Polar Biol DOI 10.1007/s00300-012-1241-0

ORIGINAL PAPER

Pelagic fish and zooplankton species assemblages in relation to water mass characteristics in the northern Bering and southeast Chukchi seas

Lisa Eisner • Nicola Hillgruber • Ellen Martinson • Jacek Maselko

Received: 18 June 2012 / Revised: 22 August 2012 / Accepted: 23 August 2012 Ó Springer-Verlag 2012

Abstract This research explores the distributions and were dominated by bivalve larvae and copepods (Centro- community composition of pelagic species in the sub-Arctic pages abdominalis, Oithona similis, Pseudocalanus sp.). and Arctic waters of the northern Bering and central and Pelagic community composition was related to environ- southern Chukchi seas during September 2007 by linking mental factors, with highest correlations between bottom pelagic zooplankton and fish assemblages to water masses. salinity and large zooplankton taxa, and latitude and fish Juvenile saffron cod (Eleginus gracilis), polar cod (Bore- species. These data were collected in a year with strong ogadus saida), and shorthorn sculpin (Myoxocephalus northward retreat of summer sea ice and therefore provide a scorpius) were most abundant in warm, low salinity Alaska baseline for assessing the effects of future climate warming Coastal Water (ACW) of the central Chukchi Sea, charac- on pelagic ecosystems in sub-Arctic and Arctic regions. terized by low chlorophyll, low nutrients, and small zoo- plankton taxa. Adult Pacific herring (Clupea pallasii) were Keywords Arctic Á Bering Sea Á Chukchi Sea Á more abundant in the less stratified Bering Strait waters and Community composition Á Water mass characteristics Á in the colder, saltier Bering Shelf Water of the northern Zooplankton distribution Á Polar cod Á Pelagic fish Bering and southern Chukchi seas, characterized by high chlorophyll, high nutrients, and larger zooplankton taxa. Juvenile pink (Oncorhynchus gorbuscha) and chum Introduction (O. keta) salmon were most abundant in the less stratified ACW in the central Chukchi Sea and Bering Strait. Abun- In recent years, sub-Arctic and Arctic marine ecosystems dances of large zooplankton were dominated by copepods have been experiencing the effects of substantial climate (Eucalanus bungii, Calanus glacialis/marshallae, Metridia change. Under increasing greenhouse gas scenarios, the pacifica) followed by euphausiids (juvenile Thysanoessa Arctic is predicted to be ice-free in summer by 2050 and raschii and unidentified taxa), whereas small zooplankton sea surface temperatures (SSTs) to increase by as much as 10 °C from 2000 to 2100 (Arzel et al. 2006; Lin et al. 2006; Stroeve et al. 2008; Wang and Overland 2009). Potential & L. Eisner ( ) Á E. Martinson Á J. Maselko increases in resource extraction and ship travel make it Auke Bay Laboratories, Alaska Fisheries Science Center, National Marine Fisheries Service (NOAA), Ted Stevens critical to collect baseline data on fisheries resources and Marine Research Institute, 17109 Pt. Lena Loop Road, their relationships between ocean and ecosystem compo- Juneau, AK 99801, USA nents in these high-latitude regions (Grebmeier et al. 2010). e-mail: [email protected] A better understanding of the water mass characteristics N. Hillgruber responsible for the distribution and abundance of pelagic School of Fisheries and Ocean Sciences, University of Alaska fish is an essential first step in interpreting the effects of Fairbanks, 17101 Pt. Lena Loop Road, Juneau, AK 99801, USA climate change on the pelagic fish communities in this region. N. Hillgruber Institute of Fisheries Ecology, Thu¨nen Institut, While the distribution and abundance of demersal fish Wulfsdorfer Weg 204, 22926 Ahrensburg, Germany assemblages in the northern (N.) Bering and southeastern 123 Polar Biol

Chukchi seas have been explored in several disjointed exists between AW and BSW, which promotes the forma- studies (Lowry and Frost 1981; Barber et al. 1997; Cui tion of a combined water mass, the Bering Shelf Anadyr et al. 2009; Norcross et al. 2010), a similar exploration of Water (BSAW) characterized by high primary and sec- the pelagic fish fauna to date is missing. For example, little ondary production (McRoy et al. 1972; Alton 1974; Stoker is known about the ecology and habitat preferences of polar 1978, 1981; Grebmeier 1987; Grebmeier et al. 1988; cod (Boreogadus saida) in the N. Bering and southern (S.) Springer 1988; Walsh et al. 1989). While high primary and central (C.) Chukchi seas (Quast 1974; Lowry and production is fueled by a high and continuous supply of Frost 1981; Barber et al. 1997; Cui et al. 2009; Norcross nitrogen from AW (Grebmeier et al. 1988), high secondary et al. 2010). Another gadoid species that has attracted even production is largely due to the transport of oceanic zoo- less scientific attention is the saffron cod (Eleginus graci- plankton northward into the Chukchi Sea (Springer et al. lis). While in recent years, this species appears to have 1989; Weingartner 1997). undergone a remarkable range extension into the northern Zooplankton distributions are generally associated with Gulf of Alaska (Johnson et al. 2009), knowledge about the water masses (e.g., Hopcroft et al. 2010). However, the biology and ecology and even taxonomy of this species is distribution of pelagic early life stages of fishes may be limited or outdated (Wolotira 1985; Johnson et al. 2009). more closely tied to bathymetry (e.g., Duffy-Anderson Even fewer studies specifically targeted the pelagic larval et al. 2006), topography and current patterns (Doyle et al. and juvenile stages of these ecologically important Arctic 2002), and to water mass characteristics (e.g., Norcross taxa (Quast 1974; Welch et al. 1992, 1993; Norcross et al. et al. 2010, Siddon et al. 2011). The strong contrasting 2010, Parker-Stetter et al. 2011). A better understanding of physical and biological characteristics of water masses in these small pelagic fishes, however, is important because of the northeastern Bering Sea and Chukchi Sea are expected their trophic link to many piscivorous predators, such as to define distinctly differing habitat characteristics for seabirds (Springer et al. 1984; Piatt et al. 1989; Gaston pelagic zooplankton and fish taxa in these regions. There- et al. 2003) and marine mammals (Seaman et al. 1982). fore, this study is directed at identifying the main water Juvenile stages of Pacific salmon (Oncorhynchus spp.) and masses and characterizing patterns in pelagic zooplankton whitefish (Coregonus spp.) are also parts of these pelagic and fish distributions in response to environmental vari- fish assemblages in Arctic and sub-Arctic waters and are ables associated with these water masses. Specifically, we important subsistence resources of local communities examined nutrient concentrations, biomass, size fraction- (Jarvela and Thorsteinson 1999). ation and production of phytoplankton, light attenuation, In the N. Bering and the C. Chukchi and S. Chukchi seas, large and small zooplankton abundance, and pelagic fish several water masses can be discerned, which are likely to abundance. The main objectives of this study are to (1) impact the distribution of pelagic zooplankton and fish, characterize the physical and biological properties of the namely Alaska Coastal Water (ACW), Bering Shelf Water major water masses in the study area; (2) identify single (BSW), and Anadyr Water (AW). These water masses have species and community composition of zooplankton and a north–south orientation (Coachman et al. 1975), with fish in relation to water masses and geographic location; ACW on the east, BSW in the middle, and AW on the west. and (3) examine whether the distributional patterns of The ACW originates along the coast over the inner shelf in zooplankton and pelagic fish communities are related to the eastern Bering Sea; it develops annually from the input environmental parameters. The data used in this study are of river water and melting ice from western Alaskan rivers the result of a northern extension of an ongoing Bering and its temperature increases rapidly through spring and Aleutian Salmon International Survey (BASIS) program summer from about 0 to 10 °C (Springer et al. 1984). The initiated by the North Pacific Anadromous Fisheries BSW originates on the middle Bering Shelf, south of St. Commission to study the effect of climate change and Lawrence Island. The AW originates from the Gulf of variability on Bering Sea pelagic ecosystems (Farley Anadyr at depth along the continental slope of the Bering 2009). Sea (Springer et al. 1989). The ACW is less saline (\*31.8–32.2), warmer (2–13 °C), and has lower con- centrations of nutrients and chlorophyll a than BSW and Methods AW (Coachman and Shigaev 1992; Weingartner 1997). In contrast, BSW and AW are cooler (0–10 °C), more saline Sample collection, laboratory analysis, and processing (BSW:*31.8–33; AW:*32.3–33.3), and have substan- tially higher chlorophyll a and nutrient concentrations A fisheries oceanography survey was conducted aboard the (Sambrotto et al. 1984; Walsh et al. 1989; Coachman and NOAA ship R/V Oscar Dyson in the S. and C. Chukchi Sea Shigaev 1992; Weingartner 1997). While strong frontal and N. Bering Sea, September 4–17 2007 (Fig. 1). Station gradients separate ACW and BSW, only a gradual interface spacing was 38–55 km from latitude 70°N–64°N and 123 Polar Biol

