Enhanced deposition to pits: A local food source for benthos

Particle deposition experiments using mimics of biogenous negative relief (“pits”) and low-excess-density particles in a small annular flume indicate a significantly enhanced deposition rate (number of particles per time) compared to smooth, flat patches of the same diameter. This study included flow visualizations as well as observations of particle residence times, particle concentrations in the pits, and particle fluxes to the pits from the main flow. Experimental conditions of particle concentration, shear velocity, and particle settling velocity mimicked the dynamic characteristics (low excess density and large size) of organic-rich flocs and flow conditions in the subtidal and deep sea where biogenous pits are common features. Results suggest that pits provide benthic organisms an important capture mechanism for such flocs. Flow visualizations concur qualitatively with previously reported results for two-dimensional cavity flow, with unique features due to the conical shape of the pits. When the Rouse number (settling velocity/shear velocity) was much less than 1, pit deposition rate increased with increasing pit aspect ratio (AR = depth/diameter; ranging from 0.25 to 2) and always exceeded deposition to a flat patch of comparable diameter. For the single aspect ratio tested (AR = 0.5) under conditions of increasing turbulence, deposition to the pit increased under transitional flow, but then decreased to near zero when conditions reached fully rough flow. Relative enhancement of deposition to this pit decreased with increased ambient bed roughness since gravel beds also effectively collect particles. Particle concentration inside pits decreased weakly with pit aspect ratio but greatly increased with increasing roughness Reynolds number. Particle residence time increased somewhat with pit aspect ratio but decreased significantly with increasing roughness Reynolds number. Particle flux into pits from the main flow increased with both increasing aspect ratio and increasing roughness Reynolds number. Enhancement of food supply to pit inhabitants thus depends on the flow regime

1-D vertical mixing/biogeochemical Regional Ocean Modeling System (ROMS) output of October 2010 - March 2011 of the Amundsen Sea Polynya, modeled at twelve bloom stations.

Dataset: INSPIRE 1-D ROMS model output1-D vertical mixing/biogeochemical Regional Ocean Modeling System (ROMS) output of October 2010 - March 2011 of the Amundsen Sea Polynya, modeled at twelve bloom stations. Data are 3-hourly averages, and saved in NetCDF files. In the NetCDF files, data are distributed over a 6x6 grid with 30 depths (ranging from the surface down to 210 m, with higher resolution near the surface). ocean_avg.nc files are the standard model output, while files named ocean_avg_sensitivity_lowWW.nc are from runs using a lower winter water initial dissolved iron concentration. For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/765252NSF Office of Polar Programs (formerly NSF PLR) (NSF OPP) OPP-1443657, NSF Office of Polar Programs (formerly NSF PLR) (NSF OPP) OPP-1443604, NSF Office of Polar Programs (formerly NSF PLR) (NSF OPP) OPP-1443315, NSF Office of Polar Programs (formerly NSF PLR) (NSF OPP) OPP-144356

Nitrogen fixation rates from samples collected in the Chukchi Sea, Arctic Ocean near Barrow, Alaska in August of 2011 (ArcticNITRO project)

Dataset: Arctic Nitrogen Fixation RatesThis dataset provides rates of nitrogen fixation for the coastal Chukchi Sea near Barrow, Alaska. Nitrogen fixation supplies ‘new’ nitrogen to the global ocean and supports primary production and impacts global biogeochemical cycles. Historically, nitrogen fixation in marine waters was considered a predominantly warm water process but this and other recent studies have shown that nitrogen fixation is occurring at low rates in polar waters. This dataset reports rates of 3.5 – 17.2 nmol N L-1 d-1 in the ice-free coastal Alaskan Arctic. Additional investigations of high-latitude marine diazotrophic physiology are required to refine these N2 fixation estimates. For a complete list of measurements, refer to the supplemental document 'Field_names.pdf', and a full dataset description is included in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: http://www.bco-dmo.org/dataset/701789NSF Arctic Sciences (NSF ARC) PLR-090983

Microbial Community Response to Terrestrially Derived Dissolved Organic Matter in the Coastal Arctic

