Aquaculture, the fastest growing food sector, is expected to expand to produce an additional 30 million metric tons of fish by 2030, thus filling the gap in supplies of seafood for humans. Salmonids aquaculture exploits the vast majority of fishmeal and fish oil rendered from ocean-dwelling forage fish. Most forage fish diverted to these commodities are human-food grade, and all are primary prey for marine predators. Rising costs, price volatility, and environmental sustainability concerns of using these commodities for aquaculture feed are driving the global search for alternatives, including marine microalgae originating from the base of marine food webs but produced in culture. We report the first evaluation of two marine microalgae, Nannochloropsis sp. and Isochrysis sp., for their potential to fully replace fishmeal and fish oil in diets of rainbow trout ( Oncorhynchus mykiss ), an important model for all salmonid aquaculture. We conducted a digestibility experiment with dried whole cells of Nannochloropsis sp. and Isochrysis sp., followed by a growth experiment using feeds with different combinations of Nannochloropsis sp., Isochrysis sp., and Schizochytrium sp. We found that digestibilities of crude protein, crude lipid, amino acids, fatty acids, omega 3 polyunsaturated fatty acids (n3 PUFA), docosahexaenoic acid (DHA), n6 (omega 6) PUFA in Isochrysis sp. were significantly higher than those in Nannochloropsis sp. Digestibility results suggest that for rainbow trout diets Isochrysis sp. is a better substitute for fishmeal and fish oil than Nannochloropsis sp. The lower feed intake by fish fed diets combining multiple microalgae, compared to fish fed the reference diet, was a primary cause of the growth retardation. In trout fillets, we detected an equal amount of DHA in fish fed fish-free diet and reference diet. This study suggests that Isochrysis sp. and Schizochytrium sp. are good candidates for DHA supplementation in trout diet formulations.

Although the role of deep-sea corals in supporting biodiversity is well accepted, their ability to recover from anthropogenic impacts is still poorly understood. An important component of recovery is the capacity of corals to grow back after damage. Here we used data collected as part of an image-based long-term monitoring program that started in the aftermath of the Deepwater Horizon oil spill to develop a non-destructive method to measure in situ growth rates of Paramuricea spp. corals and characterize the impact of the spill on growth. About 200 individual coral colonies were imaged every year between 2011 and 2017 at five sites (three that were impacted by the spill and two that were not). Images were then used to test different methods for measuring growth. The most effective method was employed to estimate baseline growth rates, characterize growth patterns, estimate the age of every colony, and determine the effects of impact and coral size on growth. Overall growth rates were variable but low, with average annual growth rates per site ranging from 0.14 to 2.5 cm/year/colony. Based on coral size and growth rates, some colonies are estimated to be over two thousand years old. While coral size did not have an influence on growth, the initial level of total impact in 2011 had a significant positive effect on the proportion of new growth after 2014. However, growth was not sufficient to compensate for branch loss at one of the impacted sites where corals are expected to take an average of 50 years to grow back to their original size. The non-destructive method we developed could be used to estimate the in situ growth rates on any planar octocoral, and would be particularly useful to follow the recovery of corals after impact or assess the effectiveness of Marine Protected Areas.

We present a complete description of the depth distribution of marine snow in Orca Basin (Gulf of Mexico), from sea surface through the pycnocline to within 10 m of the seafloor. Orca Basin is an intriguing location for studying marine snow because of its unique geological and hydrographic setting: the deepest ~200 m of the basin are filled with anoxic hypersaline brine. A typical deep ocean profile of marine snow distribution was observed from the sea surface to the pycnocline, namely a surface maximum in total particle number and midwater minimum. However, instead of a nepheloid (particle-rich) layer positioned near the seabed, the nepheloid layer in the Orca Basin was positioned atop the brine. Within the brine, the total particle volume increased by a factor of 2–3 while the total particle number decreased, indicating accumulation and aggregation of material in the brine. From these observations we infer increased residence time and retention of material within the brine, which agrees well with laboratory results showing a 2.2–3.5-fold reduction in settling speed of laboratory-generated marine snow below the seawater-brine interface. Similarly, dissolved organic carbon concentration in the brine correlated positively with measured colored dissolved organic matter (r 2 = 0.92, n = 15), with both variables following total particle volume inversely through the pycnocline. These data indicate the release of dissolved organic carbon concomitant with loss in total particle volume and increase in particle numbers at the brine-seawater interface, highlighting the importance of the Orca Basin as a carbon sink.

