Nutrient and phytoplankton changes in Roundaway Lake after a managed hydrologic drawdown

Author(s): Lizotte, Jr., R.; Taylor, J.; Locke, M.

A late summer drawdown of Roundaway Lake in the Mississippi Delta was initiated to help improve downstream flows within the Big Sunflower River. Although such water releases are important to help improve downstream water quality, a better understanding of the changes and processes within the contributing water bodies is needed to aid water resource management decisions. Consequently, we examined the influence of the lake drawdown on surface water nutrients and phytoplankton within the system before, during, and after drawdown. Lake drawdown was initiated on September 15, 2015 and was completed within six days when outflow was <5% of peak flows and channel water depths decreased by 40%, 28%, and 82% at upstream, lake, and downstream sites, respectively. Surface water samples for nutrient and phytoplankton analysis were collected on days -7, -1, 0, 1, 2, 3, 6, 13, 20, 28, and 34 at all three sites. Nutrients included soluble reactive phosphorus (SRP), dissolved inorganic nitrogen (NH4-N, NO2-N, and NO3-N), total nitrogen (TN), total phosphorus (TP), and dissolved organic carbon (DOC). Phytoplankton measurements were: chlorophyll a and in-vivo chlorophyll (phytoplankton biomass); in-vivo phycocyanin (blue-green algal biovolume); chlorophyll:phycocyanin ratio (Chl:PC, blue-green algal bloom); phytoplankton photosynthetic activity (Chl Fv/Fm); and blue-green algal photosynthetic activity (PC Fv/Fm). Surface water nutrient responses were modest with the exception of SRP. Bimodal changes in SRP occurred with decreases at all sites during the initial drawdown phase (days -1 to 6) followed by increasing SRP levels 13 to 34 days after drawdown. No clear trends were observed for dissolved inorganic nitrogen while TN increased with time and changes in turbidity and TP varied with turbidity. Regression analysis identified some of the physical and chemical factors driving phytoplankton responses to the drawdown. Chlorophyll a was associated with TN and N:P ratios (R2 = 0.571), in-vivo chlorophyll was associated with depth, TN, and N:P ratios (R2 = 0.785), and in-vivo phycocyanin was associated with SRP, TN, and N:P ratios (R2 = 0.750). Blue-green algal blooms (Chl:PC) were associated with SRP, NO3-N, and C:N ratios (R2 = 0.645). Chl Fv/Fm and PC Fv/Fm were associated with C:N ratios (R2 = 0.351) and NO3-N + C:N ratios (R2 = 0.338), respectively. These results will provide valuable information that will help water resource managers make better informed decisions to sustain surface water resources and ecosystem integrity of Mississippi Delta rivers and lakes.

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