Sediment diatom assemblages of Mountain Lake, a subalpine ecosystem, Giles County, Virginia

Castanea, Jun 2002 by Cawley, Jon C, Parker, Bruce C, Hufford, Terry L

ABSTRACT

Mountain Lake, Virginia is a unique, natural, oligotrophic subalpine ecosystem in the southern Appalachians. Its diatom flora, based on these and earlier studies of plankton tows and plankton settling chambers, was sparse with few species. By contrast, we found 66 diatom taxa (25 genera) in the recent sediments of Mountain Lake. Twelve of these 66 taxa are new records for the inland waters of Virginia. Hierarchical cluster analysis distinguishes seven diatom assemblages in this lake, divided between shallow (primarily pennate) and deep-water (primarily centric) assemblages. Shallow diatom assemblages are further defined by differences in epipelic, epiphytic, and possibly nutrient-sensitive forms. The ratio of planktonic to benthic (littoral) forms has proven useful as an indicator of water depths from sediment. These findings provide the first set of diatom data for future limnological and paleolimnological studies in Mountain Lake.

INTRODUCTION

Comparative studies of diatom assemblages along with bottom sediment variations have proven useful for evaluating lake environmental and nutrient conditions (e.g., Brugam 1983; Engstrom et al. 1985; Anderson 1990a, 1990b; Hall and Smol 1999). Diatom analysis of sediments also enables delineation of individual assemblages within lakes (Anderson 1986, 1990a, 1990b; Cameron 1995). Distribution and composition of assemblages depends on nutrient, depth, light conditions, substrate type, seasonal variations, additions from plankton fallout, and other environmental variables (Anderson 1990a). Diatom assemblages and numbers, when compared with lake sediment core samples, can be used to infer past environmental and depth conditions within lakes (HAkansson et al. 1998, Wolin and Duthie 1999).

This study assesses the diatoms and sedimentology in relatively recent (Ekman dredge) sediment samples from Mountain Lake, a natural, oligotrophic subalpine lake located in the Appalachian fold belt of Southwestern Virginia. Our chief objective was to provide baseline data for longer historical records obtained ultimately from ^sup 14^C-dated sediment cores.

Site Description

Mountain Lake is the only natural lake of significant size in the unglaciated Southern Appalachians. In addition to its unique origin associated with a fault or fracture feature (Cawley et al. 2001), the lake has exhibited significant depth fluctuations through time. Historically it has been called either "Mountain Lake" or "Salt Pond" depending on its full or not full state, respectively. With water-depth changes, environmental conditions within the lake also may have changed significantly through time. Mountain Lake has been considered relatively pristine and oligotrophic since it was first studied by Hutchinson and Pickford (1932) (Cawley et al. 1999).

Mountain Lake lies near the top of Salt Pond Mountain (37deg21'56"N, 80deg31'39"W), in Giles County, Virginia, at an elevation of 1180 m. When full, the lake is approximately 900 m long and 250 m wide, forming an elongate club-shaped body oriented from south to northwest (Figure 1). The lake is 33 m deep at its deepest point at the north end; this deepest portion consists of a narrow crack or crevice feature on the lake bottom. A much larger area at the north end of the lake is approximately 24 m deep, shallowing to the south (Cawley et al. 2001).

Topography delineates the recharge area of Mountain Lake, which remains relatively isolated both physically and hydrologically by the surrounding steep mountain peaks. Five small individual streams enter the lake, all ephemeral except one (stream I-4, Figure 1). Because of the largely undisturbed nature of the sediments, and the lack of permanent stream inputs, Mountain Lake should provide a good sedimentary record for analysis.

MATERIALS AND METHODS

Field Description, Sampling, and Storage Protocol

Sediment grab samples were taken from 16 sites in the lake along four east-west transects (4 sample locations in each transect) (Figure 1). Samples were taken using a standard Ekman Dredge (Wildco, Wildlife Supply Company, Saginaw, Michigan, model #196 BIO). Mudroch and Azcue (1995) have described in detail the Ekman sampler and its operation.

Sample collection included field descriptions and Muncel color determinations, while each sample remained in the dredge. Then, a 25 mm diameter glass tube was inserted vertically into the sample, resulting in a "mini-core" which was examined for textural or color differences from top to bottom.

The top 5 cm of undisturbed sediment, estimated to be 50-70 years accumulation (Parker et al. 1975), was sampled from the top of each Ekman grab for detailed analysis, and stored in clean, labeled (transect/sample #, depth, location) 1 liter plastic Ziploc bags. Samples were transported on ice to Virginia Tech and refrigerated. Three subsamples were taken for each location: one for wet/dry analysis and dry sieve analysis, a second for diatom analysis, and a third for pollen analysis. [Pollen data not included]