Polar Biology

, Volume 29, Issue 10, pp 837–848 | Cite as

Distribution and structure of macrobenthic fauna in the eastern Laptev Sea in relation to environmental factors

  • Matthias Steffens
  • Dieter Piepenburg
  • Michael K. Schmid
Original Paper

Abstract

The Laptev Sea still ranks among the less known regions of the world’s ocean. Here, we describe the distribution and composition of macrobenthic communities of the eastern shelf and identify key environmental control factors. Samples were collected from dredge catches carried out at 11 stations at depths between 17 and 44 m in August/September 1993 during the TRANSDRIFT I cruise of the Russian R/V “Ivan Kireev.” A total of 265 species were identified from the samples, mostly crustaceans (94). Species numbers per station ranged from 30 to 104. Macrobenthic community distribution clearly showed a depth zonation, consisting of a “Shallow” zone (<20 m), dominated by the crustaceans Mysis oculata (Mysidacea) and Saduria entomon (Isopoda) as well as molluscs, an “Intermediate” zone (20–30 m), characterised by a clear dominance of the bivalve Portlandia arctica, and a “Deep” zone (>30 m) with bivalves P. arctica and Nuculoma bellotii as well as brittle stars Ophiocten sericeum and Ophiura sarsi being most abundant. According to a correlation analysis between faunal and environmental data a combination of duration of ice cover and water depth, respectively, showed the highest affinity to macrobenthic distribution. We conclude that the food input to the benthos, which is largely related to ice-cover regime, and the stress due to the pronounced seasonal salinity variability, which is primarily related to water depth, are prime determinants of macrobenthic community distribution and major causes of the prominent depth zonation in the Laptev Sea. Within the depth zones, sediment composition seems to be most significant in controlling the patterns in the distribution of the benthic fauna.

Introduction

The Laptev Sea is a truly high-Arctic region characterised by extreme climatic conditions and, due to its isolated location north of mid-Siberia, least affected by Atlantic and Pacific water masses (Zenkevitch 1963) in the Arctic. To a large extent, it is a very shallow shelf sea the hydrography of which is shaped by long-lasting ice cover and pronounced salinity fluctuations, caused by the seasonally pulsed riverine inflow of freshwater, primarily through the discharge of the Lena River (Timokhov 1994).

Despite the remoteness and difficult accessibility of the Laptev Sea, first benthic investigations can be dated back to Nordenskjøld’s Northeast Passage in 1878 (Sirenko and Piepenburg 1994). Later, there were a number of Russian (1912–1914) and Soviet expeditions, primarily during the 1930s, that encompassed studies of the bottom fauna. In 1973, Golikov and co-workers started a first inventory of the littoral habitats of the Laptev Sea, using a scuba-diving approach (Golikov 1990). In the 1990s, international interdisciplinary research programs focused on this rather secluded Arctic sea because of its significance as one of the major “ice factories” in the Arctic, leading to a considerable increase in our knowledge about the Laptev Sea system (Kassens et al. 1999).

The benthos of the Laptev Sea shelf has repeatedly been reported to be generally poorer than in other Eurasian-Arctic seas in terms of diversity, abundance and biomass (Zenkevitch 1963; Golikov 1990; Sirenko and Piepenburg 1994; Sirenko 2001). This scarcity was attributed to the severe climate in the Laptev Sea, as well as to the variable salinities caused by the pronounced fluvial dilution during summer.

Generally, there are only a few parameters—such as salinity (in shallow waters), sediment texture and food availability—that play the most important role in determining the faunistic composition of Arctic benthic communities (Dayton 1990). However, the relative significance of the various factors can differ considerably among habitats and regions. For instance, in the Laptev Sea the macrobenthic distribution patterns have repeatedly been shown to be primarily controlled by the near-bottom salinities (Petryashov 1994; Petryashov et al. 1999; Gukov et al. 1999) while sediment composition is apparently of secondary importance (Gukov et al. 1999). In other polar regions, other factors have been identified as paramount control agents, e.g. high sedimentation rates in riverine or glacial estuaries (Syvitski et al. 1989; Jørgensen et al. 1999), ice scouring in littoral shallow-water habitats (Conlan et al. 1998) or advection of organic matter from adjacent more productive areas (Grebmeier and Barry 1991; Feder et al. 1994a, b, 2005; Gili et al. 2001).

