Isotopic turnover of polar cod (Boreogadus saida) muscle determined through a controlled feeding experiment

Polar cod (Boreogadus saida) is an important trophic link within Arctic marine food webs and is likely to experience diet shifts in response to climate change. One important tool for assessing organism diet is bulk stable isotope analysis. However, key parameters necessary for interpreting the temporal context of stable isotope values are lacking, especially for Arctic species. This study provides the first experimental determination of isotopic turnover (as half-life) and trophic discrimination factors (TDFs) of both δ13C and δ15N in adult polar cod muscle. Using a diet enriched in both 13C and 15N, we measured isotopic turnover times of 61 and 49 days for δ13C and δ15N, respectively, with metabolism accounting for >94% of the total turnover. These half-life estimates are valid for adult polar cod (>3 years) experiencing little somatic growth. We measured TDFs in our control of 2.6‰ and 3.9‰ for δ13C and δ15N, respectively, and we conclude that applying the commonly used TDF of ~1‰ for δ13C for adult polar cod may lead to misrepresentation of dietary carbon source, while the use of 3.8‰ for δ15N is appropriate. Based on these results, we recommend that studies investigating seasonal shifts in the diet of adult polar cod sample at temporal intervals of at least 60 days to account for isotopic turnover in polar cod muscle. Although isotopic equilibrium was reached by the fish in this study, it was at substantially lower isotope values than the diet. Additionally, the use of highly enriched algae in the experimental feed caused very high variability in diet isotope values which precluded accurate calculation of TDFs from the enriched fish. As a result of the challenges faced in this study, we discourage the use of highly enriched diets for similar experiments and provide recommendations to guide the design of future isotopic turnover experiments.publishedVersio

Weak seasonality in benthic food web structure within an Arctic inflow shelf region

The Arctic Ocean is characterized by pronounced seasonality in the quantity and quality of organic matter exported from the surface ocean. While it is well established that changes in food availability can alter the abundance, biomass and function of benthic organisms, the impact on food web structure is not well studied. We used bulk carbon and nitrogen stable isotope analysis to assess the quantity and quality of sediment organic matter and structure of the benthic food web in four seasons within the Northern Barents Sea (76°N − 82 °C). Despite a highly seasonal vertical flux, we found that the organic carbon and chlorophyll-a content of surface sediments was seasonally stable, suggesting a lack of seasonality in food availability at the seafloor. However, organic biomarkers indicate that the quality of sediment organic matter increased to a maximum in August and December, up to 6 months after the spring bloom. The seasonal stability of food quantity was mirrored in food-web structure (e.g., total isotopic range, number of trophic levels) which did not change significantly across sampling periods. We expected that suspension and deposit feeders would respond more readily to seasonal changes in food quality compared to predators. However, we observed no significant seasonal changes in the trophic levels or isotopic niche areas of benthic functional groups. The centroids of isotopic niches of all benthic functional groups shifted seasonally by <2 ‰ along the δ13C-axis, suggesting minimal shifts in carbon resource use. Because the northern Barents Sea experiences significant changes in seasonal sea ice cover, we expected that stable-isotope ratios of benthic organisms would show an increased consumption of sympagic-derived organic matter through less negative δ13C values in early spring and summer. However, only two taxa (the soft coral Gersemia spp. and bivalves in the family Yoldiidae) showed 13C-enrichment in spring or summer consistent with the assimilation of sympagic-derived organic matter, despite previous evidence suggesting widespread use of this carbon source. Overall, our results show that there is an apparent de-coupling in time between pelagic processes and benthic food-webs in which the accumulation and assimilation of high-quality organic matter occurs for benthos during the fall and early winter months when there is little to no fresh organic matter generated at the surface. This temporal mismatch highlights the importance of considering the timescales over which components of the marine ecosystem respond to short-term environmental changes and the methods employed to assess seasonality.Weak seasonality in benthic food web structure within an Arctic inflow shelf regionpublishedVersio

Origin of marine invertebrate larvae on an Arctic inflow shelf

Many benthic invertebrate taxa possess planktonic early life stages which drift with water currents and contribute to dispersal of the species, sometimes reaching areas beyond the current ranges of the adults. Until recently, it had been difficult to identify planktonic larvae to species level due to lack of distinguishing features, preventing detection of expatriate species. Here, we used DNA metabarcoding of the COI gene to obtain species-level identification of early life stages of benthic invertebrates in zooplankton samples from the Barents Sea and around Svalbard, where, regionally, large volumes of warm Atlantic Water enter the Arctic from the south. We compared the larval community in the water column to the adult community on the seafloor to identify mismatches. In addition, we implemented particle tracking analysis to identify the possible areas of origin of larvae. Our results show that 30-45% of larval taxa—largely polychaetes and nudibranchs—were not local to the sampling area, though most were found nearby in the Barents Sea. In the particle tracking analysis, some larvae originating along the Norwegian coast were capable of reaching the northwest coast of Svalbard within 3 mo, but larvae found east of Svalbard had a more constrained possible area of origin which did not extend to the Norwegian coast. This study highlights largely regional-scale larval connectivity in the Barents Sea but demonstrates the potential for some long-lived larval taxa to travel to Svalbard and the Barents Sea from further south

