Abundance, biomass and community structure of epipelagic zooplankton in the Canada Basin

Abundance, biomass and community structure of epipelagic zooplankton in the Canada Basin Abstract Changing environmental conditions such as decreasing sea ice cover impact Arctic zooplankton. In the Canada Basin, zooplankton surveys have seldom been done due to its traditionally thick, year-round ice cover. Here, we describe the zooplankton community of the Canada Basin before the two recent sea ice minima (2007 and 2012). Zooplankton were sampled from the upper 100 m during August and September of the years 2003–2006 using a 150-μm mesh net to determine species composition, abundance and biomass. To describe the zooplankton community and its relation to the environment, we used Bray–Curtis similarity, and then applied hierarchical clustering, non-parametric multidimensional scaling and the BEST BIO-ENV routine. The most abundant zooplankton species in all years were smaller copepods such as Oithona similis and Microcalanus pygmaeus. Biomass was dominated by larger copepod species such as Calanus hyperboreus and Calanus glacialis. For the non-copepod zooplankton, the pteropod Limacina helicina and the larvacean Fritillaria borealis were typically the most abundant species. The non-copepod biomass was dominated by the chaetognath Eukrohnia hamata and L. helicina, while F. borealis contributed relatively little to the overall biomass despite its high numbers. Zooplankton communities differed between shelf/slope and basin stations. We found no obvious interannual changes in community structure over our short 4-year observation period, with community structure influenced to a small degree by environmental factors. INTRODUCTION The Canada Basin is a deep, ice-covered basin located in the central Arctic Ocean. Zooplankton within the Arctic basins are intricately tuned to the basin’s primary production cycle (Smith and Schnack-Schiel, 1990). While the Canada Basin was historically been covered year-round by thick multiyear ice, sea ice extent and concentration have declined rapidly within the past decades, reaching a record minimum during summer 2007 only to have it exceeded recently during summer 2012 (Comiso, 2012; Parkinson and Comiso, 2013). The sea ice meltwater represents the major freshwater influx to the Arctic that is supplemented by river discharge, with both sources increasing over time (Yamamoto-Kawai et al., 2009). Water temperatures in the Canada Basin already showed a warming trend from 1993 to 2008 (Jackson et al., 2010) while freshening of the Beaufort Gyre, as well as a deepening of the nutricline and chlorophyll maximum, have also been observed between the years 2003 and 2009 (McLaughlin and Carmack, 2010). Such detectable changes in the environmental conditions typically have an impact on zooplankton communities (Richardson, 2008), and this may be particularly true in the Arctic (Gradinger et al., 2010; Nelson et al., 2014). Zooplankton in the Canada Basin has been studied only sporadically due to its traditionally thick, year-round ice cover. Studies during the last century established rudimentary community composition and seasonal cycles but were seldom synoptic or repeated (Johnson, 1963; Pautzke, 1979; Thibault et al., 1999; Ashjian et al., 2003). In contrast, the Beaufort Sea slope region experienced more extensive activities often related to oil and gas exploration (Hufford et al., 1974; Horner and Murphy, 1985; Hopky et al., 1994a,,b, c). Even the more recent studies have been concentrated in the more coastal Beaufort Sea and shelf areas (Darnis et al., 2008; Lane et al., 2008; Walkusz et al., 2008, 2010, 2013; Smoot and Hopcroft, 2017a), with less focus on the central Canada Basin (Hopcroft et al., 2005; Kosobokova and Hopcroft, 2010; Hunt et al., 2014). At present, indications are that many species are shared between the Arctic’s major basins (Kosobokova et al., 2011). The fauna contains a mixture of endemic Arctic species, viable species shared with other ocean basins and species that are advected from the Pacific such as Eucalanus bungii, Pseudocalanus newmani, Metridia pacifica and Neocalanus flemingeri (Hopcroft et al., 2005; Kosobokova and Hopcroft, 2010; Hunt et al., 2014) that are not thought to be viable in the Arctic or have little reproductive success (Matsuno et al., 2015; Wassmann et al., 2015). The abundance and biomass within the epipelagic upper 100 m of the water column are typically dominated by copepods, whereby the smaller bodied species such as Oithona similis, Microcalanus pygmaeus and Triconia borealis make up the bulk of the abundance, while larger bodied endemic copepods such as Calanus hyperboreus, Calanus glacialis, Metridia longa and Paraeuchaeta glacialis dominate the biomass (Kosobokova and Hopcroft, 2010). Non-copepod abundance is frequently dominated by the larvaceans Fritillaria borealis and Oikopleura vanhoeffeni as well as the pteropod Limacina helicina that can at times contribute a significant percentage to the biomass (Hopcroft et al., 2005; Kosobokova and Hopcroft, 2010; Hunt et al., 2014). Zooplankton species are also advected from the shelf into the basin within eddies (Carmack and Macdonald, 2002; Llinás et al., 2009), and hence meroplankton typical for shelf communities, such as cirripedia cyprids and echinoderm larvae, can occasionally be found far into the basin (Hunt et al., 2014). A freshening and warming of the Canada Basin due to climate change has implications for the zooplankton community. The epipelagic large-bodied endemic copepods are probably at greatest risk of stress or competition from the advected subarctic species. Hunt et al., (2014) suggested that there has already been a decrease in abundance of species that are typical for the Arctic and Subarctic, such as O. similis, L. helicina, M. pygmaeus, and F. borealis during 2007 and 2008. An earlier sea ice retreat and a shrinking sea ice extent combined with the freshening of the Beaufort Gyre (McLaughlin and Carmack, 2010) could potentially increase primary productivity (Arrigo and Van Dijken, 2015) and zooplankton biomass in the basins (Hunt et al., 2014) as has already been demonstrated for the Chukchi Sea shelf (Ershova et al., 2015). Here we present new data describing the epipelagic zooplankton abundance, biomass and community structure in the Canada Basin for 2003 to 2006 to fill in spatial and temporal gaps. By describing the basin’s zooplankton community before the two recent summer sea ice minima of 2007 and 2012, we hope to build a better foundation for comparison of zooplankton communities before and after major environmental changes to establish how they may be affected. METHOD Zooplankton sampling and taxonomic analysis Our study area encompassed much of the Canada Basin to as far north as 80°N latitude (Fig. 1). Samples were collected during August and September of 2003 to 2006 (day and night) aboard the Canadian Coast Guard vessel Louis S. St-Laurent. Bongo nets with a mouth diameter of 60 cm and 150-μm mesh size were deployed vertically in the upper 100 m of the water column. During 2003 and 2004, the nets were equipped with General Oceanics flowmeters to measure the volume of filtered water, while during 2005 and 2006, OceanTest flowmeters were used. On three occasions, flowmeters iced-up and gave false readings. In these cases, a filtration efficiency of 100% was assumed because there was insufficient phytoplankton to impact filtration efficiency. A total of 60 samples were analyzed: 23 from 2003, 23 from 2004, only 4 from 2005 (due to wire-time constraints) and 10 from 2006. Upon collection, the samples were preserved in seawater with 4% formaldehyde. Fig. 1. View largeDownload slide Map of the study area in the Canada Basin. Multiyear stations include 2003, 2004, 2005 and 2006 stations, if applicable. Fig. 1. View largeDownload slide Map of the study area in the Canada Basin. Multiyear stations include 2003, 2004, 2005 and 2006 stations, if applicable. In the lab, collections were subsampled using a Folsom splitter (Harris et al., 2000), with smaller subsamples (e.g. 1/128th) used to identify very abundant species (such as O. similis) and larger subsamples used for the rarer species. The full sample was generally analyzed for cnidarians, amphipods, chaetognaths and larger copepods. Animals were enumerated and measured using the ZoopBiom software (Roff and Hopcroft, 1986). For the more prominent species, up to 100 individuals per species were measured with remaining individuals in the aliquot simply enumerated. The developmental stage of larger copepods and the sex of adults were also recorded. We used prosome length to differentiate between C. glacialis and C. hyperboreus during early developmental stages (copepodite stages CI–CIII). For individuals where early life stages could not be distinguished between species, they were grouped according to their genus. In order to calculate dry weights (DW), we applied length–weight relationships according to Hopcroft et al. (2005) that were species–specific or from morphologically similar species. The ZoopBiom software employs subsample fractions and volume filtered to calculate both abundance and biomass, along with their size spectra, for each taxa. Here we used arithmetic bins of 50-μm width for creating the size spectra and summed the spectra for all copepod taxa within a sample. These sample–specific spectra were then averaged for each study year. It is notable that samples from 2 years (2004 and 2006) of our study were collected concurrently with the work by Hunt et al., (2014) that employed a coarser mesh size (256 μm). Environmental data Concurrent temperature (°C), salinity (PSU) and oxygen (mL/L) data were obtained using a SBE 911 plus (Sea-Bird Electronics Inc.) (McLaughlin et al., 2009). Means were calculated for 0–50 m and 0–100 m depth layers. We derived the Euclidean distance to the coastline from World Vector Shoreline data from the National Oceanic and Atmospheric Administration (NOAA) by using ArcMap 10.1 and the Geospatial Modelling Environment (GME). The coastline was imported into ArcMap 10.1 and the “Euclidean distance tool” applied to derive a raster layer displaying the Euclidean distance to the coastline. After that, the “isectpntrst”-command in GME was applied to spatially correlate the stations with the corresponding Euclidean distance value. Statistical analysis Statistical analysis was conducted using the PRIMER (Version6) (Clarke and Warwick, 2001), R (Version 3.2.3) and ArcMap 10.1 software. Analysis was completed using all species within a sample unless stated otherwise. We applied a fourth–root transformation of abundance and biomass data and calculated the Bray–Curtis similarity index (Bray and Curtis, 1957). Differences and patterns in the zooplankton community between stations or years were detected using weighted average hierarchical cluster analysis and non-parametric multidimensional scaling (nMDS) based