144
Land-Ocean Systems in the Siberian Arctic: Dynamics and History
investigated from a "benthic view", focussing on pigment concentrations and microbial
activities in the surface sediments of the Laptev Sea slope.
Material and methods
During two Arctic expeditions (ARK IXl4 in 1993 and ARK XIII in 1995) with RV Polarstem
to the Laptev Sea, sediment samples were collected in summer 1993 and 1995 from 23 stations
along three transects and from 40 stations along six transects respectively. Water depths ranged
from 37 to 3831 m. Samples were taken with a multiple corer (Barnett et aI., 1984) and were
transferred immediately to a refrigerated laboratory (0 0C) after recovery. Subsamples from the
surface sediment (0-1 cm) were shock-frozen at -80°C for later analyses of the pigment
concentrations. After extraction with acetone (90 %) from the sediments, chlorophyll a and
phaeopigments were determined flu oro metrically with a Turner fluorometer according to
Shuman and Lorenzen (1975). The sum of chlorophyll a and phaeopigments is expressed as
chlorophyll a equivalents as described by Thiel (1982). Activity of the enzyme ~-glucosidase,
cleaving ~-glycosides like cellulose, was measured immediately after subsampling at in situ
temperature (0 0C). The maximum velocity (V max) of the enzyme was determined at saturation
level of the methylumbelliferone (MUF) labelled substrate MUF-~-glucoside according to
Boetius and Lochte (1994).
Results and discussion
During summer in polar waters, the light availability is controlled by snow and ice cover.
Primary production is drastically reduced under ice in comparison to the open water (Sakshaug
and Slagstad, 1991; Wassmann et aI., 1991; Smith 1995). In the Laptev Sea, the extent of the
ice cover varies strongly between seasons and also interannually (Eicken et aI., 1997). On an
annual average, the eastern section has less ice cover during summer because of the generally
prevailing wind direction and by the inflow of warmer water from the river Lena. In 1993, the
eastern Laptev Sea was ice-free and the western section was ice-covered during summer
(Boetius and Damm, 1997). In 1995, the shelf edge of the Laptev Sea was ice-free during the
whole summer and only the eastern part of the slope was ice-covered (Figure 1). Data from the
investigations in 1995 in the Laptev Sea indicate that the 1-2 m thick ice cover prevented the
build-up of high phytoplankton standing stocks in the water column: Primary production in the
ice-free waters reached 340 mg C m- 2 d- i ; total production under one square meter of sea ice
was less than 20 mg C d- i (Grossmann and Gleitz, 1997). Accordingly, nutrient concentrations
were mostly higher under sea ice than in open water (Nalbandov, 1997). Nutrient exhaustion
was measured only at the shelf and slope stations along transect H in 1993 (Luchetta et aI.,
1994), where a phytoplankton bloom occurred along the receding ice edge. In 1995, no such
bloom was observed during the cruise (Bartel, 1997), but at some of the shallower stations
nutrient concentrations were low (Nalbandov, 1997).
Average concentrations of chlorophyll a equivalents in the sediments of the ice-free Arctic
slope were comparable to those from continental slopes in temperate oceanic regions
(Pfannkuche, 1985; Bovee et aI., 1990; Scheibe 1990). However, they were considerably
lower under the permanent ice cover in the central Arctic basin than those reported from the
central NE-Atlantic abyssal plains at similar depths (Pfannkuche, 1992). During both Laptev
Sea expeditions, chlorophyll a equivalents (Figure 2) and the potential activity of the enzyme ~glucosidase (Figure 3) in the surface sediments (0-1 cm) decreased with increasing water depth
(100-3000 m) ten-fold and six-fold along the ice-free transects in 1993 and 1995 respectively.
Land-Ocean Systems in the Siberian Arctic: Dynamics and History
investigated from a "benthic view", focussing on pigment concentrations and microbial
activities in the surface sediments of the Laptev Sea slope.
Material and methods
During two Arctic expeditions (ARK IXl4 in 1993 and ARK XIII in 1995) with RV Polarstem
to the Laptev Sea, sediment samples were collected in summer 1993 and 1995 from 23 stations
along three transects and from 40 stations along six transects respectively. Water depths ranged
from 37 to 3831 m. Samples were taken with a multiple corer (Barnett et aI., 1984) and were
transferred immediately to a refrigerated laboratory (0 0C) after recovery. Subsamples from the
surface sediment (0-1 cm) were shock-frozen at -80°C for later analyses of the pigment
concentrations. After extraction with acetone (90 %) from the sediments, chlorophyll a and
phaeopigments were determined flu oro metrically with a Turner fluorometer according to
Shuman and Lorenzen (1975). The sum of chlorophyll a and phaeopigments is expressed as
chlorophyll a equivalents as described by Thiel (1982). Activity of the enzyme ~-glucosidase,
cleaving ~-glycosides like cellulose, was measured immediately after subsampling at in situ
temperature (0 0C). The maximum velocity (V max) of the enzyme was determined at saturation
level of the methylumbelliferone (MUF) labelled substrate MUF-~-glucoside according to
Boetius and Lochte (1994).
Results and discussion
During summer in polar waters, the light availability is controlled by snow and ice cover.
Primary production is drastically reduced under ice in comparison to the open water (Sakshaug
and Slagstad, 1991; Wassmann et aI., 1991; Smith 1995). In the Laptev Sea, the extent of the
ice cover varies strongly between seasons and also interannually (Eicken et aI., 1997). On an
annual average, the eastern section has less ice cover during summer because of the generally
prevailing wind direction and by the inflow of warmer water from the river Lena. In 1993, the
eastern Laptev Sea was ice-free and the western section was ice-covered during summer
(Boetius and Damm, 1997). In 1995, the shelf edge of the Laptev Sea was ice-free during the
whole summer and only the eastern part of the slope was ice-covered (Figure 1). Data from the
investigations in 1995 in the Laptev Sea indicate that the 1-2 m thick ice cover prevented the
build-up of high phytoplankton standing stocks in the water column: Primary production in the
ice-free waters reached 340 mg C m- 2 d- i ; total production under one square meter of sea ice
was less than 20 mg C d- i (Grossmann and Gleitz, 1997). Accordingly, nutrient concentrations
were mostly higher under sea ice than in open water (Nalbandov, 1997). Nutrient exhaustion
was measured only at the shelf and slope stations along transect H in 1993 (Luchetta et aI.,
1994), where a phytoplankton bloom occurred along the receding ice edge. In 1995, no such
bloom was observed during the cruise (Bartel, 1997), but at some of the shallower stations
nutrient concentrations were low (Nalbandov, 1997).
Average concentrations of chlorophyll a equivalents in the sediments of the ice-free Arctic
slope were comparable to those from continental slopes in temperate oceanic regions
(Pfannkuche, 1985; Bovee et aI., 1990; Scheibe 1990). However, they were considerably
lower under the permanent ice cover in the central Arctic basin than those reported from the
central NE-Atlantic abyssal plains at similar depths (Pfannkuche, 1992). During both Laptev
Sea expeditions, chlorophyll a equivalents (Figure 2) and the potential activity of the enzyme ~glucosidase (Figure 3) in the surface sediments (0-1 cm) decreased with increasing water depth
(100-3000 m) ten-fold and six-fold along the ice-free transects in 1993 and 1995 respectively.