A re-evaluation of Mountain Lake, Giles County, Virginia: Lake origins, history and environmental systems.

摘要

This project included the following goals: (1) To review and assess the geomorphology and lake morphometry of Mountain Lake, Giles County, Virginia with regard to its age and origin. This included production of an updated bathymetric map of Mountain Lake using Sonar imaging of the lake bottom. (2) To evaluate present trophic conditions in the lake waters. This analysis included the first-reported nutrient conditions for input streams to the lake and rainwater. (3) To collect representative "modern" bottom sediment samples and to analyze these sediment samples for sedimentological characteristics, diatoms, and terrestrial pollen. This analysis focussed on present environmental conditions in the lake, and the determination of modern diatom thanatocommunities. (4) To collect complete bottom sediment cores from the lake. Coring was done using a diver-assisted manual coring device designed specifically for this project. (5) To analyze Mountain Lake sediment cores for sedimentology, age determination, and temporal differences in sediment characteristics, diatoms and pollen. This analysis focussed on interpretation and documentation of environmental changes through the lake's history.; Primary discharge from the lake presently occurs through a leaky subterranean pathway associated with the deepest, crevice-like portion of the lake. This discharge results in the crevice drain not filling shut with sediment despite its location within the lowest portion of the lake. The lake structure, crevice, and subterranean drain are associated with a regional lineation feature represented in part by the path of Salt Pond Drain and a small input stream ("I-4") to the lake. Initial damming was caused by downdrop of overlying rock. The damming is not complete, and the rate of discharge through time is controlled, in part, by regional tectonic events and by a balance of hydrologic conditions and sedimentation factors. The present lake is generally oligotrophic in nature, with phosphorus representing the major limiting nutrient. Rainfall presently represents the largest source of nutrient to the lake.; Present diatom flora in Mountain Lake includes 66 individual taxa, representing 25 genera. Of these, 12 forms or species have not been reported in Virginia inland waters prior to this project. The diatoms reflect the oligotrophic and circumneutral nature of the lake. At least seven diatom thanatocommunities can be defined in the lake, based on taxa, delineated by depth and nutrient conditions. The ratio metric of planktonic to littoral diatoms can be used to estimate past water depths in the lake from bottom sediment.; An orange clay layer at 5 cm from the modern sediment/water interface represents human intervention in lake history, namely the hotel and road building in the early 20th Century. The age of the lake is greater than 6000 years. Specific 14C from sediment produced dates of 1860 +/- 100, 4220 +/- 50 and 6160 +/- 70 bp. Within this interval, at least 6 extended periods of low or empty lake level occurred (at approximately 100, 400, 900, 1200, 1800, and 4200 yrs bp). Several of these low intervals are likely to correspond with cool dry conditions co-incident with solar minima events. When the lake has been low or empty, it has tended to develop Sphagnum bog conditions with the low lake surrounded by open or wooded meadows. Terrestrial flora surrounding the lake appears to have remained relatively similar through 6100 years, although red spruce originally accompanied hemlock.