Pteropods are a crucial group of pelagic fauna in all oceans, playing a key role in the trophic relations in food chains of polar and sub-polar regions. They often create swarms and are an important food source for e.g. baleen whales, but also for several species of commercially important fish and seabirds (Conover & Lalli 1972; Larson & Harbison 1989). The highest biodiversity of these organisms was recorded in tropical regions, however, their highest abundance was observed in cooler waters (e.g. van der Spoel 1967; Lalli & Gilmer 1989; Comeau et al. 2009; Howard et al. 2011). Due to their fragile shells and the associated difficulties in their examination, current knowledge about these organisms is still limited. The studied pteropods belong to two orders: thecosomes that form a shell, and gymnosomes that have no shell at the adult stage. Both orders differ in their food preferences, and thus play different roles in the Arctic marine ecosystem (Lalli & Gilmer 1989). Thecosomes mostly feed on phytoplankton, although small zooplankton is also a substantial component of their diet (Bernard et al. 2009). Thecosomes use a large, spherical external mucous web to collect a wide range of different food types e.g. tintinnids, dinoflagellates and diatoms, which is likely a key adaptation allowing them to live as holoplankton (Gilmer & Harbison 1986; 1991). Adult gymnosomes are predators and feed only on thecosomatous pteropods, but juvenile forms feed on phytoplankton and very small suspensions, which results in their aggregations in more fertile coastal waters (Mileykovsky 1970).
There are no data on the abundance and the trophic role of pteropods in the Barents Sea, but their importance in the food web for the Arctic, North Atlantic and Pacific regions is well documented.
Pteropods are also important for the functioning of the ocean biochemical cycle. Sediment trap studies have shown that pteropods are the major source of the carbonate flux (> 50%) into the ocean’s interior in the polar regions. Those pelagic snails make a contribution to the vertical flux of carbon through the production of fecal pellets, mucous flocs and rapid settling of aragonite shells upon their death (Howard et al. 2011). The thecosomes
In recent decades, oceans warming and other changes associated with the climate change are among the challenges faced by fragile aquatic organisms such as pteropods in this rapidly changing environment (Mucci 1983; Comeau 2009). According to the forecasts by the Intergovernmental Panel on Climate Change (IPCC 2001; Johannessen et al. 2004), the temperature in the central Arctic environment will increase by about 3-4°C during the next 50 years, which emphasizes the importance of research in the Arctic and subarctic environment. The increasing sea temperature could potentially lead to a decrease in the abundance of cold-water Arctic species that cannot adapt quickly to this challenge, which will ultimately affect the overall Arctic biodiversity (Grebmeier 2012; Bluhm et al. 2009; Weslawski et al. 2000). Moreover, pteropods are reliant on aragonite – saturated habitats for their shell formation, however, laboratory studies on these organisms from the Southern Ocean suggest that pteropod shell dissolution will occur rapidly as polar waters become undersaturated with aragonite (Fabry et al. 2008; Richardson 2008; Bednarsek et al. 2012a,b; Roberts et al. 2014).
The aim of this study was to provide information about summer species abundance, community composition and first size distributions of observed pteropods, and to examine possible environmental preferences (temperature, salinity, chlorophyll concentration) of these animals in the area of the western Barents Sea and the West Spitsbergen Current.
The study area is located between the Archipelago of Svalbard and the northern coast of Norway, in the western part of the Barents Sea. According to Reygondeau et al. (2013), this area can extend in August to the Atlantic Subarctic Biogeochemical Province. The region is strongly influenced by the West Spitsbergen Current and the North Cape Current flowing along the Norwegian coast (Loeng 1991). The West Spitsbergen Current (WSC) is a continuation of the Norwegian Atlantic Current and transports warmer and more saline Atlantic water from the Norwegian Sea into the Arctic Ocean (Piechura et al. 2001; Cottier et al. 2005). The area south of Spitsbergen (74°N) (Hisdal 1998) is under the influence of both the West Spitsbergen Current and the coastal South Cape Current that carries cold, less saline Arctic-type water from the northeast Barents Sea to the West Spitsbergen Shelf. These two distinct external water masses are usually separated on the shelf by the Polar Front (Saloranta & Svendsen 2001). In addition, waters from the Norwegian Sea contribute to the environmental conditions in our study area (Loeng et al. 1997; Walczowski et al. 2012) (Fig. 1).