Fig. 1 Stations sampled by the R/V Oscar Dyson, 4–17 September 2007. Dashed line is the international date line. Bathymetry contours are every 50 m. Latitudinal regions are circled

longitude 164°W–172°W, on the US side of the date line. (Gordon et al. 1994). Chlorophyll a samples were stored at Bottom depths ranged from 25 to 60 m. The surface mixed -70 °C and analyzed with a Turner Designs (TD-700) layer depth ranged from 10 to 30 m. laboratory fluorometer (Parsons et al. 1984). In situ fluo- Oceanographic data were obtained from conductivity– rometric data were calibrated with discrete chlorophyll temperature–depth (CTD) vertical profiles from the surface a samples to estimate chlorophyll a concentrations to 5–10 m above the bottom using a SBE (Seabird Elec- (r2 = 0.78). At a subset of stations, we conducted stable tronics Inc.) 911plus CTD with auxiliary sensors for isotope (13C) primary production experiments following chlorophyll a fluorescence (Wet Labs Wet-star) and light standard protocols (Dugdale and Goering 1967). attenuation (Wet Labs Wet-Star and Wet Labs C-Star, used Zooplankton samples were collected with two gear types to estimate relative particle concentration). Data were and mesh sizes to estimate biomass of large and small processed using standard SBE processing software, quality zooplankton taxa. For small taxa, zooplankton samples checked to remove data spikes, and 1-m binned. were collected with a 160-lm mesh 50 cm diameter Juday To estimate nutrient availability and phytoplankton net equipped with a General Oceanics flowmeter. The net biomass, size structure, and primary productivity (PP), we was cast and retrieved vertically at 1 m s-1 to within 2 m collected seawater samples from Niskin bottles attached to of the bottom, retrieved and rinsed. Zooplankton samples the CTD. Water was analyzed for nutrients (nitrate, nitrite, were sorted on board to the lowest taxonomic level and phosphate, silicate, and ammonium), chlorophyll a (totals developmental stage feasible (Volkov and Murphy 2007), using Whatman GF/Fs, and size fractions [10 lm using and water column densities (No. m-2) were determined Millipore Isopore polycarbonate membrane filters), and using flow meter counts. Zooplankton biomass (wet weight phytoplankton carbon uptake (on-deck incubation experi- in mg m-3) for each taxon was estimated from densities ments). Nutrient samples were immediately filtered with a using mean weights from the literature (Volkov and Mur- 0.2-lm polycarbonate filter, stored at -20 °C, and ana- phy 2007). For large taxa, zooplankton samples were col- lyzed at a shore-based facility using colorimetric protocols lected with a 505-lm mesh, 60 cm diameter bongo net

123 Polar Biol equipped with a General Oceanics flowmeter located above Temperature (T) and salinity (S) were used to determine the bridle. Double oblique tows were conducted from water mass cluster groups. We extracted surface (mean for surface to 5–10 m off the bottom, nets rinsed, and the top 8 m) and bottom (mean for bottom 8 m) T and S mea- contents preserved in 5 % buffered formalin–seawater surements from CTD profiles, normalized T and S data solution. At a shore-based facility, samples were sorted to separately, then split data into four columns (surface and the lowest possible taxonomic level and species-specific bottom T and S). CLUSTER analysis with SIMPROF wet weights were measured (Coyle et al. 2008). For sub- (similarity profile) tests was used to group the normalized sequent data analysis, zooplankton taxa were designated as surface and bottom T and S data. Within each cluster group, ‘‘large’’ if individual wet weights were C0.25 mg or the surface and bottom water masses were characterized ‘‘small’’ if individual wet weights were \0.25 mg. Abun- based on their T and S ranges as ACW, BSW, and AW as dance for large taxa was determined from bongo net defined in prior studies (Coachman and Shigaev 1992; samples, while small taxa abundance was determined from Weingartner 1997). Juday net samples. Large taxa primarily included large Differences in physical and biological oceanographic copepods, larvaceans (Oikopleura sp.), chaetognaths, eup- parameters among water mass cluster groups and latitudi- hausiids (juveniles and furcilia), amphipods, and cnidari- nal groups (C. Chukchi Sea, S. Chukchi Sea, and N. Bering ans. Small taxa primarily consisted of small copepods, Sea) were compared using ANOVA followed by Tukey’s larvaceans (Fritillaria sp.), cladocerans, pteropods (Lima- tests for multiple comparisons. Latitudinal separations cina helicina), polychaetes, and various meroplankton taxa were set at 64.0–65.5°N, 66–68°N, and 68.5–70°Nto (e.g., bivalve larvae, cirripedia). For small taxa, we stratify the sampling area into the N. Bering Sea, S. removed one outlier located at 70°N, 168°W, the first Chukchi Sea, and C. Chukchi Sea, respectively (Fig. 1). station sampled, since raw abundances in the Juday sample Oceanographic parameters included surface and bottom T, were very low at this station and sample collection was S, dissolved inorganic nitrogen (DIN), phosphate (PO4), considered questionable. silicate (SiO4), surface chlorophyll a, water column inte- Pelagic fish were captured at each station with a Can- grated chlorophyll a, mean fraction chlorophyll trawl 300 midwater trawl with a mean horizontal spread of a [ 10 lm, and mean light attenuation. Contour maps of 54 m, mesh size of 1.2 cm, rigged to sample the top 12 m oceanographic characteristics were produced using the of the water column. Trawls were towed at 7.8–8.5 km h-1 inverse distance routine in Geostatistical Analyst in Arc- for 30 min and sampled a mean area of 0.224 km2. Upon Map (version 9.3.1). retrieval of the trawl, the whole catch was immediately Distributions of the most common zooplankton and fish sorted. Fish were identified to species, counted, and taxa were plotted. Mean abundance of large zooplankton, weighed to the nearest 1.0 g wet weight. For large catches, small zooplankton, and fish for each water mass cluster a random subsample was sorted to taxa, counted, weighed, grouping was listed (Tables 1, 2). We used ArcMap with and the results were extrapolated to estimate the total catch natural breaks (Jenk’s classification) to produce distribu- by taxa. All fish were measured in fork length to the nearest tion plots (bubble plots) of common zooplankton and fish 1.0 mm. Abundances (No. km-2) were estimated for all taxa (taxa making up [95 % of total abundances) and PP. fish taxa. Significant differences in community composition between water masses were determined with ANOSIM Statistical analyses based on Bray-Curtis similarity indices. For pairs of water mass cluster groups, we computed an ANOSIM R value, a We conducted several nonparametric analyses using PRI- measure of the overall difference between groups based on MER-E Version 6.1 (Clarke and Warwick 2001; Clarke a given distance measure matrix. For R = 0, there is no and Gorley 2006) as well as standard parametric analyses. difference, whereas R = 1 indicates groups are non-over- Water masses and natural groupings of zooplankton and lapping. These analyses were conducted on square root fish were defined using PRIMER CLUSTER analysis. (sqrt) transformed abundance data to balance the influence Differences in environmental variables between water of rare and abundant species (Clarke and Warwick 2001). masses and latitudinal regions were analyzed with the Abundances of large zooplankton, small zooplankton, and analysis of variance (ANOVA) followed by Tukey’s tests. fish taxa were compared between pairs of water mass Relationship between water mass, geographic location, and groups. zooplankton and pelagic fish community composition was Similarities in community composition within water evaluated with PRIMER analysis of similarity routine mass groups were compared using SIMPER (similarity (ANOSIM). To determine the environmental parameters percentages routine) with similarities between stations that best explain distributional patterns of zooplankton and within a group ranging from 0 for no overlap to 100 % for fish communities, we used PRIMER BIO-ENV analysis. complete overlap. We also examined natural species 123 Polar Biol

Table 1 Statistical differences in oceanographic characteristics between water mass cluster groups (A–F) Water mass cluster A D B F E C ANOVA Tukey Surface ACWlowS ACWhighS ACWlowS ACWlowS BSW BSAW Bottom ACWlowS ACWhighS BSW BSAW BSAW BSAW FP Sig. at P \ 0.05

N 2 8 10 10 8 1 Ts 10.83 8.73 11.23 9.70 7.38 2.58 Tb 10.68 6.40 6.27 2.56 3.47 1.97 Ss 30.10 31.44 30.10 31.54 32.31 32.55 Sb 30.37 31.67 32.07 32.44 32.59 32.56 DINs 3.66 1.09 0.44 0.70 3.17 14.96 7.23 0.0001 E [ B, C [ BDEF DINb 0.56 2.53 2.97 10.42 15.60 21.36 16.57 0.0001 CEF [ ABD Ps 0.50 0.47 0.29 0.36 0.57 1.44 5.8 0.001 CE [ B, C [ F Pb 0.55 0.74 0.70 1.36 1.63 1.94 14.37 0.0001 CEF [ ABD Sis 8.27 8.04 4.38 6.11 14.40 40.22 5.34 0.001 E [ B, C [ BF

Sib 8.92 11.17 7.40 23.02 33.18 46.76 8.7 0.0001 CEF [ B, E [ D Chs 0.85 1.07 1.05 1.19 4.70 1.12 8.59 0.0001 E [ ABDF IntChwc 16.75 39.55 25.94 36.58 67.25 35.44 3.19 0.001 E [ CDF Ch10wc 0.46 0.53 0.24 0.38 0.41 0.60 4.08 0.005 D [ B Attwc 1.75 0.78 0.36 0.40 0.77 0.39 3.73 0.009 A [ BF

Mean values (untransformed) of oceanographic parameters shown for each water mass cluster group (ACWlowS:Alaska Coastal Water, low salinity, ACWhighS:Alaska Coastal Water, high salinity, BSW: Bering Shelf Water, BSAW: Bering Shelf Anadyr Water). Prior to statistical analysis, transformations (lnX?1) were conducted for DIN (dissolved inorganic nitrogen), Si (silicate), Ch (chlorophyll a), IntCh (integrated chlorophyll a), and ln(x) for Att (light attenuation), but not for T (temperature) and S (salinity) or P (phosphate). Lower case italic s indicates surface data, b indicates bottom, and wc indicates water column. Differences in oceanographic characteristics were tested with the analysis of variance (ANOVA), displaying F values and probability (P). N number of samples

Table 2 Statistical differences in oceanographic characteristics between latitude regions: central Chukchi Sea (CCh), southern Chukchi Sea (SCh), and northern Bering Sea (NBer) Regions CCh SCh NBer ANOVA Tukey FP Sig. at P \ 0.05

