Southeastern Bering Sea Research Articles

Climate-mediated changes in zooplankton community structure for the eastern Bering Sea

Zooplankton are critical to energy transfer between higher and lower trophic levels in the eastern Bering Sea ecosystem. Previous studies from the southeastern Bering Sea shelf documented substantial differences in zooplankton taxa in the Middle and Inner Shelf Domains between warm and cold years. Our investigation expands this analysis into the northern Bering Sea and the south Outer Domain, looking at zooplankton community structure during a period of climate-mediated, large-scale change. Elevated air temperatures in the early 2000s resulted in regional warming and low sea-ice extent in the southern shelf whereas the late 2000s were characterized by cold winters, extensive spring sea ice, and a well-developed pool of cold water over the entire Middle Domain. The abundance of large zooplankton taxa such as Calanus spp. (C. marshallae and C. glacialis), and Parasagitta elegans, increased from warm to cold periods, while the abundance of gelatinous zooplankton (Cnidaria) and small taxa decreased. Biomass followed the same trends as abundance, except that the biomass of small taxa in the southeastern Bering Sea remained constant due to changes in abundance of small copepod taxa (increases in Acartia spp. and Pseudocalanus spp. and decreases in Oithona spp.). Statistically significant changes in zooplankton community structure and individual species were greatest in the Middle Domain, but were evident in all shelf domains, and in both the northern and southern portions of the eastern shelf. Changes in community structure did not occur abruptly during the transition from warm to cold, but seemed to begin gradually and build as the influence of the sea ice and cold water temperatures persisted. The change occurred one year earlier in the northern than the southern Middle Shelf. These and previous observations demonstrate that lower trophic levels within the eastern Bering Sea respond to climate-mediated changes on a variety of time scales, including those shorter than the commonly accepted quasi-decadal time periods. This lack of resilience or inertia at the lowest trophic levels affects production at higher trophic levels and must be considered in management strategy evaluations of living marine resources.

Integrated assessment of the carbon budget in the southeastern Bering Sea

During the primary field program for the Bering Ecosystem Study (2008–2010), independent seasonal estimates of net primary production (NPP), net community production (NCP), vertical export production (Cexp), and benthic carbon consumption (BCC) were used to construct a shelf-wide carbon budget for the southeastern Bering Sea. Here, we quantify the annual production, utilization, and transport of NPP for the southeastern shelf region of the Bering Sea (spatially partitioned into Outer, Middle, and Coastal Domains). We observed that approximately 25% and 30% of NPP on the shelf is exported horizontally from the Middle and Outer Domains, respectively. This horizontal transport was the dominant mode of carbon export in the Outer Domain, exceeding Cexp by more than 30gCm−2yr−1 (99gCm−2yr−1 compared to 67gCm−2yr−1, respectively). In the Middle Domain, Cexp was more prominent than lateral transport (65gCm−2yr−1 and 46gCm−2yr−1, respectively), and vertically exported carbon was more efficiently recycled in this Domain than in the Outer Domain (53% and 32% of Cexp respectively). In the Coastal Domain, lateral transport was a source of carbon to the bottom layer, with estimated input of carbon exceeding NPP by as much as 54gCm−2yr−1. While the source of this additional carbon is unknown, one possible source is transport from the Middle Domain during wind events that induce coastal convergence. Overall, the combined carbon reservoir attributed to burial and transport in the Middle and Outer Domains is similar to a previous budget for this region (47%; Walsh and McRoy, 1986), although some qualitative differences are apparent. The data presented here indicate a more pelagic character in the Outer Domain relative to the Middle Domain, and that the Middle and Coastal Domain carbon budgets are balanced only when combined.

