394
Land-Ocean Systems in the Siberian Arctic: Dynamics and History
of open water (Smol, 1988), shallow water or periphytic taxa are favoured. Planktonic taxa
characteristic for deeper water appear to be more abundant during warmer periods (see also
Pienitz et aI., 1995; Wunsam et aI., 1995).
Ice cover, summer surface temperature and thermal regime have an overwhelming influence
on overall diatom assemblage in polar regions (Smol, 1988), while shifts in individual taxa
within these communities may reflect other limnological variables.
Taking these facts into consideration, prominent changes in the ratio of euplanktonic to nonplanktonic taxa (Ep/Np) were used to subdivide the Lama Lake sequence (Figure 7). Three
sections were clearly distinguished and may indicate subunits in terms of the lake's average
annual surface temperature and the amplitude of its fluctuation. These parameters are connected
with the duration of the ice-cover season and the character of mixing regime.
The first subunit with Ep/Np values below 1 coincided in range roughly with DAZ-l. Hahne
and Melles (1997) record high NAP values (non-arboreal pollen), mainly Artemisia and
Cyperaceae and place this unit in the Younger Dryas chronozone (Khotinsky, 1984). The most
distinctive feature in the diatom assemblages, besides the high percentage of small periphytic
diatoms and the predominance of taxa with alkaliphilous and oligotrophic affinities, is a
planktonic complex unique in the whole sequence, consisting of Cyclotella rossii, Aulacoseira
islandica and A. subarctica. The temperature tolerance of Cyclotella rossii, accounting for 20%
of the diatom assemblage, is high. In a transect spanning subarctic Fennoscandian lakes, it is
placed at the lower end of the temperature range (Weckstrom et aI., 1997) while in the North
Canadian training set (Pienitz et aI., 1995) C. rossii increases in abundance with temperature.
For the co-occurring Aulacoseira islandica and A. subarctica, growth under low light and
temperature conditions, survival in unfavourable conditions through the formation of resting
spores, and a start of annual production under the ice (resulting in a strong vernal peak) are
recorded (e.g. Popovskaya, 1977; Stoermer, 1993). Especially in large turbulent lakes (e.g. the
Great Lakes, the large Swedish lakes, Lake Baikal), A. islandica is observed in considerable
abundance (Popovskaya, 1977; Stoermer, 1983; Willen, 1984). Due to the meroplanktonic life
form, Aulacoseira sp. are favoured by turbulence in the water column (Lund, 1954).
In the second subunit, the Ep/Np values exceeded 1 and reached maximum values of 2.5,
meaning a predominance of euplanktonic diatoms. This section comprises the pollen
assemblage zones L6 to L9, related to the chronozones Preboreal, Boreal, Atlantic and
Subboreal (Hahne and Melles, 1997).
Strong fluctuations of the Ep/Np values along with a distinctive species succession within the
euplanktonic complex suggest further subdivisions described by the applied local diatom
assemblage zones (DAZ). As emphasized by Smol (1988), such shifts in individual taxa within
the diatom communities in polar regions may well reflect changes in environmental variables,
such as pH, nutrient content or conductivity, whose influence in such climates are weaker than
those of climate and ice cover. The distinctive maxima of individual planktonic diatom taxa
could be related to conditions favouring the growth of certain species. When grouping these
abundant species according to their ecological preferences, the proportion of the groupings in
which they are included is consequently determined to a high degree. Thus, it is considered
reasonable to discuss changes in the nutrient content of Lama Lake with respect to the species
succession within the planktonic complex (see section on trophy above). Strong similarities to
changes in catchment vegetation, as recorded from palynological investigations (Hahne and
Melles, 1997), are evident.
Parallel to the Preboreal warming, indicated by a rapid increase of arboreal pollen mainly of
the Betula exilis-type, diatom concentration increases and planktonic taxa progressively
predominate the diatom assemblages in DAZ-2. Cyclotella gordonensis, a small-sized coolwater species found in lakes extremely poor in nutrients (Kling and Hakansson, 1988), shows
Land-Ocean Systems in the Siberian Arctic: Dynamics and History
of open water (Smol, 1988), shallow water or periphytic taxa are favoured. Planktonic taxa
characteristic for deeper water appear to be more abundant during warmer periods (see also
Pienitz et aI., 1995; Wunsam et aI., 1995).