on the Bray–Curtis matrix. To take spatial variability into account, we applied a permutational multivariate analysis of variance (PERMANOVA) using the Adonis function in R (Vegan package) using 5000 permutations. We applied two factors to the function: year and region, whereby the region was split into sampling stations that had a bottom depth above 1100 m versus deeper stations. Species that showed up only once in the entire dataset were excluded from the PERMANOVA. For Microcalanus, Metridia and Pseudocalanus, the copepodites and adults were pooled as Microcalanus spp., Metridia spp. and Pseudocalanus spp., respectively for the hierarchical cluster analysis, nMDS and PERMANOVA. We used the SIMPER routine to define the similarity percentage between clusters, and which species were driving the grouping/clustering. For the SIMPER analysis, any species that contributed less than 70% to the within–group similarity was excluded. We tested for a possible increase in mean community abundance and biomass from 2003 to 2006 using the Spearman’s rank correlation coefficient (ρ; “cor.test” command in R). To test for significant differences between the annual mean abundance and biomass on species level, the data were log–transformed and analysis of variance (ANOVA) was conducted in R. If applicable, Tukey tests were used to determine which years were significantly different. The year 2005 was excluded from these analyses because of the small sample size during that year. To relate environmental data to community patterns, we employed Primer’s BEST BIO-ENV routine after normalizing the physical variables. RESULTS Environmental conditions Overall, 2003 and 2004 were more similar in their environmental conditions compared to 2005 and 2006. All years had a core of low salinity towards the center of the study area related to dynamics of the Beaufort Gyre. During 2003 and 2004, the northern stations were more saline than 2005 and 2006 (Fig. 2). A small tongue of elevated salinity occurred at the southern stations during 2003 and 2005. Fig. 2. View largeDownload slide Average (surface to 100 m) for salinity (PSU), temperature (°C) and oxygen (mL/L) plots for the years 2003–2006. Zooplankton sampling stations in 2005 indicated by white circles. Fig. 2. View largeDownload slide Average (surface to 100 m) for salinity (PSU), temperature (°C) and oxygen (mL/L) plots for the years 2003–2006. Zooplankton sampling stations in 2005 indicated by white circles. The average temperature for the upper 100 m decreased with increasing latitude during all years. Overall, temperatures were colder further offshore, and in 2003 on the eastern and western edges. During 2003, 2005 and 2006, the core of the study area had temperatures between −1 and −0.5°C. This temperature range occurred further south for 2004 compared to 2003. Oxygen concentrations during 2003 and 2004 were lower in the North relative to the core of the study area. The same pattern was observed during 2006, with the difference that the southern part of the study area showed a lower oxygen concentration compared to the other years. Oxygen concentration during 2005 was relatively high, with the highest concentration in the Northeast. General abundance and biomass In total, 50 taxonomic categories were found during 2003–2006, of which 27 were copepods (Supplementary Table SI). Of the other categories, three were euphausiids, four amphipods, four hydrozoans, two larvaceans, two pteropods, one ctenophore, an isopod, an annelid and then larvae of several different taxonomic groups. The mean community abundance and biomass varied 1.5-fold across years, being highest in 2006 (Table I), but there was no significant difference between the years (P-value abundance = 0.207; P-value biomass = 0.411) and no correlation between the years, mean abundance and biomass (Spearman’s rank correlation coefficient abundance: P-value = 0.27, ρ = 0.15; biomass: P-value = 0.65, ρ = 0.06). The PERMANOVA showed that there was no significant interaction between the factors year and region (Table II). Both factors were significant (year: P = 0.0002; region: P = 0.015) for abundance and for biomass, only the year was significant (P = 0.0002). Copepods dominated the abundance (Fig. 3) and biomass (Fig. 4) for all years. They made up around 90% of the abundance during 2003, 2005 and 2006, and 84% in 2004, contributing up to 88% (2003) of community biomass. The abundance of non-copepod zooplankton was dominated by larvaceans, mainly F. borealis, during 2003, 2004 and 2006 (6, 12 and 9%, respectively). For 2005, non-copepod zooplankton abundance was dominated by pteropods (8%), but since this observation was based on only four stations, it was not appropriate to compare it statistically to the abundance of other years. The category of “others” (Fig. 3) consisted of polychaete larvae, isopods, cnidarians, meroplankton, amphipods, euphausiids, chaetognaths and ostracods, each of them contributing <0.5% to the overall abundance. Non-copepod biomass was dominated by chaetognaths for all years (2003 = 7%, 2004 = 11%, 2005 = 8%, 2006 = 11%) (Fig. 4) with Eukrohnia hamata contributing much more than Parasagitta elegans except at the most nearshore sites. The other major invertebrate predator of zooplankton in this system was the medusa Aglantha digitale. Table I: Average abundance and biomass of zooplankton in the Canada Basin from 2003 to 2006 Year Dates # of samples Abundance (ind. m−3) ± SE Biomass (mg DW m−3) ± SE 2003 08/11–09/02 23 687 ± 62 13 ± 1.1 2004 08/09–08/30 23 736 ± 91 11 ± 1 2005 08/03–08/25 4 834 ± 263 12 ± 1.6 2006 08/10–09/8 10 1010 ± 202 17 ± 2.5 Year Dates # of samples Abundance (ind. m−3) ± SE Biomass (mg DW m−3) ± SE 2003 08/11–09/02 23 687 ± 62 13 ± 1.1 2004 08/09–08/30 23 736 ± 91 11 ± 1 2005 08/03–08/25 4 834 ± 263 12 ± 1.6 2006 08/10–09/8 10 1010 ± 202 17 ± 2.5 Values rounded to the nearest whole number. SE, standard error . Table I: Average abundance and biomass of zooplankton in the Canada Basin from 2003 to 2006 Year Dates # of samples Abundance (ind. m−3) ± SE Biomass (mg DW m−3) ± SE 2003 08/11–09/02 23 687 ± 62 13 ± 1.1 2004 08/09–08/30 23 736 ± 91 11 ± 1 2005 08/03–08/25 4 834 ± 263 12 ± 1.6 2006 08/10–09/8 10 1010 ± 202 17 ± 2.5 Year Dates # of samples Abundance (ind. m−3) ± SE Biomass (mg DW m−3) ± SE 2003 08/11–09/02 23 687 ± 62 13 ± 1.1 2004 08/09–08/30 23 736 ± 91 11 ± 1 2005 08/03–08/25 4 834 ± 263 12 ± 1.6 2006 08/10–09/8 10 1010 ± 202 17 ± 2.5 Values rounded to the nearest whole number. SE, standard error . Table II: PERMANOVA output for abundance (ab) and biomass (bm) using the factors year and region df SS ab MS ab R2 ab P ab SS bm MS bm R2 bm P bm Year 2 0.14 0.07 0.1 0.0002 0.26 0.13 0.14 0.0002 Region 1 0.05 0.05 0.04 0.02 0.04 0.04 0.02 0.24 Year:region 2 0.02 0.02 0.01 0.54 0.03 0.03 0.02 0.4 Residuals 51 1.21 0.02 0.84 1.59 0.03 0.8 df SS ab MS ab R2 ab P ab SS bm MS bm R2 bm P bm Year 2 0.14 0.07 0.1 0.0002 0.26 0.13 0.14 0.0002 Region 1 0.05 0.05 0.04 0.02 0.04 0.04 0.02 0.24 Year:region 2 0.02 0.02 0.01 0.54 0.03 0.03 0.02 0.4 Residuals 51 1.21 0.02 0.84 1.59 0.03 0.8 Table shows degrees of freedom (df), sum of squares (SS), mean squares (MS), R2 and P-value (P). Table II: PERMANOVA output for abundance (ab) and biomass (bm) using the factors year and region df SS ab MS ab R2 ab P ab SS bm MS bm R2 bm P bm Year 2 0.14 0.07 0.1 0.0002 0.26 0.13 0.14 0.0002 Region 1 0.05 0.05 0.04 0.02 0.04 0.04 0.02 0.24 Year:region 2 0.02 0.02 0.01 0.54 0.03 0.03 0.02 0.4 Residuals 51 1.21 0.02 0.84 1.59 0.03 0.8 df SS ab MS ab R2 ab P ab SS bm MS bm R2 bm P bm Year 2 0.14 0.07 0.1 0.0002 0.26 0.13 0.14 0.0002 Region 1 0.05 0.05 0.04 0.02 0.04 0.04 0.02 0.24 Year:region 2 0.02 0.02 0.01 0.54 0.03 0.03 0.02 0.4 Residuals 51 1.21 0.02 0.84 1.59 0.03 0.8 Table shows degrees of freedom (df), sum of squares (SS), mean squares (MS), R2 and P-value (P). Fig. 3. View largeDownload slide Relative abundance (ind. m−3) of major zooplankton groups in the Canada Basin for 2003–2006. Lower panel range is trimmed to increase resolution. Fig. 3. View largeDownload slide Relative abundance (ind. m−3) of major zooplankton groups in the Canada Basin for 2003–2006. Lower panel range is trimmed to increase resolution. Fig. 4. View largeDownload slide Relative biomass (mg DW m−3) of zooplankton groups in the Canada Basin for 2003–2006. Lower panel range is trimmed to increase resolution. Fig. 4. View largeDownload slide Relative biomass (mg DW m−3) of zooplankton groups in the Canada Basin for 2003–2006. Lower panel range is trimmed to increase resolution. The size spectra of copepod abundance and biomass showed expected patterns with the smaller species being the most abundant, but biomass being highest for large copepods where a series of modes occurred largely dominated by Calanus stages (Fig. 5). The peak in abundance and biomass for the size of ~300–450 μm was formed by O. similis, M. copepodites and T. borealis. The species that contributed most to the peak in abundance around 3000 μm were C. glacialis and C. hyperboreus, where the latter was also responsible for the peaks around 5000 and 7000 μm. For the biomass size spectra, the species responsible for the 3000 μm peak were C. glacialis, C. hyperboreus, M. longa and P. glacialis. For the 5000 μm peak, C. hyperboreus and P. glacialis were the main drivers and only C. hyperboreus for the 7000 μm peak. The size spectra were similar throughout all years, with the exception of a shift in the 3000 μm and the 5000 μm peaks in 2006, for which C. glacialis and C. hyperboreus were responsible. Fig. 5. View largeDownload slide Size spectra of copepod prosome length (μm) for abundance (A) and biomass (B) in the Canada Basin during 2003–2006 with prosome length ranges for O. similis (O. sim.), Microcalanus sp. (Micro.), C. glacialis (C. gla.) and C. hyperboreus (C. hy.). Fig. 5. View largeDownload slide Size spectra of copepod prosome length (μm) for abundance (A) and biomass (B) in the Canada Basin during 2003–2006 with prosome length ranges for O. similis (O. sim.), Microcalanus sp. (Micro.), C. glacialis (C. gla.) and C. hyperboreus (C. hy.). Species-specific abundance The zooplankton community for most stations consisted mostly of species common to the Arctic such as C. hyperboreus, C. glacialis, M. longa, T. borealis, O. similis, M. pygmaeus, P. glacialis and F. borealis, with O. similis and M. pygmaeus the most abundant in all years (Supplementary Table SI and Fig. 6). The species C. glacialis, M. longa, M. pygmaeus, O. similis, Scolecithricella minor, Paraheterorhabdus norvegicus, Pseudocalanus sp. copepodite, Eukrohnia hamata, Themisto libellula, T. abyssorum and A. digitale were significantly different (P-value ≤ 0.05) between some years (Supplementary Table SI). Pacific species that were advected to the Canada Basin, such as Eucalanus bungii and Neocalanus flemingeri, were observed sporadically, typically with only one or two specimens per sample. Fig. 