Map of species composition in the study area
A total of 16 samples were collected horizontally at eight locations with a HydroBios Bongo net (0.6 m aperture diameter, 500 μm mesh size) and a flowmeter attached to the net. The CTD (Conductivity, Temperature, Depth) device (Sea-Bird Electronics, Inc. SBE 911 plus) was deployed to measure temperature, salinity and depth in the study area at each station. The research was conducted between 70°59.997’N 19°53.924’E and 75°42.002’N 17°32.797’E in August 2011 aboard the R/V Oceania, owned by the Institute of Oceanology Polish Academy of Sciences. The horizontal haul duration was 15 minutes at each station with a speed of 1.5 knots, and the average sampling depth was 122 m (Table 1). The collected samples were preserved in 4% borax – buffered formalin solution. The whole collection volume was analyzed for all samples, with the exception of two stations (V31 and V4), which was split, because the specimen abundance was very high.
Sampling details with the specification of sampling stations *Net obliquity = line obliquity of the towing net
Station
Longitude
Latitude
Sampling date
Time
Max haul depth (m)
Filtered water volume (m3)
Net obliquity (°)
Water mass
V31
75°42.002′N
17°32.797′E
10.08.2011
6:04:00
153
2128
50
Atlantic
V26
74°57.001′N
18°25.047′E
10.08.2011
12:50:00
49
2115
45
Arctic
V23
74°41.972′N
18°39.208′E
10.08.2011
15:30:00
57
1621
45
Arctic
V19
74°09.897′N
19°09.453′E
10.08.2011
20:00:00
49
2381
45
Arctic
V15
73°29.932′N
19°19.694′E
11.08.2011
1:00:00
212
4099
45
Atlantic
V13
73°00.056′N
19°27.466′E
11.08.2011
6:10:00
212
4778
45
Atlantic
V9
71°59.990′N
19°41.012′E
11.08.2011
15:00:00
141
3754
45
Atlantic
V4
70°59.997′N
19°53.924′E
11.08.2011
20:00:00
106
4246
45
Atlantic
Ranges of
Station
ind. 1000 m-3
V31
177
44
-
V26
12584
446
-
V23
1317
26
15
V19
1632
2
22
V15
56
134
4
V13
1213
136
1
V9
-
1640
4
V4
44
51826
267
Shell diameters of shelled species were determined by measuring the shell diagonally from the end of the outer whorl with an ocular micrometer (Nikon SMZ 800). Ontogenetic stages of
The relationship between the abundance of pteropods’ ontogenetic stages and the following environmental variables was examined: sampling and station depth, temperature (mean over the sampling depth), and salinity (mean), as well as chlorophyll concentrations in August and the mean from April-August. Chlorophyll data were obtained using MODIS Satellite from the NASA Ocean Color Web with the spatial resolution of 4 × 4 km. Constrained ordination techniques were applied using CANOCO 5 (ter Braak & Šmilauer 2012). To this purpose, we used redundancy analysis (RDA) following the square root transformation of the abundance data. The environmental variables were ranked according to their quantitative importance by interactive forward selection, based on the Monte Carlo permutation test (ter Braak & Prentice 1988). Permutation test parameters were set as a hierarchical design, with stations defined as spilt plots to avoid pseudoreplications of environmental data, which had been collected once at a given station.
