Abstract
A key principle of agricultural best management practices (BMPs) is to improve water quality by reducing agricultural-sourced nutrients and associated eutrophication. Long-term (1998–2016) lake summer trophic state index (TSI) trends of an agricultural watershed with agricultural best management practices (BMPs) were assessed. Structural BMPs included vegetative buffers, conservation tillage, conservation reserve, a constructed wetland, and a sediment retention pond. TSI included Secchi visibility (SD), chlorophyll a (Chl), total phosphorus (TP), and total nitrogen (TN). Summer TSI 1977 was >80 in 1998–1999 (hypertrophic) and decreased over the first 10 years to TSI 1977 ≈ 75 (eutrophic). TSI 1977 decrease and changing TSI deviations coincided with vegetative buffers, conservation tillage, and conservation reserve. The TSI(SD) decrease (>90 to <70) coincided with vegetative buffers and TSI(TP) decrease (>90 to <75) coincided primarily with conservation tillage and the sediment retention pond. TSI(Chl) increase (<60 to >70) coincided with conservation tillage and vegetative buffer. Results indicate watershed-wide BMPs can modestly decrease summer trophic state through increased water transparency and decreased TP, but these changes are off-set by increases in chlorophyll a to reach a new stable state within a decade. Future research should assess algal nutrient thresholds, internal nutrient loading, and climate change effects.
Highlights
One of the great global challenges of using modern mechanized agricultural practices over the last century and into the 21st century has been the increase in eutrophication of surface waters within agricultural landscapes [1,2,3,4]
While eutrophication can be measured by assessing algae, water clarity, and nutrient levels, outside of the current study, documented changes in trophic status directly related to best management practices (BMPs) are still limited
The present study measured long-term (19-year) changes in summer eutrophication in an agriculturally influenced lake with a variety of BMPs placed in the watershed
Summary
One of the great global challenges of using modern mechanized agricultural practices over the last century and into the 21st century has been the increase in eutrophication of surface waters within agricultural landscapes [1,2,3,4]. The nutrient limitation concept has been successfully applied to deeper lakes (>3 m), difficulties in managing eutrophication and rehabilitation in shallow lakes occur as a result of the surface water algal biomass being significantly influenced by fish and/or zooplankton biomass [1,7,8,9,10]. Despite these difficulties, decreasing nutrient concentrations in lakes through reduction in N and P loads is still viewed as an important first step in mitigating eutrophication and lake rehabilitation [7]. TSI has been utilized for lake management of water use (e.g., drinking water, fish, and wildlife use) [11,12], assist in lake ecoregional nutrient criteria development [13,14], assess the impacts of land-use, such as agriculture, on lake trophic state [3,4,15], and assess lake restoration success or recovery from storm events [16,17]
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