6. View largeDownload slide Proportional abundance plots (ind. m−3) of copepods in the Canada Basin 2003–2006. Microcalanus copepodites and adults combined. Fig. 6. View largeDownload slide Proportional abundance plots (ind. m−3) of copepods in the Canada Basin 2003–2006. Microcalanus copepodites and adults combined. Microcalanus pygmaeus showed a pattern with higher abundance in the basin than towards the coast. Calanus hyperboreus also generally displayed larger numbers in the basin than towards the coast. For 2003, 2004 and 2006, the lower abundances of C. hyperboreus, C. glacialis and M. longa coincided with the region of lower salinity and higher oxygen concentration within the Beaufort Gyre (Figs 2 and 6). For 2003, 2005 and 2006, M. longa displayed higher abundances towards the shelf than in the Basin, while during 2004, abundances tended to be lower at a cluster of stations toward the north central basin. While C. hyperboreus, C. glacialis and M. longa had higher abundances towards the western Beaufort Sea (especially in 2004), O. similis and M. pygmaeus had elevated abundances towards the eastern Beaufort Sea, where the water was fresher due to the Beaufort Gyre. During 2003, 2004 and 2006, F. borealis was less abundant in the south of our study area, towards the shelf break. Beyond that, no obvious spatial pattern of abundance was observed for F. borealis or L. helicina (Fig. 7). For most species, the distance to the coastline, bottom depth and mean salinity (surface to 100 m) each explained less than 8% of the variance (Table III). For C. hyperboreus, the distance to the coast explained 30% of the variance and salinity explained 36% of the variance. For O. similis, 21% of the variance was accounted for by salinity (Table III). Fig. 7. View largeDownload slide Proportional abundance plots (ind. m−3) of F. borealis and L. helicina in the Canada Basin 2003–2006. Fig. 7. View largeDownload slide Proportional abundance plots (ind. m−3) of F. borealis and L. helicina in the Canada Basin 2003–2006. Table III: Relationship between zooplankton abundance and distance to coast, bottom depth and mean salinity for the upper 100 m of the Canada Basin from 2003 to 2006 Species r2 coast r2 depth r2 salinity Calanus glacialis 0.05 0.01 0.02 Calanus hyperboreus 0.3 0.006 0.36 Metridia longa 0.11 0.02 0.08 Microcalanus sp. 0.0006 0.046 0.015 Oithona similis 0.00002 <0.001 0.21 Fritillaria borealis 0.04 0.14 0.002 Limacina helicina 0.08 0.0015 0.004 Species r2 coast r2 depth r2 salinity Calanus glacialis 0.05 0.01 0.02 Calanus hyperboreus 0.3 0.006 0.36 Metridia longa 0.11 0.02 0.08 Microcalanus sp. 0.0006 0.046 0.015 Oithona similis 0.00002 <0.001 0.21 Fritillaria borealis 0.04 0.14 0.002 Limacina helicina 0.08 0.0015 0.004 Abundance data were log-transformed. Table III: Relationship between zooplankton abundance and distance to coast, bottom depth and mean salinity for the upper 100 m of the Canada Basin from 2003 to 2006 Species r2 coast r2 depth r2 salinity Calanus glacialis 0.05 0.01 0.02 Calanus hyperboreus 0.3 0.006 0.36 Metridia longa 0.11 0.02 0.08 Microcalanus sp. 0.0006 0.046 0.015 Oithona similis 0.00002 <0.001 0.21 Fritillaria borealis 0.04 0.14 0.002 Limacina helicina 0.08 0.0015 0.004 Species r2 coast r2 depth r2 salinity Calanus glacialis 0.05 0.01 0.02 Calanus hyperboreus 0.3 0.006 0.36 Metridia longa 0.11 0.02 0.08 Microcalanus sp. 0.0006 0.046 0.015 Oithona similis 0.00002 <0.001 0.21 Fritillaria borealis 0.04 0.14 0.002 Limacina helicina 0.08 0.0015 0.004 Abundance data were log-transformed. Community structure The hierarchical cluster analysis using abundance data showed one large group (group F: 37 samples) and five smaller groups (group A: 3 samples; group B: 2 samples; group C: 6 samples; group D: 2 samples; group E: 4 samples) and six single samples (hereafter called outliers except for station 28A) formed at 67–81% similarities. The large group F consisted of 17 samples from 2003, 14 samples from 2004, 5 samples from 2006 and 2 samples from 2005 (Fig. 8). The second largest group was group C with a total of six samples from all years except 2003. Most of the multiyear stations clustered together in groups C, D and F, indicating that community structure was fairly similar throughout the years for these stations (Fig. 8). However, some multiyear stations were by themselves as outliers or in separate groups. The nMDS (2D stress: 0.22; 3D stress: 0.16) reinforced this pattern. SIMPER analysis revealed that most of the similarity within groups C and F was due to O. similis, Microcalanus and calanoid nauplii. Most of the dissimilarity between group C and F was due to Triconia, O. similis and Microcalanus. Fig. 8. View largeDownload slide Hierarchical clustering analysis of fourth–root transformed abundance (Bray–Curtis similarity in %, solid lines: outliers (O); broken lines: multiple stations), nMDS plot and spatial distribution of zooplankton abundance groups in the Canada Basin 2003–2006. Spatial maps with 100 and 1000 m depth contours. Fig. 8. View largeDownload slide Hierarchical clustering analysis of fourth–root transformed abundance (Bray–Curtis similarity in %, solid lines: outliers (O); broken lines: multiple stations), nMDS plot and spatial distribution of zooplankton abundance groups in the Canada Basin 2003–2006. Spatial maps with 100 and 1000 m depth contours. The hierarchical cluster analysis of biomass showed seven groups that formed at 62–74% similarity. The majority of the samples were sorted into two larger groups (group B: 32 samples and group C: 21 samples), one smaller group (group A: 3 samples) and 4 samples that were by themselves (hereafter called outliers except for station 28A) (Fig. 9). This pattern was also presented in the nMDS (2D stress: 0.19; 3D stress: 0.14). Groups B and C contained samples from all years. The majority of samples in group C were from 2004 (11 samples) followed by 2006 (6 samples), 2005 (3 samples) and only 1 sample was from 2003. The majority of samples in group B were from 2003 (19 samples), followed by 2004 (10 samples) and 2005 and 2006 only contributed 1 and 2 samples, respectively, to group B. Most of the Basin stations from 2003 to 2006 were within groups B and C. The main differences between these two groups were a lower mean biomass of M. longa, T. libellula and Parasagitta elegans in 2003 compared to 2004, but these differences were only significant for M. longa (P ≤ 0.001; Supplementary Table SI). Group A consisted of three stations from 2003, all of which were located on the southwestern part of the study area. Group A in the biomass analysis consists of the exact same samples as group A in the abundance analysis. The similarity in the abundance of these three samples was due to Pseudocalanus, O. similis and calanoid nauplii. The similarity for biomass was driven by C. glacialis, E. hamata and C. hyperboreus. Fig. 9. View largeDownload slide Hierarchical clustering analysis of fourth–root transformed biomass (Bray–Curtis similarity %, solid lines: outliers (O); broken lines: multiple stations), nMDS plot and spatial distribution of zooplankton biomass groups in the Canada Basin 2003–2006. Spatial maps with 100 and 1000 m depth contour. Fig. 9. View largeDownload slide Hierarchical clustering analysis of fourth–root transformed biomass (Bray–Curtis similarity %, solid lines: outliers (O); broken lines: multiple stations), nMDS plot and spatial distribution of zooplankton biomass groups in the Canada Basin 2003–2006. Spatial maps with 100 and 1000 m depth contour. Sample 28A was displayed as a single sample in terms of biomass and abundance (Figs 8 and 9) and was located the closest to the mouth of the Mackenzie River, which influences the species composition and abundance. Compared to other groups, it had a very low abundance of F. borealis and C. glacialis and was the only station where the neritic Centropages abdominalis was observed. The sample CABOSs was also located closer to the shelf than most stations. It is notable that CABOSs was a multiyear station that was sampled during all years, and whereas the other three CABOS samples clustered in group C for biomass, the sample from 2006 was by itself. The CABOSs sample was one of only two samples where E. bungii was observed and M. longa and T. libellula were absent from the sample. The absence of M. longa and T. libellula during 2006 contributed the most to the dissimilarity between the CABOSs sample and group C according to the SIMPER analysis. According to the BEST analysis, a combination of mean temperature for the upper 100 m, distance to coastline and bottom depth were the best environmental variables to explain the variance for community structure based on abundance (ρ = 0.356). Sea surface temperature was the best variable to explain the variance for biomass (ρ = 0.293) (Table IV). Adding salinity and oxygen to the models did not improve the relationships (Table IV). Table IV: BEST BIO-ENV analysis of zooplankton community structure in the Canada Basin 2003–2006 to temperature (T), salinity (S), oxygen (O), distance to coastline (C) and bottom depth (B) Surface 0−50 m 0−100 m Abundance T (0.323) B (0.2) B (0.2) C, B (0.288) T, B (0.297) T, B (0.332) T, C, B (0.306) T, C, B (0.342) T, C, B (0.356) T, O, C, B (0.314) T, S, C, B (0.354) T, S, C, B, (0.351) T, S, O, C, B (0.315) T, S, O, C, B (0.338) T, S, O, C, B (0.318) Biomass T (0.293) B (0.214) B (0.214) T, B (0.249) T, B (0.236) T, B (0.267) T, O, B (0.253) T, S, B (0.256) T, C, B, (0.265) T, O, C, B (0.235) T, S, C, B (0.252) T, S, C, B (0.252) T, S, O, C, B (0.225) T, S, O, C, B (0.242) T, S, O, C, B (0.233) Surface 0−50 m 0−100 m Abundance T (0.323) B (0.2) B (0.2) C, B (0.288) T, B (0.297) T, B (0.332) T, C, B (0.306) T, C, B (0.342) T, C, B (0.356) T, O, C, B (0.314) T, S, C, B (0.354) T, S, C, B, (0.351) T, S, O, C, B (0.315) T, S, O, C, B (0.338) T, S, O, C, B (0.318) Biomass T (0.293) B (0.214) B (0.214) T, B (0.249) T, B (0.236) T, B (0.267) T, O, B (0.253) T, S, B (0.256) T, C, B, (0.265) T, O, C, B (0.235) T, S, C, B (0.252) T, S, C, B (0.252) T, S, O, C, B (0.225) T, S, O, C, B (0.242) T, S, O, C, B (0.233) Best combinations explaining clustering for abundance and biomass are in bold. ρ is given in parentheses. Table IV: BEST BIO-ENV analysis of zooplankton community structure in the Canada Basin 2003–2006 to temperature (T), salinity (S), oxygen (O), distance to coastline (C) and bottom depth (B) Surface 0−50 m 0−100 m Abundance T (0.323) B (0.2) B (0.2) C, B (0.288) T, B (0.297) T, B (0.332) T, C, B (0.306) T, C, B (0.342) T, C, B (0.356) T, O, C, B (0.314) T, S, C, B (0.354) T, S, C, B, (0.351) T, S, O, C, B (0.315) T, S, O, C, B (0.338) T, S, O, C, B (0.318) Biomass T (0.293) B (0.214) B (0.214) T, B (0.249) T, B (0.236) T, B (0.267) T, O, B (0.253) T, S, B (0.256) T, C, B, (0.265) T, O, C, B (0.235) T, S, C, B (0.252) T, S, C, B (0.252) T, S, O, C, B (0.225) T, S, O, C, B (0.242) T, S, O, C, B (0.233) Surface 0−50 m 