Vertical distribution of mesozooplankton in the coastal Canadian Beaufort Sea in summer

Journal of Marine Systems 127 (2013) 26–35

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Vertical distribution of mesozooplankton in the coastal Canadian Beaufort Sea in summer Wojciech Walkusz a, b,⁎, William J. Williams c, Slawomir Kwasniewski b a b c

Freshwater Institute, Fisheries and Oceans Canada, 501 University Crescent, Winnipeg, MB, R3T 2N6, Canada Institute of Oceanology, Polish Academy of Sciences, Powstancow Warszawy 55, 81–712 Sopot, Poland Institute of Ocean Sciences, Fisheries and Oceans Canada, 9860 West Saanich Road, Sidney, BC, V8L 4B2, Canada

a r t i c l e

i n f o

Article history: Received 27 July 2011 Received in revised form 4 January 2012 Accepted 12 January 2012 Available online 21 January 2012 Keywords: Zooplankton Vertical distribution Canadian Arctic Beaufort Sea Plume Mackenzie River

a b s t r a c t This paper contributes to baseline knowledge of lower trophic levels that is necessary to monitor the impact of oil and gas development on the Canadian Beaufort Sea ecosystem. As part of the Nahidik Program, the vertical distribution of mesozooplankton was studied along two transects in the coastal Canadian Beaufort Sea in the summer of 2009. Mesozooplankton was collected with 153 μm conical net in two hydrologically distinct layers – the upper layer which was fresher and warmer due to the Mackenzie River runoff, and the lower layer which was colder and more saline. Two separate mesozooplankton assemblages were distinguished in the individual layers. The average zooplankton abundance in the two layers was 3120 ± 2860 ind. m− 3 and 4200 ± 5550 ind. m− 3 in the upper and lower layer, respectively. The upper layer was largely inhabited by meroplanktonic Polychaeta (752 ± 1038 ind. m− 3) and Bivalvia larvae (228 ± 307 ind. m − 3) as well as by youngest stages of Pseudocalanus spp. (245 ± 499 ind. m− 3). Conversely, the lower layer was mainly occupied by typical marine taxa such as Calanus glacialis (95 ± 76 ind. m− 3), C. hyperboreus (27 ± 12 ind. m − 3) and Triconia borealis (111 ± 81 ind. m − 3). Oithona similis, a widely distributed eurytopic cyclopoid copepod, showed no consistent pattern of vertical distribution (280 and 291 ind. m − 3, in the lower and upper layer, respectively). Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.

1. Introduction Coastal waters of the Beaufort Sea are an important habitat for fish communities and constitute a vital area for the subsistence harvest of the local people (Usher, 2002). This area has recently gained significant attention from the scientific community due to its potential use in the oil and gas exploration. Its interest in local ecology has resulted in a number of publications that describe both the environmental settings and various ecosystem components in the area (e.g. Carmack and Macdonald, 2002; Carmack et al., 2004; Conlan et al., 2008; Walkusz et al., 2010). The Canadian Beaufort Shelf and Sea are under strong influence from the fresh water input of the Mackenzie River, with its annual discharge of ~ 300 km 3 (Milliman and Meade, 1983) of highly turbid water (annual load of 127 10 6 tonnes of sediment; O'Brien et al., 2006). The resulting turbid Mackenzie plume is not only sizable in the nearshore Beaufort Sea but it can, under favourable wind

⁎ Corresponding author. Tel.: + 1 2049845541; fax: + 1 2049842403. E-mail address: [email protected] (W. Walkusz).

conditions, extend up to 400 km into the ocean. It also displays great temporal and spatial variability due to wind action and the presence of ice (Carmack and Macdonald, 2002). The combination of freshwater input and lack of tidal mixing causes the plume to be relatively thin with strong vertical stratification at its base due to the sharp salinity gradient there. This salinity/density gradient is additionally enhanced by heat absorption in the highly turbid surface waters (Carmack et al., 2004). The river plume has been shown to have a tremendous impact on both abiotic and biotic components of the oceanic ecosystem. The high suspended solids content has a controlling effect on the underwater radiance and consequently on photosynthesis (Carmack et al., 2004; Retamal et al., 2008). In addition, riverine discharge causes formation of ecological zones as one transits from the inshore brackish waters, through the intermediate, to the offshore oceanic waters (Walkusz et al., 2010). Each of these ecological zones is characterized by typical phytoplankton, zooplankton and ichthyoplankton communities that change from brackishadapted species towards oceanic species offshore (Retamal et al., 2008; Walkusz et al., 2010; Wong et al., 2013-this issue). The aim of this paper is to describe summer vertical distribution of mesozooplankton over the shallow Canadian Beaufort Shelf that is strongly influenced by the Mackenzie River discharge. We hypothesise that the stratified water column with evident surface layer will have a direct effect on the mesozooplankton distribution.