N 12 15 12 Ts 10.75 9.31 8.12 8.49 0.001 CCh [ NBer Tb 4.82 5.51 4.44 0.58 0.567 Ss 30.79 31.20 31.69 3.84 0.031 NBer [ CCh Sb 32.21 32.14 32.03 0.33 0.719 DINs 0.48 0.95 3.37 6.73 0.003 NBer [ CCh, SCh DINb 4.82 7.82 9.99 0.5 0.611 Ps 0.36 0.31 0.61 11.87 0.0001 NBer [ CCh, SCh Pb 0.94 1.03 1.27 1.2 0.313 Sis 3.31 8.18 13.55 20.33 0.0001 NBer, SCh [ CCh Sib 9.45 18.83 25.61 5.66 0.007 NBer [ CCh Chs 0.79 2.81 1.88 3.17 0.054 SCh [ CCh IntChwc 22.79 47.37 47.11 6.66 0.003 NBer, SCh [ CCh Ch10wc 0.30 0.38 0.48 3.91 0.029 NBer [ CCh Attwc 0.21 0.74 0.83 21.65 0.0001 NBer, SCh [ CCh Mean values (untransformed) of oceanographic parameters shown for each latitudinal region. Prior to statistical analysis, transformations (lnX ? 1) were conducted for DIN (dissolved inorganic nitrogen), Si (silicate), Ch (chlorophyll a), IntCh (integrated chlorophyll a), and ln(x) for Att (light attenuation), but not for T (temperature) and S (salinity) or P (phosphate). Lower case italic s indicates surface data, b indicates bottom, and wc indicates water column. Differences in oceanographic characteristics were tested with analysis of variance (ANOVA), displaying F values and probability (P). N number of samples

123 Polar Biol composition of groupings of zooplankton and fish taxa south with significantly warmer and less saline surface using the CLUSTER analysis with SIMPROF tests, which water in the C. Chukchi Sea; however, no significant lati- allowed us to describe spatial variations in community tudinal differences were seen for bottom T and S (Table 2). composition over the whole survey region, irrespective of Latitudinal variations were partially reflected in varia- water mass. tions in water mass distribution. ACWlowS/BSW (group B) The environmental factors that best explained the was primarily seen at inshore stations in the Chukchi Sea, community composition of zooplankton and fish abun- but was not found in the N. Bering Sea. ACWlowS/ dance were determined with BIO-ENV analysis within the ACWlowS (group A) was detected at only two stations BEST routine, which searches over subsets of environ- (Figs. 2a, b, 3a, c). ACWhighS/BSAW (group F) was seen at mental variables to determine ones that best explain the several offshore stations throughout the survey area, spatial variations in community composition. Spearman whereas ACWhighS/ACWhighS (group D) was found pri- rank correlation coefficients (q), which estimate the mea- marily nearshore in the N. Bering Sea and Bering Strait. sure of agreement between the environmental and taxo- BSW/BSAW (group E) was observed offshore in the nomic data sets (with 0 indicating no similarity and 1 S. Chukchi and N. Bering seas, but not further north in the perfect agreement), were computed for all combinations of C. Chukchi Sea. Finally, BSAW/BSAW (group C) was variables. Environmental variables included latitude, lon- detectable only at one station north of St. Lawrence Island. gitude, station depth, and the oceanographic parameters Overall, for surface and bottom combined, ACWlowS was listed above that were used for water mass and latitude seen only in the Chukchi Sea with the exception of one comparisons. Nutrients, chlorophyll a, and light attenuation station inshore in the NBS, whereas ACWhighS, BSW, and were ln(x ? 1) or ln(x) transformed, and all data were BSAW were seen throughout the study region. normalized prior to BIO-ENV analysis. Vertical sections of T and S were used to visualize the vertical structure of surface and bottom water masses within each cluster group (Fig. 4). These plots show that Results surface mixed layer depths were shallow and increased from onshore (*10 m depth) to offshore (*20 m depth) Physical characterization of water masses (Fig. 4a–c). Longitudinal variations occurred between in the N. Bering and Chukchi seas surface and deepwater masses, with fronts located further offshore (westward) for surface compared to deepwater Six separate water masses were identified from the cluster masses (Fig. 4a–c). By latitude, lower salinity and higher analysis (SIMPROF test at 5 % significance) within the N. temperature surface waters occurred between 66–67°N and Bering Sea and the S. and C. Chukchi Sea regions. Cluster 69–70°N (Fig. 4d) possibly due to closer proximity to groups were classified by surface and bottom water masses shore. including two ranges of ACW, one with lower salinity

(ACWlowS) and one with higher salinity (ACWhighS), BSW Biological characterization of water masses and BSAW (AW, Anadyr water and BSW combined). in the N. Bering and Chukchi seas Surface/bottom water mass cluster groups (Table 1) included three groups with high water column stratification The main surface water masses not only differed signifi-

(B = ACWlowS/BSW, F = ACWhighS/BSAW, and cantly in their physical characteristics, but also displayed E = BSW/BSAW) and three groups with low stratification large differences in biological properties, such as nutrients,

(A = ACWlowS/ACWlowS,D= ACWhighS/ACWhighS, and chlorophyll a, and particle concentrations (Table 1). Sur- C = BSAW/BSAW) (Fig. 3a). ACWhighS was character- face nutrients increased from onshore to offshore with DIN ized by T of *6–11 °C (with two outliers at 4–5 °C) and S values above limiting levels ([1 lM) in water mass cluster of *31–32. BSW had T of *5–9 °C and S of *31.8–32.5 groups C and E located along the western edges of our with one outlier at 31.6. BSAW had T of -1to4°C and survey area (Figs. 2, 3a). Water mass cluster group C with S of 32.3–33.0 with one outlier at T = 5, S = 32.1. surface BSAW had the highest DIN, followed by group E Over the survey area, SST ranged from 2.6 to 11.9 °C with surface BSW and groups B, D, and F with surface and sea surface salinity (SSS) from 29.5 to 32.6; bottom ACW (Table 1). As with T and S, surface nutrients (DIN,

T ranged from -0.9 to 10.8 °C and bottom S from 30.0 to SiO4,PO4) showed latitudinal gradients, with increases in 32.9 (Fig. 2). Higher T was observed in the lower S waters nutrient concentrations from north to south (Table 2). and vice versa (Figs. 2a, b,3b; Table 1). At both the surface Bottom nutrients had a similar spatial pattern as surface and bottom, temperature generally decreased and salinity nutrients, with highest values further offshore in bottom increased from onshore to offshore (Figs. 2,4). In addition, BSAW (clusters C, E, F); bottom nutrients throughout our mean surface T decreased and S increased from north to survey area were always above 1 lM (Fig. 2). Bottom DIN 123 Polar Biol

Fig. 2 Contours of surface a temperature (°C), b salinity, c dissolved Water mass designations are Alaska Coastal Water low S (ACW low inorganic nitrogen (DIN, lM), and d silicate (lM); bottom e temper- S) and high S (ACW high S), Bering Shelf Water (BSW) and Bering ature, f salinity and g DIN, h silicate; i surface chlorophyll a (chla, Shelf Anadyr Water combined (BSAW). Surface/Bottom water -3 -3 -1 mg m ) with surface primary productivity (mg C m d ); water masses are A = ACWlowS/ACWlowS,B= ACWlowS/BSW, -2 column j integrated chlorophyll a (mg m ), k mean percent large C = BSAW/BSAW, D = ACWhighS/ACWhighS,E= BSW/BSAW, chla (ratio [10 lm/total chla), and l mean light attenuation coeffi- F = ACWhighS/BSAW cient (m-1). Water mass cluster group is shown for each station. and SiO4 were higher in cluster groups C, E, and F than in correlations seen for bottom compared to surface nutrients groups A, B, and D (Table 1), indicating that bottom and for PO4 compared to SiO4 (Pearson correlation for DIN BSAW had higher nutrient concentrations than other water compared to PO4 and SiO4 = 0.87, 0.69 for surface and masses. DIN covaried with PO4 and SiO4, with higher 0.95 and 0.88 for bottom nutrients).

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Fig. 3 a Water mass groupings from cluster analysis of surface (top 8 m) and bottom (bottom 8 m) temperature and salinity (TS) data. Water mass designations are Alaska Coastal Water low S (ACW low S) and high S (ACW high S), Bering Shelf Water (BSW), and Bering Shelf Anadyr Water combined (BSAW). Surface/Bottom water masses are A = ACWlowS/ ACWlowS,B= ACWlowS/BSW, C = BSAW/BSAW, D = ACWhighS/ACWhighS, E = BSW/BSAW, and F = ACWhighS/BSAW. b Surface and bottom water TS characteristics for all stations. Cluster groups and water mass designations (ovals) are indicated

Latitudinal variations were only significant for bottom larger in surface ACWhighS (group D) than in surface silicate with decreasing concentrations observed from N. ACWlowS (group B) (Table 1). The smallest phytoplankton Bering Sea to the C. Chukchi Sea (Table 2). Surface and were in group B, which also had low mean concentrations integrated water column chlorophyll a and PP were also of surface DIN (\1 lM). Phytoplankton also showed lati- relatively high in areas with high surface nutrients, par- tudinal variations with larger phytoplankton found in the N. ticularly in water mass group E, where surface chlorophyll Bering than in the S. and C. Chukchi and higher chloro- a was significantly higher than in all other groups (Fig. 2; phyll a concentrations found in the N. Bering or S. Chukchi Table 1). Only stations located northwest of St. Lawrence than in C. Chukchi (Table 2). Mean water column light Island (Groups C, F, and D) represented exceptions with attenuation was highest, indicating more particles in the relatively high surface nutrients and low chlorophyll water, in the N. Bering and S. Chukchi seas than in the C. a (Table 1; Fig. 2). Chukchi Sea (Table 2), with significantly higher values in Over the survey area, moderate correlations were seen group A than in B and F (Table 1). The high chlorophyll for surface chlorophyll a with surface DIN and SiO4 a in the offshore regions of the S. Chukchi (group E) likely (Pearson correlation = 0.33 and 0.51, respectively). High account for the higher attenuation of light in these regions, correlations between surface and integrated chlorophyll whereas high sediment loads associated with river input a (Pearson correlation = 0.70) suggest that phytoplankton may account for high attenuation in the nearshore regions peak concentrations were mainly in surface waters. Based in the N. Bering Sea (group A and D) (Yukon River input) on the mean water column ratios of [10 lm/total chloro- and to a lesser extent in the S. Chukchi Sea (Noatak and phyll a, a higher percentage of large phytoplankton were Kobuk River input) (Figs. 1, 2; Table 1). Light attenuation located nearshore in the N. Bering Sea, at the innermost was positively correlated with integrated chlorophyll a and station at 70°N (group D, ACWhighS/ACWhighS), and in the surface SiO4 (Pearson correlation = 0.54 and 0.53, high chlorophyll a region in the S. Chukchi Sea (group E, respectively) and negatively correlated with latitude BSW/BSAW) (Fig. 2). Phytoplankton particle sizes were (Pearson correlation =-0.57).

