Water quality parameters of a Nebraska reservoir differ between drought and normal conditions

Increased anthropogenic disturbance to watersheds and rivers is exacerbating the effects of oscillating wet-dry periods common in the Midwestern United States, consequently impacting the maintenance and health of lake and reservoir systems. A change in flow regime should affect water quality parameters in reservoirs, but few data exist to document such changes. During a four-year drought, the mean water volume of Harlan County Reservoir, in south-central Nebraska, dropped more than 50%, changing this irrigation reservoir from a lotic to a lentic system. We monitored changes in chlorophyll a, turbidity, temperature and dissolved oxygen over 7 months (April–October) during 2003–2006 (drought conditions) and 2007–2009 (normal conditions) at 15 locations. Chlorophyll a and turbidity were both significantly greater during drought conditions in most months. Dissolved oxygen predominately decreased during drought conditions, while water temperature did not change. Chlorophyll a and turbidity were also greater in the upper (river inflow) than the lower (near dam) reaches of the reservoir during the drought, but were similar during normal years. The observed substantial increases in chlorophyll a levels caused the reservoir to be classified as hypereutrophic, using the Trophic State Index, during drought years but eutrophic during normal years. These data emphasize the need to include precipitation and inflow patterns when examining water quality parameters and assigning trophic status to a particular water body.