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Land-Ocean Systems in the Siberian Arctic: Dynamics and History
Factor 4 (Thalassiosira nordenskioeldii assemblage) explains only 5.8% of the total variance.
The dominant species is Thalassiosira nordenskioeldii (marine-neritic species), and T.
antarctica (marine-neritic) is of secondary importance. This factor shows highest factor
loadings in some samples of the central and northeastern parts of the Laptev Sea shelf (Figure
8).
Diatom sediment assemblages and their relationship to oceanography
Based on the factor model of diatom sediment assemblages explained above the Laptev Sea
shelf can be subdivided into three provinces (Figure 8): the eastern province with dominating
Chaetoceros spp. and brackish water species, the central shelf province of which sediments are
mainly characterized by ice algae and a patchy occurrence of Thalassiosira nordenskioeldii, and
the coastal province where freshwater taxa are the dominant components. This distribution
pattern approximately traces the position of the polynya during winter months (Figure 8;
Reimnitz et aI., 1994; Dethleff, 1995). North of the polynya the sediments are dominated by the
ice algae and T. nordenskioeldii assemblages, whereas the sediments south of this polynya are
characterized by the freshwater diatoms and Chaetoceros assemblages.
Statistically, the ice algae assemblage is the most significant factor in the Laptev Sea and
explains 43.9% of the total variance. This assemblage particularly dominates the sediments of
the central shelf to the north of the wintry polynya (Figure 8). Characteristic for this region is
also the patchy dominance of the marine-brackish planktic diatom Thalassiosira
nordenskioeldii which is a typical species of the spring phytoplankton bloom in polar waters
(Paasche, 1975). A similar co-occurrence pattern of ice algae and T. nordenskioeldii in surface
sediments is also reported from the Bering and Chukchi Seas (Sancetta, 1981; Schandelmeier
and Alexander, 1981). A reason for this could be an ice algae and T. nordenskioeldii bloom that
occurs along the ice edge which retreats to the north during ice melting in spring and early
summer. In sediments from deeper regions (e.g. the continental slope) ice algae show generally
a low abundance, due to strong dissolution of the thin and delicate valves of the ice algae
(Cremer, 1998). Thus, the co-occurrence of ice algae and T. nordenskioeldii assemblages
points to shallow shelf regions with a seasonal ice cover and ice edge blooms during ice melting
in spring.
The freshwater diatoms assemblage is the second-significant factor and explains 19.9% of the
total variance. This assemblage predominantly consists of a freshwater diatom flora rich in
genera (see Table 2) and species. However, few species occur with higher abundances, e.g.
Aulacoseira subarctica, Asterionella formosa, Diatoma tenuis (Cremer, 1998). The freshwater
diatom flora is characteristic for coastal sediments and especially for the region to the east of the
Lena delta (Figure 8). In front of the Yana and Anabar Rivers diatom sediment assemblages not
only consist of freshwater diatoms but of other, marine and brackish water species (Figure 8).
The distribution of freshwater diatoms in sediments of the Laptev Sea reflects the intensity of
riverine freshwater supply by the rivers on the one hand and the mixing of river water with
brackish-marine water masses from the shelf on the other hand. In all, the occurrence of
freshwater diatoms in high quantities points to a strong riverine freshwater supply in the
vicinity of river mouths and river deltas.
The eastern region of the Laptev Sea shelf is dominated by the Chaetoceros assemblage. This
assemblage consists particularly of Chaetoceros resting spores. Arctic-marine-epibenthic
species of Nitzschia and Thalassiosira hyperborea are of secondary importance. The regions
showing a main significance of the Chaetoceros assemblage correspond with regions where
salinity of surface waters is reduced to brackish conditions due to strong river water influx
during summer (Figures 2 and 8). Resting spores of Chaetoceros are the most abundant
diatoms in sediments of the Laptev Sea. The chain forming vegetative stages of this genus
Land-Ocean Systems in the Siberian Arctic: Dynamics and History
Factor 4 (Thalassiosira nordenskioeldii assemblage) explains only 5.8% of the total variance.