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(a) ACWH ACWL ACWH ACWL

ACWH ACWH

ACWH ACWH

BSW BSW BSAW BSAW

(b) BSW ACWH ACWL BSW ACWH ACWL

BSAW BSW BSAW BSW

(c) BSAW BSW ACWL BSAW BSW ACWL ACWH ACWH ACWH ACWH

ACWL ACWL ACWH ACWH

BSAW BSAW

(d) ACWL BSW ACWL ACWL BSW ACWL ACWH ACWH ACWH ACWH

ACWL ACWL BSW BSW BSW BSW

BSAW BSAW ACWH ACWH

Fig. 4 Vertical slices of temperature (T) and salinity (S) for east–west transects along a 70°N, b 67.5°N, c 64.5°N and d north–south transect along *168°W, graphed in Ocean Data View. Surface and bottom water masses are labeled

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Single species abundances in relation to water masses chaetognath S. elegans was the most widespread species and geographic location with moderate abundances in all water mass cluster groups, while the amphipod Parathemisto sp. was least prevalent Zooplankton sampling yielded copepods, larvaceans, with low abundances in groups C, E, F, and D; it was chaetognaths, euphausiids, amphipods, cnidarians, cteno- absent in groups A and B. phores, polychaetes, and meroplankton (Table 3). Taxa Small zooplankton taxa were more evenly distributed groups responsible for [90 % of the total abundance for across water masses with distributions generally highest large zooplankton included copepods (59 %), euphausiids inshore in the C. and S. Chukchi Sea, although high con- (25 %), and larvaceans (6 %), and for small zooplankton centrations of O. similis, C. abdominalis, and copepod meroplankton (52 %) and copepods (39 %) (Table 4) with nauplii were also observed in the N. Bering Sea (Fig. 5k– meroplankton making up 49 % of the total abundance, p). The dominant small zooplankton taxa had the highest followed by copepods (40 %) for large and small taxa abundances in groups with surface ACW (B: ACWlowS/ combined. In contrast, large and small copepods accounted BSW, D: ACWhighS/ACWhighS and F: ACWhighS/BSAW) for the highest overall wet weight biomass (62 %), fol- and lowest in group C (surface and bottom BSAW) (Fig. 6; lowed by euphausiids (22 %). Large zooplankton taxa Table 3). Small copepods, O. similis and C. abdominalis, accounted for almost twice as much of the total biomass as had the highest concentrations in group B (ACWlowS/BSW) small taxa (63 % large compared to 37 % small), although with O. similis also in high concentrations in group D small taxa were slightly more abundant (45 % large and (surface and bottom ACWhighS). Pseudocalanus sp. 55 % small) (Table 4). Among large taxa, copepods were occurred in moderate abundances in E, F, B, and D. most abundant in the N. Bering Sea followed by C. Polychaetes were most abundant in groups B, F, and E, but Chukchi and S. Chukchi, whereas euphausiids were most not present in group C. Bivalve larvae, the most numerous abundant in the S. Chukchi Sea followed by C. Chukchi meroplankton, were most abundant in group F (ACWhighS/ and N. Bering (Table 4). The ten most abundant large BSAW) and B (ACWlowS/BSW), followed by groups E, D, zooplankton taxa making up 95.7 % of the total abundance and A. Copepod nauplii were seen only in low abundances were large copepods (Eucalanus bungii, Calanus glacialis/ in groups E, F, B, D and absent in other groups. marshallae, Metridia pacifica), euphausiids (juveniles of Fish surface trawls yielded mostly juvenile stages of fish Thysanoessa raschii and unidentified furcilia and juveniles), species, resulting in a total of 120,515 individuals chaetognaths (Sagitta elegans), amphipods (Parathemisto belonging to 29 taxa (Table 5). Only seven fish species sp.), cnidarians (Aglantha digitale), and larvaceans (Oiko- made up 97.2 % of the total abundance, namely, in pleura sp.). The six most abundant small taxa making up descending order, juvenile saffron cod (E. gracilis, 55.9 % 95.7 % of the total abundance were small copepods of total), adult Pacific herring (13.7 %), juvenile shorthorn (Centropages abdominalis, Oithona similis, Pseudocalanus sculpin (Myoxocephalus scorpius, 9 %), juvenile polar cod sp., and unidentified naupliar stages), polychaetes (uni- (7.6 %), juvenile pink salmon (O. gorbuscha, 4.3 %), adult dentified trochophores), and bivalve larvae. Zooplankton Pacific sand lance (3.9 %), and juvenile chum salmon taxa varied along north/south and onshore/offshore gradi- (3.1 %) (Table 5). All other fish species made up \1%of ents and between water mass cluster groups (Table 3; the total catch and were therefore excluded from further Fig. 5). distributional analysis. Large taxa were concentrated further offshore in higher Horizontal distribution of the major seven fish taxa salinity water masses (Fig. 5a–j). The dominant large varied notably with latitude and water mass cluster. Fish zooplankton taxa were in highest abundances in water mass were captured in surface water masses, so fish distributions cluster groups with bottom BSAW (C: BSAW/BSAW, are expected to be more strongly associated with surface E: BSW/BSAW, and F: ACWhighS/BSAW) and lowest in rather than bottom water masses. Both gadoid species, group A (surface and bottom ACWlowS), which only had saffron cod and polar cod, as well as juvenile shorthorn one large taxon, S. elegans. Large copepods (E. bungii, sculpins and adult Pacific sand lance were captured mostly M. pacifica, and C. marshallae) and the larvacean Oiko- above 68°N (Fig. 7), but species distribution also varied pleura sp. had the highest concentrations in cluster groups with water mass cluster (Fig. 6; Table 5). Saffron cod C, E, and F (bottom BSAW), although E. bungii was also juveniles were widely distributed in nearshore waters, with present in medium levels in group D (surface and bottom highest mean abundances in water mass groups with sur-

ACWhighS) (Fig. 6; Table 1). For euphausiids, unidentified face ACW (B: ACWlowS/BSW, D: ACWhighS/ACWhighS, furcilia and T. raschii had relatively high abundances in and F: ACWhighS/BSAW), similar to small zooplankton groups E and F, whereas unidentified juveniles were (Fig. 6). In contrast, polar cod was most abundant in group observed in relatively high numbers in group E. The cni- F (ACWhighS/BSAW; Table 5). Virtually, no juvenile darian A. digitale had highest abundance in group D. The gadoids occurred in groups E and C (BSW/BSAW and 123 Polar Biol

Table 3 Mean zooplankton abundances (No. m-2) by water mass structure groupings for large (lg) and small (sm) taxa and sums of mean abundance for each taxonomic group (bold data) Taxa group Size Water mass cluster A D B F E C Surface ACWlowS ACWhighS ACWlowS ACWlowS BSW BSAW Bottom ACWlowS ACWhighS BSW BSAW BSAW BSAW Taxa

Copepod lg Calanus glacialis/mar. 212 1,703 1,208 2,565 2,205 2,499 Epilabidocera amphitrites 505 960 283 51 0 0 Eucalanus bungii 94 13,108 6,979 21,124 37,055 37,485 Metridia pacifica 0 1,843 394 13,053 10,599 29,988 Neocalanus cristatus 0 0 2 131 163 10 Neocalanus flemingeri 0 0 0 49 105 469 Neocalanus plumchrus 0 37 15 386 553 1,718 Neocalanus sp. 0 128 157 175 235 625 Large copepods 800 17,800 9,000 37,535 50,915 72,800 sm Acartia hudsonica 0 83 5,619 0 0 0 Acartia longiremis 0 0 0 258 0 0 Centropages abdominalis 19,831 54,934 94,947 58,581 65,810 11,096 Copepoda (nauplii) 3,059 27,660 18,655 13,696 17,847 0 Eurytemora pacifica 293 0 361 0 0 0 Metridia sp. (copepodites) 0 1,435 0 1,533 4,649 46,313 Microcalanus pygmaeus 0 0 622 925 536 50,172 Microsetella sp. 0 344 361 1,370 2,521 0 Neocalanus sp. (copepodites) 0 1,742 0 228 1,905 483 Oithona plumifera 0 0 602 0 0 0 Oithona similis 25,121 154,208 223,268 91,322 73,113 77,188 Oncea borealis 0 1,926 0 932 5,218 0 Pseudocalanus sp. 5,956 27,125 38,505 36,947 41,249 9,166 Racovitzanus antarcticus 0 0 0 488 0 0 Scolecithricella sp. 0 0 0 116 0 0 Small copepods 54,000 269,000 383,000 206,000 213,000 194,000 Larvacean lg Oikopleura sp. 0 1,839 334 3,109 6,382 8,747 sm Fritillaria sp. 0 25,715 903 369 0 11,578 Larvaceans 0 27,600 1,200 3,500 6,400 20,300 Cladoceran sm Evadne sp. 0 921 1,516 229 0 0 Podon sp. 3,522 1,220 2,005 688 0 0 Cladocerans 3,500 2,100 3,500 900 0 0 Chaetogn. lg Eukrohnia hamata 0 32 18 156 312 156 Parasagitta elegans 2,464 2,076 1,039 2,083 1,899 2,811 Chaetognaths 2,500 2,100 1,100 2,200 2,200 3,000 Euphausiid lg Euphausiacea (calyptopis) 0 0 0 0 62 0 Euphausiacea (furcilia) 52 645 2,720 16,155 10,683 3,124 Euphausiacea (juv) 37 888 86 1,071 4,788 2,499 Thysanoessa inermis 0 16 8 110 15 0 Thysanoessa raschii 0 711 121 7,704 2,212 0 Thysanoessa sp. 0 0 0 225 1,466 0 sm Euphausiacea eggs 0 0 0 2,880 0 0 Euphausiids 100 2,300 2,900 28,100 19,200 5,600