0924-7963/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2012.01.001

W. Walkusz et al. / Journal of Marine Systems 127 (2013) 26–35

2. Materials and methods The study was conducted during a cruise on the CCGS Nahidik to the Canadian Beaufort Shelf from 8 to 16 August, 2009. Mesozooplankton were collected along two transects, GRY and DAL, at 4 and 3 stations in each one, respectively (Fig. 1; Table 1). Transect GRY was located on the eastern boundary of the Mackenzie Trough and encompassed depths from 23 to 104 m. Transect DAL ran perpendicularly to the Tuktoyaktuk Peninsula and covered depths between 22 and 118 m. Although zooplankton do not perform daily vertical migrations during the midnight sun period (Blachowiak-Samolyk et al., 2006), our sampling was consistently performed during the morning hours (7–8 am MDT). At each station, CTD casts were completed prior to the zooplankton sampling. Mesozooplankton samples were collected with a conical zooplankton net (0.25 m 2 mouth area, 1.95 m length, 153 μm mesh size, 0.5 m s − 1 hauling speed) towed vertically. Four net samples were collected at each station: two from the entire water column and two from above the pycnocline, which was determined from the CTD measurements prior to the net sampling. The samples were preserved in a 4% borax-buffered formaldehyde solution in filtered sea water. Qualitative–quantitative examination of samples in the laboratory followed standard procedures (Harris et al., 2000). Taxonomic identification was to species, otherwise to the lowest possible level. Copepodid stages of some copepods were also distinguished. In the case of Calanus glacialis and C. hyperboreus, the method used in Walkusz et al. (2010) to discriminate between the copepodid stages was applied. Abundances of mesozooplankton in this paper (ind. m − 3) are the mean values from the two samples collected in each layer. Abundance data for the lower layer (under the pycnocline) were obtained by subtracting the abundance of the upper layer (above the pycnocline) from the abundance in the entire water column. The Authors are aware that the mesh size (153 μm) used in this study is to coarse to draw strong conclusions on abundance and distribution of smaller taxa such as Rotatoria. The PRIMER package (Clarke and Warwick, 1994) was used for similarity assessment between the samples (hierarchical, agglomerative clustering (Cluster); non-metrical Multi-Dimensional Scaling (MDS)). SIMPER routine was used to determine dissimilarity percentages between the distinguished groups of samples. To reduce the emphasis of the most abundant taxa, the abundance (ind. m− 3) was log10 transformed prior to analysis. In order to assess species diversity in each layer Simpson's and Shannon diversity indices were calculated. Cross-

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Table 1 List of stations sampled during the 2009 CCGS Nahidik cruise and basic environmental characteristics of the sampled water layers. Station Position

Date Depth Layer extent (m) (m)

GRY 5 70.105 N 137.793 W GRY 3 69.914 N 137.261 W GRY 2 69.845 N 137.072 W GRY 1 69.736 N 136.773 W DAL 5 71.279 N 131.202 W DAL 3 70.955 N 130.995 W DAL 1 70.456 N 130.813 W

08Aug 13Aug 12Aug 12Aug 15Aug 16Aug 16Aug

Mean salinity

Mean temperature (°C)

