Kienel: Late Weichselian to Holocene Diatom Succession in a Sediment Core
399
for changes in the morphology of the lake basin either. The climatic changes for the western
Siberian region recorded by, for example, Khotinsky (1984), Velichko et al. (1997) and
Astakhov (1997), as compiled in Figure 11, are clearly reflected in the diatom assemblages of
Lama Lake.
The correlation of the abundance of euplanktonic diatom taxa with that of arboreal pollen can
be supplemented by the sediment's total organic matter content. An overwhelming influence of
climate (via the lake's summer surface temperatures) on the composition of diatom assemblages
in subpolar and polar regions is generally recorded (e.g. Smol, 1988). On this basis, the
euplanktonic/non-planktonic diatom ratio (Ep/Np) is used to infer trends in the lake's average
annual surface temperature (LST) and the amplitude of its variations.
The distinctive maxima of certain diatom taxa and their specific ecological preferences for
thermal and trophic conditions, as well as the succession of planktonic species maxima, enable
us to assess these parameters in the lake development. These results are substantiated by
corresponding results from the palynology, sedimentology and geochemistry of the lake's
sediments. However, a much higher temporal resolution is needed to resolve timing effects in
the responses of terrestrial and aquatic ecosystems.
The lowest LST -average, a high temperature fluctuation amplitude and a low and unstable
trophic status in the DAZ-I (coinciding in range with the Younger Dryas, regionally termed
Norilsk Stadial) are inferred from the lowest Ep/Np ratio, an abundance in species with high
survival capacity and a maximum percentage in trophy-tolerant species. During DAZ-2
(Preboreal), the LST-average increases while the amplitude remains low because of increased
precipitation (Velichko et aI., 1997). The trophic status is assessed as oligotrophic according to
the proportions in trophic tolerance groupings. A moderate LST -amplitude on the highest
average and the highest trophic status of Lama Lake are inferred for DAZ-3 and 4 (Boreal),
supported by the high proportions of alpha-meso to eutrophic diatom taxa and maxima in the
total organic carbon content of the sediments, arboreal pollen abundance, diatom concentration,
and Ep/Np ratio. It is possible that nutrient enrichment occurred in marginal areas of the lake.
The high abundance of planktonic taxa with annual maxima in the summer epilimnion could be
indicative of a pronounced summer stratification of the lake. A rapid decrease in average LST
marks the end of this period and is reflected in a sudden retreat in the Ep/Np ratio and a lower
trophic status (DAZ-S). Favourable climatic conditions are inferred for the remaining Atlantic
time (DAZ-6), during which average LST and amplitude are both considered high. Diatom
concentration and the Ep/Np ratio attain higher values again, while the proportions within the
trophic tolerance groupings fluctuate, suggesting unstable trophic conditions. A subsequent
decrease in the Ep/Np ratio during the DAZ-7 (Subboreal), indicates a general decreasing trend
in the average LST . The species composition of the diatom assemblage remains stable, leading
to stable proportions in the ecological preference groupings. These trends are terminated by a
rapid decrease in the average LST. The Ep/Np ratio and the diatom concentrations drop to levels
comparable to Preboreal times, which is presumably a response to the early Subatlantic cooling.
Subsequently, the average LST fluctuates strongly, based on the values of the Ep/Np ratio. The
,little climate optimum' (dated to 1000 yr. BP; Velichko et ai, 1997) could coincide with a rapid
increase of the Ep/Np ratio in DAZ-9. The high proportions of trophy-tolerant oligotrophic taxa
could indicate unstable trophic conditions at a low level.
A recent change in material supply is indicated by a distinct change in the diatom assemblage
and may originate from pollution from the Norilsk smelter complex. Heavy metal
concentrations have increased in the sediments since the 1930s (Hagedorn et aI., this volume).
399
for changes in the morphology of the lake basin either. The climatic changes for the western
Siberian region recorded by, for example, Khotinsky (1984), Velichko et al. (1997) and
Astakhov (1997), as compiled in Figure 11, are clearly reflected in the diatom assemblages of
Lama Lake.
The correlation of the abundance of euplanktonic diatom taxa with that of arboreal pollen can
be supplemented by the sediment's total organic matter content. An overwhelming influence of
climate (via the lake's summer surface temperatures) on the composition of diatom assemblages
in subpolar and polar regions is generally recorded (e.g. Smol, 1988). On this basis, the
euplanktonic/non-planktonic diatom ratio (Ep/Np) is used to infer trends in the lake's average
annual surface temperature (LST) and the amplitude of its variations.
The distinctive maxima of certain diatom taxa and their specific ecological preferences for
thermal and trophic conditions, as well as the succession of planktonic species maxima, enable
us to assess these parameters in the lake development. These results are substantiated by
corresponding results from the palynology, sedimentology and geochemistry of the lake's
sediments. However, a much higher temporal resolution is needed to resolve timing effects in
the responses of terrestrial and aquatic ecosystems.
The lowest LST -average, a high temperature fluctuation amplitude and a low and unstable
trophic status in the DAZ-I (coinciding in range with the Younger Dryas, regionally termed
Norilsk Stadial) are inferred from the lowest Ep/Np ratio, an abundance in species with high
survival capacity and a maximum percentage in trophy-tolerant species. During DAZ-2
(Preboreal), the LST-average increases while the amplitude remains low because of increased
precipitation (Velichko et aI., 1997). The trophic status is assessed as oligotrophic according to
the proportions in trophic tolerance groupings. A moderate LST -amplitude on the highest
average and the highest trophic status of Lama Lake are inferred for DAZ-3 and 4 (Boreal),
supported by the high proportions of alpha-meso to eutrophic diatom taxa and maxima in the
total organic carbon content of the sediments, arboreal pollen abundance, diatom concentration,
and Ep/Np ratio. It is possible that nutrient enrichment occurred in marginal areas of the lake.
The high abundance of planktonic taxa with annual maxima in the summer epilimnion could be
indicative of a pronounced summer stratification of the lake. A rapid decrease in average LST
marks the end of this period and is reflected in a sudden retreat in the Ep/Np ratio and a lower
trophic status (DAZ-S). Favourable climatic conditions are inferred for the remaining Atlantic
time (DAZ-6), during which average LST and amplitude are both considered high. Diatom
concentration and the Ep/Np ratio attain higher values again, while the proportions within the
trophic tolerance groupings fluctuate, suggesting unstable trophic conditions. A subsequent
decrease in the Ep/Np ratio during the DAZ-7 (Subboreal), indicates a general decreasing trend
in the average LST . The species composition of the diatom assemblage remains stable, leading
to stable proportions in the ecological preference groupings. These trends are terminated by a
rapid decrease in the average LST. The Ep/Np ratio and the diatom concentrations drop to levels
comparable to Preboreal times, which is presumably a response to the early Subatlantic cooling.
Subsequently, the average LST fluctuates strongly, based on the values of the Ep/Np ratio. The
,little climate optimum' (dated to 1000 yr. BP; Velichko et ai, 1997) could coincide with a rapid
increase of the Ep/Np ratio in DAZ-9. The high proportions of trophy-tolerant oligotrophic taxa
could indicate unstable trophic conditions at a low level.
A recent change in material supply is indicated by a distinct change in the diatom assemblage
and may originate from pollution from the Norilsk smelter complex. Heavy metal
concentrations have increased in the sediments since the 1930s (Hagedorn et aI., this volume).