Major phosphorus fractions in the Des Moines River and their differential retention by Saylorville Lake

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Date
2023-05
Authors
Noack, Benjamin
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Crumpton, William G
Isenhart, Thomas M
Hall, Steven J
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Ecology, Evolution, and Organismal Biology
Abstract
Elevated phosphorous (P) loads are a major concern for rivers in agricultural regions because of the role of P in eutrophication, excessive algal growth, and water quality impairment. With respective to eutrophication, water quality monitoring typically focuses on total P (TP) and dissolved reactive P (DRP). However, there are numerous operationally defined P fractions, some of which could have equal or greater importance to eutrophication and water quality impairment than TP and DRP. For the current study, nine distinct P fractions were measured in an effort to understand the relative importance of the major fractions of dissolved and particulate P in a large, agriculturally impacted river and how these fractions are affected by a large, flood control reservoir. Samples were collected bi-weekly to monthly for five years at the inflow and outflow from Saylorville Lake, a flood control reservoir on the Des Moines River in central Iowa. Samples were assayed to determine the dissolved and particulate as well as molybdate-reactive and non-reactive fractions of P entering and exiting the reservoir. Fraction-specific partial yields were calculated and a simplified mass balance framework used to evaluate whether Saylorville Lake acted as a source or sink for each P fraction over the period of analysis. Our results indicate that the majority of P entering and exiting the reservoir tends to consist of reactive fractions, with most of the non-reactive P in particulate forms. In general, the reservoir acts as a sink for particulate fractions of P and total suspended solids as well as total non-reactive P, which is largely comprised of particulate non-reactive P.
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