While a number of taxonomic and zoogeographic studies have been conducted in the eastern Laptev Sea, Russian research mainly focused on the study of benthic communities in shallow coastal bays (Gukov 1989, 1991, 1992a, 1996, 1997; Sidorov and Gukov 1992; Gukov et al. 1999) and on the delineation of zoogeographic (Sirenko and Piepenburg 1994; Sirenko et al. 1995; Petryashov et al. 1999) and trophic zones (Gukov 1995, 1998). Since most of these studies were based on grab samples, they may have underestimated the fraction of mobile epifauna such as amphipods or mysids due to their avoidance behaviour with regard to grab sampling devices. Photographic surveys in the Laptev Sea covered the epibenthic megafauna (Piepenburg and Schmid 1997). Additional research efforts were geared towards the impact of single environmental parameters such as freshwater influence (Gukov 1992b, 1993) on the macrobenthos. A community analysis of the macrobenthic fauna in relation to an extensive set of environmental parameters had not been carried out to date. Therefore, the motivation of the present study was to contribute to the existing knowledge about the Laptev Sea benthos by deploying towed gear which has a better catch efficiency for mobile epifauna than grabs and to relate macrobenthic distribution patterns to a broad set of environmental parameters. In the following we use the term “macrobenthos” for describing the assemblages of “large” benthic organisms, i.e. according to the well-established scientific usage (Gage and Tyler 1991), for those seafloor organisms that are large enough to be retained on sieves with a mesh size of 0.5 mm (macrobenthos sensu strictu, mostly infaunal) or to be visible in seabed images and/or to be caught by towed sampling gear (megabenthos sensu strictu, mostly epifaunal).

The objectives of this study are to (1) describe the macrobenthic distribution patterns in the eastern Laptev Sea, (2) delineate macrobenthic communities by means of multivariate statistics and (3) identify key environmental factors controlling the distribution and composition of macrobenthic fauna.

Study area

The Laptev Sea is the smallest of the Russian Arctic seas, covering an area of about 498,000 km2 between the Taimyr Peninsula and the New Siberian Islands (Fig. 1). About three quarters of the Laptev Sea belong to the shelf with comparably low water depths (Holmes and Creager 1974). The average depth is 53 m, and very shallow banks are less than 20 m deep (Timokhov 1994). The shelf bed is furrowed by submarine valleys extending from the mouths of the large rivers flowing into the Laptev Sea (Fig. 1).
Fig. 1

Laptev Sea. Map showing locations of dredge stations during the TRANSDRIFT I cruise of R/V “Ivan Kireev” in 1993. Depth contours in metres

The hydrography of the Laptev Sea is characterised by the discharge of large rivers leading to reduced salinities in the surface water layer and enhanced ice formation. Most important in this respect is the Lena, discharging about 520 km3 year−1 freshwater which is equivalent of 65% of the total annual freshwater inflow to the Laptev Sea (Timokhov 1994; Rachold et al. 1996). This leads to a very variable and patchy distribution of hydrographic conditions, which in turn results in steep ecological gradients. The southeastern Laptev Sea is characterised by a stable two-layer stratification of water masses, featuring a brackish-water top layer during summer (Timokhov 1994).

The Laptev Sea is covered with sea ice throughout almost 9 months, from the beginning of October until mid-June (Bareiss et al. 1999). In winter, a fast-ice belt covers the southern and southeastern Laptev Sea, extending from the coastline 500 km northwards approximately until the 20-m depth contour. North of this belt, there is a narrow flaw lead, separating the fast-ice from the drift-ice region.