0−100 m Abundance T (0.323) B (0.2) B (0.2) C, B (0.288) T, B (0.297) T, B (0.332) T, C, B (0.306) T, C, B (0.342) T, C, B (0.356) T, O, C, B (0.314) T, S, C, B (0.354) T, S, C, B, (0.351) T, S, O, C, B (0.315) T, S, O, C, B (0.338) T, S, O, C, B (0.318) Biomass T (0.293) B (0.214) B (0.214) T, B (0.249) T, B (0.236) T, B (0.267) T, O, B (0.253) T, S, B (0.256) T, C, B, (0.265) T, O, C, B (0.235) T, S, C, B (0.252) T, S, C, B (0.252) T, S, O, C, B (0.225) T, S, O, C, B (0.242) T, S, O, C, B (0.233) Best combinations explaining clustering for abundance and biomass are in bold. ρ is given in parentheses. DISCUSSION Community structure, abundance and biomass The zooplankton community in the Canada Basin consisted mostly of species characteristic for the Arctic such as C. hyperboreus, C. glacialis, M. longa, O. similis, M. pygmaeus, F. borealis and L. helicina (Johnson, 1956; Conover and Huntley, 1991; Auel and Hagen, 2002; Hopcroft et al., 2005; Lane et al., 2008), whereby small copepods dominated the abundance and larger bodied copepods dominated the biomass. These patterns are consistent with previous studies conducted in the Canada Basin and Beaufort Sea slope (Darnis et al., 2008; Kosobokova and Hopcroft, 2010; Hunt et al., 2014; Smoot and Hopcroft, 2017a) and were reflected in our copepod size spectra (Fig. 5), which displayed the pattern typical for Arctic basin copepods (Hopcroft et al., 2005). Non-copepod abundance was dominated by larvaceans, except in 2005 when L. helicina dominated the non-copepod abundance. However, due to the small sample size in 2005, we cannot be certain whether pteropods were unusually dominant in the basin, although Kosobokova and Hopcroft (2010) also reported pteropods being more abundant than larvaceans slightly earlier in that same year. The major contribution of chaetognaths and hydrozoans to non-copepod biomass is also consistent with previous studies (Hopcroft et al., 2005; Kosobokova and Hopcroft, 2010). The Canada Basin zooplankton community is very homogenous spatially and has an insignificant interannual variability during our observation window, possibly due to the relatively long life cycles of most predominant species like C. hyperboreus (2–4 years) (Hirche, 1997; Broms et al., 2009), C. glacialis (2 years) (Kosobokova, 1999). The main patterns observed with the hierarchical clustering analysis and nMDS for abundance and biomass (Figs 8 and 9) confirm the observation of spatial homogeneity, with two large groups that incorporated samples from all years (mostly Basin samples). However, despite no statistical significant interannual differences in overall mean abundance and biomass, biomass cluster C consisted predominantly of 2004 samples and cluster B of samples from 2003. The PERMANOVA also revealed that the year has an effect on the zooplankton community. This could be due to a difference in sea ice concentration and the environmental conditions associated with it. During 2004, the ice edge was further north than during 2003 with a higher concentration of first-year ice observed in the southern Canada Basin and Beaufort Sea compared to 2004 (National Ice Center: Weekly chart products; http://nsidc.org/data/bist/). A sea ice retreat beyond the shelf break can lead to increased wind-driven upwelling at the shelf break (Carmack and Chapman, 2003), which brings nutrient-rich water into the surface layers and leads to increased production. Besides observing the spatial homogeneity of the Canada Basin zooplankton community, we found the same distinctions between the shelf/slope and the basin stations as in Hunt et al. (2014). The BEST analysis supports the influence of station depth on differences between zooplankton communities, since the best models for abundance included bottom depth. The basin stations were characterized by the general absence of shelf taxa, although stations in the western basin had low numbers of meroplankton, which suggests some transport from the shelf into the basin by eddies (Llinás et al., 2009). The clusters of shelf/slope stations (abundance: group A; biomass: group A) were defined by higher Pseudocalanus abundance and the presence of cirripedia nauplii and cyprids, which are all characteristics for water masses influenced by shelf/slope waters (Smoot and Hopcroft, 2017a). That station 28A was not included into the shelf/slope cluster, but displayed as a single sample for abundance as well as biomass likely reflects an influence by runoff from the Mackenzie River. This is further supported by the observation of the neritic copepod Centropages abdominalis and echinoderm larvae (Walkusz et al., 2010; Smoot and Hopcroft, 2017a) in the sample. The average abundance of C. glacialis and C. hyperboreus was of the same order of magnitude as previous observations in the same area (Hunt et al., 2014), and those further towards the Chukchi Sea and central Arctic (Thibault et al., 1999). While the abundance of C. hyperboreus and M. longa seemed to be impacted by the fresher water in the core of our study area, the abundance of O. similis did not appear to be substantially influenced. This may reflect the more euryhaline and eurythermic character of O. similis (Nishida, 1985; Nielsen et al., 2002) compared to Arctic endemic species. Oithona similis showed a significant increase in mean abundance (P-value = 0.05) from 2003 to 2006. However, 4 years of observation is insufficient to convincingly establish whether these trends are persistent or not. It is notable that we report higher mean abundance, but similar biomass and cluster structure, in both 2004 and 2006 than Hunt et al. (2014), who reported on samples collected concurrently in those years. This is a direct reflection of our finer mesh size of 150 μm (compared to their 236 μm) that catches earlier stages of smaller, abundant species such as O. similis and M. pygmaeus as well as copepod nauplii (Gallienne and Robins, 2001; Hopcroft et al., 2005) that contribute little to community biomass. Our mean biomass for all years was slightly higher than the 9.6 mg m−3 (Hopcroft et al., 2005) reported for 2002 using a similar methodological approach. The non-significant Spearman’s rank correlation coefficients indicate that there is no significant change in abundance or biomass from 2003 to 2006, but if we combine our observation with those preceding (Hopcroft et al., 2005) and partially overlapping (Hunt et al., 2014) our observation period, it becomes clear that there has been an increasing trend in mean biomass from 2002 to 2008 (except in 2004). Historic comparison Historic datasets for comparison to our findings are limited for the Canada Basin due to the remoteness of the area as well as the ice cover. Most previous data come from ice-stations such as Drift Station Alpha (Johnson, 1963), T-3 ice islands (Scott, 1969), NP-22 and NP-23 (Kosobokova, 1982) and the Surface Heat Budget of the Arctic Ocean (SHEBA) (Ashjian et al., 2003). The extremely low abundances suggested at Drift Station Alpha are most likely due to incomplete descriptions of methodology and shall be ignored. The T-3 ice island copepod mean abundance for August 1966 to 1969 were 105 ind. m−3 (mesh size 215-μm, sampling depth up to 100 m) (Scott, 1969), about 5–10-fold lower than our mean abundance, in part due to differences in collection mesh size. The mean abundance observed from the ice-stations NP-22 and NP-23 during August and September 1975 and 1977 was 349 ind. m−3 (mesh size 180 μm). The NP-22 stations were located further north in the central Arctic compared to our study. Our data (811 ind. m-3 mean abundance for 2003, 2004 and 2006) were much closer to the mean abundance for the zooplankton community reported for SHEBA of 591 ind. m−3 (Ashjian et al., 2003) for August and early September in 1998 (mesh size 150-μm, sampling depth: 100 m, lifestage “eggs” omitted) and that of Smoot and Hopcroft (2017b) of about 744 ind. m−3 along the Beaufort Slope for August and September 2012–2014 (mesh size 150 μm, sampling depth 100 m). Overall, there is a suggestion of increasing mean abundance when comparing the historic and recent datasets. However, caution must be exercised comparing contemporary data to historical data due to differing sampling techniques, areas, as well as changing taxonomy (e.g. Johnson, (1956) reported C. finmarchicus instead of C. glacialis, since C. glacialis was not described by Jaschnov until 1955 (Jaschnov, 1955)). Nevertheless, increasing copepod biomass has also been suggested to occur in the adjoining Chukchi Sea (Ershova et al., 2015). The lack of historic data that are comparable to our study, and the suggestion of increasing abundance and biomass highlight the need for the Canada Basin to be sampled more regularly and with consistent methods. CONCLUSION The epipelagic zooplankton communities of the Canada Basin are dominated by copepods both in number and in biomass. We found that there was no obvious interannual change in community structure over our short 4-year observation period, with community structure influenced to a small degree by environmental factors. When our observations are combined with contemporary studies (Hopcroft et al., 2005; Hunt et al., 2014) and historical data, both abundance and biomass have displayed increasing long-term trends. SUPPLEMENTARY DATA Supplementary material is available at Journal of Plankton Research online. ACKNOWLEDGEMENTS We thank the Department of Fisheries and Oceans (DFO) and Fiona McLaughlin for providing sampling opportunities and the crew of the Canadian Coast Guard Icebreaker Louis S. St-Laurent as well as the students and staff collecting the zooplankton samples. We also thank Cheryl Clarke-Hopcroft, Chris Stark and Caitlin Smoot for support with the taxonomic questions. Funding This work was supported by the National Science Foundation (NSF) [OPP-0909571]; The Arctic Ocean Diversity (ArcOD) Project of the Census of Marine Life (CoML); and a University of Alaska Fairbanks graduate fellowship; and contributes to the Circumpolar Biodiversity Monitoring Program (CBMP) through National Oceanic and Atmospheric Administration (NOAA)/Cooperative Institute for Alaska Research (CIFAR) awards [NA08OAR4320751, NA13OAR4320056]. DATA ARCHIVING https://arcticdata.io/catalog/#view/doi:10.18739/A27S4S REFERENCES Arrigo , K. R. and Van Dijken , G. L. 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Oceanogr. , 139 , 42 – 65 . Google Scholar CrossRef Search ADS Yamamoto-Kawai , M. , McLaughlin , F. A. , Carmack , E. C. , Nishino , S. , Shimada , K. and Kurita , N. ( 2009 ) Surface freshening of the Canada Basin, 2003–2007: river runoff versus sea ice meltwater . J. Geophys. Res. Oceans , 114 , 2156 – 2202 . Google Scholar CrossRef Search ADS Author notes Corresponding editor: Roger Harris © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Plankton Research Oxford University Press

Abundance, biomass and community structure of epipelagic zooplankton in the Canada Basin