Warming at nearly twice the global rate, higher than average air temperatures are the new \u27normal\u27 for Arctic ecosystems. This rise in temperature has triggered hydrological and geochemical changes that increasingly release carbon-rich water into the coastal ocean via increased riverine discharge, coastal erosion, and the thawing of the semipermanent permafrost ubiquitous in the region. To determine the biogeochemical impacts of terrestrially derived dissolved organic matter (tDOM) on marine ecosystems we compared the nutrient stocks and bacterial communities present under ice-covered and ice-free conditions, assessed the lability of Arctic tDOM to coastal microbial communities from the Chukchi Sea, and identified bacterial taxa that respond to rapid increases in tDOM. Once thought to be predominantly refractory, we found that similar to 7% of dissolved organic carbon and similar to 38% of dissolved organic nitrogen from tDOM was bioavailable to receiving marine microbial communities on short 4 - 6 day time scales. The addition of tDOM shifted bacterial community structure toward more copiotrophic taxa and away from more oligotrophic taxa. Although no single order was found to respond universally (positively or negatively) to the tDOM addition, this study identified 20 indicator species as possible sentinels for increased tDOM. These data suggest the true ecological impact of tDOM will be widespread across many bacterial taxa and that shifts in coastal microbial community composition should be anticipated

Antarctic sea ice carbon dioxide system and controls

In austral summer, from December 2008 to January 2009, we investigated the sea-ice carbon dioxide (CO(2)) system and CO(2) controls in the Amundsen and Ross Seas, Antarctica. We sampled seawater, brine and sea ice for the measurements of total alkalinity (A(T)), total inorganic carbon (DIC), pH, inorganic nutrients, particulate organic carbon (POC) and nitrogen (PON), chlorophyll a, pigments, salinity and temperature. Large variability in all measured parameters was observed in time and space due to the complex sea-ice dynamics. We discuss the controls of the sea-ice CO(2) system, such as brine rejection, biological processes, calcium carbonate (CaCO(3)) precipitation/dissolution and CO(2) exchange. Most (80 to 90%) of the DIC loss was due to brine rejection, which suggests that the sea ice acted as an efficient DIC sink from 0.8 and 2.6 mol m(-2) yr(-1) (9.6-31 g C m(-2) yr(-1)). The remaining change in DIC was to a large extent explained by net biological production. The A(T):DIC ratio in the sea ice was higher than in the under-ice water (UIW), with ratios reaching 1.7, which indicated CaCO(3) precipitation and concomitant DIC loss in the sea ice. Elevated A(T):DIC ratios and carbonate concentrations were also observed in the UIW, which reflect the solid CaCO(3) rejected from the ice during melt. The potential for uptake of atmospheric CO(2) in the mixed layer increased by approximately 56 mu atm due to the combined effect of CaCO(3) precipitation during ice formation, and ice melt in summer

The influence of light and Water mass on bacterial population dynamics in the Amundsen Sea Polynya

Abstract Despite being perpetually cold, seasonally ice-covered and dark, the coastal Southern Ocean is highly productive and harbors a diverse microbiota. During the austral summer, ice-free coastal patches (or polynyas) form, exposing pelagic organisms to sunlight, triggering intense phytoplankton blooms. This strong seasonality is likely to influence bacterioplankton community composition (BCC). For the most part, we do not fully understand the environmental drivers controlling high-latitude BCC and the biogeochemical cycles they mediate. In this study, the Amundsen Sea Polynya was used as a model system to investigate important environmental factors that shape the coastal Southern Ocean microbiota. Population dynamics in terms of occurrence and activity of abundant taxa was studied in both environmental samples and microcosm experiments by using 454 pyrosequencing of 16S rRNA genes. We found that the BCC in the photic epipelagic zone had low richness, with dominant bacterial populations being related to taxa known to benefit from high organic carbon and nutrient loads (copiotrophs). In contrast, the BCC in deeper mesopelagic water masses had higher richness, featuring taxa known to benefit from low organic carbon and nutrient loads (oligotrophs). Incubation experiments indicated that direct impacts of light and competition for organic nutrients are two important factors shaping BCC in the Amundsen Sea Polynya

Summer comes to the Southern Ocean: how phytoplankton shape bacterioplankton communities far into the deep dark sea