Surface sediments of the eastern Laptev Sea are dominated by silt and clay (“fines”) reflecting its role as a major depositing area for sediments discharged by the Lena and Yana Rivers (Lindemann 1994). Rachold et al. (1996) estimate the Lena to transport a sediment load of 21×106 t a−1 into the Lena delta and—while most of it is deposited within the Lena delta—approximately 10–17% is transported into the Laptev Sea. The majority of riverine fine sediments that reach the sea are apparently deposited in the submarine valleys while the seabed of the shallow shoals feature higher proportions of sand (Lindemann 1994).

Materials and methods

Field sampling

Samples were taken during the TRANSDRIFT I cruise of the Russian R/V “Ivan Kireev” to the Laptev Sea in 1993. Macrobenthic animals were collected from dredge catches carried out at 11 stations in the eastern Laptev Sea at depths between 14 and 44 m between August and September 1993 (Table 1). The dredge had an opening of 1×0.5 m and a mesh size of 5 mm. Dredge hauls were conducted at a constant ship speed of 1 kn and lasted an average of 10 min.
Table 1

Stations during the TRANSDRIFT I cruise of R/V “Ivan Kireev” in 1993. Station label, sampling date, geographic start positions of dredge hauls, water depth (m), bottom-water temperature (°C), bottom-water salinity, weight percentage (%) of “fines” (clay and silt) in surface sediments, average percentage (%) of the period of the year (1978–1991) during which the station was covered by sea ice, species numbers (S), total standardised abundances (ind. 300 m2)

Station

Sampling date

Geographic position

Water depth (m)

Bottom-water temperature (°C)

Bottom-water salinity

Fines (weight-%)

Average ice-cover duration (% of year)

No. of species

Total abundance (ind. 300 m−2)

13

22 August 1993

73°03.8′N 139°24.2′E

17

−1.18

29.05

69.3

77.5

31

2,500

16

23 August 1993

73°00.2′N 131°30.6′E

28

−1.26

29.31

98.9

72.9

32

7,300

21

20 August 1993

73°29.9′N 131°39.9′E

25

−1.17

27.41

94.9

74.4

30

1,800

42

15 August 1993

74°31.3′N 127°27.5′E

34

−1.54

32.31

67.3

67.1

49

10,600

44

17 August 1993

74°24.5′N 131°01.3′E

30

−1.20

28.06

23.9

71.5

87

2,000

46

17 August 1993

74°29.8′N 134°03.2′E

14

0.03

25.70

20.3

75.8

39

500

48

18 August 1993

74°30.0′N 137°05.0′E

22

−1.46

29.90

90.2

79.4

76

3,800

49

18 August 1993

74°29.9′N 139°41.3′E

25

−1.24

30.16

39.9

79.9

48

130

53

04 September 1993

74°58.7′N 129°46.6′E

40

−1.57

32.82

95.0

70.0

38

3,800

70

07 September 1993

75°18.3′N 129°32.6′E

44

−1.69

33.19

76.7

70.7

104

6,200

73A

01 September 1993

75°48.9′N 134°23.2′E

43

−0.08

31.81

85.0

71.8

56

17,000

Dredge stations were distributed along three transects in order to cover longitudinal and latitudinal gradients (Fig. 1): one longitudinal transect following the Eastern Lena Valley along about 130°E, another longitudinal one following the Yana Valley, and a latitudinal transect between the Lena Delta and Kotelnyy Island along about 74°30′N.

Environmental parameters

Bottom-water temperatures and salinities were recorded with a CTD at the same stations at which the dredge was deployed. Hydrographic data were made available by Hölemann (Alfred Wegener Institute for Polar and Marine Research Bremerhaven, Germany) and Dmitrenko (Arctic and Antarctic Research Institute St Petersburg, Russia).