Comparison of spring-time phytoplankton community composition in two cold years from the western Gulf of Alaska into the southeastern Bering Sea

The Bering Sea is a highly productive ecosystem providing the main oceanographic connection between the North Pacific and Arctic oceans. The atmospheric connection with the Arctic Ocean leads to seasonal sea ice formation in the Bering Sea, the areal extent and timing of retreat of which have important implications for primary productivity and phytoplankton community composition in this region. Hydrographic data from cruises and satellite sea ice and sea surface temperature data in spring 2011 and 2012 suggest classification of these years as relatively warmer and colder years, respectively. Locations in the western Gulf of Alaska (Pavlof Bay), at the north end of an eastern pass through the Aleutian Islands (Unimak Pass), and on the continental shelf of the Bering Sea (M2) were visited in both years. Stratification was apparent on the shelf in 2012, while the water column was comparatively well-mixed at other locations in both years. Phytoplankton biomass was highest in 2011 overall and specifically on the shelf in both years, while minimal biomass was measured within the well-mixed Unimak Pass in 2012. Surface phytoplankton size distributions included substantial contributions of picoplankton (<3µm) in 2011 (21–35%), while micro- (20–200µm) and nanoplankton (3–20µm) comprised 79% and 95% of biomass in Pavlof Bay and at M2, respectively, in 2012. Analyses of similarity revealed spatial variability in the phytoplankton assemblages within each year (2011: R=0.588, p<0.004; 2012: R=0.646, p<0.004). Additionally, between-year variability had a strong and significant effect on differences between assemblages across all locations (R=0.579, p<0.0003), likely masking differences between sites when years were grouped (R=0.134, p<0.079). These differences were likely driven by the dominance (up to 75% in Unimak Pass) of the colonial prymnesiophyte Phaeocystis sp. at all sites in 2011, resulting in reduced community diversity, compared to more widespread abundance of large diatoms of the genera Thalassiosira, Thalassiothrix, and Chaetoceros in 2012. The current study shows that within-regime differences in phytoplankton community composition between years can be almost as great as that between locations with vastly different oceanographic settings. High between- and multi-year variability may combine with a potential overall decline in sea ice in the Bering Sea to produce significant changes in the productive base of the food web with potential cascading effects on higher, economically and ecologically important trophic levels.

Descriptions of eggs of snailfishes (family Liparidae) from the Bering Sea and eastern North Pacific Ocean

Eggs of liparid fishes were collected by bottom trawl in the southeastern Bering Sea and removed from an oceanographic mooring cable off the coast of British Columbia, Canada. Eggs were large with embryos most commonly in late flexion to postflexion stages. Based on meristic counts and collection localities, eggs were identified as Elassodiscus tremebundus, Paraliparis rosaceus, and Careproctus sp. (either Careproctus colletti or Careproctus melanurus). Eggs of E. tremebundus are oval with a smooth, opaque chorion. Eggs of P. rosaceus are spherical and the chorion is sculptured with a uniformly pebbled texture. Eggs of Careproctus sp. are slightly oval and the chorion is smooth and opaque. Embryos of all species are unpigmented. A pelvic disk is present in embryos of E. tremebundus, but it is rudimentary: a small patch of tissue that was visible only after embryos were cleared and stained. Adults of the genus Paraliparis do not have a pelvic disk and no disk was observed in developing embryos. Embryos of Careproctus sp. have a large disk with rays visible in later stages. Hatching glands are present on the head and nape of embryos of all three species. Hatching glands of E. tremebundus are separated into bands across the snout, along the jaws and opercle, and in a patch on the nape, while those of P. rosaceus and Careproctus sp. are evenly distributed over the head and nape. This is the first full description of the embryonic stages of these taxa.

Spawning phenology and geography of Aleutian Islands and eastern Bering Sea Pacific cod (Gadus macrocephalus)