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Table 3 continued Taxa group Size Water mass cluster A D B F E C Surface ACWlowS ACWhighS ACWlowS ACWlowS BSW BSAW Bottom ACWlowS ACWhighS BSW BSAW BSAW BSAW Taxa

Amphipod lg Aceroides sp. 53 0 0 0 0 0 Ampelisca sp. 01 0000 Amphipoda 186 22 0 0 0 0 Monoculodes sp. 106 77 365 107 0 0 Parathemisto sp. 0 242 222 608 720 781 Themisto libellula 00 346620 Themisto pacifica 0 34 9 23 205 0 Stenothoidae 0 0 0 30 0 0 Hyperia medusarum 00 0070 Hyperoche medusarum 00 0010 Amphipods 300 400 600 800 1,000 800 Cnidarian lg Aegina sp. 0 0 0 110 0 0 Aglantha digitale 201 3,948 500 855 833 937 Cnidaria 0 2 3 2 0 0 Coryne principes 0 0 62 0 0 0 Cormorpha flammea 08 0000 Leptomedusae (unident.) 0 1 7 0 1 0 Melicertum octocostatum 02 8520 Melicertum sp.01 0100 Obelia sp. 42 0 0 0 0 0 Proboscidactyla flavicirrata 00 2000 Cnidarians 200 4,000 600 1,000 800 900 Ctenophore lg Ctenophora (unident.) 0 0 10 8 22 0 Pteropod lg Clione limacina 0 0 5 6 18 156 sm Limacina helicina (juv) 0 2,408 2,839 414 1,447 19 Pteropods 0 2,400 2,800 400 1,500 200 Polychaete lg Polynoidae 0 0 0 0 0 0 Syllidae 0 0 2 0 0 0 Tomopteris sp.00 0100 sm Polychaeta (troch) 13,456 12,015 56,898 63,080 60,422 0 Polychaetes 13,500 12,000 56,900 63,100 60,400 0 Meroplank. lg Ammodytes hexapterus 00 0100 Argis lar 35 0 0 0 0 0 Chionoecetes sp. 178 172 96 161 230 49 Cottidae 0 0 0 1 0 0 Crangonidae 383 0 111 0 0 0 Diastylis sp. 70 0000 Eudorellopsis sp. 27 0 0 0 0 0 Fish eggs 0 1 0 0 0 0 Fish (juvenile) 0 0 1 0 0 0 Fish (larvae) 260 49 19 3 16 0 Flatfish (juv) 0 0 1 0 0 0 Hippolytidae (juv) 0 1 0 0 8 0 Hippolytidae (zoea) 0 0 2 4 0 0 Hyas sp. (megalopa) 21 3 0 43 7 0

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Table 3 continued Taxa group Size Water mass cluster A D B F E C Surface ACWlowS ACWhighS ACWlowS ACWlowS BSW BSAW Bottom ACWlowS ACWhighS BSW BSAW BSAW BSAW Taxa

Mysida (juv) 0 0 1 0 0 0 Neomysis rayii 70 0000 Paguridae (glaucothoe) 0 0 8 6 7 0 Paguridae (zoea) 74 100 52 25 1 0 Pandalidae (zoea) 0 0 0 15 0 0 Pandalus sp. 51 0000 Perigonimus sp.00 0000 Pleuronectidae 31 7 9 3 1 0 Asteroidea/bipinnaria 0 3,521 1,882 0 0 0 sm Cirripedia (cyprid) 587 3,884 5,845 2,851 198 0 Cirripedia (nauplii) 8,203 8,748 2,792 2,399 1,516 11,578 Echinodermata (larvae) 0 5,297 2,783 705 0 0 Bivalve larvae 134,908 157,679 561,429 426,551 227,053 23,156 Meroplankton 145,000 179,000 575,000 433,000 229,000 35,000

Table 4 Percentages of Abundance Total (%) Combined biomass zooplankton abundance (No. -2 m ) by taxa group for large, Large (%) Small (%) Large (%) Small (%) small, and all taxa combined (total) Copepods 59 39 40 30 32 Euphausiids 25 \1222 Chaetognaths 4 0 \18 Meroplankton 2 52 49 4 Amphipods 1 0 \11 Cnidarians 3 0 \11 Larvaceans 6 1 1 1 Polychaetes \176 1 Column total 100 100 100 63 37 CCh (%) SCh (%) NBer (%)

Percent of large taxa abundance by region Copepods 46 40 73 Percentages of total biomass for Euphausiids 25 51 10 all taxa combined are shown for Chaetognaths 8 2 4 comparison. Percentages of Meroplankton 2 1 2 large taxa abundance by geographic region (central Amphipods \121 Chukchi Sea (CCh), southern Cnidarians 11 1 2 Chukchi Sea (SCh), and Larvaceans 7 3 8 northern Bering Sea (NBer)) are included Column total 100 100 100

BSAW/BSAW, respectively). Shorthorn sculpin juveniles gadoid distribution, with peak abundance below 68°N were primarily caught in water mass groups B (ACWlowS/ (Fig. 7). Adult herring occurred mostly offshore within BSW) and F (ACWhighS/BSAW), while adult Pacific sand water mass groups D (surface and bottom ACWhighS) and lance were virtually absent in all groups but F (Fig. 6). E, and other than juvenile Chinook salmon (O. tshawyts- Adult Pacific herring distribution differed markedly from cha), was the only taxon caught in group C (Fig. 6;

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Fig. 5 Zooplankton abundance (No. m-2) for common large taxa digitale, j Oikopleura sp., and small taxa, k Centropages spp. and a Eucalanus bungii and b Thysanoessa raschii, c Sagitta elegans, l Oithona similis, m Pseudocalanus sp., n bivalve larvae, o Poly- d Euphausiacea furcilia, e Euphausiacea juveniles, f Calanus gla- chaete, and p Copoda nauplii cialis/marshallae, g Parathemisto sp., h Metridia pacifica, i Aglantha

Table 3). In contrast to the above latitudinal distributions, between water mass cluster groups were seen between juvenile pink and chum salmon were widely distributed group A (ACWlowS/ACWlowS) and all other groups throughout our study area with highest concentrations in (ANOSIM, R from 0.67 to 1.00) and between B (ACW- less stratified ACW water of groups A and D (Figs. 3,4,6). lowS/BSW) and E (BSW/BSAW) (ANOSIM, R = 0.65). Significant differences were also found between groups B Community composition in relation to water masses and D, B and F, D and E (ANOSIM, R = 0.23–0.44, and geographic location Table 6). Similarity of large taxa within groups (among stations within a water mass group) ranged from 20 % Zooplankton community composition for large taxa (group A) to 44–51 % (groups D, B, F) to 60 % (group (reflected by the abundance and relative proportion of E). In contrast, the abundance of small zooplankton taxa each taxon) varied among water mass cluster groups was more evenly distributed across water masses with the (Table 6). For large zooplankton, the largest differences largest differences between water mass cluster group B

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Fig. 5 continued

(ACWlowS/BSW) and other water mass groups, with B and A 24–29 % (group F, A, and D) to 37 % (group E) and showing the largest differences, followed by B and E, then B 52 % (group B). and D (ANOSIM R = 0.60, 0.36 to 0.29, respectively, The natural groupings of the zooplankton and fish taxa Table 6). For small zooplankton taxa, similarity within water determined using cluster analysis (SIMPROF test at 5 % mass groups ranged from 49–50 % (groups D and F) to level of significance) showed a likeness to water mass 58–59 % (group A and E) to 70 % (group B). cluster groupings or latitudinal gradients. Large zoo- Fish community composition varied with water mass plankton taxa had seven groups that showed cross-shelf group. Significant differences in fish community compo- gradients corresponding to water mass grouping and dis- sition occurred only between water mass cluster group B tinct latitudinal variations (Figs. 3b, 8a). Small zooplank-

(ACWlowS/BSW) and other groups, with large differences ton taxa had six groups, with less distinct longitudinal and between B and A, B and E followed by B and D latitudinal gradients than seen for large zooplankton (ANOSIM, R = 0.73, 0.64 0.44, respectively, Table 6). (Fig. 8b). Fish taxa had five groups (Fig. 8c). The geo- Similarity within water mass groups ranged from graphic location of fish groups suggests that communities

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Fig. 5 continued varied primarily along latitudinal gradients with water (q = 0.59) indicating that salinity could explain 59 % of masses being only of secondary importance (Fig. 8c). the variability in large zooplankton community composi- tion. Surface salinity was also correlated with large zoo- Distribution of zooplankton and pelagic fish in relation plankton community composition, albeit to a lesser extent to environmental factors (q = 0.37, Table 7). Separate BEST analyses for bottom salinity within each geographic region yielded even higher Large zooplankton community composition was clearly correlations, q = 0.76, 0.80, and 0.68, for N. Bering Sea, related to bottom water mass. For large zooplankton S. Chukchi Sea, and C. Chukchi Sea, respectively. abundances, the highest correlation of zooplankton com- Small zooplankton community composition did not munity composition with environmental variables (BEST correlate well with our suite of environmental variables. routine) was with bottom salinity, bottom SiO4, and lon- The highest correlation for small zooplankton abundances gitude (Spearman rank coefficient, q = 0.65, Table 7). The was with bottom salinity, surface DIN, mean water column highest single variable correlation was with bottom salinity light attenuation, water column mean ratio of[10 lm/total

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Fig. 6 Mean abundance of large zooplankton, small zooplankton, and fish for water mass cluster groups (a–f). Taxa shown account for 96, 96, and 97 % of total abundance over our survey grid for large zooplankton, small zooplankton, and fish, respectively

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Table 5 Mean abundance (No. km-2) of fish by water mass cluster groups (A–F) with surface and bottom water masses shown Common name Water mass group A D B F E C Total Length Surface over bottom ACWlowS ACWhighS ACWlowS ACWhighS BSW BSAW caught (mm) Scientific name ACWlowS ACWhighS BSW BSAW BSAW BSAW

