260
Land-Ocean Systems in the Siberian Arctic: Dynamics and History
described by Sidorov and Gukov (1992). This is an indication of a complete consumption of
oxygen caused by a stable stratification of the water column and decomposition of organic
matter.
The measured low concentration of dissolved inorganic phosphorus is a feature typical of all
large Siberian river systems. In contrast, silicon concentrations are high. This reflects the
inflow of phosphorous-depleted and silicon-enriched ground water which is the major water
source during the winter season (Simanchuk, 1938; Zubakina, 1974, 1979; Rusanov et aI.,
1979). The very low concentration of SPM in the river « 1 mg/l) can be explained by low
current velocities and the complete absence of sediment input from runoff.
Although, the freshwater discharge during the winter season is low, inflow of freshwater
influences the thermohaline structure of the Laptev Sea near the delta. This is indicated by the
presence of a water layer with reduced salinity below 10 that is separated by a pycnocline from
the underlying more brackish water. Like salinity, the silicon concentration above the
pycnocline is also a result of the mixing of river water with the brackish water of the SE Laptev
Sea. Low suspended matter concentrations in river water and the missing turbulence in the icecovered SE Laptev Sea can explain the low SPM concentrations in the water column off the
delta.
The beginning of the spring thaw
The first indications of the beginning spring thaw and the resulting increase in water discharge
of the Lena river were observed during May 31 to June 5 in the Bykovskaya Channel. During
this period, the water level within the river increased and the concentration of silicon decreased
to its typical summer concentration of approximately 2100 /lg/l. The decrease in silicon
concentration is caused by a change in the sources of the river water. During winter the major
inflow comes from ground water while, on the contrary, terrestrial runoff is the dominate
source in summer (Gordeev and Sidorov, 1993). In the course of the summer, silicon is also
consumed to some extend by freshwater diatoms. Increasing turbulence at the riverbed and the
input of particles with the runoff are the causes for an increase in SPM concentrations recorded
during this time period.
While the silicon concentration in the river decreased the silicon concentration in the surface
water layer near the river mouth increased significantly. This is accompanied by a salinity
below 1 and the formation of a distinct halocline in the central part of the N -S transect near the
river mouth.
Low salinity in combination with high concentrations of silicon, low SPM concentrations
andmedium oxygen saturation levels suggest that with the onset of the breakup winter water is
flushed from the river channels to the sea, forming a wedge of freshwater between the ice
cover and the brackish water of the SE Laptev Sea.
The hydrochemical and thermohaline structure of the water column beneath the pycnocline
showed no significant changes. Only the oxygen saturation in the near bottom layer increased.
This might be caused by an increased turbulence in the saline water which is also supported by
a slight increase of suspended matter concentrations beneath the pycnocline. However, the main
oxygen distribution pattern did not change.
Beginning of the high discharge period (spring-summer situation)
The third oceanographic survey along the 130 0 30'E transect revealed a completely new
situation. The thickness of the freshwater wedge increased by 4-5 m. At station 10 near the
Trofimovskaya river mouth the freshwater layer - with a salinity between 0.15-0.22 and a
temperature about 0 °C - reached a thickness of 9.5 m. The suspended matter concentrations
measured in the freshwater layer off the delta near the Bykovskaya branch and within the
Land-Ocean Systems in the Siberian Arctic: Dynamics and History
described by Sidorov and Gukov (1992). This is an indication of a complete consumption of
oxygen caused by a stable stratification of the water column and decomposition of organic
matter.
The measured low concentration of dissolved inorganic phosphorus is a feature typical of all
large Siberian river systems. In contrast, silicon concentrations are high. This reflects the
inflow of phosphorous-depleted and silicon-enriched ground water which is the major water
source during the winter season (Simanchuk, 1938; Zubakina, 1974, 1979; Rusanov et aI.,
1979). The very low concentration of SPM in the river « 1 mg/l) can be explained by low
current velocities and the complete absence of sediment input from runoff.
Although, the freshwater discharge during the winter season is low, inflow of freshwater
influences the thermohaline structure of the Laptev Sea near the delta. This is indicated by the
presence of a water layer with reduced salinity below 10 that is separated by a pycnocline from
the underlying more brackish water. Like salinity, the silicon concentration above the
pycnocline is also a result of the mixing of river water with the brackish water of the SE Laptev
Sea. Low suspended matter concentrations in river water and the missing turbulence in the icecovered SE Laptev Sea can explain the low SPM concentrations in the water column off the
delta.
The beginning of the spring thaw
The first indications of the beginning spring thaw and the resulting increase in water discharge
of the Lena river were observed during May 31 to June 5 in the Bykovskaya Channel. During
this period, the water level within the river increased and the concentration of silicon decreased
to its typical summer concentration of approximately 2100 /lg/l. The decrease in silicon
concentration is caused by a change in the sources of the river water. During winter the major
inflow comes from ground water while, on the contrary, terrestrial runoff is the dominate
source in summer (Gordeev and Sidorov, 1993). In the course of the summer, silicon is also
consumed to some extend by freshwater diatoms. Increasing turbulence at the riverbed and the
input of particles with the runoff are the causes for an increase in SPM concentrations recorded
during this time period.
While the silicon concentration in the river decreased the silicon concentration in the surface
water layer near the river mouth increased significantly. This is accompanied by a salinity
below 1 and the formation of a distinct halocline in the central part of the N -S transect near the
river mouth.
Low salinity in combination with high concentrations of silicon, low SPM concentrations
andmedium oxygen saturation levels suggest that with the onset of the breakup winter water is
flushed from the river channels to the sea, forming a wedge of freshwater between the ice
cover and the brackish water of the SE Laptev Sea.
The hydrochemical and thermohaline structure of the water column beneath the pycnocline
showed no significant changes. Only the oxygen saturation in the near bottom layer increased.
This might be caused by an increased turbulence in the saline water which is also supported by
a slight increase of suspended matter concentrations beneath the pycnocline. However, the main
oxygen distribution pattern did not change.
Beginning of the high discharge period (spring-summer situation)
The third oceanographic survey along the 130 0 30'E transect revealed a completely new
situation. The thickness of the freshwater wedge increased by 4-5 m. At station 10 near the
Trofimovskaya river mouth the freshwater layer - with a salinity between 0.15-0.22 and a
temperature about 0 °C - reached a thickness of 9.5 m. The suspended matter concentrations
measured in the freshwater layer off the delta near the Bykovskaya branch and within the