Pacific cod (Gadus macrocephalus) is an economically and ecologically important species in the southeastern Bering Sea and Aleutian Islands, yet little is known about the spawning dynamics of Pacific cod in these regions. To address this knowledge gap, we applied a gross anatomical maturity key for Pacific cod to describe temporal and spatial patterns of reproductive status over three winter spawning seasons: 2005, 2006, and 2007. Maturity status of female Pacific cod was assessed by fishery observers during sampling of commercial catches and used to construct maps showing spawning activity in the Bering Sea and Aleutian Islands. Most spawning activity was observed on the Bering Sea shelf and Aleutian Island plateaus between 100 and 200m depth. Data for those days when a high percentage of spawning stage fish were observed were used to identify areas with concentrations of spawning fish. Spawning concentrations were identified north of Unimak Island, in the vicinity of the Pribilof Islands, at the shelf break near Zhemchug Canyon, and adjacent to islands in the central and western Aleutian Islands along the continental shelf. The spawning season was found to begin in the last days of February or early March and extend through early to mid-April. Variation in spawning time (averaging ~10 days between years) may have been associated with a change from warm (2005) to cold (2007) climate conditions during the study period. Our information on Pacific cod spawning patterns will help inform fishery management decisions, models of spawning and larval dispersal and the spatial structure of the stock.

Numerical study on tidal mixing along the shelf break in the Green Belt in the southeastern Bering Sea

Tidal mixing and its associated iron and nutrients flux from a subsurface layer along the shelf break has been considered as one of the key processes to maintain the high summertime biological productivity in the Green Belt in the southeastern Bering Sea. In the present study, tidal mixing near the shelf break is examined with a three‐dimensional high‐resolution numerical model to quantify the enhanced mixing and to reveal the underlying physical mechanisms based on the different characteristics of diurnal and semidiurnal internal waves. Strong turbulent mixing with energy dissipation rates of over 1 × 10−8 W/kg is reproduced along the Green Belt in the numerical model forced by barotropic diurnal and/or semidiurnal tides at open boundaries. The energy dissipation off the shelf break near the sea surface is enhanced due to the semidiurnal internal waves, whereas the dissipation near the bottom of the shelf break is enhanced by the diurnal topographically trapped waves. An additional experiment with stratification representative of winter conditions shows a significant influence of stratification on the energy dissipation off the shelf break due to the change in features of both diurnal and semidiurnal internal waves. Low‐mode semidiurnal baroclinic energy flux from the Aleutian Passes is shown to increase the energy dissipation along the shelf slope between Pribilof and Zhemchug Canyons. These results suggest that the strong vertical mixing along the shelf break induced by the diurnal and semidiurnal tides both play important roles in maintaining the iron and nutrients supply along the Green Belt.

Feeding ecology of age-0 walleye pollock (Gadus chalcogrammus) and Pacific cod (Gadus macrocephalus) in the southeastern Bering Sea

Walleye pollock (Gadus chalcogrammus) and Pacific cod (Gadus macrocephalus) are of particular economic and ecological importance in the southeastern Bering Sea. The spatial and temporal overlap of early life stages of both species may explain their strongly correlated recruitment trends. Pelagic larvae and juveniles were collected during four research cruises in May, July and September of 2008, an exceptionally cold year, and their stomach contents were examined. Feeding success and diet composition of walleye pollock and Pacific cod were consistently different in spring, summer, and fall. Pacific cod larvae and juveniles always consumed larger and progressively fewer prey items per stomach than walleye pollock; this difference was particularly pronounced in the fall. Our data suggest that co-occurring early life stages of walleye pollock and Pacific cod were dividing prey resources rather than competing for them, at least during the exceptionally cold conditions in 2008 in the southeastern Bering Sea.

Associations between North Pacific right whales and their zooplanktonic prey in the southeastern Bering Sea