Sedimentological data about surface seabed sediments, taken concomitantly during the TRANSDRIFT I cruise, was provided by Lindemann (1994). The weight percentages of silt (2–63 μm grain size) and clay (<2 μm grain size) were summarised under the term “fines.”

We considered variability in ice coverage as a proxy of food input to the benthos, as it is well established that ice cover strongly affects the production and sedimentation of organic matter to the seabed (Grebmeier and Barry 1991).

Since sea-ice concentration data of the study area were not available to us for the year 1993, we used a data set where the covered time period (ranging from 1978 to 1991) most closely matched the year of sampling. We assumed the sea-ice concentrations obtained for this time period to be representative in terms of our study.

Ice conditions in the study area were based on the relative portion of the year, averaged across the period from 1978 to 1991, during which a location is covered by sea ice. Satellite data to calculate the percentages were made available by the US Geological Survey (Schweitzer 1995).

Sample processing

Catches were sieved on board through 1 mm mesh size and preserved in a 4% buffered seawater–formalin solution for further analyses.

In the laboratory in Kiel, specimens were counted and identified to putative species or the lowest possible taxon, using keys by Sars (1891), Mortensen (1927), Gaevskaya (1948), D’yakonov (1950, 1954), Klekowski and Weslawski (1991, 1992), Kirkegaard (1992, 1996) and Hartmann-Schröder (1996).

Data analysis

The type of sampling gear used has a great influence, and largely determines the outcome of an inventory of benthic communities (Holme and McIntyre 1984). Dredges do not yield quantitative data on benthic abundance because the sampling efficiency of the gear is unknown and differs among taxa and body sizes. Also, as a rule, the swept area of the gear cannot be determined with sufficient precision. Therefore, we used relative rather than absolute abundance values for the community analyses, as these would not change among sites, hence allowing for valid between-station comparisons.

As a consequence, our dredge results cannot be directly compared to those of benthic investigations based on grab samples (Petryashov 1994; Petryashov et al. 1999) but are used to supplement previous grab surveys by targeting an otherwise neglected fraction of the macrobenthic communities.

Specimen counts (not including nematodes and colonial organisms) were calculated to a swept area of 300 m2. Swept area was estimated by combining information of dredge width, trawl time and ship speed. Total abundances are presented in order to depict gross differences (orders of magnitude) between stations but are not used in the further analysis for the reasons explained above.

Species numbers were determined for each station. As measures of species diversity and evenness, ln-based Shannon–Wiener (Shannon and Weaver 1949) and Pielou indices (Pielou 1966), respectively, were calculated for each station (not including nematodes and colonial organisms).

Multivariate statistics were applied to perform community analyses by means of the software package PRIMER v5.01 (Clarke and Warwick 1994). In order to quantify similarities between stations in faunistic composition (Q analysis) Bray–Curtis indices were calculated based on the √√–transformed abundances of 72 genera (those containing all species occurring at at least two stations). To characterise similarities between genera in distribution patterns across stations (R analysis), the set of 72 genera was reduced further to only those 36 genera with an abundance share of at least 1% at at least one station, prior to computation of Bray–Curtis indices.

Cluster analysis and multidimensional scaling (MDS) were used to produce visual presentations of the similarity patterns between stations and genera (Clarke and Warwick 1994). Applying the explorative approach proposed by Clarke and Warwick (1994) distinct faunistic zones (i.e. station groups) and assemblages (i.e. genera groups) were delineated. From the inspection of dendrograms and MDS plots a “shade matrix,” i.e. a so-called “community table,” was derived, containing the stations and genera sorted according to their resemblances and depicting the abundance of each genus at each station, which helps to identify dominant and discriminator species of each assemblage.