Saffron cod Eleginus gracilis 98 8,727 17,624 8,133 0 0 67,535 79 Pacific herring (adult) Clupea pallasii 892 4,231 262 349 3,867 82 16,502 200 Shorthorn sculpin Myoxocephalus 6 265 1,813 2,736 43 0 10,412 51 scorpius Polar cod Boreogadus saida 120 550 634 3,304 2 0 9,230 68 Pink salmon Oncorhynchus 1,076 1,541 442 248 197 0 5,169 175 gorbuscha Pacific sand lance Ammodytes hexapterus 0 0 10 2,108 2 0 4,769 75 Chum salmon Oncorhynchus keta 685 1,034 278 110 412 0 3,784 196 Slender eelblenny Lumpenus fabricii 0 3 277 287 0 0 1,124 75 Capelin Mallotus villosus 0 32 148 24 117 0 638 102 Pacific herring Clupea pallasii 831 0 0 0 0 0 381 92 Rainbow smelt Osmerus mordax 652 0 0 0 0 0 299 101 Longhead Dab (larvae) Limanda proboscidea 2 5 58 22 7 0 199 34 Sockeye salmon Oncorhynchus nerka 2 3 0 13 49 0 125 210 Chinook salmon Oncorhynchus 21 38 0 0 0 5 80 228 tshawytscha Bering flounder (larvae) Hippoglossoides 00 327006337 robustus Three-spined Gasterosteus aculeatus 0 0 19 0 0 0 42 39 stickleback (adult) Coho salmon Oncorhynchus kisutch 47 7 0 0 0 0 35 302 Arctic lamprey (adult) Lethenteron 0 3 0 1 6 0 20 383 camtschaticum Chum salmon Oncorhynchus keta 0 2 0 3 6 0 20 653 (immature) Chum salmon Oncorhynchus keta 0 1 0 0 8 0 17 691 (maturing) Crested sculpin (adult) Blepsias bilobus 0 2 5 0 0 0 13 107 Bering wolfish Anarhichas orientalis 0 0 4 0 1 0 9 176 Slender eelblenny Lumpenus fabricii 19 0 0 0 0 0 9 169 (adult) Chinook salmon Oncorhynchus 2 1 0 0 2 0 6 677 (immature) tshawytscha Greenland halibut Reinhardtius 01 0200572 (larvae) hippoglossoides Arctic Cod (larvae) Boreogadus saida 00 0200438 Nine-spined stickleback Pungitius pungitius 90 0000459 (adult) Pacific Sand lance Ammodytes hexapterus 0 0 0 1 0 0 4 146 (adult) Crested Sculpin Blepsias bilobus 01 1000392 Sockeye salmon Oncorhynchus nerka 0 0 0 1 1 0 3 373 (immature) Prowfish Zaprora silenus 0 0 0 0 0 0 2 117 Shorthorn Sculpin Myoxocephalus 0 0 1 0 0 0 2 177 (adult) scorpius Atka mackerel Pleurogrammus 0 0 0 0 1 0 1 161 monopterygius Pond Smelt Hypomesus olidus 2 0 0 0 0 0 1 110

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Table 5 continued Common name Water mass group A D B F E C Total Length Surface over bottom ACWlowS ACWhighS ACWlowS ACWhighS BSW BSAW caught (mm) Scientific name ACWlowS ACWhighS BSW BSAW BSAW BSAW

Rainbow Smelt (adult) Osmerus mordax 2 0 0 0 0 0 1 220 Saffron Cod (adult) Eleginus gracilis 2 0 0 0 0 0 1 180 Salmon shark Lamna ditropis 0 1 0 0 0 0 1 1,854 Starry flounder (adult) Platichthys stellatus 2 0 0 0 0 0 1 330 Walleye pollock Theragra 0 1 0 0 0 0 1 115 chalcogramma Total number of fish caught for all stations combined and mean length. Fish taxa are listed in the order of their total abundance. All fish listed were juveniles, unless stated differently chlorophyll a (chla10), and station depth (Spearman rank Discussion coefficient, q = 0.39, Table 7). The highest single variable correlation was with chla10 (q = 0.23). The highest two- Characteristics of water masses in the N. Bering value correlation included chla10 and surface DIN and Chukchi seas (q = 0.31). These data suggest that there may be a mod- erate relationship between small zooplankton community Four separate water masses (within six water mass cluster composition and surface DIN and phytoplankton taxa groups) were identified within the N. Bering Sea and the S. variations (reflected by changes in chlorophyll a size and C. Chukchi Sea regions. These results are in contrast to fraction ratios). Within each geographic region, separate previous studies that concluded that the N. Bering Sea and BEST analysis of water mass properties (T and S) with southeastern Chukchi Sea regions are generally influenced small zooplankton showed that bottom salinity had corre- by only three dominant water masses, including the ACW, lations of q = 0.37, 0.14, and 0.37 for N. Bering Sea, the BSW, and the AW (Coachman et al. 1975; Springer S. Chukchi Sea, and C. Chukchi Sea, respectively. By region, et al. 1984). Our analysis of surface water detected two the highest single value correlation for small zooplankton components of the ACW, a nearshore low S component was for light attenuation (q = 0.44) for the N. Bering, sur- (ACWlowS) and more offshore high S component face SiO4 (q = 0.48) for the S. Chukchi, and surface PO4 (ACWhighS), in addition to the BSW and BSAW, with the (q = 0.46) for the C. Chukchi Sea. Thus, correlations of latter being generally located the furthest offshore in the N. small zooplankton abundances with environmental factors Bering Sea and S. Chukchi Sea. Similarly, an intermediate were higher by region than for all regions combined. water mass between ACW and BSW, termed transitional Pelagic fish community composition was highly corre- water (TW), was described for the Chukchi Sea (Hopcroft lated with latitude suggesting that the distributional ranges of et al. 2010) with the mean over top 50 m for S ranging fish taxa were primarily the result of geographic location and from 31.3 to 32.0 and for T from 4.5 to 8.0. Thus, the TW only secondarily due to water masses and associated habitat has similar S but lower T ranges than our ACWhighS. The preferences. For fish abundances, the highest correlation was ACW originates along the Alaskan coast over the inner gained for three variables namely latitude, mean water col- shelf in the southeastern Bering Sea and moves northward umn light attenuation, and surface SiO4 (q = 0.68) through Bering Strait, and into the Chukchi Sea, generally (Table 7). Latitude alone could explain 59 % of the vari- hugging the Alaska coastline. The low S component of ability in fish community composition (q = 0.59). For SST ACW is most likely due to coastal river discharges with alone (data not shown), a moderate correlation (q = 0.29) large contributions from the Yukon River (Weingartner was obtained, suggesting a secondary relationship with sur- 1997), which provides most of the freshwater from late face water mass TS characteristics. For mean water column May through September (USGS 2008). In contrast, the light attenuation alone, a moderate correlation (q = 0.39) BSW (and BSAW) most likely represents a mix of winter- was also found, suggesting that water clarity may be associ- formed Bering Sea shelf water and deepwater from the ated with fish community composition. Separate BEST Gulf of Anadyr (Weingartner 1997). analyses within each geographic region yielded single cor- Water masses encountered in our survey showed con- relations with surface temperature or salinity of q = 0.37, siderable variability in biological (nutrients, chlorophyll a, 0.21, and 0.41, for N. Bering Sea, S. Chukchi Sea, and C. PP, and light attenuation) as well as physical characteris- Chukchi Sea, respectively. tics. Surface nutrients, surface and integrated chlorophyll

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Fig. 7 Fish abundance (No. m-2)ofa saffron cod, b polar cod, c shorthorn sculpin, d adult sand lance, e adult herring, f chum salmon, and g pink salmon

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Table 6 Differences between Water mass cluster pairs Large zooplankton Small zooplankton Fish water mass cluster groups for communities of large and small Surface A: ACW B: ACW 0.87* 0.60* 0.73* zooplankton (No. m-2) and fish lowS lowS (No. km-2) using PRIMER Bottom ACWlowS BSW analysis of similarity routine D: ACWhighS 0.67* 0.00 -0.04

(ANOSIM) on square root ACWhighS transformed (sqrt) abundance F: ACW 0.77* 0.00 -0.01 data highS BSAW E: BSW 1.00* 0.21 0.23 BSAW

D: ACWhighS B: ACWlowS 0.30* 0.29* 0.44*

ACWhighS BSW

F: ACWhighS 0.09 0.06 0.03 R The ANOSIM test statistic, ,is BSAW shown. R varies roughly from 0 (no difference) to 1 (all E: BSW 0.44* 0.17* 0.02 dissimilarities between groups BSAW are larger than any B:ACWlowS F: ACWhighS 0.23* 0.14* 0.13 dissimilarities within groups). BSW BSAW BSAW/BSAW, a single station, was not shown and was not E: BSW 0.65* 0.36* 0.64* significantly different among BSAW groups F: ACWhighS E: BSW -0.03 -0.05 0.12 * Significant differences BSAW BSAW (P \ 0.05)

Fig. 8 Natural groupings of taxa from CLUSTER routine in PRIMER for sqrt transformed abundances of a large zooplankton, b small zooplankton, and c fish a concentrations, and PP were highest in regions with nutrients advected northward from Anadyr Strait continu- surface BSW (cluster group E) in the N. Bering Sea and ously fuel PP after ice melt in June until the onset of winter S. Chukchi Sea. These regions located downstream of the storm mixing in September (Sambrotto et al. 1984), thus Anadyr Strait are characterized by decreased turbulences yielding high yearly PP estimates of 250–300 g C m-2 y-1 and high phytoplankton biomass (Springer and McRoy in BSAW in the 1980s (Sambrotto et al. 1984; Grebmeier 1993). Previous studies indicated that in BSAW, the et al. 1988; Springer 1988; Walsh et al. 1989). Surface

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Table 7 Correlation factors between environmental variables and the observed community structure for large zooplankton sqrt (No. m-2), small zooplankton sqrt (No. m-2), and fish sqrt (No. km-2) using BEST BIO-ENV analysis (Spearman Rank correlation) No. of variables Highest corr.