Due to the seriously endangered status of North Pacific right whales Eubalaena japonica, an improved understanding of the environmental factors that influence the species' distribution and occurrence is needed to better assess the effects of climate change and industrial activities on the population. Associations among right whales, zooplankton, and the physical envi- ronment were examined in the southeastern Bering Sea during the summers of 2008 and 2009. Sampling with nets, an optical plankton counter, and a video plankton recorder in proximity to whales as well as along cross-isobath surveys indicated that the copepod Calanus marshallae is the primary prey of right whales in this region. Acoustic detections of right whales from sonobuoys deployed during the cross-isobath surveys were strongly associated with C. marshallae abun- dance, and peak abundance estimates of C. marshallae in 2.5 m depth strata near a tagged right whale ranged as high as 10 6 copepods m �3 . The smaller Pseudocalanus spp. was higher in abun- dance than C. marshallae in proximity to right whales, but significantly lower in biomass. High concentrations of C. marshallae occurred in both the surface and bottom layers of the highly stratified water column, but there was no evidence of diel vertical migration. Instead, occurrence of C. marshallae in the bottom layer was associated with elevated near-bottom light attenuance and chlorophyll fluorescence, suggesting C. marshallae may aggregate at depth while feeding on resuspended phytodetritus. Despite the occasional presence of strong horizontal gradients in hydrographic properties, no association was found between C. marshallae and either fronts or phytoplankton distribution.

Seasonal variation in the cross-shelf distribution of seabirds in the southeastern Bering Sea

We tested the hypothesis that the distribution of seabird species' associations across the southeastern Bering Sea shelf reflects the underlying ecology of four bathymetrically-defined hydrographic domains: the Inner or Coastal Shelf Domain (depth (Z)<50m), the Middle Shelf Domain (50m<Z<100m), the Outer Shelf Domain (100m<Z<200m), and the Shelf-Slope Domain (200m<Z<3000m). The domains differ in stratification, which intensifies from winter to summer and breaks down in the fall. To examine seabird distributions with respect to these domains in multiple seasons, we quantified the cross-shelf distribution of species with respect to water depth using a 37-year database. We then used a multivariate tree analysis to group species with similar depth-use distributions, and mapped these clusters against the hydrographic domains. There were three patterns of seabird depth use: an inshore, shallow-water group in summer and fall, but not winter and spring, which conformed roughly to the Inner Shelf Domain; a group of species that were distributed widely across the Middle and Outer Shelf Domains, and a third group of species that occupied the outer portion of the Outer Shelf Domain and the Shelf-Slope Domain. The multivariate tree analysis revealed close correspondence between the seabird-derived domains and the bathymetrically-defined Outer Shelf and Shelf-Slope domains in spring and to a lesser extent in summer. In summer and fall, and to a lesser extent in spring, the seabird groupings showed a differentiation between the Inner Shelf Domain and the Middle Shelf Domain. Seabird-derived differentiation between the Shelf-Slope Domain and the Outer Shelf Domain was strongest in spring and summer. These seasonal patterns likely reflected the seasonal variation in the hydrographic differentiation of the bathymetrically-defined domains. Cross-validation of the multivariate tree analysis showed that the portion of seabird distribution patterns explained by the tree analysis was smallest in winter (when there is no stratification on the middle and inner shelves) and greatest in summer (when stratified water columns result in hydrographically defined domains), as would be expected under our hypothesis. We also examined hypotheses predicting why pursuit diving seabirds most often forage in shallow water whereas surface-foraging (surface-seizing) seabirds are more common over deep offshore waters. The hypothesis for regionally enhanced primary production as a driving factor was not supported for the inshore foraging seabirds but was supported for those foraging over shelf-slope waters.

Nested scales of spatial heterogeneity in juvenile walleye pollock Theragra chalcogramma in the southeastern Bering Sea

We sought to characterize the distribution of juvenile walleye pollock Theragra chalcogramma in an area of intense predatorprey interactions and to describe habitat features that lead to their observed distributions. The distribution of juvenile walleye pollock around the Pribilof Islands in the southeastern Bering Sea in 2008 and 2009 was patchy, at spatial scales rang- ing from a few meters to several 10s of kilometers. These patches, and the spaces between them, were hierarchically nested with small, dense patches clustered together into larger aggregations which were further aggregated over the region sampled. Vertical physical habitat structure affected the vertical distribution of juvenile pollock similarly in both years. However, despite largely similar physical characteristics in both years, there were significant differences between years in the horizontal distribution, patch structure, and abundance of juvenile pollock. In neither year did biological or physical environment characteristics explain the broad-scale variability in the horizontal distribution of juvenile pollock, but at small horizontal scales a behavioral compo- nent was evident, with fish changing their group coherence as conditions changed despite consis- tent group sizes. It is clear that if we are to understand the processes that drive the distribution of juvenile pollock, we must consider multiple scales of heterogeneity. Only then will we begin to understand the role of pollock in the ecosystem and be able to predict the consequences of large sources of variability such as climate change, a critical question in the rapidly changing physical and biological environment of the Bering Sea.