In an attempt to explain the multispecies distribution patterns, they were correlated with the recorded set of environmental factors by means of the BIO-ENV approach (Clarke and Ainsworth 1993). This explorative method correlates the biotic pattern (represented in the faunistic station similarity matrix), using the weighted Spearman rank correlation (ρS), with the resemblances (Euclidean distances) between stations computed for each single environmental parameter separately (k=1) as well as for all possible combinations of them (k=2, 3, ..., n). It is then possible to identify the environmental parameter or the subset of environmental parameters that correlates best with—and, by implication, may influence most—the distribution of the macrobenthic assemblages. Longitude and ice cover as well as bottom-water salinity and water depth were highly correlated (r=0.918 and 0.829, respectively). We, therefore, excluded longitude and salinity from the BIO-ENV analysis according to Clarke and Ainsworth (1993). Oceanographic measurements in the Laptev Sea showed that water depth is a proxy for salinity variability: salinity was highly variable at shallow depths and less variable at greater depths (Timokhov 1994; Dmitrenko et al. 1995).

Results

A total of 265 putative macrobenthic species was identified in the samples. Most species were crustaceans (94), followed by polychaetes (67), molluscs (43), echinoderms (14) and cnidarians (7) (a complete species list is available from the first author on request).

Local species numbers (i.e. species numbers per station) ranged from 30 (stn 21) to 104 (stn 70) (Table 1).

Abundance varied widely between 130 ind. 300 m−2 southeast of Kotelnyy Island (stn 49) to 17,000 ind. 300 m−2 northwest of Kotelnyy Island (stn 73A) (Table 1).

The Q analysis of faunistic station similarities classified the 11 stations into three groups that corresponded to distinct depth zones (Fig. 2). The R analysis of inter-genus resemblances allocated the 36 selected abundant genera to 4 genera groups (Fig. 3). The “shade matrix” (Fig. 4) of stations and genera reveals that one genera group is mainly associated with the “Shallow” zone while two other groups had their highest abundances in the “Intermediate” and “Deep” zones, respectively. Some species could not be allocated to a certain group: they showed no clear resemblance pattern because they were either rather rare or occurred in all depth zones with similar abundance. The “Shallow” zone (<20 m) was dominated by the crustacean species Mysis oculata (Mysidacea) and Saduria entomon (Isopoda), as well as by molluscs. The stations of the “Intermediate” zone (20–30 m) were characterised by a clear dominance of the bivalve Portlandia arctica. The bivalve Nuculoma bellotii was abundant and the brittle stars Ophiocten sericeum and Ophiura sarsi were common to abundant at a few stations in the “Deep” zone (30–44 m). Within depth zones, faunistic composition as well as diversity and evenness is strongly related to sediment composition (Fig. 5). Stations with high “fines” proportions are dominated by bivalves and are very poor in diversity and evenness. Stations with relatively high sand proportions are more diverse are generally characterised by crustaceans at shallow depths, while in greater depths echinoderms prevail.
Fig. 2

Pattern of station similarities in macrobenthic composition (Q analysis). Bray–Curtis similarities were calculated between station pairs based on the relative abundance of each genus at each station. a Dendrogram, b multidimensional scaling (MDS) plot (stress=0.1), c map showing the spatial distribution of station groups determined from dendrogram and MDS plot. Different shadings indicate grouping of stations at three depths

Fig. 3

Pattern of generic similarities in distribution across stations (R analysis). Only those 36 macrobenthic genera that had an abundance share of at least 1% at at least one station were included in the analysis. Bray–Curtis similarities were calculated between pairs of genera based on the relative abundance of each genus at each station. a Dendrogram, b multidimensional scaling plot (stress=0.2). Different shadings indicate grouping of genera. Group labels “Shallow,” “Intermediate” and “Deep” indicate which genus group is typical for a particular station group from Fig. 2

Fig. 4

Shade matrix of 11 stations and 36 macrobenthic genera that were selected for R analysis. Stations and genera are re-sorted according to their resemblance determined by Q and R analysis, respectively, to show the distribution of genera groups (assemblages) across station groups (depth zones). Relative abundances (%) of genera in dredge catches are depicted by symbols of increasing size. F frequencies of occurrence across stations