Journal of Plankton Research , Volume Advance Article (4) – Jul 11, 2018

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Abstract

Abstract Changing environmental conditions such as decreasing sea ice cover impact Arctic zooplankton. In the Canada Basin, zooplankton surveys have seldom been done due to its traditionally thick, year-round ice cover. Here, we describe the zooplankton community of the Canada Basin before the two recent sea ice minima (2007 and 2012). Zooplankton were sampled from the upper 100 m during August and September of the years 2003–2006 using a 150-μm mesh net to determine species composition, abundance and biomass. To describe the zooplankton community and its relation to the environment, we used Bray–Curtis similarity, and then applied hierarchical clustering, non-parametric multidimensional scaling and the BEST BIO-ENV routine. The most abundant zooplankton species in all years were smaller copepods such as Oithona similis and Microcalanus pygmaeus. Biomass was dominated by larger copepod species such as Calanus hyperboreus and Calanus glacialis. For the non-copepod zooplankton, the pteropod Limacina helicina and the larvacean Fritillaria borealis were typically the most abundant species. The non-copepod biomass was dominated by the chaetognath Eukrohnia hamata and L. helicina, while F. borealis contributed relatively little to the overall biomass despite its high numbers. Zooplankton communities differed between shelf/slope and basin stations. We found no obvious interannual changes in community structure over our short 4-year observation period, with community structure influenced to a small degree by environmental factors. INTRODUCTION The Canada Basin is a deep, ice-covered basin located in the central Arctic Ocean. Zooplankton within the Arctic basins are intricately tuned to the basin’s primary production cycle (Smith and Schnack-Schiel, 1990). While the Canada Basin was historically been covered year-round by thick multiyear ice, sea ice extent and concentration have declined rapidly within the past decades, reaching a record minimum during summer 2007 only to have it exceeded recently during summer 2012 (Comiso, 2012; Parkinson and Comiso, 2013). The sea ice meltwater represents the major freshwater influx to the Arctic that is supplemented by river discharge, with both sources increasing over time (Yamamoto-Kawai et al., 2009). Water temperatures in the Canada Basin already showed a warming trend from 1993 to 2008 (Jackson et al., 2010) while freshening of the Beaufort Gyre, as well as a deepening of the nutricline and chlorophyll maximum, have also been observed between the years 2003 and 2009 (McLaughlin and Carmack, 2010). Such detectable changes in the environmental conditions typically have an impact on zooplankton communities (Richardson, 2008), and this may be particularly true in the Arctic (Gradinger et al., 2010; Nelson et al., 2014). Zooplankton in the Canada Basin has been studied only sporadically due to its traditionally thick, year-round ice cover. Studies during the last century established rudimentary community composition and seasonal cycles but were seldom synoptic or repeated (Johnson, 1963; Pautzke, 1979; Thibault et al., 1999; Ashjian et al., 2003). In contrast, the Beaufort Sea slope region experienced more extensive activities often related to oil and gas exploration (Hufford et al., 1974; Horner and Murphy, 1985; Hopky et al., 1994a,,b, c). Even the more recent studies have been concentrated in the more coastal Beaufort Sea and shelf areas (Darnis et al., 2008; Lane et al., 2008; Walkusz et al., 2008, 2010, 2013; Smoot and Hopcroft, 2017a), with less focus on the central Canada Basin (Hopcroft et al., 2005; Kosobokova and Hopcroft, 2010; Hunt et al., 2014). At present, indications are that many species are shared between the Arctic’s major basins (Kosobokova et al., 2011). The fauna contains a mixture of endemic Arctic species, viable species shared with other ocean basins and species that are advected from the Pacific such as Eucalanus bungii, Pseudocalanus newmani, Metridia pacifica and Neocalanus flemingeri (Hopcroft et al., 2005; Kosobokova and Hopcroft, 2010; Hunt et al., 2014) that are not thought to be viable in the Arctic or have little reproductive success (Matsuno et al., 2015; Wassmann et al., 2015). The abundance and biomass within the epipelagic upper 100 m of the water column are typically dominated by copepods, whereby the smaller bodied species such as Oithona similis, Microcalanus pygmaeus and Triconia borealis make up the bulk of the abundance, while larger bodied endemic copepods such as Calanus hyperboreus, Calanus glacialis, Metridia longa and Paraeuchaeta glacialis dominate the biomass (Kosobokova and Hopcroft, 2010). Non-copepod abundance is frequently dominated by the larvaceans Fritillaria borealis and Oikopleura vanhoeffeni as well as the pteropod Limacina helicina that can at times contribute a significant percentage to the biomass (Hopcroft et al., 2005; Kosobokova and Hopcroft, 2010; Hunt et al., 2014). Zooplankton species are also advected from the shelf into the basin within eddies (Carmack and Macdonald, 2002; Llinás et al., 2009), and hence meroplankton typical for shelf communities, such as cirripedia cyprids and echinoderm larvae, can occasionally be found far into the basin (Hunt et al., 2014). A freshening and warming of the Canada Basin due to climate change has implications for the zooplankton community. The epipelagic large-bodied endemic copepods are probably at greatest risk of stress or competition from the advected subarctic species. Hunt et al., (2014) suggested that there has already been a decrease in abundance of species that are typical for the Arctic and Subarctic, such as O. similis, L. helicina, M. pygmaeus, and F. borealis during 2007 and 2008. An earlier sea ice retreat and a shrinking sea ice extent combined with the freshening of the Beaufort Gyre (McLaughlin and Carmack, 2010) could potentially increase primary productivity (Arrigo and Van Dijken, 2015) and zooplankton biomass in the basins (Hunt et al., 2014) as has already been demonstrated for the Chukchi Sea shelf (Ershova et al., 2015). Here we present new data describing the epipelagic zooplankton abundance, biomass and community structure in the Canada Basin for 2003 to 2006 to fill in spatial and temporal gaps. By describing the basin’s zooplankton community before the two recent summer sea ice minima of 2007 and 2012, we hope to build a better foundation for comparison of zooplankton communities before and after major environmental changes to establish how they may be affected. METHOD Zooplankton sampling and taxonomic analysis Our study area encompassed much of the Canada Basin to as far north as 80°N latitude (Fig. 1). Samples were collected during August and September of 2003 to 2006 (day and night) aboard the Canadian Coast Guard vessel Louis S. St-Laurent. Bongo nets with a mouth diameter of 60 cm and 150-μm mesh size were deployed vertically in the upper 100 m of the water column. During 2003 and 2004, the nets were equipped with General Oceanics flowmeters to measure the volume of filtered water, while during 2005 and 2006, OceanTest flowmeters were used. On three occasions, flowmeters iced-up and gave false readings. In these cases, a filtration efficiency of 100% was assumed because there was insufficient phytoplankton to impact filtration efficiency. A total of 60 samples were analyzed: 23 from 2003, 23 from 2004, only 4 from 2005 (due to wire-time constraints) and 10 from 2006. Upon collection, the samples were preserved in seawater with 4% formaldehyde. Fig. 1. View largeDownload slide Map of the study area in the Canada Basin. Multiyear stations include 2003, 2004, 2005 and 2006 stations, if applicable. Fig. 1. View largeDownload slide Map of the study area in the Canada Basin. Multiyear stations include 2003, 2004, 2005 and 2006 stations, if applicable. In the lab, collections were subsampled using a Folsom splitter (Harris et al., 2000), with smaller subsamples (e.g. 1/128th) used to identify very abundant species (such as O. similis) and larger subsamples used for the rarer species. The full sample was generally analyzed for cnidarians, amphipods, chaetognaths and larger copepods. Animals were enumerated and measured using the ZoopBiom software (Roff and Hopcroft, 1986). For the more prominent species, up to 100 individuals per species were measured with remaining individuals in the aliquot simply enumerated. The developmental stage of larger copepods and the sex of adults were also recorded. We used prosome length to differentiate between C. glacialis and C. hyperboreus during early developmental stages (copepodite stages CI–CIII). For individuals where early life stages could not be distinguished between species, they were grouped according to their genus. In order to calculate dry weights (DW), we applied length–weight relationships according to Hopcroft et al. (2005) that were species–specific or from morphologically similar species. The ZoopBiom software employs subsample fractions and volume filtered to calculate both abundance and biomass, along with their size spectra, for each taxa. Here we used arithmetic bins of 50-μm width for creating the size spectra and summed the spectra for all copepod taxa within a sample. These sample–specific spectra were then averaged for each study year. It is notable that samples from 2 years (2004 and 2006) of our study were collected concurrently with the work by Hunt et al., (2014) that employed a coarser mesh size (256 μm). Environmental data Concurrent temperature (°C), salinity (PSU) and oxygen (mL/L) data were obtained using a SBE 911 plus (Sea-Bird Electronics Inc.) (McLaughlin et al., 2009). Means were calculated for 0–50 m and 0–100 m depth layers. We derived the Euclidean distance to the coastline from World Vector Shoreline data from the National Oceanic and Atmospheric Administration (NOAA) by using ArcMap 10.1 and the Geospatial Modelling Environment (GME). The coastline was imported into ArcMap 10.1 and the “Euclidean distance tool” applied to derive a raster layer displaying the Euclidean distance to the coastline. After that, the “isectpntrst”-command in GME was applied to spatially correlate the stations with the corresponding Euclidean distance value. Statistical analysis Statistical analysis was conducted using the PRIMER (Version6) (Clarke and Warwick, 2001), R (Version 3.2.3) and ArcMap 10.1 software. Analysis was completed using all species within a sample unless stated otherwise. We applied a fourth–root transformation of abundance and biomass data and calculated the Bray–Curtis similarity index (Bray and Curtis, 1957). Differences and patterns in the zooplankton community between stations or years were detected using weighted average hierarchical cluster analysis and non-parametric multidimensional scaling (nMDS) based on the Bray–Curtis matrix. To take spatial