concentrations determine lake TP, and develop a conceptual model describing the relationship between P inputs and lake TP. Based on this model, we develop a new index to quantify the contribution of internal nutrient loading to lake TP based 20 on flow-weighted mean DP concentrations of inflows, lake TP, and water residence time. Our results indicate that in many cases management should prioritize dissolved over particulate P inputs to lakes.