Lower

Upper Lower Upper Lower

104

95–40

40–0

32.70

19.35

− 1.39 7.54

Upper

52

48–20

20–0

32.37

18.33

− 1.37 8.21

36

32–20

20–0

32.33

19.32

− 1.37 8.52

23

19–10

10–0

31.83

22.05

− 1.23 8.50

118

110–20 20–0

32.46

28.51

− 1.35 3.02

54

51–20

20–0

32.36

28.66

− 1.23 4.45

22

20–10

10–0

31.63

28.24

− 0.66 5.99

section visualizations of the hydrological parameters were prepared using Ocean Data View software package (Schlitzer, 2011). 3. Results At all stations, the water column was vertically stratified (Figs. 2 and 3) with fresher, warmer water in the surface layer (called the upper layer hereafter) and cooler, more saline water underneath the pycnocline (called the lower layer hereafter) (Table 1). Stations along the GRY transect were under stronger influence of the plume that was demonstrated by significantly lower salinities in the upper layer (~ 19) compared to the upper layer on the DAL transect (~28.5; paired t-test, P b 0.05). The upper layer temperatures were lower on the DAL (below 6 °C) transect than along the GRY transect (above 7.5 °C) which was also another indication of weaker river influence (paired t-test, P b 0.05; insignificant at stations GRY 5 and DAL 5). Overall, there were 56 taxa found in the mesozooplankton samples of which 35 were identified to the species and 4 to genus level (Table 2). A majority of the mesozooplankton consisted of Crustacea

Fig. 1. Map of the study area with locations of transects and sampling stations.

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Fig. 2. Cross-shelf sections of temperature and salinity along the GRY transect. CTD cast locations are marked by vertical lines.

(29 taxa) with Copepoda being the most diverse group (18 species). Other taxa included Cirripedia, Polychaeta, Bivalvia, Echinodermata, Euphausiacea and Ostracoda, however these zooplankters were not identified to the species level. There were more taxa found in the lower than upper layer (46 vs. 40, respectively), which was reflected

in higher values of Shannon (1.34 vs. 1.06) and Simpson (0.46 vs. 0.22) indices. The mesozooplankton in the upper layer was different from the one in the lower layer at the 53% similarity level (Fig. 4) (ANOSIM R = 0.59, P = 0.004). Subsequent SIMPER analysis revealed that the

Fig. 3. Cross-shelf sections of temperature and salinity along the DAL transect. CTD cast locations are marked by vertical lines. The sections are only partially gridded due to widely spaced CTD casts across the shelf.

W. Walkusz et al. / Journal of Marine Systems 127 (2013) 26–35

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Table 2 List of taxa found in the upper and lower layer, respectively. Values in bold denote taxa of which abundances (ind. m− 3) were statistically different in the two layers (paired Student's t-test; P b 0.05; data square root transformed prior to analysis). Group

Taxon

Rotifera Hydrozoa medusae

Rotatoria Aeginopsis laurentii Aglantha digitale Bougainvillia sp. Eumedusa birulai Euphysa flammea Halitholus yoldiaarcticae Obelia sp. Rathkea octopunctata Sarsia princeps Hydromedusae n. det. Dimophyes arctica Beroë cucumis Mertensia ovum Polychaeta larvae Ostracoda Cirripedia larvae Acartia bifilosa A. clausii A. longiremis Acartia sp. I-V Calanus glacialis C. hyperboreus Eurytemora herdmanii Eurytemora hirundoides Eurytemora spp. CI-CV Jaschnovia brevis Metridia longa Microcalanus spp. Oithona similis Pseudocalanus spp. Scolecithricella minor Temora sp. Triconia borealis Aetideidae spp. CI-CIII Cyclopoida n. det. Harpacticoida n. det. Copepoda nauplii Podon leuckartii Apherusa glacialis Onisimus glacialis Themisto abyssorum T. libellula Amphipoda n. det. Euphausiacea calyptopis Isopoda n. det. Pandalus borealis zoea Hyas coarctatus zoea Clione limacina Limacina helicina Bivalvia larvae Echinodermata larvae Eukrohnia hamata Sagitta elegans Fritillaria borealis Oikopleura vanhoeffeni

Siphonophora Ctenophora Polychaeta Ostracoda Cirripedia Copepoda

Cladocera Amphipoda

Euphausiacea Isopoda Decapoda Pteropoda Bivalvia Echinodermata Chaetognatha Appendicularia