Fig. 5

Distribution of biotic and environmental parameters among stations and station groups (depth zones). a Relative abundance (%) shares of major taxa, b species numbers (S), Shannon–Wiener diversities, Pielou evenness values, c average percentage (%) of the period of the year (1978–1991) during which the station was covered by sea ice, d bottom-water temperature (°C) and salinity, e weight percentage (%) of “fines” (clay and silt) in surface sediments, f water depth (m)

The BIO-ENV comparison between faunal and environmental data indicated that of all single parameters (Table 2, k=1) the length of ice cover showed the highest affinity to macrobenthic distribution (ρS=0.42), followed by water depth (ρS=0.39) and proportion of fines in surface sediments (ρS=0.26). The faunal patterns were even better correlated to a combination of ice cover and depth (ρS=0.61) or ice cover, depth and fines (ρS=0.50) whereas combinations of other or more parameters yielded lower ρS values (Table 2).
Table 2

BIO-ENV analysis. Results of the correlation analysis of the relationship between macrobenthic distribution patterns and environmental factors (ice cover, water depth, fines and bottom-water temperature). Combinations of environmental factors, k at a time, yielding Spearman rank correlation coefficients (ρS) between biotic and environmental resemblance data. Within each k category, parameters—or parameter combinations—are sorted according to their ρS values. Bold type indicates the best combination (maximum ρS)

k

ρS

Best variable combinations

1

0.42

Ice cover

   

0.39

Water depth

   

0.26

Fines

   

0.13

Bottom-water temperature

   

2

0.61

Ice cover

Water depth

  

0.46

Water depth

Fines

  

0.44

Ice cover

Fines

  

0.35

Water depth

Bottom-water temperature

  

0.33

Ice cover

Bottom-water temperature

  

0.21

Fines

Bottom-water temperature

  

3

0.50

Ice cover

Water depth

Fines

 

0.48

Ice cover

Water depth

Bottom-water temperature

 

0.37

Water depth

Fines

Bottom-water temperature

 

0.34

Ice cover

Fines

Bottom-water temperature

 

4

0.42

Ice cover

Water depth

Fines

Bottom-water temperature

Discussion

Species numbers

In terms of species diversity, the Laptev Sea has long been regarded as one of the poorest regions in the Arctic (Zenkevitch 1963; Golikov 1990). However, this notion was revised when the Russian–German expeditions since the early 1990s and the identification efforts of specialised taxonomists added numerous additional species known to occur in the Laptev Sea. While Zenkevitch (1963) reported a total of 522 invertebrate species, Sirenko and Piepenburg (1994) gave a figure of 1,084, and Petryashov et al. (1999) increased the tally to 1,234. Most of these (1,114) belong to the macrozoobenthos (i.e. macrobenthos, excluding plants) (Sirenko 2001). Our dredge inventory of the macrozoobenthic fauna provided a total of 265 species. A grab survey carried out concomitantly during the TRANSDRIFT I cruise in 1993 (Fahl et al. 2001) yielded about 200 macrozoobenthic species. The current estimate of the total macrozoobenthic species number for the Laptev Sea is still smaller than that of the Barents Sea (2,081) but higher than the figures reported for the East Siberian Sea (824) and the Chukchi Sea (935) (Sirenko 2001).

