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

Tabellaria fenestrata var. fenestrata within Mountain Lake exhibited a range of discernible forms, from a heavy, ribbed form through the nominate morphology. Similarly, a small distinctive notched teratological form of Cymbella affinis var affinis was present in the shallow south end of the lake accompanied by the regular form of C. affinis.

Table 3 presents Mountain Lake diatom count data. Hierarchical cluster analysis of these data reveals seven individual cluster groups or diatom assemblages, two of which are subgroupings of a larger group. These groupings are located spatially on the lake map (Figure 2). The cluster groups show primary divisions apparent between shallow, intermediate, and deep-water diatom groups.

At the shallow south end of the lake, most of the samples were relatively similar, dominated by pennate forms. Even when full, this portion of the lake will have water depths of

The shallow group (samples 1, 2, 3, 4, and 8) was delineated by larger numbers of epiphytic and epipelic diatoms. This assemblage overall included Gomphonema, Pinnularia, Eunotia, and Cymbella species. The shallow group was further subdivided into two smaller groups. The first of these (samples 1 and 2) corresponded to shallow (

The second group (samples 3 and 4) was delineated largely by epiphytic diatoms closely associated with the macrophytes Nitella and Elodea in the shallow (

Since the mid-1980s, Nitella beds within Mountain Lake had been declining, replaced in part by Ceratophyllum demersum L., a vascular freshwater weed species associated with more eutrophic conditions (Beaty and Parker 1994). At the time of our sampling, Ceratophyllum was well-established in the region of the lake near our sampling point 7 (Figure 1). Our sediment sample 7 contained large amounts of living and decayed Ceratophyllum material. This sample was associated with a low diatom diversity, nearly 100% Cocconeis placentula, an epiphyte on Ceratophyllum. The preferred epiphytic habit of C. placentula is well-documented in the earlier literature (Hutchinson 1975). In 1997 and 1998, Ceratophyllum decreased in the lake, as the waters returned to an increasingly oligotrophic state due to phosphate limitation (Cawley et al. 1999).

Samples 5, 6, 7, and 8, although not grouped by the cluster analysis, all occurred at mid--depths between 6 and 13 m. An average ratio of planktonic to benthic diatoms for these samples was 5.6: 1, about an order of magnitude higher than the average ratios of the shallower samples.

The two intermediate deep groups (6, 9, 10, and 12) (14 and 15) were similar, both representing relatively steeper rock (geologically lying on sandstone rather than shale) and fine organic sand to silt-covered bottom conditions between 10 and 21 m. These groups were primarily represented by Cyclotella stelligera, Nitzschia sinuata, Stephanodiscus alpinus, and Tabellaria fenestrata. In addition, a number of Fragilaria species were present, and an occasional shallower water form. The average ratio of planktonic to benthic forms in this region was 15: 1, about a threefold increase over the mid-depth sample ratios.