Large zooplankton 1Sb 0.59 Tb 0.46 DINb 0.46 Ss 0.37 2Sb, Sib 0.62 Sb, Lon 0.62 Sb, DINb 0.60 Sb, Ss 0.59 3Sb, Sib, Lon 0.65 Sb, DINb, Lon 0.64 Sb, Tb, Sib 0.62 Sb, Tb, Lon 0.61 Small zooplankton 1 Ch10 0.23 Sb 0.20 Att 0.17 DINs 0.17 2 DIN, Ch10 0.31 Ch10, Sb 0.31 Ch10, Ps 0.29 Ch10, Att 0.28 3 Att, Ch10, DINs 0.35 DINs, Ch10, Sb 0.35 Att, Ch10, Ps 0.34 Ps, Ch10, Sb 0.34 5 Att, Ch10, DINs, Depth, Sb 0.39 Fish 1 Lat 0.59 Att 0.39 Sis 0.39 Lon 0.31 2 Lat, Att 0.63 Lat, Sis 0.60 Lat, Ts 0.58 Lat, DINs 0.55 3 Lat, Att, Sis 0.66 Lat, Att, Ts 0.64 Lat, Lon, Att 0.64 Lat, Att, DINs 0.62 Variables shown include temperature at the surface (Ts), bottom (Tb), salinity at the surface (Ss) and bottom (Sb), latitude (Lat), longitude (Lon), mean water column ratio chlorophyll a [10 lm/total (Ch10), mean water column light attenuation (Att), phosphate at surface (Ps), silicate at surface (Sis) and bottom (Sib), DIN at surface (DINs) and bottom (DINb), and bottom depth (Depth). The top four correlations are shown for 1, 2, and 3 variable combinations. The best set of variables, out of a possible 10, that explain community composition are in bold. The numbers indicate the strength of the correlation for each set of variables. All correlations are significant (P \ 0.01). Station at 70°N 168°W removed from analysis

nutrient concentrations, surface and integrated chlorophyll may affect the foraging behavior and success of visual a concentrations, and PP were lowest in ACW, which is not predators, such as zooplanktivorous fish. unexpected as nutrients are stripped from the water column by the PP during spring in ACW and outside of the Anadyr Species and community composition in relation plume region in BSW; mean PP here was 0.53 g C m-2 d-1 to water mass and geography with a yearly estimate of 78 g C m-2 y-1 for 1985–1989 (Springer and McRoy 1993). Distributions of large zooplankton species in this study In addition to differences in PP, the composition of were related to water masses in the southeastern Chukchi phytoplankton taxa also varied between BSW and ACW in Sea, and similar results were also shown previously (Lane the Chukchi Sea. Large chain-forming diatoms have been et al. 2008; Hopcroft et al. 2010; Matsuno et al. 2011). For observed within high chlorophyll a regions, while smaller example, copepods made up large percentages of the total taxa such as phytoflagellates observed in low nutrient zooplankton abundance in 2004, with the large oceanic waters outside of the Anadyr plume region (Springer and copepod taxa E. bungii, M. pacifica, and Neocalanus spp. McRoy 1993). Our surface and water column size-frac- dominating the offshore, more saline water masses (Hop- tionated chlorophyll a data partially support these obser- croft et al. 2010). In this study, we also found E. bungii and vations with larger particles in BSW than ACW in the M. pacifica to be most abundant in BSAW. Since all these S. Chukchi Sea (Fig. 2). taxa represent sub-Arctic Pacific species that are common River inputs also impacted water mass properties. High to the Bering Sea ecosystem, their distribution suggests a light attenuation in the N. Bering and S. Chukchi Sea northward advection with BSAW. While in the current nearshore ACW is likely related to high sediment loads study E. bungii and M. pacifica were also among the most from the Yukon River that flow into Norton Sound in the abundant large zooplankton taxa, Neocalanus spp. were N. Bering Sea and possibly from the Noatak and Kobuk substantially less abundant, suggesting that these taxa had rivers that flow into Kotzebue Sound in the S. Chukchi Sea already settled out of the Bering Sea and AW and were (Fig. 1). In contrast, the high light attenuation in the S. therefore not transported northwards. Chukchi BSW is due at least partly to high concentrations In contrast to the large oceanic copepods, small copepod of phytoplankton indicated by the high chlorophyll a val- taxa were more evenly distributed across water masses ues in this area. The observed variations in water clarity (Pseudocalanus sp.) or were found in higher concentrations may have ecological implications for pelagic taxa as they in ACW and TW than in BSW (C. abdominalis and

