3%
Land-Ocean Systems in the Siberian Arctic: Dynamics and History
Fragilaria construens var. venter is especially flexible and adapted to use high temporary
nutrient concentrations occurring also in the pelagic zone of oligotrophic lakes (e.g. Pienitz et
aI., 1995, Voigt, 1997). It is possible that F. construens var. venter partly replaces the small
planktonic Cyclotella sp. during thermally and trophically unfavourable periods, which would
corroborate its opportunistic life strategy.
The pollen maxima of Larix and Alnus, accompanied by a rapid decrease of Betula exilis,
provide the palynological evidence to infer the onset of a strong warming trend culminating in
the optimal climate conditions for the region (Khotinsky, 1984), placed in the Boreal
chronozone. Further increasing diatom concentrations, along with higher proportions of
oligo/beta-mesotrophic taxa, were recorded for DAZ-3. The planktonic diatom complex is
predominated by Cyclotella comensis, associated with higher nitrate concentrations (Stoermer et
aI., 1985; Pappas and Stoermer, 1995) and an annual maximum during summer stratification
(Stoermer, 1993).
Contemporary to the highest AP values, diatom concentration and proportions of
euplanktonic diatom taxa and diatom taxa placed in the alpha-mesol eutrophic tolerance group
reach their maxima in the sequence. The occurrence of Cyclotella ocellata in the sequence is
also exceptional, as it is the most abundant planktonic diatom in DAZ-4. In the North Canadian
training set, C. ocellata plots at the upper end of temperature gradient (Pienitz et aI, 1995).
Stoermer (1993) records the annual maximum during summer stratification and a requirement
for higher phosphorus content of the water. TP optima from phosphorus training sets range
from 10.5 (Wunsam and Schmidt, 1995) to 19.5 /lg/l (Reavie et aI., 1995).
From palynological evidence, the transition to the Atlantic is marked by a slight decrease in
arboreal pollen relative to cooler periods (Khotinsky, 1984; Hahne and Melles, 1997). In
comparison, the diatom assemblage appears to respond more intensely to early Atlantic
,Novosagorskiy - cooling' (8000-7900 yr. BP; Khotinsky, 1984), when diatom concentrations
decreased rapidly (10-20 v/mm). Abundant Cymbella spp. and Fragilaria pinnata were
responsible for the Ep/Np ratio decreases. The contemporary low species number, high
diversity, and maximum evenness of the assemblages can be interpreted in several ways. First,
they may simply result from dissolution and the differing susceptibility in different species,
given the poor preservation of the observed valves. Second, these features could also be
responses to cooler conditions. Diversity decreased while evenness increased, since less
species found suitable conditions and co-occurred in more even proportions.
Subsequently, all diatom-inferred parameters stabilized again. On a species level, a second
maximum in Aulacoseira islandica was followed by the maximum abundance of Cyclotella
radiosa.
Within DAZ-6, the Ep/Np ratio fell compared to the Boreal diatom assemblages. Species
proportions in the planktonic complex fluctuated strongly. However, Cyclotella comensis
always predominates and attains maximum abundance in the sequence. The proportions of taxa
with preferences to oligo/beta-mesotrophic and circumneutral conditions peaked.
The following period is placed in the Subboreal on the basis of pollen records and is
characterized by a general decrease in Ep/Np ratio. In the first cool period (4600-4100 y. BP,
Khotinsky, 1984), when Picea pollen strongly decreased, a strong increase of small Fragilaria
spp. (mainly F. pinnata) was observed. These species have been found to dominate in alkaline,
glacial habitats (e.g. Haworth, 1976). Higher abundances may also be related to their
tychoplanktonic and thus flexible life form. Beyond this feature, diatom assemblages are stable
in terms of diversity and distribution in pH preference and nutrient tolerance groupings.