Zonation

The most conspicuous feature in the spatial distribution of macrofauna is the pronounced depth zonation, although the depth range covered by our investigation was rather narrow (14–44 m). This finding corroborates the basic conclusions of previous benthos investigations in the Laptev Sea (Piepenburg and Schmid 1997; Petryashov et al. 1999). In fact, the zonation pattern presented here largely matches the biogeographic regions reported by Petryashov et al. (1999). The “Shallow” assemblage features species which are known to tolerate pronouncedly lowered salinities (e.g. M. oculata and S. entomon), while the “Intermediate” stations are characterised by “polyhaline-Arctic” water masses (bottom-water salinities 18–30) and indicator species (e.g. P. arctica) (Petryashov et al. 1999). Most of the “Deep” stations are under the influence of “poly-euhaline-Arctic” water (bottom-water salinity range 30–32), indicated by the enhanced abundances of echinoderms and the bivalve N. bellotii. Only our deepest station is in the transition zone to the “euhaline-Arctic” region (bottom-water salinities >32), which is characterised by the presence of species such as, e.g. O. sarsi, Ctenodiscus crispatus and Munnopsis typica (Petryashov et al. 1999).

Based on a photographic survey of the epibenthic megafauna, Piepenburg and Schmid (1997) reported essentially the same abundance distribution pattern as we do here. They found that on shallow shoals of the Laptev Sea, at a water depth less than 30 m, epibenthic abundance was low. However, from the slopes of the shelf valleys deeper than 30 m, not subject to reduced and fluctuating salinities and possible ice gouging impacts, they recorded very high densities of the brittle star O. sericeum. These high densities are similar to maximum values found for O. sericeum on shelf banks in the Barents Sea (Piepenburg and Schmid 1996a) or the Belgica Bank off Northeast Greenland (Piepenburg and Schmid 1996b). These results imply that there is no significant difference between the Greenland, Barents and Laptev Sea shelves in terms of ophiuroid abundance or biomass (Piepenburg 2000), except for the very shallow Laptev Sea shoals.

Environmental control

Given the narrow depth range, the clear zonation pattern in the distribution of macrobenthic abundance and community composition in the southern Laptev Sea is not controlled by water depth per se (i.e. hydrostatic pressure) but by environmental factors that are strongly correlated with water depth. The fauna of the “Shallow” assemblage, characterised by a rather low diversity and high proportions of mobile species, is thought to be primarily influenced by environmental stress (as discussed in Feder et al. 1994a, b), caused by the input of freshwater and sediments from the discharge of the Lena and Yana Rivers (Timokhov 1994) or by the direct scouring impacts of either anchor ice or grounding icebergs or pack-ice floes (Lindemann 1994; Lindemann et al. 1995) as was found for other benthic shelf habitats (Gutt et al. 1996; Conlan et al. 1998; Peck et al. 1999). The “Intermediate” and “Deep” assemblages are probably primarily determined by higher and more stable salinities as well as sediment grain size (Feder et al. 1994b). Stations under the influence of fluvially modified water masses showed high proportions of fine sediments and a very pronounced dominance of a few detritovorous bivalves, such as P. arctica and N. bellotii. Similar findings were reported by other studies in the Laptev Sea (Gukov 1989, 1998) as well as other polar areas (Syvitski et al. 1989; Jørgensen et al. 1999). High sedimentation rates of inorganic matter are thought to effectively impede the settlement of suspension-feeding organisms (Moore 1977; Wlodarska-Kowalczuk and Pearson 2004). At stations with freshwater influence, species numbers and diversities were as low as those of the “Shallow” assemblage. High diversities were recorded at stations 44 and 48 which are located in the transition zone between “polyhaline-Arctic” and “poly-euhaline-Arctic” water masses (Petryashov et al. 1999), probably because species of different zoogeographic distribution type can co-exist here. In addition, Lindemann et al. (1995) found a maximum number of ice gouges at the western flank of the Stolbovoy Bank near station 44. This may indicate that ice scouring exerts a disturbance regime at this location that causes an enhanced local diversity (Peck et al. 1999; Gutt and Piepenburg 2003), which is in accordance with the intermediate disturbance hypothesis by reducing the effects of competitive exclusion of species (Huston 1979).