Conservative and non-conservative variations of total alkalinity on the southeastern Bering Sea shelf

Recent observations of calcium carbonate (CaCO3) mineral undersaturations on the Bering Sea shelf have prompted new interest in the physical and biological factors that control the inorganic carbon system in the region. Understanding of the dynamics that influence the spatio-temporal variability of total alkalinity (TA) – one major component of the seawater carbonate system – has been constrained by limited historical data collected across the shelf, and the consensus has been that TA is largely conservative. However, the recently documented undersaturated conditions have the potential to cause substantial non-conservative variability in TA in this region through the dissolution of carbonate minerals. In order to quantify the contribution of carbonate mineral precipitation and dissolution to variability in TA on the southeastern Bering Sea shelf, we examined seasonal observations of TA that were made between 2008 and 2010 as part of the BEST-BSIERP Bering Sea Project. Conservative influences accounted for most of the variability in TA concentrations, with well-constrained mixing dominating in spring and summer of 2008. Bering Shelf Water (BSW) contained a constant ratio of TA to salinity, while river discharge (RW) added TA relative to salinity at a predictable rate. Although substantial organic carbon production and denitrification can cause some non-conservative variation in TA concentrations (a maximum of ~15μmolkgSW−1 combined), carbonate mineral dissolution and precipitation were shown to be the most important processes responsible for non-conservative TA–salinity relationships. CaCO3 uptake by the dominant pelagic phytoplankton calcifier (i.e., coccolithophores) was shown to alter TA concentrations by as much as 59μmolkgSW−1. Evidence for shallow-water CaCO3 mineral dissolution was also observed, which caused TA concentrations to increase by as much as 36μmolkgSW−1. Therefore, contrary to our previous understanding, the non-conservative physico-biogeochemical factors observed in this study play an important role in controlling the ocean carbon cycle of the Bering Sea shelf.

Emergence of the Arctic Themisto libellula (Amphipoda: Hyperiidae) on the southeastern Bering Sea shelf as a result of the recent cooling, and its potential impact on the pelagic food web

AbstractPinchuk, A. I., Coyle, K. O., Farley, E. V., and Renner, H. M. 2013. Emergence of the Arctic Themisto libellula (Amphipoda: Hyperiidae) on the southeastern Bering Sea shelf as a result of the recent cooling, and its potential impact on the pelagic food web. – ICES Journal of Marine Science, 70: 1244–1254. The eastern Bering Sea shelf experienced a sequence of warm years after a regime shift in the late 1970s. Following a series of unusually warm years in the early 2000s, the climate shifted again in 2007 to a series of extremely cold years that were marked by intense ice coverage and late ice retreat. Spatial and temporal variability in zooplankton communities during the recent cold period was investigated as part of the collaborative BEST-BSIERP program. An increasing presence of the Arctic hyperiid Themisto libellula, which had not been reported from the southeastern Bering Sea since the 1970s, was observed in the Middle Shelf Domain, indicating a developing structural shift in the zooplankton community in response to continuous cold conditions. Simultaneously, T. libellula became an increasingly dominant prey in the diets of zooplanktivorous fish and seabirds, demonstrating the important role for T. libellula in the pelagic food web. Our analysis suggests that T. libellula is capable of controlling copepod populations, thus it may become a potential contributor to top-down regulation of Calanus spp. in the eastern Bering Sea.