Origin of marine invertebrate larvae on an Arctic inflow shelf

Many benthic invertebrate taxa possess planktonic early life stages which drift with water currents and contribute to dispersal of the species, sometimes reaching areas beyond the current ranges of the adults. Until recently, it had been difficult to identify planktonic larvae to species level due to lack of distinguishing features, preventing detection of expatriate species. Here, we used DNA metabarcoding of the COI gene to obtain species-level identification of early life stages of benthic invertebrates in zooplankton samples from the Barents Sea and around Svalbard, where, regionally, large volumes of warm Atlantic Water enter the Arctic from the south. We compared the larval community in the water column to the adult community on the seafloor to identify mismatches. In addition, we implemented particle tracking analysis to identify the possible areas of origin of larvae. Our results show that 30-45% of larval taxa—largely polychaetes and nudibranchs—were not local to the sampling area, though most were found nearby in the Barents Sea. In the particle tracking analysis, some larvae originating along the Norwegian coast were capable of reaching the northwest coast of Svalbard within 3 mo, but larvae found east of Svalbard had a more constrained possible area of origin which did not extend to the Norwegian coast. This study highlights largely regional-scale larval connectivity in the Barents Sea but demonstrates the potential for some long-lived larval taxa to travel to Svalbard and the Barents Sea from further south.publishedVersio

Overexploitation, Recovery, and Warming of the Barents Sea Ecosystem During 1950–2013

The Barents Sea (BS) is a high-latitude shelf ecosystem with important fisheries, high and historically variable harvesting pressure, and ongoing high variability in climatic conditions. To quantify carbon flow pathways and assess if changes in harvesting intensity and climate variability have affected the BS ecosystem, we modeled the ecosystem for the period 1950–2013 using a highly trophically resolved mass-balanced food web model (Ecopath with Ecosim). Ecosim models were fitted to time series of biomasses and catches, and were forced by environmental variables and fisheries mortality. The effects on ecosystem dynamics by the drivers fishing mortality, primary production proxies related to open-water area and capelin-larvae mortality proxy, were evaluated. During the period 1970–1990, the ecosystem was in a phase of overexploitation with low top-predators’ biomasses and some trophic cascade effects and increases in prey stocks. Despite heavy exploitation of some groups, the basic ecosystem structure seems to have been preserved. After 1990, when the harvesting pressure was relaxed, most exploited boreal groups recovered with increased biomass, well-captured by the fitted Ecosim model. These biomass increases were likely driven by an increase in primary production resulting from warming and a decrease in ice-coverage. During the warm period that started about 1995, some unexploited Arctic groups decreased whereas krill and jellyfish groups increased. Only the latter trend was successfully predicted by the Ecosim model. The krill flow pathway was identified as especially important as it supplied both medium and high trophic level compartments, and this pathway became even more important after ca. 2000. The modeling results revealed complex interplay between fishery and variability of lower trophic level groups that differs between the boreal and arctic functional groups and has importance for ecosystem management

Reconstructing EUV spectrum of star forming regions from millimeter recombination lines of HI, HeI, and HeII

The extreme ultraviolet (EUV) spectra of distant star-forming regions cannot be probed directly using either ground- or space-based telescopes due to the high cross-section for interaction of EUV photons with the interstellar medium. This makes EUV spectra poorly constrained. The mm/submm recombination lines of H and He, which can be observed from the ground, can serve as a reliable probe of the EUV. Here we present a study based on ALMA observations of three Galactic ultra-compact HII regions and the starburst region Sgr B2(M), in which we reconstruct the key parameters of the EUV spectra using mm recombination lines of HI, HeI and HeII. We find that in all cases the EUV spectra between 13.6 and 54.4 eV have similar frequency dependence: L_{\nu}~ \nu^{-4.5 +/- 0.4}. We compare the inferred values of the EUV spectral slopes with the values expected for a purely single stellar evolution model (Starburst99) and the Binary Population and Spectral Synthesis code (BPASS). We find that the observed spectral slope differs from the model predictions. This may imply that the fraction of interacting binaries in HII regions is substantially lower than assumed in BPASS. The technique demonstrated here allows one to deduce the EUV spectra of star forming regions providing critical insight into photon production rates at \lambda < 912 A and can serve as calibration to starburst synthesis models, improving our understanding of star formation in distant universe and the properties of ionizing flux during reionization.Comment: v1: 18 pages, 5 figures, 1 table, 1 appendix v2: BPASS version 2.1 -> 2.2.1, SB99 tracks explicitly added to figures, evolution with metallicity added, few points clarified. 20 pages, 6 figures, 1 table. Accepted to Ap