An important finding of the BIO-ENV analysis is that the ice-cover regime (taken as a proxy of food supply to the benthos) is as important as a determinant of macrobenthic community distribution and structure as water depth (largely a proxy of the salinity regime). This finding supports previous conclusions that the pelagic–benthic coupling (Graf 1992), i.e. the export of organic matter from the sea surface layer to the seabed, strongly affects the benthic systems, particularly in high-latitude seas (Petersen and Curtis 1980), and spatio-temporal patterns in ice cover, by mediating the production and coupling regimes, are also reflected in the distribution of benthic assemblages (Grebmeier and Barry 1991; Piepenburg et al. 2001).

A comparison of phytoplankton, zooplankton and benthic distribution patterns suggested that different processes control the communities in the water column and at the seafloor of the Laptev Sea (Fahl et al. 2001). The authors concluded that benthic distribution is affected by fluvial input and meso-scale current patterns rather than by large-scale biogeographical or oceanographic features and that the most important factor is bottom-water salinity, which in turn is closely correlated with water depth. However, they also concluded that the influence of the sediment structure, commonly invoked as a prime benthic community determinant (Snelgrove and Butman 1994), is apparently less significant, because grain-size distribution is rather homogeneous in the Laptev Sea (Lindemann 1994). Our findings, in contrast, point to a relatively high importance of sediment composition for benthic community structures. Furthermore, Fahl et al. (2001) reported that no correlation between nutrients—and related food supply to the benthos—and benthic biomass was found, even though the high input of nutrients by the Lena sustains a relatively high phytoplankton biomass near the delta. The low biomass was probably a result of the high sediment load and freshwater inflow near the Lena River that limit benthic colonisation. This situation differs clearly from that in the northeastern Bering and Chukchi Seas where high benthic abundance and biomass is explained by high primary productivity (300 g C m−2 a−1) caused by upwelling and northward movement of nutrient-rich Anadyr water masses through the Bering Strait (Grebmeier and Barry 1991; Feder et al. 1994b).

Conclusions

In general, our findings largely corroborate the conclusions found by previous studies (see Sect. “Introduction”). Low and seasonally variable salinities (parameterised by water depth) at depths <20 m coincide with the occurrence of an impoverished fauna, mainly consisting of few euryhaline brackish-water species. This differs pronouncedly from the macrobenthic assemblages at greater depths, characterised by higher and more stable salinities and presence of stenohaline taxa such as ophiuroids. We conclude that the salinity regime, either directly or acting in concert with stress agents such as high sedimentation rates or disturbance events such as ice gouging, is a prime determinant of macrobenthic distribution in the Laptev Sea. Compared to most other Arctic shelf seas the Laptev Sea appears to be special in that it is very shallow and high stress levels, primarily related to the huge impacts of riverine inflows, are of particular ecological significance.

Notes

Acknowledgements

We thank the captain and crew of the Russian vessel “Ivan Kireev” as well as the participants of the TRANSDRIFT I expedition in 1993 for their help during the cruise. Thanks to I. Richling (Kiel) for identification of mollusc species. V. Wiese (Cismar) kindly provided species data for non-bivalve molluscs. Thanks also to F. Lindemann (Kiel) for sedimentological data, as well as to I.A. Dmitrenko (St Petersburg) and J. Hölemann (Bremerhaven) for providing CTD data. Moreover, the help of H. Feder (Fairbanks) and an anonymous reviewer in amending the manuscript was of great value and is gratefully acknowledged. Our study is a contribution to the interdisciplinary Russian–German project “Laptev Sea System” which was supported by grant 03PL009A of the German Federal Ministry of Research and Technology to M. Spindler (Kiel). This paper is based on M.S.’s master thesis.

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Copyright information

© Springer-Verlag 2006

Authors and Affiliations

  • Matthias Steffens
    • 1
  • Dieter Piepenburg
    • 2
  • Michael K. Schmid
    • 1
  1. 1.Institut für Polarökologie der Universität KielKielGermany
  2. 2.Akademie der Wissenschaften und der Literatur MainzInstitut für Polarökologie der Universität KielKielGermany

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