Nutrient and algal responses to a managed drawdown in an agricultural riverine lake

Authors: Lizotte, Richard; Taylor, Jason; MURDOCK, JUSTIN - Tennessee Technological University; Locke, Martin

Submitted to: Chemistry and Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/30/2023
Publication Date: 4/20/2023
Citation: Lizotte Jr, R.E., Taylor, J.M., Murdock, J.N., Locke, M.A. 2023. Nutrient and algal responses to a managed drawdown in an agricultural riverine lake. Chemistry and Ecology. 39(4):319–336. https://doi.org/10.1080/02757540.2023.2198511.
DOI: https://doi.org/10.1080/02757540.2023.2198511

Interpretive Summary: As we improve our understanding of how best to use water management practices to reduce environmental impacts on nutrients and algae to sustain water quality, an improved understanding of the effects of water use on water quality is needed. To do this, we assessed how shallow lake nutrients and algae responded to a rapid (days to weeks) decrease in water level during late summer to early fall. The decrease in water level was due to a drawdown used to improve flow in the downstream Big Sunflower River. We observed how nitrogen and phosphorus concentrations first decreased as water levels decreased and then increased as water levels continued to decrease and then stabilize. Algae responses were more complex and were linked to changes in nutrients and light levels but only indirectly linked to changes in water level. Our study showed that lowering water levels in a shallow lake during late summer to early fall can have minimal impacts on lake water quality. Our results are of interest to water management, regulatory and other agencies and farming stakeholders by providing additional information to improve and sustain water quality and overall environmental quality using water management practices.

Technical Abstract: The water level of a shallow impounded riverine lake was lowered to help improve downstream flows and water quality within the Big Sunflower River in the agriculturally dominated Mississippi Delta. A study was conducted to better understand how nutrient and algal processes within shallow lakes respond to relatively rapid water level changes to guide water resource management decisions. The drawdown was initiated in late summer and completed within six days, ending when outflow was <5% of peak flows. Channel water depths were reduced from 28-82%. Surface water nutrients (phosphorus, nitrogen and carbon) and water column algal measurements (chlorophyll a, in-vivo chlorophyll, in-vivo phycocyanin, chlorophyll:phycocyanin ratio, and photosynthetic activity) were measured at three sites on days -7, -1, 0, 1, 2, 3, 6, 13, 20, 28, and 34, relative to the start of the drawdown. Nitrogen and phosphorus responses were bimodal, showing a decrease during the initial drawdown phase followed by increasing levels in the ensuing weeks. Algal responses showed complex interactions between nutrients and changes in light availability that were often indirectly related to hydrologic changes. Late summer to early fall drawdowns in small impoundments used to improve downstream flows can have minimal water quality impacts to the upstream impounded systems.