Regional patterns of bioturbation and iron and manganese reduction in the sediments of the southeastern Bering Sea

Regional patterns of iron (Fe) and manganese (Mn) reduction rates across the shelf and slope of the southeastern Bering Sea, as well as the relative importance of these pathways in sedimentary organic matter remineralization, were investigated during the spring and summer of 2009. Reduction rates of Fe and Mn were calculated using depth profiles of solid-phase iron and manganese oxide concentrations and bioturbation coefficients, Db, determined from profiles of excess 234Th. Iron reduction was found to be a significant pathway for carbon mineralization across the shelf, with an average rate of 1.74mmolm−2d−1. However, Fe reduction rates higher than 6mmolm−2d−1 were calculated, and a significant regional pattern was observed, with highest rates found on the northern shelf, and dropping toward the south and offshore. Conversely, Mn oxide reduction was found to be of minor significance, with low reduction rates in all regions, averaging only 0.09mmolm−2d−1 across the shelf, and accounting for no more than 5% of total carbon oxidation in any region. These results indicate that Fe oxide reduction is a significant pathway for carbon remineralization in the northern and middle-shelf regions, where organic matter deposition rates and benthic biomass are high. Additionally, this work provides insight into the potential role of sedimentary iron reduction as a source of bioavailable Fe in this region.

Correlation between recruitment and fall condition of age-0 pollock (Theragra chalcogramma) from the eastern Bering Sea under varying climate conditions

Fishery managers require an understanding of how climate influences recruitment if they are to separate the effects of fishing and climate on production. The southeastern Bering Sea offers opportunities to understand climate effects on recruitment because inter-annual oscillations in ice coverage set up warm or cold conditions for juvenile fish production. Depth-averaged temperature anomalies in the Bering Sea indicate the past nine years have included three warm (2003–2005), an average (2006), and five cold (2007–2011) years. We examined how these climatic states influenced the diet quality and condition (size, energy density and total energy) of young-of-the-year (YOY) pollock (Theragra chalcogramma) in fall. The implications of fall condition were further examined by relating condition prior to winter to the number of age-1 recruits-per-spawner the following summer (R/S). The percentage of lipid in pollock diets was threefold higher in cold years compared with warm years, but stomach fullness did not vary. Consequently, fish energy densities were 33% higher in cold years (P<0.001) than in warm years. In contrast, neither fish size (P=0.666), nor total energy (P=0.197) varied with climatic condition. However, total energy was significantly (P=0.007) and positively correlated with R/S (R2=0.736). We conclude that recruitment to age-1 in the southeastern Bering Sea is improved under environmental conditions that produce large, energy dense YOY pollock in fall.

A multivariate analysis of observed and modeled biophysical variability on the Bering Sea shelf: Multidecadal hindcasts (1970–2009) and forecasts (2010–2040)

Coupled physical/biological models can be used to downscale global climate change to the ecology of subarctic regions, and to explore the bottom-up and top-down effects of that change on the spatial structure of subarctic ecosystems—for example, the relative dominance of large vs. small zooplankton in relation to ice cover. Here we utilize a multivariate statistical approach to extract the emergent properties of a coupled physical/biological hindcast of the Bering Sea for years 1970–2009, which includes multiple episodes of warming and cooling (e.g. the recent cooling of 2005–2009), and a multidecadal regional forecast of the coupled models, driven by an IPCC global model forecast of 2010–2040. Specifically, we employ multivariate empirical orthogonal function (EOF) analysis to derive the spatial covariance among physical and biological timeseries from our simulations. These are compared with EOFs derived from spatially gridded measurements of the region, collected during multiyear field programs. The model replicates observed relationships among temperature and salinity, as well as the observed inverse correlation between temperature and large crustacean zooplankton on the southeastern Bering Sea shelf. Predicted future warming of the shelf is accompanied by a northward shift in both pelagic and benthic biomass.