Lower layer

Upper layer

Average

SD

Average

SD

90 1 24

203 1 42

525 1 29 b1

958 2 27 b1

b1

b1

b1 7

1 19

b1 b1 4 b1

b1 b1 8 b1

b1 12 b1 b1 b1 40 b1 11 b1

b1 31 b1 b1 1 69 b1 11 1

b1

b1

752

1038

3 13 95 27 1 2 4 b1 10 48 280 1665 b1

5 24 76 12 2 6 6 b1 8 56 278 3072 1

5 2 2 b1 4 20 2 1 b1 1

2 5 6 b1 6 21 2 1 1 2

b1 3 291 245

b1 4 119 499

111 b1 b1 1 1535

81 b1 b1 1 2271

b1 20

1 33

b1

b1

b1 b1 b1 b1 b1

b1 b1 b1 b1 b1

540 1 b1

312 4 b1

b1

b1

b1 b1 b1

b1 b1 b1

1 18 228 18 3 2 42 322

1 43 307 21 8 3 52 361

b1 b1 1 18 45 18 b1 1 9 115

b1 b1 1 19 50 28 b1 2 20 181

differences between the two groupings were due to higher abundances of meroplanktonic polychaeta larvae, Rotatoria, small (trunk length b 5 mm) Oikopleura spp. and young stages of Pseudocalanus spp. (Table 3) in the upper layer (Table 2). While small (umbrella height b 5 mm) Aglantha digitale, Fritillaria borealis and Obelia sp. showed affinity to the upper layer, older stages of Calanus glacialis (CV and adult females), C. hyperboreus CIV and larger Oikopleura spp. were more abundant in the lower layer. Taxa such as Oithona similis and stages CIII/CIV of C. glacialis did not show any depth preference. The sample from the upper layer at station DAL 5 was an outlier due to its lower abundance of Rotatoria and higher abundance of Triconia borealis in comparison with other upper layer samples.

Fig. 4. Results of a) cluster analysis and b) multidimensional scaling, performed on log10 transformed zooplankton abundance data.

Mesozooplankton composition differed significantly vertically, horizontally, and between transects. Calanus glacialis was the most abundant in the lower layer at all stations (Fig. 5). Copepodid stages CV and adult females (AF) were generally found at the deepest, shelf break stations while younger stages predominated at shallow ones, towards the coast (Fig. 6). Similarly, C. hyperboreus was more numerous in the lower layer, but was also absent at all in the upper layer at the shallowest stations, closest to the shore (Fig. 5). The majority of its population consisted of copepodid stages CIV, CV and AF; stages CII and CIII were found predominantly at the outermost stations on the GRY transect (Fig. 6). Pseudocalanus spp. was abundant in the lower layer with markedly higher abundances at station GRY 1 (Fig. 5). In general, younger developmental stages of Pseudocalanus (CI-CIII) prevailed at all stations, however, a greater percentage of older copepodids were noted along DAL transect. Oithona similis was more abundant in the upper layer at sea-ward stations GRY 3,

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Table 3 The result of SIMPER analysis identifying taxa that primarily accounted for the distinction of the lower and upper layers identified in the study. Taxon

Polychaeta larvae Rotatoria Oikopleura spp. b5 mm Pseudocalanus spp. CI Pseudocalanus spp. CII A. digitale b 5 mm Pseudocalanus spp. CV Triconia borealis Pseudocalanus spp. CIII Microcalanus spp. Pseudocalanus spp. CIV Bivalvia veliger Fritillaria borealis C. hyperboreus CIV Hydromedusae larvae C. glacialis CI Echinodermata larvae C. glacialis CII C. glacialis CV Oikopleura spp. = >5 mm Pseudocalanus spp. AF L. helicina b5 mm Oikopleura spp. = >10 mm Copepoda nauplii Acartia spp. CI-CV Aglantha digitale = > 5 mm C. glacialis CIV C. glacialis AF M. longa AF Oithona similis C. glacialis CIII Obelia sp.

Bray Curtis dissimilarity Contribution (%)

Cumulative (%)

4.43 4.01 3.64 3.43 3.12 3.11 3.05 3.02 3.01 2.94 2.86 2.85 2.83 2.62 2.62 2.06 2.00 1.96 1.93 1.91 1.90 1.82 1.78 1.66 1.57 1.56 1.55 1.47 1.45 1.40 1.34 1.21

4.43 8.44 12.08 15.51 18.63 21.74 24.78 27.80 30.81 33.75 36.61 39.46 42.30 44.92 47.54 49.59 51.59 53.56 55.48 57.39 59.29 61.11 62.89 64.55 66.12 67.68 69.23 70.69 72.14 73.55 74.89 76.10