Reconstructing the EUV Spectrum of Star-forming Regions from Millimeter Recombination Lines of H I, He I, and He II

The extreme ultraviolet (EUV) spectra of distant star-forming regions cannot be probed directly using either ground- or space-based telescopes due to the high cross section for interaction of EUV photons with the interstellar medium. This makes EUV spectra poorly constrained. The millimeter/submillimeter recombination lines of H and He, which can be observed from the ground, can serve as a reliable probe of the EUV. Here we present a study based on ALMA observations of three Galactic ultracompact H II regions and the starburst region Sgr B2(M), in which we reconstruct the key parameters of the EUV spectra using millimeter recombination lines of H I, He I, and He II. We find that in all cases the EUV spectra between 13.6 and 54.4 eV have similar frequency dependence: L_ ν ∝ ν^(-4.5±0.4). We compare the inferred values of the EUV spectral slopes with the values expected for a purely single stellar evolution model (Starburst99) and the Binary Population and Spectral Synthesis code (BPASS). We find that the observed spectral slope differs from the model predictions. This may imply that the fraction of interacting binaries in H II regions is substantially lower than assumed in BPASS. The technique demonstrated here allows one to deduce the EUV spectra of star-forming regions, providing critical insight into photon production rates at λ ⩽ 912 Å, and can serve as calibration to starburst synthesis models, improving our understanding of star formation in the distant universe and the properties of ionizing flux during reionization

The Bolocam Galactic Plane Survey IV: 1.1 and 0.35 mm Dust Continuum Emission in the Galactic Center Region

The Bolocam Galactic Plane Survey (BGPS) data for a six square degree region of the Galactic plane containing the Galactic center is analyzed and compared to infrared and radio continuum data. The BGPS 1.1 mm emission consists of clumps interconnected by a network of fainter filaments surrounding cavities, a few of which are filled with diffuse near-IR emission indicating the presence of warm dust or with radio continuum characteristic of HII regions or supernova remnants. New 350 {\mu}m images of the environments of the two brightest regions, Sgr A and B, are presented. Sgr B2 is the brightest mm-emitting clump in the Central Molecular Zone and may be forming the closest analog to a super star cluster in the Galaxy. The Central Molecular Zone (CMZ) contains the highest concentration of mm and sub-mm emitting dense clumps in the Galaxy. Most 1.1 mm features at positive longitudes are seen in silhouette against the 3.6 to 24 {\mu}m background observed by the Spitzer Space Telescope. However, only a few clumps at negative longitudes are seen in absorption, confirming the hypothesis that positive longitude clumps in the CMZ tend to be on the near-side of the Galactic center, consistent with the suspected orientation of the central bar in our Galaxy. Some 1.1 mm cloud surfaces are seen in emission at 8 {\mu}m, presumably due to polycyclic aromatic hydrocarbons (PAHs). A ~0.2\degree (~30 pc) diameter cavity and infrared bubble between l \approx 0.0\degree and 0.2\degree surrounds the Arches and Quintuplet clusters and Sgr A. The bubble contains several clumpy dust filaments that point toward Sgr A\ast; its potential role in their formation is explored. [abstract truncated]Comment: 76 pages, 22 figures, published in ApJ: http://iopscience.iop.org/0004-637X/721/1/137

Reconstructing the EUV Spectrum of Star-forming Regions from Millimeter Recombination Lines of HI, HeI, and HeII

The extreme ultraviolet (EUV) spectra of distant star-forming regions cannot be probed directly using either ground- or space-based telescopes due to the high cross section for interaction of EUV photons with the interstellar medium. This makes EUV spectra poorly constrained. The millimeter/submillimeter recombination lines of H and He, which can be observed from the ground, can serve as a reliable probe of the EUV. Here we present a study based on ALMA observations of three Galactic ultracompact H II regions and the starburst region Sgr B2(M), in which we reconstruct the key parameters of the EUV spectra using millimeter recombination lines of H I, He I, and He II. We find that in all cases the EUV spectra between 13.6 and 54.4 eV have similar frequency dependence: Ln n-4.5+0.4. We compare the inferred values of the EUV spectral slopes with the values expected for a purely single stellar evolution model (Starburst99) and the Binary Population and Spectral Synthesis code (BPASS). We find that the observed spectral slope differs from the model predictions. This may imply that the fraction of interacting binaries in H II regions is substantially lower than assumed in BPASS. The technique demonstrated here allows one to deduce the EUV spectra of star-forming regions, providing critical insight into photon production rates at l * 912A, and can serve as calibration to starburst synthesis models, improving our understanding of star formation in the distant universe and the properties of ionizing flux during reionization.L.M.’s stipend at the IAS is provided by the Friends of the Institute for Advanced Study. A part of this work was conducted while L.M. was supported by the SOS NRAO program and as a Groce Fellow at Caltech. L.M. is grateful to Dr. David and Barbara Groce for their kindness and support. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administratio