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The influence of legacy P on lake water quality in a Midwestern agricultural watershed

Melissa Motew, Xi Chen, Eric G. Booth, Stephen R. Carpenter, Pavel Pinkas, Samuel C. Zipper, Steven P. II Loheide, Simon D. Donner, Kai Tsuruta, Peter A. Vadas, Christopher J. Kucharik
Ecosystems 2017 v. no. pp. 1-15
agricultural soils, agricultural watersheds, climate, climate change, eutrophication, fertilizer application, hydrologic models, lakes, landscapes, manure spreading, mathematical models, phosphorus, pollution load, probability, rain, sediments, summer, surface water, terrestrial ecosystems, turbidity, water quality, Wisconsin
Decades of fertilizer and manure applications have led to a buildup of phosphorus (P) in agricultural soils and sediments, commonly referred to as legacy P. Legacy P can provide a long-term source of P to surface waters where it causes eutrophication. Using a suite of numerical models, we investigated the influence of legacy P on water quality in the Yahara Watershed of southern Wisconsin, USA. The suite included Agro-IBIS, a terrestrial ecosystem model; THMB, a hydrologic and nutrient routing model; and the Yahara Water Quality Model which estimates water quality indicators in the Yahara chain of lakes. Using five alternative scenarios of antecedent P storage (legacy P) in soils and channels under historical climate conditions, we simulated outcomes of P yield from the landscape, lake P loading, and three lake water quality indicators. Legacy P had a significant effect on lake loads and water quality. Across the five scenarios for Lake Mendota, the largest and most upstream lake, average P yield (kg ha−1) varied by −41 to +22%, P load (kg y−1) by −35 to +14%, summer total P (TP) concentration (mg l−1) by −25 to +12%, Secchi depth (m) by −7 to +3%, and the probability of hypereutrophy by −67 to +34%, relative to baseline conditions. The minimum storage scenario showed that a 35% reduction in present-day loads to Lake Mendota corresponded with a 25% reduction in summer TP and smaller reductions in the downstream lakes. Water quality was more vulnerable to heavy rainfall events at higher amounts of P storage and less so at lower amounts. Increases in heavy precipitation are expected with climate change; therefore, water quality could be protected by decreasing P reserves.