123 Polar Biol

O. similis). Bivalve larvae were also found in high con- and C. Chukchi Sea were evaluated separately. Natural centrations in ACW, but were absent in other water masses zooplankton cluster groups varied with both latitude and in 2004 (Hopcroft et al. 2010). Euphausiids contributed longitude in the current study. Matsuno et al. (2011) also significantly to community biomass (21 %) and abundance found that different natural zooplankton cluster groups (2 %) in our study with highest relative abundances found were located in the S. Chukchi compared to the N. Chukchi in the S. Chukchi Sea, while they occurred only in low Sea (i.e., clusters varied with latitude) and secondarily from concentrations (10 % biomass and 0.1 % of total abun- offshore to onshore (with longitude). Hopcroft et al. (2010) dance) in 2004 (Hopcroft et al. 2010); as in the current found onshore to offshore variations in zooplankton cluster study, however, euphausiids were concentrated in higher groups, and these variations mimicked spatial variations in salinity water masses and absent from ACW (Hopcroft water mass over the top 50 m. et al. 2010). It should also be noted that abundances pre- Zooplankton size varied with water mass. Larger zoo- sented here are likely underestimated, since our sampling plankton taxa were found in BSAW, whereas smaller was restricted to daytime only, when euphausiids are zooplankton taxa were generally found in the ACW or known to form a layer near the bottom (Coyle and Pinchuk more evenly distributed across water masses for the current 2002). Prior studies observed high and variable abundances study and in prior research (Lane et al. 2008; Hopcroft of barnacle larvae in the southeastern Chukchi Sea (Hop- et al. 2010). Small phytoplankton generally prevalent in croft et al. 2010; Matsuno et al. 2011). In contrast, no ACW in the Chukchi Sea may be grazed on by small barnacle larvae were collected in this study, possibly zooplankton taxa, but may be unable to sustain large because previous studies were conducted one to two zooplankton taxa (Kobari et al. 2008). Accordingly, the months earlier (July–August) and barnacle larvae might large zooplankton taxa may be partially sustained by larger have already settled out at the time of our sampling. Also, phytoplankton in BSAW. Previous research examining the year 2007 was exceptional due to an increased annual zooplankton data collected during our survey indicates that mean transport and water temperatures of Pacific water biomass and size structure varied by geographic region. through Bering Strait (Woodgate et al. 2010). This high Total biomass was 50 % lower in the C. Chukchi Sea volume transport might have been responsible for moving compared to regions further south (Volkov and Murphy the barnacle larvae distribution further northwards and thus 2007). The biomass of large zooplankton taxa [3.3 mm, out of our study area (Matsuno et al. 2011). primarily copepods (E. bungii), chaetognaths (S. elegans), Community composition of zooplankton varies with and euphausiids (T. raschii), was highest in the N. Bering water mass (Hopcroft et al. 2010; Matsuno et al. 2011). Sea where it made up 75 % of the total biomass, compared Large zooplankton taxa, in particular, were clearly related to 50 % in the C. and S. Chukchi Sea (Volkov and Murphy to water mass, with a stronger relationship to bottom than 2007), with differences due partially to variations in water surface water mass characteristics (q = 0.59 and 0.37, for mass coverage between regions. If distributions of water bottom and surface S, respectively), suggesting that larger masses change with climate (e.g., if ACW moves north- zooplankton species may have been concentrated in bottom ward or covers more spatial area as the climate warms), water masses. The strong relationship with bottom water then the associated zooplankton communities may also mass characteristics also suggests that large Bering Sea change, with potential for an increase in the abundance of taxa, such as Eucalanus bungii, were advected in BSAW small taxa and a decrease in large taxa. A reduction in into more northern habitats (Hopcroft et al. 2010; Matsuno phytoplankton cell size with warming was observed in et al. 2011). During August 2004, zooplankton communi- Arctic waters (Li et al. 2009), due to increased surface ties were also correlated with environmental variables over temperature and freshwater input leading to increased the upper 50 m, namely temperature and density stratification and a reduction in surface nutrient availabil- (q = 0.75) (Hopcroft et al. 2010). The higher correlations ity. This may be similar to an increase in areal extent of between water mass and zooplankton community compo- ACW and would further limit prey availability for large sition found in the 2004 study may be partially due to zooplankton taxa. These changes in water mass distribution differences in geographic coverage, since sampling in 2004 could reduce the abundances or spatial extent of large lipid- extended to marine waters well west of 170°W and north of rich zooplankton and would likely disrupt Arctic food webs 70°N with a southern limit at Bering Strait (*66°N). In and limit prey availability for larger or later stages of contrast, our sampling was focused on nearshore waters planktivorous fishes, marine mammals, and seabirds. In the east of 170°W and south of 70°N, but extended further SE Bering Sea, warm climate conditions were associated south to St. Lawrence Island (64°N) into the N. Bering Sea. with smaller zooplankton in the water and in the prey base For the current study, we found improved correlations for juvenile salmon and age-0 pollock compared to cold between community composition and bottom salinity climate conditions when larger lipid-rich zooplankton were (q = 0.68–0.80) when the N. Bering Sea, S. Chukchi Sea, more prevalent (Coyle et al. 2008, 2011). Accordingly, 123 Polar Biol during cold years, forage fish such as age-0 walleye pollock encounter RCW during our study, it is impossible to had higher energy density going into winter that likely determine whether we were close to the southern extent of allowed higher overwinter survival and increased recruit- this water mass. ment (Heintz pers. comm. 2012). The most abundant fish taxon during our survey was Pelagic fish community composition correlated highly another juvenile gadoid, namely saffron cod. Saffron cod is with latitude suggesting that the distributional ranges of a shallow water species that is widely distributed in the fish taxa were primarily the result of geographic location coastal waters of the Arctic, but can also range as far south and only secondarily due to water masses and associated as the northern Gulf of Alaska (Wolotira 1985; Johnson habitat preferences. Specifically, we found the highest et al. 2009). While it has been suggested that this wide- concentrations of age-0 polar cod, saffron cod, shorthorn ranging distribution may be the result of two, externally sculpin, and adult sand lance in the C. Chukchi Sea. Polar very similar species, to date no conclusive taxonomic study cod and sand lance were least widespread across water has been conducted (Wolotira 1985). Saffron cod spawn masses (high numbers in group F) followed by shorthorn demersal eggs under coastal ice in very shallow water. sculpin (high numbers in groups F and B) and saffron cod Larvae hatch in early spring (April–May), metamorphose (high numbers in groups F, B, and D); all cluster groups after 2–3 months, and descend to the bottom by midsum- had surface ACW. In contrast, no age-0 saffron cod and mer (Wolotira 1985). Juvenile saffron cod are able to tol- shorthorn sculpin and only very few age-0 polar cod were erate low salinity water and do not undergo seasonal caught in the N. Bering Sea and S. Chukchi Sea, with migration (Wolotira 1985 and the literature cited therein), lowest polar cod counts in groups E and C (surface BSW thus explaining their predominance in nearshore water and BSAW), saltier, colder, offshore water masses. mass groups with surface ACW. The large abundance of Juvenile polar cod are known to associate in dense saffron cod during our study was noteworthy but might in schools with sea ice or to be dispersed in the midwater and part be explained by the coastal extent of our sampling move into deeper water as they grow (Frost and Lowry grid. In addition, other studies have also noted an increase 1983; Rand and Logerwell 2011) assuming a more in the abundance of saffron cod in 2007, namely large demersal life style. The association of polar cod with sur- numbers of saffron cod in the coastal waters of Prince face ACW in our study is notably different from the William Sound, northern Gulf of Alaska (Johnson et al. observed absence of demersal polar cod and pelagic larval 2009); occurrence of this species in PWS had previously polar cod in ACW in the summer of 2004 (Norcross et al. never been documented. 2010); however, as mentioned above, in 2004, the station In contrast to polar cod, saffron cod, and shorthorn grid was located mainly west of 170°W and only three sculpin, the abundances of adult Pacific herring were stations were identified as ACW. In contrast, all our sam- highest in surface BSW offshore in the N. Bering Sea and pling in the Chukchi Sea was east of 170°W suggesting that S. Chukchi Sea. Since Pacific herring were concentrated in we targeted a very different area of the Chukchi Sea. In the colder BSW, higher abundances might be found in addition, demersal fish in 2004 were sampled with a colder, higher salinity water masses in the Chukchi Sea, modified beam trawl with a comparatively small opening of outside of our survey area. Adult Pacific herring sampled \3.0 m 9 1.0 m, originally designed for the capture of during BASIS (2002–2007) in the eastern Bering Sea were crabs and juvenile flatfish taxa (Gunderson and Ellis 1986). concentrated in colder, higher salinity water masses While this sampling device has shown to be effective in (BASIS unpublished data). sampling juvenile gadoids (Gunderson and Ellis 1986; Pink and chum salmon juveniles were observed Abookire and Rose 2005; Norcross et al. 2010), it is clear throughout the survey area, with high abundances in ACW that this gear targets a very different fish assemblage than with low stratification: ACWlowS/ACWlowS in the C. was sampled in this study. Also, the occurrence of polar Chukchi Sea and ACWhighS/ACWhighS in Bering Strait. cod in the ACW component is not surprising, because this Genetic information indicates that these salmon originated species is widely abundant in Arctic waters and has pre- from different stocks. Juvenile pink and chum salmon viously been documented to also occur in coastal, near- captured in C. Chukchi Sea originated from Kotzebue shore, and even brackish waters (Craig et al. 1982; Jarvela Sound (69 %) and the Seward Peninsula to Norton Sound and Thorsteinson 1999). Previous studies found the highest (29 %) in western Alaska, while fish in Bering Strait (late summer 1989–1991) concentrations of age-0 polar originated primarily from northeastern Russia (77 %) cod near stations with cold Resident Chukchi seawater (Kondzela et al. 2009). Thus, the higher salinity ACW in (RCW) (Wyllie-Echeverria et al. 1997). The southern the Bering Strait may serve as a migration corridor for extent of RCW is often observed in bottom waters near Russian pink and chum salmon stocks, whereas Yukon 70°N–71°N with a semipermanent front formed between River pink and chum salmon juveniles may be migrating ACW and RCW (Weingartner 1997). Since we did not northward in the inshore ACWlowS. These northward 123 Polar Biol migrations may be substantial, for example, for the entire Baseline data for understanding climate change eastern Bering Sea and Southeast Chukchi Sea combined (54.5–70°N), half of the total catch for juvenile pink and Under scenarios of climate change, alterations in water mass chum salmon came from the N. Bering Sea and Chukchi characteristics and spatial extent are to be expected and may Sea (64–70°N) (Moss et al. 2009). greatly impact the Arctic and sub-Arctic pelagic ecosystem. This study provides a baseline for assessing potential future Distribution of zooplankton and pelagic fish in relation effects of climate change. The year 2007 was a year with to environmental factors exceptionally fast ice melt and record minimum ice coverage in the Arctic (NCAR 2007). In addition, this year was char- The community compositions of large zooplankton were acterized by exceptionally warm water temperatures and strongly associated with water masses with the highest cor- significantly increased annual mean water transport through relations with bottom salinity, whereas fish composition had Bering Strait (Woodgate et al. 2010). The high water volume the highest correlation with latitude. Bottom salinity or sur- transport and the high water temperatures might have been face temperature alone could explain 59 and 29 % of the responsible for some of the distributional patterns of pelagic community variability for large zooplankton and pelagic taxa observed in this study, for example, the increased forage fish, respectively, while small zooplankton had weaker numbers of pink and chum salmon in the C. Chukchi Sea. In relationships to environmental parameters. Large zooplank- the late 1980s and early 1990s, sockeye (O. nerka), pink, ton taxa are known to be advected northward into the Chukchi chum, and coho salmon (O. kisutch) were also observed in the Sea (Springer et al. 1989; Weingartner 1997). In contrast, Canadian Arctic, outside their previously known distribu- forage fish distributions can be viewed as the result of an tional range and these range extensions were suggested to interplay between spawning location and nursery area and have been related to temperature increases in Arctic waters active migration in response to habitat preferences. Similarly, (Babaluk et al. 2000). While it is not clear what might have the benthic epifaunal community composition on the Chuk- been the cause for the high abundances of saffron cod, it is chi Sea shelf was also related to latitude and substrate type, noteworthy that this species also experienced a range exten- whereas pelagic zooplankton and fish larvae communities sion in 2007, albeit to the south (Johnson et al. 2009). The were related to water mass (Bluhm et al. 2009; Hopcroft et al. exceptionally warm year of 2007 might exemplify some of 2010; Norcross et al. 2010). Lower trophic level components the physical and biological changes to be expected for Arctic may have a more direct link or a shorter response time to waters under warming climate scenarios and therefore may water mass variations than higher trophic organisms such as provide not only necessary baseline data against which to fish, although, ultimately, we expect all trophic levels to be measure future change, but might also already allow some affected by changes in lower trophic level productivity within insight into the mechanisms through which change will be a marine ecosystem. manifested in the Arctic waters of the northern Bering and Interactions between trophic levels also can vary by Chukchi seas. water mass and latitude as indicated by pelagic fish diet studies in 2007 (Volkov and Murphy 2007). The Acknowledgments We thank the Alaska Fisheries Science Center, C. Chukchi Sea nearshore regions with surface ACW Ecosystem Monitoring and Assessment program scientists Alex Andrews, Kristin Cieciel, Ed Farley, Jeanette Gann, Jennifer Lanks- (cluster B and F, D) characterized by low nutrients, low bury, Jim Murphy, and Bruce Wing, Fisheries Oceanography Coor- phytoplankton biomass, and a higher relative percentage of dinated Investigations program scientist Morgan Busby, TINRO small zooplankton, polar and saffron cod, and shorthorn Center Vladivostok scientist Anatoly Volkov, and student volunteers sculpins fed primarily on small copepods (C. abdominalis Lauren Kuehne and Jenefer Bell for collecting and processing BASIS fisheries and oceanography data. We are grateful to the captain and and Pseudocalanus sp.) and chum and pink salmon fed on crew of the NOAA ship R/V Oscar Dyson for their assistance during small fish (primarily pricklebacks, a taxon captured in our field sampling. Funding was provided by the Bering Sea Fisher- similar locations as salmon, BASIS unpublished data) man’s Association, Arctic-Yukon-Kuskokwim-Sustainable-Salmon- (Volkov and Murphy 2007). In contrast, in offshore waters Initiative, and NOAA National Marine Fisheries Service. We also thank Mike Sigler and two anonymous reviewers for their helpful of the N. Bering Sea and S. Chukchi Sea and Bering Strait suggestions for the improvements of this manuscript. Any mention of regions with surface BSW or ACWhighS (cluster E and D) trade names is for descriptive purposes only and does not reflect characterized by high nutrients, high phytoplankton bio- endorsement by the US government. mass, and large zooplankton, chum and pink salmon and adult herring fed primarily on euphausiids and other large References zooplankton (Volkov and Murphy 2007; Moss et al. 2009). Thus, pelagic fish generally preyed upon the available food Abookire AA, Rose CS (2005) Modifications to a plumb staff beam resources in the system, which in turn were influenced by trawl for sampling uneven, complex habitat. Fish Res water mass and geographic variations. 71:247–254 123 Polar Biol

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