Factors affecting low summer dissolved oxygen concentrations in Mississippi Delta bayous

Author(s): Lizotte Jr., R.; Shields Jr., F.; Locke, M.; Murdock, J.; Knight, S.

Streams in watersheds supporting intensive row-crop agriculture are vulnerable to ecological degradation due to non-point source discharge of pollutants such as nutrients. Low gradient streams such as bayous are especially susceptible due to increased water residence time, and often result in poor water quality and chronic low dissolved oxygen (DO) concentrations (hypoxia). The goal of the current study was to assess physical, chemical, and biological components affecting low DO during summer of 2011 in three Mississippi Delta bayous. Three sites were selected within each bayou: upstream channel, lake or open water in the water body mid-section, and downstream channel. Dissolved oxygen was monitored at 40 cm depth every 15 minutes for 6-7 days on alternate weeks. Stream surface water samples collected biweekly were analyzed for nutrient and chlorophyll a concentrations. Minimum daily DO levels were frequently below the State instantaneous minimum DO standard of 4 mg/L. Diel DO fluctuation (the difference between daily maximum and minimum DO concentrations) reflected large 24-h DO ranges (=10 mg/L) across all three bayous. Pearson product moment correlations showed minimum DO concentrations to be negatively correlated with total phosphorus (TP) concentrations across all habitats. Total nitrogen (TN) concentrations and dissolved organic carbon (DOC) concentrations were negatively correlated with minimum DO concentrations only in lake habitats. Diel DO fluctuation was positively correlated with water column chlorophyll a concentrations across all habitats. Upstream diel DO fluctuation was also positively correlated with water depth and TP concentrations while downstream diel DO fluctuation was positively correlated with TP but not water depth. Low summer DO concentrations and changes in diel DO fluctuations were affected by both nitrogen and phosphorus driving summer algal blooms (eutrophic to hypereutrophic conditions) in Mississippi Delta bayous. Organic carbon inputs may exacerbate DO minimums in these nutrient-rich systems. As a result, nutrient reduction in all habitats in conjunction with increased water depth in upstream habitats is necessary to improve summer DO concentrations in Mississippi Delta bayous.

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