The early Subatlantic cooling, as indicated by the ,first Holocene NAP maximum' (Hahne
and Melles, 1997), seemed to be slightly predated by changes in the diatom assemblages. The
Ep/Np ratio rapidly dropped below 'I', marking the third subunit with respect to this
Land-Ocean Systems in the Siberian Arctic: Dynamics and History
Fragilaria construens var. venter is especially flexible and adapted to use high temporary
nutrient concentrations occurring also in the pelagic zone of oligotrophic lakes (e.g. Pienitz et
aI., 1995, Voigt, 1997). It is possible that F. construens var. venter partly replaces the small
planktonic Cyclotella sp. during thermally and trophically unfavourable periods, which would
corroborate its opportunistic life strategy.
The pollen maxima of Larix and Alnus, accompanied by a rapid decrease of Betula exilis,
provide the palynological evidence to infer the onset of a strong warming trend culminating in
the optimal climate conditions for the region (Khotinsky, 1984), placed in the Boreal
chronozone. Further increasing diatom concentrations, along with higher proportions of
oligo/beta-mesotrophic taxa, were recorded for DAZ-3. The planktonic diatom complex is
predominated by Cyclotella comensis, associated with higher nitrate concentrations (Stoermer et
aI., 1985; Pappas and Stoermer, 1995) and an annual maximum during summer stratification
(Stoermer, 1993).
Contemporary to the highest AP values, diatom concentration and proportions of
euplanktonic diatom taxa and diatom taxa placed in the alpha-mesol eutrophic tolerance group
reach their maxima in the sequence. The occurrence of Cyclotella ocellata in the sequence is
also exceptional, as it is the most abundant planktonic diatom in DAZ-4. In the North Canadian
training set, C. ocellata plots at the upper end of temperature gradient (Pienitz et aI, 1995).
Stoermer (1993) records the annual maximum during summer stratification and a requirement
for higher phosphorus content of the water. TP optima from phosphorus training sets range
from 10.5 (Wunsam and Schmidt, 1995) to 19.5 /lg/l (Reavie et aI., 1995).
From palynological evidence, the transition to the Atlantic is marked by a slight decrease in
arboreal pollen relative to cooler periods (Khotinsky, 1984; Hahne and Melles, 1997). In
comparison, the diatom assemblage appears to respond more intensely to early Atlantic
,Novosagorskiy - cooling' (8000-7900 yr. BP; Khotinsky, 1984), when diatom concentrations
decreased rapidly (10-20 v/mm). Abundant Cymbella spp. and Fragilaria pinnata were
responsible for the Ep/Np ratio decreases. The contemporary low species number, high
diversity, and maximum evenness of the assemblages can be interpreted in several ways. First,
they may simply result from dissolution and the differing susceptibility in different species,
given the poor preservation of the observed valves. Second, these features could also be
responses to cooler conditions. Diversity decreased while evenness increased, since less
species found suitable conditions and co-occurred in more even proportions.
Subsequently, all diatom-inferred parameters stabilized again. On a species level, a second
maximum in Aulacoseira islandica was followed by the maximum abundance of Cyclotella
radiosa.
Within DAZ-6, the Ep/Np ratio fell compared to the Boreal diatom assemblages. Species
proportions in the planktonic complex fluctuated strongly. However, Cyclotella comensis
always predominates and attains maximum abundance in the sequence. The proportions of taxa
with preferences to oligo/beta-mesotrophic and circumneutral conditions peaked.
The following period is placed in the Subboreal on the basis of pollen records and is
characterized by a general decrease in Ep/Np ratio. In the first cool period (4600-4100 y. BP,
Khotinsky, 1984), when Picea pollen strongly decreased, a strong increase of small Fragilaria
spp. (mainly F. pinnata) was observed. These species have been found to dominate in alkaline,
glacial habitats (e.g. Haworth, 1976). Higher abundances may also be related to their
tychoplanktonic and thus flexible life form. Beyond this feature, diatom assemblages are stable
in terms of diversity and distribution in pH preference and nutrient tolerance groupings.
The early Subatlantic cooling, as indicated by the ,first Holocene NAP maximum' (Hahne
and Melles, 1997), seemed to be slightly predated by changes in the diatom assemblages. The
Ep/Np ratio rapidly dropped below 'I', marking the third subunit with respect to this