18 pages, 6 figures, 1 table, supporting information https://doi.org/10.1002/ecs2.2641During austral spring and summer, the coastal Antarctic experiences a sharp increase in primary production and a steepening of biotic and abiotic gradients that result from increased solar radiation and retreating sea ice. In one of the largest seasonally ice-free regions, the Amundsen Sea Polynya, pelagic samples were collected from 15 sites during a massive Phaeocystis antarctica bloom in 2010/2011. Along with a suite of other biotic and abiotic measurements, bacterioplankton were collected and analyzed for community structure by pyrosequencing of the 16S rRNA gene. The aims were to identify patterns in diversity and composition of heterotrophic bacterioplankton and to test mechanistic hypotheses for explaining these differences along variations in depth, water mass, phytoplankton biomass, and organic and inorganic nutrients. The overall goal was to clarify the relationship between primary producers and bacterioplankton community structure in the Southern Ocean. Results suggested that both epipelagic and mesopelagic bacterioplankton communities were structured by phytoplankton blooming in the euphotic zone. As chlorophyll a (chl-a) increased in surface waters, the abundance of surface bacterioplankton increased, but their diversity decreased. Similarity in bacterioplankton community composition between surface-water sites increased as the bloom progressed, suggesting that algal blooms may homogenize surface-water bacterioplankton communities at larger spatial scales. Below the euphotic zone, the opposite relationship was found. Mesopelagic bacterioplankton diversity increased with increasing chl-a in the overlying surface waters. This shift may be promoted by several factors including local increase in organic and inorganic nutrients from particles sinking out of the euphotic zone, an increase in niche differentiation associated with the particle flux, interactions with deep-dwelling macrozooplankton, and release from competition with primary producers. Additional multivariate analyses of bacterioplankton community structure and nutrient concentrations revealed distinct depth horizons, with bacterioplankton communities having maximum alpha and beta diversity just below the euphotic zone, while nutrient composition gradually homogenized with increasing depth. Our results provide evidence for bloom-driven (bottom-up) control of bacterioplankton community diversity in the coastal Southern Ocean and suggest mechanisms whereby surface processes can shape the diversity of bacterioplankton communities at great depthThe study was funded by the Swedish Research Council (grants to SB and LR) and the U.S. National Science Foundation through the ASPIRE project (ANT‐0839069

The Northeast Water polynya as an atmospheric CO2 sink: a seasonal rectification hypothesis

During the multidisciplinary ‘NEW92’ cruise of the United States Coast Guard Cutter (USCGC) Polar Sea to the recurrent Northeast Water (NEW) Polynya (77–81°N, 6–17°W; July–August 1992), total dissolved inorganic carbon and total alkalinity in the water column were measured with high precision to determine the quantitative impact of biological processes on the regional air-sea flux of carbon. Biological processes depleted the total inorganic carbon of summer surface waters by up to 2 mol C m−2 or about 3%. On a regional basis this depletion correlated with depth-integrated values of chlorophyll a, particulate organic carbon, and the inorganic nitrogen deficit. Replacement of this carbon through exchange with the atmosphere was stalled owing to the low wind speeds during the month of the cruise, although model calculations indicate that the depletion could be replenished by a few weeks of strong winds before ice forms in the autumn. These measurements and observations allowed formulation of a new hypothesis whereby seasonally ice-covered regions like the NEW Polynya promote a unique biologically and physically mediated “rectification” of the typical (ice free, low latitude) seasonal cycle of air-sea CO2 flux. The resulting carbon sink is consistent with other productivity estimates and represents an export of biologically cycled carbon either to local sediments or offshore. If this scenario is representative of seasonally ice-covered Arctic shelves, then the rectification process could provide a small, negative feedback to excess atmospheric CO2

Dynamic bacterial and viral response to an algal bloom at subzero temperatures

New evidence suggests that cold‐loving (psychrophilic) bacteria may be a dynamic component of the episodic bloom events of high‐latitude ecosystems. Here we report the results of an unusually early springtime study of pelagic microbial activity in the coastal Alaskan Arctic. Heterotrophic bacterioplankton clearly responded to an algal bloom by doubling cell size, increasing the fraction of actively respiring cells (up to an unprecedented 84% metabolically active using redox dye CTC), shifting substrate‐uptake capabilities from kinetic parameters better adapted to lower substrate concentrations to those more suited for higher concentrations, and more than doubling cell abundance. Community composition (determined by polymerase chain reaction/DGGE and nucleotide sequence analysis) also shifted over the bloom. Results support, for the first time with modern molecular methods, previous culture‐based observations of bacterial community succession during Arctic algal blooms and confirm that previously observed variability in pelagic microbial activity can be linked to changes in community structure. During early bloom stages, virioplankton and bacterial abundance were comparable, suggesting that mortality due to phage infection was low at that time. The virus‐to‐bacteria ratio (VBR) increased 10‐fold at the height of the bloom, however, suggesting an increased potential for bacterioplankton mortality resulting from viral infection. The peak in VBR coincided with observed shifts in both microbial activity and community structure. These early‐season data suggest that substrate and virioplankton interactions may control the active microbial carbon cycling of this region