variability into account, we applied a permutational multivariate analysis of variance (PERMANOVA) using the Adonis function in R (Vegan package) using 5000 permutations. We applied two factors to the function: year and region, whereby the region was split into sampling stations that had a bottom depth above 1100 m versus deeper stations. Species that showed up only once in the entire dataset were excluded from the PERMANOVA. For Microcalanus, Metridia and Pseudocalanus, the copepodites and adults were pooled as Microcalanus spp., Metridia spp. and Pseudocalanus spp., respectively for the hierarchical cluster analysis, nMDS and PERMANOVA. We used the SIMPER routine to define the similarity percentage between clusters, and which species were driving the grouping/clustering. For the SIMPER analysis, any species that contributed less than 70% to the within–group similarity was excluded. We tested for a possible increase in mean community abundance and biomass from 2003 to 2006 using the Spearman’s rank correlation coefficient (ρ; “cor.test” command in R). To test for significant differences between the annual mean abundance and biomass on species level, the data were log–transformed and analysis of variance (ANOVA) was conducted in R. If applicable, Tukey tests were used to determine which years were significantly different. The year 2005 was excluded from these analyses because of the small sample size during that year. To relate environmental data to community patterns, we employed Primer’s BEST BIO-ENV routine after normalizing the physical variables. RESULTS Environmental conditions Overall, 2003 and 2004 were more similar in their environmental conditions compared to 2005 and 2006. All years had a core of low salinity towards the center of the study area related to dynamics of the Beaufort Gyre. During 2003 and 2004, the northern stations were more saline than 2005 and 2006 (Fig. 2). A small tongue of elevated salinity occurred at the southern stations during 2003 and 2005. Fig. 2. View largeDownload slide Average (surface to 100 m) for salinity (PSU), temperature (°C) and oxygen (mL/L) plots for the years 2003–2006. Zooplankton sampling stations in 2005 indicated by white circles. Fig. 2. View largeDownload slide Average (surface to 100 m) for salinity (PSU), temperature (°C) and oxygen (mL/L) plots for the years 2003–2006. Zooplankton sampling stations in 2005 indicated by white circles. The average temperature for the upper 100 m decreased with increasing latitude during all years. Overall, temperatures were colder further offshore, and in 2003 on the eastern and western edges. During 2003, 2005 and 2006, the core of the study area had temperatures between −1 and −0.5°C. This temperature range occurred further south for 2004 compared to 2003. Oxygen concentrations during 2003 and 2004 were lower in the North relative to the core of the study area. The same pattern was observed during 2006, with the difference that the southern part of the study area showed a lower oxygen concentration compared to the other years. Oxygen concentration during 2005 was relatively high, with the highest concentration in the Northeast. General abundance and biomass In total, 50 taxonomic categories were found during 2003–2006, of which 27 were copepods (Supplementary Table SI). Of the other categories, three were euphausiids, four amphipods, four hydrozoans, two larvaceans, two pteropods, one ctenophore, an isopod, an annelid and then larvae of several different taxonomic groups. The mean community abundance and biomass varied 1.5-fold across years, being highest in 2006 (Table I), but there was no significant difference between the years (P-value abundance = 0.207; P-value biomass = 0.411) and no correlation between the years, mean abundance and biomass (Spearman’s rank correlation coefficient abundance: P-value = 0.27, ρ = 0.15; biomass: P-value = 0.65, ρ = 0.06). The PERMANOVA showed that there was no significant interaction between the factors year and region (Table II). Both factors were significant (year: P = 0.0002; region: P = 0.015) for abundance and for biomass, only the year was significant (P = 0.0002). Copepods dominated the abundance (Fig. 3) and biomass (Fig. 4) for all years. They made up around 90% of the abundance during 2003, 2005 and 2006, and 84% in 2004, contributing up to 88% (2003) of community biomass. The abundance of non-copepod zooplankton was dominated by larvaceans, mainly F. borealis, during 2003, 2004 and 2006 (6, 12 and 9%, respectively). For 2005, non-copepod zooplankton abundance was dominated by pteropods (8%), but since this observation was based on only four stations, it was not appropriate to compare it statistically to the abundance of other years. The category of “others” (Fig. 3) consisted of polychaete larvae, isopods, cnidarians, meroplankton, amphipods, euphausiids, chaetognaths and ostracods, each of them contributing <0.5% to the overall abundance. Non-copepod biomass was dominated by chaetognaths for all years (2003 = 7%, 2004 = 11%, 2005 = 8%, 2006 = 11%) (Fig. 4) with Eukrohnia hamata contributing much more than Parasagitta elegans except at the most nearshore sites. The other major invertebrate predator of zooplankton in this system was the medusa Aglantha digitale. Table I: Average abundance and biomass of zooplankton in the Canada Basin from 2003 to 2006 Year Dates # of samples Abundance (ind. m−3) ± SE Biomass (mg DW m−3) ± SE 2003 08/11–09/02 23 687 ± 62 13 ± 1.1 2004 08/09–08/30 23 736 ± 91 11 ± 1 2005 08/03–08/25 4 834 ± 263 12 ± 1.6 2006 08/10–09/8 10 1010 ± 202 17 ± 2.5 Year Dates # of samples Abundance (ind. m−3) ± SE Biomass (mg DW m−3) ± SE 2003 08/11–09/02 23 687 ± 62 13 ± 1.1 2004 08/09–08/30 23 736 ± 91 11 ± 1 2005 08/03–08/25 4 834 ± 263 12 ± 1.6 2006 08/10–09/8 10 1010 ± 202 17 ± 2.5 Values rounded to the nearest whole number. SE, standard error . Table I: Average abundance and biomass of zooplankton in the Canada Basin from 2003 to 2006 Year Dates # of samples Abundance (ind. m−3) ± SE Biomass (mg DW m−3) ± SE 2003 08/11–09/02 23 687 ± 62 13 ± 1.1 2004 08/09–08/30 23 736 ± 91 11 ± 1 2005 08/03–08/25 4 834 ± 263 12 ± 1.6 2006 08/10–09/8 10 1010 ± 202 17 ± 2.5 Year Dates # of samples Abundance (ind. m−3) ± SE Biomass (mg DW m−3) ± SE 2003 08/11–09/02 23 687 ± 62 13 ± 1.1 2004 08/09–08/30 23 736 ± 91 11 ± 1 2005 08/03–08/25 4 834 ± 263 12 ± 1.6 2006 08/10–09/8 10 1010 ± 202 17 ± 2.5 Values rounded to the nearest whole number. SE, standard error . Table II: PERMANOVA output for abundance (ab) and biomass (bm) using the factors year and region df SS ab MS ab R2 ab P ab SS bm MS bm R2 bm P bm Year 2 0.14 0.07 0.1 0.0002 0.26 0.13 0.14 0.0002 Region 1 0.05 0.05 0.04 0.02 0.04 0.04 0.02 0.24 Year:region 2 0.02 0.02 0.01 0.54 0.03 0.03 0.02 0.4 Residuals 51 1.21 0.02 0.84 1.59 0.03 0.8 df SS ab MS ab R2 ab P ab SS bm MS bm R2 bm P bm Year 2 0.14 0.07 0.1 0.0002 0.26 0.13 0.14 0.0002 Region 1 0.05 0.05 0.04 0.02 0.04 0.04 0.02 0.24 Year:region 2 0.02 0.02 0.01 0.54 0.03 0.03 0.02 0.4 Residuals 51 1.21 0.02 0.84 1.59 0.03 0.8 Table shows degrees of freedom (df), sum of squares (SS), mean squares (MS), R2 and P-value (P). Table II: PERMANOVA output for abundance (ab) and biomass (bm) using the factors year and region df SS ab MS ab R2 ab P ab SS bm MS bm R2 bm P bm Year 2 0.14 0.07 0.1 0.0002 0.26 0.13 0.14 0.0002 Region 1 0.05 0.05 0.04 0.02 0.04 0.04 0.02 0.24 Year:region 2 0.02 0.02 0.01 0.54 0.03 0.03 0.02 0.4 Residuals 51 1.21 0.02 0.84 1.59 0.03 0.8 df SS ab MS ab R2 ab P ab SS bm MS bm R2 bm P bm Year 2 0.14 0.07 0.1 0.0002 0.26 0.13 0.14 0.0002 Region 1 0.05 0.05 0.04 0.02 0.04 0.04 0.02 0.24 Year:region 2 0.02 0.02 0.01 0.54 0.03 0.03 0.02 0.4 Residuals 51 1.21 0.02 0.84 1.59 0.03 0.8 Table shows degrees of freedom (df), sum of squares (SS), mean squares (MS), R2 and P-value (P). Fig. 3. View largeDownload slide Relative abundance (ind. m−3) of major zooplankton groups in the Canada Basin for 2003–2006. Lower panel range is trimmed to increase resolution. Fig. 3. View largeDownload slide Relative abundance (ind. m−3) of major zooplankton groups in the Canada Basin for 2003–2006. Lower panel range is trimmed to increase resolution. Fig. 4. View largeDownload slide Relative biomass (mg DW m−3) of zooplankton groups in the Canada Basin for 2003–2006. Lower panel range is trimmed to increase resolution. Fig. 4. View largeDownload slide Relative biomass (mg DW m−3) of zooplankton groups in the Canada Basin for 2003–2006. Lower panel range is trimmed to increase resolution. The size spectra of copepod abundance and biomass showed expected patterns with the smaller species being the most abundant, but biomass being highest for large copepods where a series of modes occurred largely dominated by Calanus stages (Fig. 5). The peak in abundance and biomass for the size of ~300–450 μm was formed by O. similis, M. copepodites and T. borealis. The species that contributed most to the peak in abundance around 3000 μm were C. glacialis and C. hyperboreus, where the latter was also responsible for the peaks around 5000 and 7000 μm. For the biomass size spectra, the species responsible for the 3000 μm peak were C. glacialis, C. hyperboreus, M. longa and P. glacialis. For the 5000 μm peak, C. hyperboreus and P. glacialis were the main drivers and only C. hyperboreus for the 7000 μm peak. The size spectra were similar throughout all years, with the exception of a shift in the 3000 μm and the 5000 μm peaks in 2006, for which C. glacialis and C. hyperboreus were responsible. Fig. 5. View largeDownload slide Size spectra of copepod prosome length (μm) for abundance (A) and biomass (B) in the Canada Basin during 2003–2006 with prosome length ranges for O. similis (O. sim.), Microcalanus sp. (Micro.), C. glacialis (C. gla.) and C. hyperboreus (C. hy.). Fig. 5. View largeDownload slide Size spectra of copepod prosome length (μm) for abundance (A) and biomass (B) in the Canada Basin during 2003–2006 with prosome length ranges for O. similis (O. sim.), Microcalanus sp. (Micro.), C. glacialis (C. gla.) and C. hyperboreus (C. hy.). Species-specific abundance The zooplankton community for most stations consisted mostly of species common to the Arctic such as C. hyperboreus, C. glacialis, M. longa, T. borealis, O. similis, M. pygmaeus, P. glacialis and F. borealis, with O. similis and M. pygmaeus the most abundant in all years (Supplementary Table SI and Fig. 6). The species C. glacialis, M. longa, M. pygmaeus, O. similis, Scolecithricella minor, Paraheterorhabdus norvegicus, Pseudocalanus sp. copepodite, Eukrohnia hamata, Themisto libellula, T. abyssorum and A. digitale were significantly different (P-value ≤ 0.05) between some years (Supplementary Table SI). Pacific species that were advected to the Canada Basin, such as Eucalanus bungii and Neocalanus flemingeri, were observed sporadically, typically with only one or two specimens per sample. Fig. 6. View largeDownload slide Proportional abundance