GRY 5 and DAL 3, while at the stations closer to the shore it was more numerous in the lower layer (Fig. 5). Triconia borealis was distributed primarily in the lower layer with the exception of station GRY 5 where higher abundance was noted in the upper layer (Fig. 7). Copepoda nauplii were significantly more abundant in the lower layers at most stations, with the highest recorded abundance at GRY1. However, at GRY 5 and GRY 3 abundance was more evenly distributed between the upper and lower layers, and at DAL 3 they were more abundant in the upper layer (Fig. 7). Meroplanktonic organisms such as bivalvia and polychaeta larvae (Fig. 7) showed very similar pattern of distribution and were abundant along the GRY transect (mostly upper layer) and virtually absent on the DAL line.

4. Discussion The Mackenzie River outflow plays a major environmental role in the coastal Canadian Beaufort Sea in summer (Carmack and Macdonald, 2002). The plume is very responsive to the wind and, as a result, the actual distribution of the plume over the Canadian Beaufort Shelf is quite variable. West or northwesterly winds lead to the formation of alongshelf currents towards Amundsen Gulf and Ekman transport tends to push the plume against the Tuktoyaktuk Peninsula leading to a coastal current that flows towards Amundsen Gulf (Macdonald and Yu, 2006). During our study, a different situation took place, with winds from the northeast and east prevailing. These winds lead to alongshelf currents toward Alaska while Ekman transport advects the plume offshore. As a result we observed weakening of the plume on the DAL line and strengthening of the plume signature offshore in the Mackenzie Bay which was seen as a clearly distinct fresher layer along the GRY line.

The river discharge influences the marine habitats not only as a result of distribution of the fresh water plume but also due to the distribution of suspended solids. As a consequence, phytoplankton growth is strongly limited due to PAR irradiance attenuation (Retamal et al., 2008). This leads to the overall low primary productivity of the nearshore Beaufort Sea in comparison with the offshore part of the Mackenzie estuary (Macdonald and Yu, 2006). There is, however, greater bacterial and heterotrophic activity observed in the region more heavily influenced by the plume (Parsons et al., 1989). Contrary to many previous studies of the Arctic estuarine ecosystems, Limnocalanus macrurus – an abundant copepod of waters influenced by riverine discharge (Hirche et al., 2003; Walkusz et al., 2010) – was not recorded at all during our current study. The fact that Limnocalanus can switch from local and rare to widely-spread and one of the most abundant taxa, has also been observed in the Kara Sea (Fetzer et al., 2002) and suggests that this species is able to quickly respond to abrupt changes in the hydrological conditions e.g. intense freshening of the coastal areas. There were two types of spatial mesozooplankton variability observed in our study – horizontal and vertical. Horizontally, the most striking difference between the two studied transects was in terms of Pseudocalanus spp. and meroplanktonic organisms that were much more abundant along the GRY transect. Walkusz et al. (2010) showed that the plume intensity shapes local zooplankton communities and leads to the formation of the ecological zones across the shelf. In our case the GRY transect was under much stronger riverine influence that seems to promote advantageous conditions to the aforementioned taxa. Calanus glacialis and Pseudocalanus development was more advanced along the DAL line and this can potentially be explained by upwelling events in this area (Williams and Carmack, 2008) that bring marine water onto the shelf along with older copepodites stages which already descended to depth for overwintering (Walkusz et al., submitted for publication). Vertical variability, as we hypothesized, was caused by the strong hydrological stratification observed along both transects. This observation was confirmed by both Cluster and MDS analyses that revealed the existence of two distinct assemblages characterized by specific mesozooplankton composition. Our results are consistent with observations of Fetzer et al. (2002) who also pointed to a two-layer system in the Kara Sea, also under significant influence of freshwater runoff. They found that typical marine taxa such as Calanus glacialis, Jaschnovia. tolli, Microcalanus pygmaeus and Triconia borealis occupied deeper part of the water column. In contract with our and Fetzer et al. (2002) findings, Deubel et al. (2003) did not record any major differences between the layers in the Kara Sea and explained their findings by a wide range of salinity tolerance in the most abundant species collected (e.g. Drepanopus bungei, Limnocalanus macrurus, Pseudocalanus spp. or O. similis). The upper layer of the water column, clearly under strong influence of the Mackenzie plume, hosted a taxonomically less diverse mesozooplankton assemblage. This layer, much fresher and warmer due to the plume presence, was particularly densely populated by meroplanktonic zooplankters including Polychaeta and Bivalvia larvae as well as Rotatoria. Conlan et al. (2008) showed that the coastal Canadian Beaufort Sea is rich in benthic polychaetes and bivalves capable of producing abundant larvae. Mileikovsky (1973) and Raby et al. (1994) found similar surface dwelling planktotrophic Polychaeta and Bivalvia larvae, and they attributed this pattern to feeding behaviour. Apart from meroplanktonic larvae, Pseudocalanus spp. was abundant in a fresher environment which seems to be a consistent pattern found for this taxon (Walkusz et al., 2010). Pseudocalanus was shown to feed non-selectively provided the concentration of particles was high enough, which would definitely be the case in the Mackenzie Estuary (Emmerton et al., 2008). Although the brackish waters in the estuarine transition zone are relatively high in primary production (Retamal et al., 2008), there is additionally greater bacterial and heterotrophic productivity (Garneau et al., 2009) that

