Limnological Conditions in Lake William C. Bowen and Municipal Reservoir #1, Spartanburg County, South Carolina, August to September 2005, May 2006, and October 2006

Abstract

The U.S. Geological Survey, in cooperation with the Spartanburg Water System, conducted three spatial surveys of the limnological conditions in Lake William C. Bowen (Lake Bowen) and Municipal Reservoir #1 (Reservoir #1), Spartanburg County, South Carolina, during August to September 2005, May 2006, and October 2006. The surveys were conducted to identify spatial distribution and concentrations of geosmin and 2-methylisoborneol, common trophic state indicators (nutrients, transparency, and chlorophyll a), algal community structure, and stratification of the water column at the time of sampling. Screening tools such as the Carlson trophic state index, total nitrogen to total phosphorus ratios, and relative thermal resistance to mixing were used to help compare data among sites and among seasons. Water-column samples were collected at two depths at each selected site: a near-surface sample collected above a 1-meter depth and a lake-bottom sample collected at a depth of 2.5 to 7 meters, depending on the depth at the site. The degree of stratification of the water column was demonstrated by temperature-depth profiles and computed relative thermal resistance to mixing. Seasonal occurrence of thermal stratification (August to September 2005; May 2006) and de-stratification (October 2006) was evident in the depth profiles of water temperature in Lake Bowen. The most stable water-column (highest relative thermal resistance to mixing) conditions occurred in Lake Bowen during the August to September 2005 survey. The least stable water-column (destratified) conditions occurred in Lake Bowen during the October 2006 survey and Reservoir #1 during all three surveys. Changes with depth in dissolved oxygen (decreased with depth to near anoxic conditions in the hypolimnion), pH (decreased with depth), and specific conductance (increased with depth) along with thermal stratification indicated Lake Bowen was exhibiting characteristics common to both mesotrophic and eutrophic conditions. Nutrient dynamics were different in Lake Bowen during the May 2006 survey from those during the August to September 2005 and October 2006 surveys. Total organic nitrogen concentrations (total Kjeldahl nitrogen minus ammonia) remained relatively constant within the surveys and ranged from 0.15 to 0.36 milligram per liter during the period of study. Nitrate was the dominant inorganic species of nitrogen during May 2006. Ammonia was the dominant species during the August to September 2005 and October 2006 surveys. During the August and September 2005 survey, ammonia was detected only in bottom samples collected in the near anoxic hypolimnion, but during the October 2006 survey, ammonia was detected under destratified conditions in surface and bottom samples. In Lake Bowen, total phosphorus concentrations in bottom samples did not exhibit the dramatic, high values during the May 2006 and October 2006 surveys (0.009 to 0.014 milligram per liter) that were identified for the August to September 2005 survey (0.022 to 0.034 milligram per liter). Chlorophyll a concentrations appeared to vary with the species of inorganic nitrogen. Greater chlorophyll a concentrations were identified in samples from the May 2006 survey (6.8 to 15 micrograms per liter) than in the August to September 2005 (1.2 to 6.4 micrograms per liter) and October surveys (5.6 to 8.2 micrograms per liter) at all sites in Lake Bowen and Reservoir #1. For the three limnological surveys, surface concentrations of chlorophyll a and total phosphorus were well below established numerical criteria for South Carolina. In general, the computed trophic state indices indicated that mesotrophic conditions were present in Lake Bowen and Reservoir #1. The total nitrogen to total phosphorus ratios in Lake Bowen and Reservoir #1 were below 22:1 for the August to September 2005 survey, indicating a high probability of dominance by nitrogen-fixing cyanobacteria. Ratios during the May and October 2006 surveys at