Light absorption properties of southeastern Bering Sea waters: Analysis, parameterization and implications for remote sensing

The absorption coefficients of phytoplankton (aPHY(λ)), non-algal particles (NAP) (aNAP(λ)) and colored dissolved organic matter (CDOM) (aCDOM(λ)) were investigated and parameterized in the southeastern Bering Sea during July 2008. The absorption coefficients were well structured with respect to hydrographic and biogeochemical characteristics of the shelf. The highest values of aPHY(443) were observed offshore and the lowest values of aPHY(443) were found in the coastal domain, a low productivity region associated with limited macronutrients. Values of aDG(λ) (aCDOM(λ)+aNAP(λ)) revealed an east–west gradient pattern with higher values in the coastal domain, and lower values in the outer domain. Lower chlorophyll specific aPHY(λ) (a*PHY(λ)) observed relative to middle and lower latitude waters indicated a change in pigment composition and/or package effect, which was consistent with phytoplankton community structure. aCDOM(λ) was the dominant light absorbing coefficient at all wavelengths examined except at 676nm. Modeling of remote-sensing reflectance (Rrs(λ)) and the diffuse attenuation coefficient (Kd(λ)) from inherent optical properties revealed the strong influence of aCDOM(λ) on Rrs(λ) and Kd(λ). Good optical closure was achieved between modeled and radiometer measured Rrs(λ) and Kd(λ) with average percent difference of less than 25% and 19% respectively, except at red wavelengths. The aCDOM(λ) accounted for >50% of Kd(λ) which was vertically variable. Chlorophyll-a calculated by the NASA standard chlorophyll-a algorithm (OC4.v6) was overestimated due to higher aCDOM(λ) and underestimated due to lower a*PHY(λ) at low and high concentrations of chlorophyll-a, respectively.

Interannual changes in the zooplankton community structure on the southeastern Bering Sea shelf during summers of 1994–2009

On the southeastern Bering Sea shelf, mesozooplankton plays an important role in material transfer between primary producers and fisheries resources. The biomass of mesozooplankton in this region is known to vary annually, but little is known about annual changes in community structure and species composition. In the present study, regional and long-term changes in abundance, biomass and community structure of copepods and chaetognaths on the shelf were evaluated based on NORPAC net samples collected during summers of 1994–2009. During the study period, regime shifts occurred from high interannual variability regime (1994–1999) to low interannual variability regime with high temperature (2000–2005), then to a low interannual variability regime with low temperature (2007–2009). A total of 24 calanoid copepod species belonging to 21 genera were identified from samples. Copepod abundance ranged from 150 to 834,486inds.m−2, was greatest on the Middle shelf, and was higher in cold years, than in warm years. Copepod biomass ranged from 0.013 to 150gDMm-2, and was also higher in cold years than in warm years. Based on the results of cluster analysis, the copepod community was divided into six groups (A–F). The regional and interannual distributions of each group were distinct. Interannual changes in abundance of the dominant copepod on the Outer shelf and Middle shelf were highly significant (p<0.0001), and their abundances were negatively correlated with temperature and salinity. Interannual changes in copepod community that occurred between cold and warm years are thought to have been caused by differences in the magnitude and timing of the spring phytoplankton bloom between the two regimes. Abundance and biomass of the chaetognath Parasagitta elegans ranged from 30 to 15,180inds.m-2 and from 11 to 1559mgDMm−2, respectively. Chaetognath abundance was significantly correlated with the abundance of the dominant copepods (p<0.0001). Differences in cold and warm years may also affect recruitment of walleye pollock. We conclude that on the southeastern Bering Sea shelf, the magnitude and timing of primary production, which is related to climate change, may significantly affect how it is transferred through the food web.