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Fig. 5. Vertical distribution of C. glacialis, C. hyperboreus, Pseudocalanus spp. and O. similis (ind. m− 3; all developmental stages).

provides potential food source for grazers, or the basic consumer trophic level, such as Copepods (Poulet, 1976). Triconia borealis – a small poecilostomatoid copepod – was in many studies shown to be

strongly and positively correlated with depth (e.g. BlachowiakSamolyk et al., 2008; Walkusz et al., 2009), thus its presence in the upper layer of GRY 5 station is unexpected. It is not impossible that

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W. Walkusz et al. / Journal of Marine Systems 127 (2013) 26–35

Fig. 6. Developmental stages of C. glacialis, C. hyperboreus and Pseudocalanus spp. observed at each station and each layer.

this is a result of the sampling strategy employed and more subtle vertical distribution of the species that was not resolved by our coarse two layer sampling. Omnivore copepods such as O. similis or T. borealis make use of particles concentrating on density gradient layers (Gallienne and Robins, 2001; Gonzalez and Smetacek, 1994; Metz, 1995). Thus we think that at station GRY 5 there could be a stronger density gradient due to lower salinity in the plume influenced layer closer to the surface, than the deeper temperature density gradient that was used to delimit the sampling strata. Therefore these species could still be abundant and concentrate at depths up to 15–10 m, below the halocline, within the layer identified by us as the upper layer. This species abundance measured in this study is comparable to that observed by Hopcroft et al. (2005). It is, however, much greater than the abundance recorded in the Barents Sea (BlachowiakSamolyk et al., 2006). Since Triconia is considered omnivorous (Blachowiak-Samolyk et al., 2007) it could potentially take advantage

of high sediment waters in which elevated heterotrophic productivity occur, similarly to Pseudocalanus (Garneau et al., 2009). Typical marine taxa such as C. glacialis, C. hyperboreus and T. borealis tended to stay in colder and more saline lower layer – a behaviour that would be expected in the presence of suboptimal conditions (lower salinity and higher temperatures) in the upper layer. Although these taxa were below the euphotic zone, which was reduced due to high sediment content, they could have taken advantage of the deep chlorophyll maxima that have been reported to occur in that area (Carmack et al., 2004). A similar pattern of vertical distribution was also observed in the Barents Sea (Eilertsen et al., 1989) where C. glacialis was found to be most numerous just below the chlorophyll maximum. Kosobokova (1999) observed C. glacialis descending as a response to the warming of the surface layer. Calanus glacialis, a true arctic species, is abundant on the shelves adjacent to the Arctic Ocean (Kwasniewski et al., 2003 and

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Fig. 7. Vertical distribution of T. borealis, Copepoda nauplii, Bivalvia veligers and Polychaeta larvae (ind. m− 3).