plots (ind. m−3) of copepods in the Canada Basin 2003–2006. Microcalanus copepodites and adults combined. Fig. 6. View largeDownload slide Proportional abundance plots (ind. m−3) of copepods in the Canada Basin 2003–2006. Microcalanus copepodites and adults combined. Microcalanus pygmaeus showed a pattern with higher abundance in the basin than towards the coast. Calanus hyperboreus also generally displayed larger numbers in the basin than towards the coast. For 2003, 2004 and 2006, the lower abundances of C. hyperboreus, C. glacialis and M. longa coincided with the region of lower salinity and higher oxygen concentration within the Beaufort Gyre (Figs 2 and 6). For 2003, 2005 and 2006, M. longa displayed higher abundances towards the shelf than in the Basin, while during 2004, abundances tended to be lower at a cluster of stations toward the north central basin. While C. hyperboreus, C. glacialis and M. longa had higher abundances towards the western Beaufort Sea (especially in 2004), O. similis and M. pygmaeus had elevated abundances towards the eastern Beaufort Sea, where the water was fresher due to the Beaufort Gyre. During 2003, 2004 and 2006, F. borealis was less abundant in the south of our study area, towards the shelf break. Beyond that, no obvious spatial pattern of abundance was observed for F. borealis or L. helicina (Fig. 7). For most species, the distance to the coastline, bottom depth and mean salinity (surface to 100 m) each explained less than 8% of the variance (Table III). For C. hyperboreus, the distance to the coast explained 30% of the variance and salinity explained 36% of the variance. For O. similis, 21% of the variance was accounted for by salinity (Table III). Fig. 7. View largeDownload slide Proportional abundance plots (ind. m−3) of F. borealis and L. helicina in the Canada Basin 2003–2006. Fig. 7. View largeDownload slide Proportional abundance plots (ind. m−3) of F. borealis and L. helicina in the Canada Basin 2003–2006. Table III: Relationship between zooplankton abundance and distance to coast, bottom depth and mean salinity for the upper 100 m of the Canada Basin from 2003 to 2006 Species r2 coast r2 depth r2 salinity Calanus glacialis 0.05 0.01 0.02 Calanus hyperboreus 0.3 0.006 0.36 Metridia longa 0.11 0.02 0.08 Microcalanus sp. 0.0006 0.046 0.015 Oithona similis 0.00002 <0.001 0.21 Fritillaria borealis 0.04 0.14 0.002 Limacina helicina 0.08 0.0015 0.004 Species r2 coast r2 depth r2 salinity Calanus glacialis 0.05 0.01 0.02 Calanus hyperboreus 0.3 0.006 0.36 Metridia longa 0.11 0.02 0.08 Microcalanus sp. 0.0006 0.046 0.015 Oithona similis 0.00002 <0.001 0.21 Fritillaria borealis 0.04 0.14 0.002 Limacina helicina 0.08 0.0015 0.004 Abundance data were log-transformed. Table III: Relationship between zooplankton abundance and distance to coast, bottom depth and mean salinity for the upper 100 m of the Canada Basin from 2003 to 2006 Species r2 coast r2 depth r2 salinity Calanus glacialis 0.05 0.01 0.02 Calanus hyperboreus 0.3 0.006 0.36 Metridia longa 0.11 0.02 0.08 Microcalanus sp. 0.0006 0.046 0.015 Oithona similis 0.00002 <0.001 0.21 Fritillaria borealis 0.04 0.14 0.002 Limacina helicina 0.08 0.0015 0.004 Species r2 coast r2 depth r2 salinity Calanus glacialis 0.05 0.01 0.02 Calanus hyperboreus 0.3 0.006 0.36 Metridia longa 0.11 0.02 0.08 Microcalanus sp. 0.0006 0.046 0.015 Oithona similis 0.00002 <0.001 0.21 Fritillaria borealis 0.04 0.14 0.002 Limacina helicina 0.08 0.0015 0.004 Abundance data were log-transformed. Community structure The hierarchical cluster analysis using abundance data showed one large group (group F: 37 samples) and five smaller groups (group A: 3 samples; group B: 2 samples; group C: 6 samples; group D: 2 samples; group E: 4 samples) and six single samples (hereafter called outliers except for station 28A) formed at 67–81% similarities. The large group F consisted of 17 samples from 2003, 14 samples from 2004, 5 samples from 2006 and 2 samples from 2005 (Fig. 8). The second largest group was group C with a total of six samples from all years except 2003. Most of the multiyear stations clustered together in groups C, D and F, indicating that community structure was fairly similar throughout the years for these stations (Fig. 8). However, some multiyear stations were by themselves as outliers or in separate groups. The nMDS (2D stress: 0.22; 3D stress: 0.16) reinforced this pattern. SIMPER analysis revealed that most of the similarity within groups C and F was due to O. similis, Microcalanus and calanoid nauplii. Most of the dissimilarity between group C and F was due to Triconia, O. similis and Microcalanus. Fig. 8. View largeDownload slide Hierarchical clustering analysis of fourth–root transformed abundance (Bray–Curtis similarity in %, solid lines: outliers (O); broken lines: multiple stations), nMDS plot and spatial distribution of zooplankton abundance groups in the Canada Basin 2003–2006. Spatial maps with 100 and 1000 m depth contours. Fig. 8. View largeDownload slide Hierarchical clustering analysis of fourth–root transformed abundance (Bray–Curtis similarity in %, solid lines: outliers (O); broken lines: multiple stations), nMDS plot and spatial distribution of zooplankton abundance groups in the Canada Basin 2003–2006. Spatial maps with 100 and 1000 m depth contours. The hierarchical cluster analysis of biomass showed seven groups that formed at 62–74% similarity. The majority of the samples were sorted into two larger groups (group B: 32 samples and group C: 21 samples), one smaller group (group A: 3 samples) and 4 samples that were by themselves (hereafter called outliers except for station 28A) (Fig. 9). This pattern was also presented in the nMDS (2D stress: 0.19; 3D stress: 0.14). Groups B and C contained samples from all years. The majority of samples in group C were from 2004 (11 samples) followed by 2006 (6 samples), 2005 (3 samples) and only 1 sample was from 2003. The majority of samples in group B were from 2003 (19 samples), followed by 2004 (10 samples) and 2005 and 2006 only contributed 1 and 2 samples, respectively, to group B. Most of the Basin stations from 2003 to 2006 were within groups B and C. The main differences between these two groups were a lower mean biomass of M. longa, T. libellula and Parasagitta elegans in 2003 compared to 2004, but these differences were only significant for M. longa (P ≤ 0.001; Supplementary Table SI). Group A consisted of three stations from 2003, all of which were located on the southwestern part of the study area. Group A in the biomass analysis consists of the exact same samples as group A in the abundance analysis. The similarity in the abundance of these three samples was due to Pseudocalanus, O. similis and calanoid nauplii. The similarity for biomass was driven by C. glacialis, E. hamata and C. hyperboreus. Fig. 9. View largeDownload slide Hierarchical clustering analysis of fourth–root transformed biomass (Bray–Curtis similarity %, solid lines: outliers (O); broken lines: multiple stations), nMDS plot and spatial distribution of zooplankton biomass groups in the Canada Basin 2003–2006. Spatial maps with 100 and 1000 m depth contour. Fig. 9. View largeDownload slide Hierarchical clustering analysis of fourth–root transformed biomass (Bray–Curtis similarity %, solid lines: outliers (O); broken lines: multiple stations), nMDS plot and spatial distribution of zooplankton biomass groups in the Canada Basin 2003–2006. Spatial maps with 100 and 1000 m depth contour. Sample 28A was displayed as a single sample in terms of biomass and abundance (Figs 8 and 9) and was located the closest to the mouth of the Mackenzie River, which influences the species composition and abundance. Compared to other groups, it had a very low abundance of F. borealis and C. glacialis and was the only station where the neritic Centropages abdominalis was observed. The sample CABOSs was also located closer to the shelf than most stations. It is notable that CABOSs was a multiyear station that was sampled during all years, and whereas the other three CABOS samples clustered in group C for biomass, the sample from 2006 was by itself. The CABOSs sample was one of only two samples where E. bungii was observed and M. longa and T. libellula were absent from the sample. The absence of M. longa and T. libellula during 2006 contributed the most to the dissimilarity between the CABOSs sample and group C according to the SIMPER analysis. According to the BEST analysis, a combination of mean temperature for the upper 100 m, distance to coastline and bottom depth were the best environmental variables to explain the variance for community structure based on abundance (ρ = 0.356). Sea surface temperature was the best variable to explain the variance for biomass (ρ = 0.293) (Table IV). Adding salinity and oxygen to the models did not improve the relationships (Table IV). Table IV: BEST BIO-ENV analysis of zooplankton community structure in the Canada Basin 2003–2006 to temperature (T), salinity (S), oxygen (O), distance to coastline (C) and bottom depth (B) Surface 0−50 m 0−100 m Abundance T (0.323) B (0.2) B (0.2) C, B (0.288) T, B (0.297) T, B (0.332) T, C, B (0.306) T, C, B (0.342) T, C, B (0.356) T, O, C, B (0.314) T, S, C, B (0.354) T, S, C, B, (0.351) T, S, O, C, B (0.315) T, S, O, C, B (0.338) T, S, O, C, B (0.318) Biomass T (0.293) B (0.214) B (0.214) T, B (0.249) T, B (0.236) T, B (0.267) T, O, B (0.253) T, S, B (0.256) T, C, B, (0.265) T, O, C, B (0.235) T, S, C, B (0.252) T, S, C, B (0.252) T, S, O, C, B (0.225) T, S, O, C, B (0.242) T, S, O, C, B (0.233) Surface 0−50 m 0−100 m Abundance T (0.323) B (0.2) B (0.2) C, B (0.288) T, B (0.297) T, B (0.332) T, C, B (0.306) T, C, B (0.342) T, C, B (0.356) T, O, C, B (0.314) T, S, C, B (0.354) T, S, C, B, (0.351) T, S, O, C, B (0.315) T, S, O, C, B (0.338) T, S, O, C, B (0.318) Biomass T (0.293) B (0.214) B (0.214) T, B (0.249) T, B (0.236) T, B (0.267) T, O, B (0.253) T, S, B (0.256) T, C, B, (0.265) T, O, C, B (0.235) T, S, C, B (0.252) T, S, C, B (0.252) T, S, O, C, B (0.225) T, S, O, C, B (0.242) T, S, O, C, B (0.233) Best combinations explaining clustering for abundance and biomass are in bold. ρ is given in parentheses. Table IV: BEST BIO-ENV analysis of zooplankton community structure in the Canada Basin 2003–2006 to temperature (T), salinity (S), oxygen (O), distance to coastline (C) and bottom depth (B) Surface 0−50 m 0−100 m Abundance T (0.323) B (0.2) B (0.2) C, B (0.288) T, B (0.297) T, B (0.332) T, C, B (0.306) T, C, B (0.342) T, C, B (0.356) T, O, C, B (0.314) T, S, C, B (0.354) T, S, C, B, (0.351) T, S, O, C, B (0.315) T, S, O, C, B (0.338) T, S, O, C, B (0.318) Biomass T (0.293) B (0.214) B (0.214) T, B (0.249) T, B (0.236) T, B (0.267) T, O, B (0.253) T, S, B (0.256) T, C, B, (0.265) T, O, C, B (0.235) T, S, C, B (0.252) T, S, C, B (0.252) T, S, O, C, B (0.225) T, S, O, C, B (0.242) T, S, O, C, B (0.233) Surface 0−50 m 0−100 m Abundance T (0.323) B (0.2) B (0.2) C, B (0.288) T, B (0.297) T, B (0.332) T, C, B (0.306) T, C, B (0.342) T, C, B (0.356) T, O, C, B (0.314) T, S, C, B (0.354) T, S, C, B, (0.351) T, S, O, C, B (0.315) T, S, O, C, B (0.338) T, S, O, C, B (0.318) Biomass T (0.293) B (0.214) B (0.214) T, B (0.249) T, B (0.236) T, B (0.267) T, O, B (0.253) T, S, B (0.256) T, C, B, (0.265) T, O, C, B (0.235) T, S, C, B (0.252) T, S, C, B (0.252) T, S, O, C, B (0.225) T, S, O, C, B (0.242) T, S, O, C, B (0.233) Best combinations explaining clustering for abundance and biomass are in bold. ρ is given in parentheses. DISCUSSION Community structure, abundance and biomass The zooplankton community in the Canada Basin consisted mostly of species characteristic for the Arctic such as C. hyperboreus, C. glacialis, M. longa, O. similis, M. pygmaeus, F. borealis and L. helicina (Johnson, 1956; Conover and Huntley, 1991; Auel and Hagen, 2002; Hopcroft et al., 2005; Lane et al., 2008), whereby small copepods dominated the abundance and larger bodied copepods dominated the biomass. These patterns are consistent with previous studies conducted in the Canada Basin and Beaufort Sea slope (Darnis et al., 2008; Kosobokova and Hopcroft, 2010; Hunt et al., 2014; Smoot and Hopcroft, 2017a) and were reflected in our copepod size spectra (Fig. 5), which displayed the pattern typical for Arctic basin copepods (Hopcroft et al., 2005). Non-copepod abundance was dominated by larvaceans, except in 2005 when L. helicina dominated the non-copepod abundance. However, due to the small sample size in 2005, we cannot be certain whether pteropods were unusually dominant in the basin, although Kosobokova and Hopcroft (2010) also reported pteropods being more abundant than larvaceans slightly earlier in that same year. The major contribution of chaetognaths and hydrozoans to non-copepod biomass is also consistent with previous studies (Hopcroft et al., 2005; Kosobokova and Hopcroft, 2010). The Canada Basin zooplankton community is very homogenous spatially and has an insignificant interannual variability during our observation window, possibly due to the relatively long life cycles of most predominant species like C. hyperboreus (2–4 years) (Hirche, 1997; Broms et al., 2009), C. glacialis (2 years) (Kosobokova, 1999). The main patterns observed with the hierarchical clustering analysis and nMDS for abundance and biomass (Figs 8 and 9) confirm the observation of spatial homogeneity, with two large groups that incorporated samples from all years (mostly Basin samples). However, despite no statistical significant interannual differences in overall mean abundance and biomass, biomass cluster C consisted predominantly of 2004 samples and cluster B of samples from 2003. The PERMANOVA also revealed that the year has an effect on the zooplankton community. This could be due to a difference in sea ice concentration and the environmental conditions associated with it. During 2004, the ice edge was further north than during 2003 with a higher concentration of first-year ice observed in the southern Canada Basin and Beaufort Sea compared to 2004 (National Ice Center: Weekly chart products; http://nsidc.org/data/bist/). A sea ice retreat beyond the shelf break can lead to increased wind-driven upwelling at the shelf break (Carmack and Chapman, 2003), which brings nutrient-rich water into the surface layers and leads to increased production. Besides observing the spatial homogeneity of the Canada Basin zooplankton community, we found the same distinctions between the shelf/slope and the basin stations as in Hunt et al. (2014). The BEST analysis supports the influence of station depth on differences between zooplankton communities, since the best models for abundance included bottom depth. The basin stations were characterized by the general absence of shelf taxa, although stations in the western basin had low numbers of meroplankton, which suggests some transport from the shelf into the basin by eddies (Llinás et al., 2009). The clusters of shelf/slope stations (abundance: group A; biomass: group A) were defined by higher Pseudocalanus abundance and the presence of cirripedia nauplii and cyprids, which are all characteristics for water masses influenced by shelf/slope waters (Smoot and Hopcroft, 2017a). That station 28A was not included into the shelf/slope cluster, but displayed as a single sample for abundance as well as biomass likely reflects an influence by runoff from the Mackenzie River. This is further supported by the observation of the neritic copepod Centropages abdominalis and echinoderm larvae (Walkusz et al., 2010; Smoot and Hopcroft, 2017a) in the sample. The average abundance of C. glacialis and C. hyperboreus was of the same order of magnitude as previous observations in the same area (Hunt et al., 2014), and those further towards the Chukchi Sea and central Arctic (Thibault et al., 1999). While the abundance of C. hyperboreus and M. longa seemed to be impacted by the fresher water in the core of our study area, the abundance of O. similis did not appear to be substantially influenced. This may reflect the more euryhaline and eurythermic character of O. similis (Nishida, 1985; Nielsen et al., 2002) compared to Arctic endemic species. Oithona similis showed a significant increase in mean abundance (P-value = 0.05) from 2003 to 2006. However, 4 years of observation is insufficient to convincingly establish whether these trends are persistent or not. It is notable that we report higher mean abundance, but similar biomass and cluster structure, in both 2004 and 2006 than Hunt et al. (2014), who reported on samples collected concurrently in those years. This is a direct reflection of our finer mesh size of 150 μm (compared to their 236 μm) that catches earlier stages of smaller, abundant species such as O. similis and M. pygmaeus as well as copepod nauplii (Gallienne and Robins, 2001; Hopcroft et al., 2005) that contribute little to community biomass. Our mean biomass for all years was slightly higher than the 9.6 mg m−3 (Hopcroft et al., 2005) reported for 2002 using a similar methodological approach. The non-significant Spearman’s rank correlation coefficients indicate that there is no significant change in abundance or biomass from 2003 to 2006, but if we combine our observation with those preceding (Hopcroft et al., 2005) and partially overlapping (Hunt et al., 2014) our observation period, it becomes clear that there has been an increasing trend in mean biomass from 2002 to 2008 (except in 2004). Historic comparison Historic datasets for comparison to our findings are limited for the Canada Basin due to the remoteness of the area as well as the ice cover. Most previous data come from ice-stations such as Drift Station Alpha (Johnson, 1963), T-3 ice islands (Scott, 1969), NP-22 and NP-23 (Kosobokova, 1982) and the Surface Heat Budget of the Arctic Ocean (SHEBA) (Ashjian et al., 2003). The extremely low abundances suggested at Drift Station Alpha are most likely due to incomplete descriptions of methodology and shall be ignored. The T-3 ice island copepod mean abundance for August 1966 to 1969 were 105 ind. m−3 (mesh size 215-μm, sampling depth up to 100 m) (Scott, 1969), about 5–10-fold lower than our mean abundance, in part due to differences in collection mesh size. The mean abundance observed from the ice-stations NP-22 and NP-23 during August and September 1975 and 1977 was 349 ind. m−3 (mesh size 180 μm). The NP-22 stations were located further north in the central Arctic compared to our study. Our data (811 ind. m-3 mean abundance for 2003, 2004 and 2006) were much closer to the mean abundance for the zooplankton community reported for SHEBA of 591 ind. m−3 (Ashjian et al., 2003) for August and early September in 1998 (mesh size 150-μm, sampling depth: 100 m, lifestage “eggs” omitted) and that of Smoot and Hopcroft (2017b) of about 744 ind. m−3 along the Beaufort Slope for August and September 2012–2014 (mesh size 150 μm, sampling depth 100 m). Overall, there is a suggestion of increasing mean abundance when comparing the historic and recent datasets. However, caution must be exercised comparing contemporary data to historical data due to differing sampling techniques, areas, as well as changing taxonomy (e.g. Johnson, (1956) reported C. finmarchicus instead of C. glacialis, since C. glacialis was not described by Jaschnov until 1955 (Jaschnov, 1955)). Nevertheless, increasing copepod biomass has also been suggested to occur in the adjoining Chukchi Sea (Ershova et al., 2015). The lack of historic data that are comparable to our study, and the suggestion of increasing abundance and biomass highlight the need for the Canada Basin to be sampled more regularly and with consistent methods. CONCLUSION The epipelagic zooplankton communities of the Canada Basin are dominated by copepods both in number and in biomass. We found that there was no obvious interannual change in community structure over our short 4-year observation period, with community structure influenced to a small degree by environmental factors. When our observations are combined with contemporary studies (Hopcroft et al., 2005; Hunt et al., 2014) and historical data, both abundance and biomass have displayed increasing long-term trends. SUPPLEMENTARY DATA Supplementary material is available at Journal of Plankton Research online. ACKNOWLEDGEMENTS We thank the Department of Fisheries and Oceans (DFO) and Fiona McLaughlin for providing sampling opportunities and the crew of the Canadian Coast Guard Icebreaker Louis S. St-Laurent as well as the students and staff collecting the zooplankton samples. We also thank Cheryl Clarke-Hopcroft, Chris Stark and Caitlin Smoot for support with the taxonomic questions. Funding This work was supported by the National Science Foundation (NSF) [OPP-0909571]; The Arctic Ocean Diversity (ArcOD) Project of the Census of Marine Life (CoML); and a University of Alaska Fairbanks graduate fellowship; and contributes to the Circumpolar Biodiversity Monitoring Program (CBMP) through National Oceanic and Atmospheric Administration (NOAA)/Cooperative Institute for Alaska Research (CIFAR) awards [NA08OAR4320751, NA13OAR4320056]. DATA ARCHIVING https://arcticdata.io/catalog/#view/doi:10.18739/A27S4S REFERENCES Arrigo , K. R. and Van Dijken , G. L. 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Oceanogr. , 139 , 42 – 65 . Google Scholar CrossRef Search ADS Yamamoto-Kawai , M. , McLaughlin , F. A. , Carmack , E. C. , Nishino , S. , Shimada , K. and Kurita , N. ( 2009 ) Surface freshening of the Canada Basin, 2003–2007: river runoff versus sea ice meltwater . J. Geophys. Res. Oceans , 114 , 2156 – 2202 . Google Scholar CrossRef Search ADS Author notes Corresponding editor: Roger Harris © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

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Journal of Plankton ResearchOxford University Press

Published: Jul 11, 2018

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