Vertical distributions of the early life stages of walleye pollock (Theragra chalcogramma) in the Southeastern Bering Sea

The present study examines vertical distributions of the early life stages of walleye pollock (Theragra chalcogramma) in the Southeastern Bering Sea to assess ontogenetic and diel vertical migration in relation to development and habitat. Walleye pollock demonstrated a decrease in the depth of occurrence following hatching, indicating an ontogenetic change in vertical distribution. Eggs occurred deepest in the water column and early juveniles occurred shallowest. Vertical distributions were related to the date of collection, water column depth, and thermocline depth. Non-feeding stages (eggs and yolksac larvae, <4.5mm standard length [SL]) did not exhibit diel vertical migration. Feeding larvae exhibited diel vertical migration, although patterns varied between two feeding stages. Preflexion stage larvae (4.5–9.9mm SL) were concentrated between 10 and 20m during the day and deeper at night. Postflexion stage larvae (flexion and postflexion, 10.0–24.5mm SL) underwent regular diel migrations (0–20m, night; 10–40m, day). Low sample sizes limited our ability to assess diel vertical migration in early juveniles, but this stage tends to occur in the upper 20m of the water column, regardless of time of day. These results suggest that vertical distributions and diel migration potentially are driven by prey availability at sufficient light levels for preflexion larvae to feed and a trade-off between prey access and predation risk for postflexion larvae. Vertical distributions of eggs and preflexion larvae varied with habitat examined (on the continental shelf versus over the continental slope). Vertical distributions of walleye pollock eggs, yolksac larvae, and preflexion larvae in the Bering Sea are different from distributions in other ecosystems, which can impact transport and modeling efforts.

Prey Patch Patterns Predict Habitat Use by Top Marine Predators with Diverse Foraging Strategies

Spatial coherence between predators and prey has rarely been observed in pelagic marine ecosystems. We used measures of the environment, prey abundance, prey quality, and prey distribution to explain the observed distributions of three co-occurring predator species breeding on islands in the southeastern Bering Sea: black-legged kittiwakes (Rissa tridactyla), thick-billed murres (Uria lomvia), and northern fur seals (Callorhinus ursinus). Predictions of statistical models were tested using movement patterns obtained from satellite-tracked individual animals. With the most commonly used measures to quantify prey distributions - areal biomass, density, and numerical abundance - we were unable to find a spatial relationship between predators and their prey. We instead found that habitat use by all three predators was predicted most strongly by prey patch characteristics such as depth and local density within spatial aggregations. Additional prey patch characteristics and physical habitat also contributed significantly to characterizing predator patterns. Our results indicate that the small-scale prey patch characteristics are critical to how predators perceive the quality of their food supply and the mechanisms they use to exploit it, regardless of time of day, sampling year, or source colony. The three focal predator species had different constraints and employed different foraging strategies – a shallow diver that makes trips of moderate distance (kittiwakes), a deep diver that makes trip of short distances (murres), and a deep diver that makes extensive trips (fur seals). However, all three were similarly linked by patchiness of prey rather than by the distribution of overall biomass. This supports the hypothesis that patchiness may be critical for understanding predator-prey relationships in pelagic marine systems more generally.

Sea ice impacts on spring bloom dynamics and net primary production in the Eastern Bering Sea

In the Eastern Bering Sea, changes in sea ice have been implicated in recent major upper‐trophic level shifts. However, the underlying relationships between sea ice and primary producers have not been well tested. Here, we combine data from multiple satellite platforms, reanalysis model results and biophysical moorings to explore the dynamics of spring and summer primary production in relation to sea ice conditions. In the northern Bering Sea, sea ice consistently retreated in late spring, leading to ice‐edge phytoplankton blooms in cold (0–1 °C) waters. However, in the southeastern Bering Sea, sea ice retreat was far more irregular. Although this did not significantly alter bloom timing, late retreat led to blooms at the ice‐edge while early retreat led to blooms in open waters that were warmer (≤5.4 °C) and &gt;70% more productive. Early sea ice retreat also led to higher productivity in summer, likely due to weaker thermal stratification. Overall, annual net primary production during warm years of early sea ice retreat was enhanced by 40–50% compared to years with late sea ice retreat in the southeastern Bering Sea. These findings suggest the potential for future sea ice loss to enhance overall carrying capacity of the southeastern Bering Sea ecosystem. Consistently warm blooms in the future may also channel more energy flow toward the pelagic, rather than benthic, environment. To date, however, neither sea ice extent nor the timing of its retreat have undergone long‐term changes in the Eastern Bering Sea.