A Re-evaluation of the Trophic State of Mountain Lake, Virginia, USA

Abstract

ABSTRACT Limnological studies of Mountain Lake, Giles County, Virginia were conducted from June to October of 1997 and April to October of 1998 to evaluate the lake's trophic state, which was considered to be in an early stage of eutrophication from the late 1980s to early 1990s. Orthophosphate-P levels averaged 1.5μg/L in 1997 and 2.2μg/L in 1998, a return from 27.0μg/L in 1990 to early and mid 1980s levels. Ammonium-N also dropped, but nitrate-N increased in 1997–98 compared to 1990. The N:P ratio of 143:1 in 1997 and 244:1 in 1998 compared to 9.4:1 in 1990 clearly indicated P as the major limiting nutrient. Dissolved silica, hardness, alkalinity, and pH in the lake were similar to earlier studies. Phytoplankton taxa and cell densities were similar to those of the mid-1980s. Mountain Lake is thus oligotrophic and presently shows no signs of eutrophication. Input streams feeding the lake were also low in key nutrients, but precipitation constituted a major source of P, N, and acidity to the lake.