|SHIELDS, F. - Retired ARS Employee
|MURDOCK, JUSTIN - Tennessee Technical University
Submitted to: Mississippi Water Resources Research Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 12/1/2013
Publication Date: N/A
Interpretive Summary: Dissolved oxygen concentrations were measured in three low flow streams (bayous) in agricultural watersheds within the Mississippi Delta that showed chronic low dissolved oxygen concentrations (hypoxia). To try to determine the cause of bayou hypoxia, water depth, nutrients, and algae (as chlorophyll a) were measured at three sites in each of the three bayous simultaneously with dissolved oxygen to see if and how these variables correlated. Results of the study showed that lowest daily dissolved oxygen concentrations decreased when total phosphorus concentrations increased at all bayou sites. At deeper water sites (lake habitats), lowest daily dissolved oxygen concentrations also decreased when total nitrogen and dissolved organic carbon concentrations increased. Daily dissolved oxygen fluctuation (the difference between the daily maximum and daily minimum concentrations) increased with increasing algae at all sites. Upstream daily dissolved oxygen fluctuation also increased when when water depth and total phosphorus concentrations increased. Downstream daily dissolved oxygen fluctuation increased when total phosphorus increased and water depth decreased. Bayou hypoxia was affected by both nitrogen and phosphorus causing summer algal blooms (eutrophication) with the addition of dissolved organic carbon worsening these conditions. For these reasons, nutrient reduction and increased upstream water depth is needed to improve summer dissolved oxygen concentrations in Mississippi Delta bayous.
Technical Abstract: Streams in watersheds supporting intensive row-crop agriculture are vulnerable to ecological degradation due to non-point source pollutants such as nutrients. Low gradient streams such as bayous are especially susceptible to pollutants due to increased water residence time, and they often exhibit 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 related to DO during summer of 2011 in three Mississippi Delta bayous. Three sites were selected to represent longitudinal zonation within each bayou: upstream channel, mid-section lake, and downstream channel. Dissolved oxygen was monitored at each site at 40 cm depth at 15 minute intervals 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 Mississippi State instantaneous minimum DO standard of 4 mg/L. Daily maximum and minimum DO concentrations fluctuated more than10 mg/L in all three bayous. Pearson Product Moment correlations showed minimum DO concentrations to be negatively correlated with total phosphorus (TP) across all habitats. Total nitrogen (TN) and dissolved organic carbon (DOC) were negatively correlated with minimum DO concentrations only in lake habitats. Diel DO fluctuation was positively correlated with water column chlorophyll a across all habitats. Upstream diel DO fluctuation was also positively correlated with water depth and TP while downstream diel DO fluctuation was positively correlated with TP but negatively correlated with water depth. Low summer DO concentrations and magnitude of 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.