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references therein). In the Beaufort Sea it has been recorded in greatest numbers in the areas further offshore but was also found closer to shore (Walkusz et al., 2010). Other studies in the river influenced regions (e.g. Kara Sea, Fetzer et al., 2002) show that C. glacialis can temporarily withstand conditions of lower salinity and higher temperature but its center of occurrence is purely marine environment (Thibault et al., 1999). We think that the distribution pattern of C. glacialis found in our study supports its characteristic as arctic marine shelf species. Increasing abundance of the species towards the shore in the lower layer can be the result of species avoidance of the fresher, warmer surface layer and narrowing the species hospitable ambit due to decreasing depth. Such a circumstance, although unfavourable for the species, can be positive for its predators such as fish or bowhead whales (Walkusz et al., 2013-this issue). Another cold water species, C. hyperboreus, is found widely in the Arctic (Conover, 1988) with the centers of its distribution located in deep basins of the Greenland Sea and Arctic Ocean (Hirche, 1997; Richter, 1994). Abundances recorded here are comparable to those observed by Thibault et al. (1999) in the cross Arctic Ocean transect (max. 63 ind. m − 3). They were, however, much higher when compared to Fetzer et al. (2002) observations from fresh water influenced Kara Sea. Kosobokova et al. (1998) showed that C. hyperboreus abundances increased offshore in areas under strong river runoff and that notion has also been confirmed in the Canadian Beaufort Sea (Walkusz et al., 2010). We believe that C. hyperboreus is an expatriate species that is being brought on the Mackenzie Shelf through upwelling. And although the water temperature and salinity in the lower layer are still within species tolerance limits, it experiences suboptimal conditions due to shallow water depths and therefore is most likely not able to thrive in the region. It is, however, most likely concentrating in lower layer due to physical narrowing of hospitable ambit, similar to what is happening to C. glacialis, and therefore may form an important element for the fish larvae diet (Walkusz et al., 2011) and as such plays a key role in the coastal ecosystem functioning. Significant variability in ambient salinity is tolerable for polyhaline taxa (Kinne, 1971), however, a majority of the mesozooplankton found in the studied region were rather typical marine organisms with fairly narrow tolerance (potentially with exception of eurytopic O. similis). Since Calanus copepods were found to be susceptible to long term exposure to low salinity (Zajaczkowski and Legezynska, 2001), we suggest that strong storms causing mixing to the bottom of the water column may cause mass mortality of zooplankton in the most inshore locations. If this assumption holds true then downward transport of dead zooplankton would be a key source of organic matter to the rich benthic communities in the shallow Beaufort Sea (Conlan et al., 2008). Knowledge on vertical distribution of mesozooplankton is of crucial importance for the major oil and gas development in the Canadian Beaufort Sea. Each part of the offshore hydrocarbons exploration process bears particular environmental consequences. Although very often these impacts are of local range, they can merge together and form zones of large-scale, cumulative disturbances (Patin, 1999). Information on which taxa may be impacted in particular water depths either during the well drilling, regular production process or during the hypothetical oil spill will help to estimate the damage and mitigate the risk associated with hydrocarbon development. Since the data presented in this paper is of a “snapshot” nature, it should be taken in with caution. They provide only information on the summer situation in the relatively limited area of the Canadian Beaufort Sea. With both regional differences in hydrology and strong seasonality, more research on zooplankton vertical distribution is required to completely fill our gap in knowledge. Acknowledgments This study is a part of, and was financially supported by, the Nahidik Program led by Donald Cobb (Department of Fisheries and Oceans,

Winnipeg) whom we wish to thank. Financial contribution was also made by the Polish Ministry of Science and Higher Education (grants 289/W-NOGAP/2008/0 and 562/W-NOGAP/2009/0). The Polish– Canadian cooperation was based on the Interchange Canada Agreement for Wojciech Walkusz. Our thanks go to the crew of the CCGS Nahidik who made the sampling possible and always created excellent working atmosphere. References Blachowiak-Samolyk, K., Kwasniewski, S., Richardson, K., Dmoch, K., Hansen, E., Hop, H., Falk-Petersen, S., Mouritsen, L.T., 2006. Arctic zooplankton do not perform diel vertical migration (DVM) during periods of midnight sun. Mar. Ecol. Prog. Ser. 308, 101–116. Blachowiak-Samolyk, K., Kwasniewski, S., Dmoch, K., Hop, H., Falk-Petersen, S., 2007. Trophic structure of zooplankton in the Fram Strait on spring and autumn 2003. Deep Sea Res. 54, 2716–2728. Blachowiak-Samolyk, K., Kwasniewski, S., Hop, H., Falk-Petersen, S., 2008. 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