Ice cover, summer surface temperature and thermal regime have an overwhelming influence
on overall diatom assemblage in polar regions (Smol, 1988), while shifts in individual taxa
within these communities may reflect other limnological variables.
Taking these facts into consideration, prominent changes in the ratio of euplanktonic to nonplanktonic taxa (Ep/Np) were used to subdivide the Lama Lake sequence (Figure 7). Three
sections were clearly distinguished and may indicate subunits in terms of the lake's average
annual surface temperature and the amplitude of its fluctuation. These parameters are connected
with the duration of the ice-cover season and the character of mixing regime.
The first subunit with Ep/Np values below 1 coincided in range roughly with DAZ-l. Hahne
and Melles (1997) record high NAP values (non-arboreal pollen), mainly Artemisia and
Cyperaceae and place this unit in the Younger Dryas chronozone (Khotinsky, 1984). The most
distinctive feature in the diatom assemblages, besides the high percentage of small periphytic
diatoms and the predominance of taxa with alkaliphilous and oligotrophic affinities, is a
planktonic complex unique in the whole sequence, consisting of Cyclotella rossii, Aulacoseira
islandica and A. subarctica. The temperature tolerance of Cyclotella rossii, accounting for 20%
of the diatom assemblage, is high. In a transect spanning subarctic Fennoscandian lakes, it is
placed at the lower end of the temperature range (Weckstrom et aI., 1997) while in the North
Canadian training set (Pienitz et aI., 1995) C. rossii increases in abundance with temperature.
For the co-occurring Aulacoseira islandica and A. subarctica, growth under low light and
temperature conditions, survival in unfavourable conditions through the formation of resting
spores, and a start of annual production under the ice (resulting in a strong vernal peak) are
recorded (e.g. Popovskaya, 1977; Stoermer, 1993). Especially in large turbulent lakes (e.g. the
Great Lakes, the large Swedish lakes, Lake Baikal), A. islandica is observed in considerable
abundance (Popovskaya, 1977; Stoermer, 1983; Willen, 1984). Due to the meroplanktonic life
form, Aulacoseira sp. are favoured by turbulence in the water column (Lund, 1954).
In the second subunit, the Ep/Np values exceeded 1 and reached maximum values of 2.5,
meaning a predominance of euplanktonic diatoms. This section comprises the pollen
assemblage zones L6 to L9, related to the chronozones Preboreal, Boreal, Atlantic and
Subboreal (Hahne and Melles, 1997).
Strong fluctuations of the Ep/Np values along with a distinctive species succession within the
euplanktonic complex suggest further subdivisions described by the applied local diatom
assemblage zones (DAZ). As emphasized by Smol (1988), such shifts in individual taxa within
the diatom communities in polar regions may well reflect changes in environmental variables,
such as pH, nutrient content or conductivity, whose influence in such climates are weaker than
those of climate and ice cover. The distinctive maxima of individual planktonic diatom taxa
could be related to conditions favouring the growth of certain species. When grouping these
abundant species according to their ecological preferences, the proportion of the groupings in
which they are included is consequently determined to a high degree. Thus, it is considered
reasonable to discuss changes in the nutrient content of Lama Lake with respect to the species
succession within the planktonic complex (see section on trophy above). Strong similarities to
changes in catchment vegetation, as recorded from palynological investigations (Hahne and
Melles, 1997), are evident.
Parallel to the Preboreal warming, indicated by a rapid increase of arboreal pollen mainly of
the Betula exilis-type, diatom concentration increases and planktonic taxa progressively
predominate the diatom assemblages in DAZ-2. Cyclotella gordonensis, a small-sized coolwater species found in lakes extremely poor in nutrients (Kling and Hakansson, 1988), shows