2.3 Diatom analysis

The following diatom assemblages could be identified in the studied sequence of Serteya 2 site (Fig. 16):

Figure 16. Serteya 2: Diatom diagram

Depth: 4.6-4.3m. Compact clayey lake mud. Rare finds of valves.

Depth: 4.3-3.9m. Clayey lake mud. Concentration of valves varies from 16 to 50 thousand per 1g of dry sediment. Among the identified 49 taxa, epiphytic species form 65-80%. The dominant species is Fragilaria construens var venter; the subdominants are Fragilaria brevistriata, F. inflata and the planktonic species, Aulacoseria italica, and A. ambigua. The total amount of planktonic valves is 7-16%. Alkaliphilous taxa typical of moderately alkaline lakes make up 90-97% of the total assemblage. Biogeographically indifferent species are in the majority. The species adapted to boreal moderately warm conditions constitute 17-26%. Cold water species are represented by those typical of Late Glacial and early Holocene lakes in Central Russia: Navicula scutelloides, Fragilaria inflata. Hence one may suggest the occurrence of a mesotrophic-moderately eutrophic lake which included a body of free water and a macrophyte coverage. The fall in productivity of planktonic species recorded at 4.1-3.9m, was supposedly due to the lowering of the lake level.

Depth: 3.9-3.2m. Clayey fine detritus lake mud. Concentration of valves varies from 0.1 to 0.5 million per 1g of dry sediment. The total number of taxa rises to 65, mostly due to the boreal species (20-30%). Fragilaria are the dominant species, with the subdominant remaining the same as in the lower unit. Fluctuation in the content of planktonic species (15-24%) shows an instability of the lake level. The appearance of planktonic species such as Cyclostephanos dubius and Stephanodiscus hantzschii, is indicative of the lake's increased trophic state. The abundance of alkaliphilous taxa (97-99%) shows its eutrophic character.

Depth: 3.2-3.0m. Clayey fine detritus lake mud. Planktonic species form 7-11%. Productivity of benthic diatoms reaches 1-2.3 million per 1g of dry sediment. The lake-level supposedly lowered.

Depth: 3.0-2.4m. Clayey fine detritus lake mud with fragmented lake shells. Productivity of diatoms reaches the highest value of 1.5-4.5 million per 1g of dry sediment. The total number of taxa rises to 78. An increased rate of planktonic species (25-35%) indicates a stable rise of lake-level. The dominant species include the peryphytic Fragilaria ssp, combined with the planktonic, Aulacoseira ambigua, A. italica, and A. granulata. The subdominants consist of the benthic and epiphyte species, Amphora ovalis, Gyrosigma attenuatum, Cocconeis placentula, Achnanthes ssp, Navicula benthic genus is represented by 10 taxa. The proportion of epiphytes diminishes to 60-72%. The rise of lake-level led to the restriction of the macrophyte coverage. This combined with the improved light condition of the water column, was beneficial for the development of planktonic and benthic species. Alkaliphilous taxa remain dominant (96-99%), Cyclostephanos dubious being subdominant. One notes also the planktonic species of Stephanodiscus hantzschii (<1%). At that time, the lake reached the maximum depth, while retaining its eutrophic character.

Depth: 2.4-1.5m. Clayey lake mud with plant detritus. Rapid decline in the rate of planktonic species (3-11%). Productivity of diatoms varies in the range of 0.7-2.0 million per 1g of dry sediment. The total number of taxa reaches 114, due to the appearance of benthic species: Pinnularia (11 taxa), Navicula (15 taxa), and Gomphomena (9 taxa). In addition to Fragilaria, the dominant-subdominant group includes Cocconeis placentula, Amphora ovalis, Achnanthes ssp. The dominance of alkaliphilous taxa shows a moderately alkaline and eutrophic character of the lake. The lake-level was lower, as in the preceding unit.

Depth: 1.5-0.9m. Lake mud and archaeological deposits. High productivity of diatoms reaching the values of 0.9-1.5 million per 1g of dry sediment. High rate of planktonic species (39-42%), markedly dropping at 0.9m. The dominant group consists of Aulacoseira ambigua and epiphyte Fragilaria ssp. The subdominants include the benthic species, Amphora ovalis, and Synedra ulna. Alkaliphilous taxa are dominant (88-90%). The total proportion of epiphytes diminishes (41-67%). Considerable rise of the lake-level reduced the macrophyte cover.

Depth: 0.9-0.5m. Lake mud with plant detritus. Diatom productivity varies in the range of 0.2-1.0 million per 1g of dry sediment. At the depth of 0.8-0.85m one notes the cells of the planktonic species, Aulacoseria ambigua, and a considerable amount of the spores of Aulacoseria ssp. The latter make up >50% of the total amount of valves at the depth of 0.55-0.85m. The dominant-subdominant group includes the epiphytes and rheophilous species, adapted to small streams and springs rich in free oxygen: Gomphonema angustatum, Meridion circulare, Eunotia preaerupta. The rate of alkaliphilous taxa diminishes to 45-75%. The increased rate of acidophilous taxa is indicative of the moderately acidic reaction. At the end of the unit the lake-level finally drops and the lake transform into low fen mire.

Depth: 0.5-0.0m. Peat accumulated in low fen mire. Diatom productivity varies in the range of 0.06-0.2 million per 1g of dry sediment. Benthic species are dominant, Eunotia preaerupta, Synedra ulna, Meridion circulare. Fragilaria ssp appears at 0.4-0.0m. One notes the occurrence of edaphic species adapted to soils and moist habitats: Pinnularia borealis, Navicula mutica. The diatoms of mire habitat show a wide diversity: Pinnularia (8 taxa), Eunotia (7 taxa); these are normally found in organic-rich slightly acid (pH<7) water bodies. Acidophilous taxa form 13-26% at a depth of 0.3-0.5m. Their rate diminishes to 6% at 0.2-0.0m. At the same depth, alkaliphilous taxa sharply increase, reaching 80-85%. The rate of planktonic species increases in the same interval from 6 to 12%. One notes the occurrence of species indicative of anthropogenic-related eutrophication: Cyclostephanos dubius, Stephanodiscus hantzschii.


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