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

Authors: MOTEW, MELISSA - University Of Wisconsin; CHEN, XI - University Of Wisconsin; BOOTH, ERIC - University Of Wisconsin; CARPENTER, STEPHEN - University Of Wisconsin; PINKAS, PAVEL - University Of Wisconsin; ZIPPER, SAM - University Of Wisconsin; LOHEIDE, STEVEN - University Of Wisconsin; DONNER, SIMON - University Of British Columbia; Vadas, Peter; KUCHARIK, CHRISTOPHER - University Of Wisconsin

Submitted to: Ecosystems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/21/2017
Publication Date: 3/10/2017
Publication URL: http://handle.nal.usda.gov/10113/5700677
Citation: Motew, M., Chen, X., Booth, E.G., Carpenter, S.R., Pinkas, P., Zipper, S., Loheide, S.P., Donner, S., Vadas, P.A., Kucharik, C. 2017. The influence of legacy P on lake water quality in a Midwestern agricultural watershed. Ecosystems. doi:10.1007/s10021-017-0125-0.

Interpretive Summary: Many watersheds have a buildup of phosphorus (P) in agricultural soils and stream and lake sediments, commonly referred to as legacy P. This stored source of P can contribute to eutrophication of surface waters (excess of nutrients that promotes algae growth and harms other aquatic life) over many years. Using computer models, we investigated the influence of legacy P on water quality in the Yahara Watershed of southern Wisconsin, USA. We used five different scenarios of P stored in soils and channels; and we simulated P loss from the landscape, P loading to lakes, and water quality impacts. The minimum P storage scenario showed that a 48% reduction in lake loads would require a 70% reduction in soil P in croplands and a 99% reduction in P stored in stream sediments. These reductions can be achieved while maintaining enough soil P for crops. 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, and effects on eutrophication could be decreased by decreasing legacy P. This study gives policy makers and producers specific information on how much management will have to reduce P in soils and sediments if desired water quality is to be achieved.

Technical Abstract: Decades of fertilizer and manure application have led to a buildup of phosphorus (P) in agricultural soils and stream and lake 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 P stored in soils and channels under historical climate conditions, we simulated outcomes of P yield from the landscape, P loading, summer total P (TP) concentration, Secchi transparency, and the probability of hypereutrophy in each of the four lakes. Across the scenarios, initial P storage was varied from recent historical levels between -92 and +143% in soils and -99 and +200% in stream sediments. Stored 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 -49 to +63%, P load (kg y-1) varied by -48 to +30%, summer TP (mg L-1) varied by -36 to +37%, Secchi depth (m) varied by -6 to +12%, and the probability of hypereutrophy varied by -79 to +65%. The minimum storage scenario showed that a 48% reduction in lake loads could be achieved in Lake Mendota and surpassed in the lower lakes with a 70% reduction in surface soil P concentration in croplands and a 99% reduction in P stored in stream sediments. These reductions can be achieved while maintaining soil P above recommended levels for crops. However, even with these reductions, the lower lakes did not reach the mesotrophic threshold for TP concentration (0.024 mg L-1) in any scenario. 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, and effects on eutrophication could be decreased by decreasing legacy P. Mitigation of the legacy P problem requires removal or immobilization of stored P in watershed soils and sediments.