The dominant species is Thalassiosira nordenskioeldii (marine-neritic species), and T.
antarctica (marine-neritic) is of secondary importance. This factor shows highest factor
loadings in some samples of the central and northeastern parts of the Laptev Sea shelf (Figure
8).
Diatom sediment assemblages and their relationship to oceanography
Based on the factor model of diatom sediment assemblages explained above the Laptev Sea
shelf can be subdivided into three provinces (Figure 8): the eastern province with dominating
Chaetoceros spp. and brackish water species, the central shelf province of which sediments are
mainly characterized by ice algae and a patchy occurrence of Thalassiosira nordenskioeldii, and
the coastal province where freshwater taxa are the dominant components. This distribution
pattern approximately traces the position of the polynya during winter months (Figure 8;
Reimnitz et aI., 1994; Dethleff, 1995). North of the polynya the sediments are dominated by the
ice algae and T. nordenskioeldii assemblages, whereas the sediments south of this polynya are
characterized by the freshwater diatoms and Chaetoceros assemblages.
Statistically, the ice algae assemblage is the most significant factor in the Laptev Sea and
explains 43.9% of the total variance. This assemblage particularly dominates the sediments of
the central shelf to the north of the wintry polynya (Figure 8). Characteristic for this region is
also the patchy dominance of the marine-brackish planktic diatom Thalassiosira
nordenskioeldii which is a typical species of the spring phytoplankton bloom in polar waters
(Paasche, 1975). A similar co-occurrence pattern of ice algae and T. nordenskioeldii in surface
sediments is also reported from the Bering and Chukchi Seas (Sancetta, 1981; Schandelmeier
and Alexander, 1981). A reason for this could be an ice algae and T. nordenskioeldii bloom that
occurs along the ice edge which retreats to the north during ice melting in spring and early
summer. In sediments from deeper regions (e.g. the continental slope) ice algae show generally
a low abundance, due to strong dissolution of the thin and delicate valves of the ice algae
(Cremer, 1998). Thus, the co-occurrence of ice algae and T. nordenskioeldii assemblages
points to shallow shelf regions with a seasonal ice cover and ice edge blooms during ice melting
in spring.
The freshwater diatoms assemblage is the second-significant factor and explains 19.9% of the
total variance. This assemblage predominantly consists of a freshwater diatom flora rich in
genera (see Table 2) and species. However, few species occur with higher abundances, e.g.
Aulacoseira subarctica, Asterionella formosa, Diatoma tenuis (Cremer, 1998). The freshwater
diatom flora is characteristic for coastal sediments and especially for the region to the east of the
Lena delta (Figure 8). In front of the Yana and Anabar Rivers diatom sediment assemblages not
only consist of freshwater diatoms but of other, marine and brackish water species (Figure 8).
The distribution of freshwater diatoms in sediments of the Laptev Sea reflects the intensity of
riverine freshwater supply by the rivers on the one hand and the mixing of river water with
brackish-marine water masses from the shelf on the other hand. In all, the occurrence of
freshwater diatoms in high quantities points to a strong riverine freshwater supply in the
vicinity of river mouths and river deltas.
The eastern region of the Laptev Sea shelf is dominated by the Chaetoceros assemblage. This
assemblage consists particularly of Chaetoceros resting spores. Arctic-marine-epibenthic
species of Nitzschia and Thalassiosira hyperborea are of secondary importance. The regions
showing a main significance of the Chaetoceros assemblage correspond with regions where
salinity of surface waters is reduced to brackish conditions due to strong river water influx
during summer (Figures 2 and 8). Resting spores of Chaetoceros are the most abundant
diatoms in sediments of the Laptev Sea. The chain forming vegetative stages of this genus