A clear pycnocline was present at almost all stations, being most pronounced at stations V4 and V9, located closest to the Norwegian coast, where temperature of surface water reached positive values (from +8°C to +11°C). Stations V23 and V31 were the only part of the study area where the typical thermocline was not observed. The thermocline usually occurred at a depth of ~ 60 m, indicating significant vertical differences in temperature and thereby in densities of water masses (Fig. 2). Based on the surface water temperature, we noticed the presence of water masses of different origin in the study area: stations V19 and V23 were cold (Arctic water masses), while stations V4, V9, V13, V15 were relatively warm and under the influence of Atlantic water masses (Table 1) (Sakshaug et al. 1994; Walczowski et al. 2012).
Variability in temperature and salinity of surface water at the sampling stations along the studied transect (the value of 200 m was used for the deeper stations and 100 m was used for the shallower stations)
Chlorophyll concentration data were also obtained for the period from April to August and the mean value for these months was also calculated (Table 3).
Chlorophyll concentration (mg m-3) in the study area
Station
Chlorophyll (mg m-3)
April
May
June
July
August
Mean
V31
7.183
8.297
0.842
1.688
0.696
3.741
V26
1.205
0.941
2.324
0.532
0.977
1.196
V23
0.480
0.885
2.004
0.392
0.765
0.905
V19
1.126
1.990
1.862
NoData
0.684
1.416
V15
1.365
3.570
2.068
0.488
0.539
1.606
V13
2.270
9.077
8.564
NoData
0.643
5.138
V9
0.783
0.951
0.962
0.269
0.467
0.687
V4
0.477
0.934
1.242
0.880
0.528
0.812
The total number of sampled specimens was 143 178 ind. 1000 m−3.
Size distributions for pteropods in the study area in August 2011 (some species were omitted due to the absence in the station area or due to the low abundance of specimens)
The RDA analysis demonstrated a significant relationship between variation in the abundance of pteropods and the environment – 84.3% (pseudo-F = 36.9, p = 0.0078). According to the interactive forward selection, only two of the tested environmental variables were significant: mean temperature (pseudo-F = 33.4, p = 0.0048), which explained 70.5% of the species variability, and chlorophyll concentrations in August (pseudo-F = 11.4, p = 0.0209), explaining 13.8%. The abundance of
Redundancy analysis (RDA) ordination plot showing the relationship between the abundance of Pteropoda ontogenetic stages and the significant environmental variables, with the proportion of total variability explained by the first two canonical axes. Note that males and females are identified based on respective sizes.
Sampling took place at the end of the Arctic summer when the primary production in this region is usually still quite high and starts to decrease. The most abundant stages of the recorded species were juveniles, while the older individuals occurred very rarely at the surveyed stations.
One of the stations (V31) in the north was significantly different from the others because it was located within the range of the eastern branch of the West Spitsbergen Current (Walczowski et al. 2013), which resulted in a high proportion of
Previous studies have shown that
As evidenced by the previous studies, the life cycle of
During our research conducted at the end of August, individuals of
The results presented in this study confirm the ability of pteropods to form large clusters especially in the coastal, surface waters and in water mixing zones (Table 1). In the polar regions, Pteropoda have previously been recorded in high densities. For example, in Kongsfjorden, i.e. one of the fjords on the western coast of Spitsbergen, and in the area of the West Spitsbergen Current,
In conclusion, we found that water temperature was the most significant abiotic factor, defined by the prevailing water masses that largely determine the distribution of pteropods. It is also a well-known fact that the species’ reproduction cycle is closely related to seasonal changes in food availability and to species biology and ecology, hence the importance of chlorophyll concentration for the variability of pteropods. The duration of this research was too short to conclude possible changes in the distribution patterns of pteropod species. However, this paper is a good introduction to further consideration about the effect of abiotic factors on the ecology of these planktonic organisms. Our research gives also the first insight into the detailed size distribution and abundance of pteropods in the specific region of the western Barents Sea, created by the Atlantic and Arctic waters.