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Why is watershed phosphorus loading so stubbornly persistent?

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Watershed phosphorus loading from watersheds often persists after significant financial investments in Best Management Practices, often frustrating watersheds managers. Managing phosphorus sources can be difficult requiring individual land owners to alter practices that may affect their livelihood. Because source management can be difficult, and outcomes of these changes are often uncertain, the traditional approach for reducing watershed phosphorus loading is to modify drainage systems to settle particulate phosphorus in settling ponds and wetlands.

One of the primary assumptions in this approach is that phosphorus is permanently sequestered once it reports to pond or wetland sediments. Furthermore, this phosphorus accumulates in pond and wetland sediments building a legacy phosphorus pool in the watershed that only increases over time. Recent evidence for both stormwater ponds and wetlands suggest that sediments expected to permanently sequester settled phosphorus are becoming saturated and are releasing phosphorus to surface waters, offsetting new efforts aimed at reducing watershed phosphorus loading. Further exacerbating the problem, legacy phosphorus is released in a dissolved form that is not removed by traditional stormwater treatment practices and can directly result in excessive algal blooms. Recent studies demonstrate that many pond and wetland sediments have abundant mobile phosphorus and the potential to offset watershed nutrient reduction efforts. Watershed modeling and management approaches must address legacy phosphorus in stormwater ponds and wetlands to achieve meaningful reductions in watershed phosphorus loading.

For more information and to hear Joe Bischoff speak about the effects of watershed phosphorus, please join us at the Drainage and Waters Conference on February 15, 2018.

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