The anthropogenic impact of polymetallic nodule harvesting in the Clarion-Clipperton Fracture Zone is expected to strongly affect the benthic ecosystem. To predict the long-term, industrial-scale impact of nodule mining on the deep-sea environment and to improve the reliability of the sediment plume model, information about the specific characteristics of deep-sea particles is needed. Discharge simulations of mining-related fine-grained (median diameter ≈ 20 μm) sediment plumes at concentrations of 35–500 mg L –1 (dry weight) showed a propensity for rapid flocculation within 10 to 135 min, resulting in the formation of large aggregates up to 1100 μm in diameter. The results indicated that the discharge of elevated plume concentrations (500 mg L –1 ) under an increased shear rate (G ≥ 2.4 s –1 ) would result in improved efficiency of sediment flocculation. Furthermore, particle transport model results suggested that even under typical deep-sea flow conditions (G ≈ 0.1 s –1 ), rapid deposition of particles could be expected, which would restrict heavy sediment blanketing (several centimeters) to a smaller fall-out area near the source, unless subsequent flow events resuspended the sediments. Planning for in situ tests of these model projections is underway.

Hydrodynamic behaviour and the transport pathways of microplastics within the ocean environment are not well known, rendering accurate predictive models for dispersal management of such pollutants difficult to establish. In the natural environment, aggregation between plastic microbeads and phytodetritus or suspended sediments in rivers and oceans further complicate the patterns of dispersal. In this laboratory study, the physical characteristics and hydrodynamic behaviour of a selection of common plastic microbeads, as used in exfoliation skincare cosmetic products, were investigated. Additionally, the potential for aggregation of these microbeads with phytodetritus and suspended sediments, as well as the subsequent sinking and resuspension behaviour of produced aggregates, were investigated with roller tanks, settling columns and erosion chamber. Physical characteristics of the plastic microbeads showed great heterogeneity, with various densities, sizes and shapes of plastic material being utilised in products designed for the same purpose. The majority of the plastics investigated were positively buoyant in both freshwater and seawater. Aggregation between plastic microbeads and phytoplankton was observed to be swift, with even extremely high concentrations of plastics being rapidly scavenged by suspended algal material. Following aggregation to sizes of 300 to 4400 μm diameter, some formerly buoyant plastics were observed to settle through the water column and enter the benthic boundary layer with settling velocities ranging between 32 and 831 m day –1 . These aggregates could be resuspended in the laboratory under critical shear velocities of 0.67–1.33 cm s –1 (free stream velocities of > 10 cm s –1 ). This rapid aggregation and subsequent settling indicates a potentially important transport pathway for these waste products, a pathway that should be considered when modelling discharge and transport of plastic microbeads and determining the ecosystems that may be at risk from exposure.

Mesoscale eddies and fronts in the ocean greatly impact lateral transport and in turn the trajectories of sinking particles. Such influence was explored for April and October 2012 in the Gulf of Mexico using numerical simulations performed with a regional model at 1-km horizontal resolution. Results are compared qualitatively to field samples from two sediment traps located at GC600 (27°22.5 N, 90°30.7 W) and AT357 (27°31.5 N, 89°42.6 W), 81 km apart. In April the traps collected a comparable amount of material, while in October the flux at GC600 greatly exceeded that at AT357. Through inverse calculations, several thousand particle trajectories were reconstructed multiple times from the ocean surface to the depth of the traps (approximately 1,000 m) using a range of sinking velocities, 20–100 m d –1 . Taken together, model results and trap data indicate that cross-shore transport of riverine input induced by mesoscale eddies, and convergence and divergence processes at the scale of a few kilometers, significantly impact the trajectory of sinking particles. The large majority of modeled particles reach the bottom faster than would be expected by their sinking speeds alone. This finding is associated with submesoscale-induced horizontal convergence in the mixed layer that aggregates particles preferentially in downwelling regions, accelerating their descent. Furthermore, this study confirms that the cone of influence of vertical fluxes is highly variable in both space and time in the presence of an energetic eddy field, especially for particles with sinking velocity of 50 m d –1 or less. It also demonstrates that the variability of vertical fluxes in the Gulf of Mexico is highly complex and can be understood only by considering the mesoscale circulation and seasonal cycle of primary productivity, which in turn are linked to riverine inputs, wind forcing and the seasonal cycle of the mixed-layer depth.

Seafloor sediment resuspension events of different scales and magnitudes and the resulting deep (>1,000 m) benthic nepheloid layers were investigated in the northern Gulf of Mexico during Fall 2012 to Summer 2013. Time-series data of size-specific in-situ settling speeds of marine snow in the benthic nepheloid layer (moored flux cameras), particle size distributions (profiling camera), currents (various current meters) and stacked time-series flux data (sediment traps) were combined to recognize resuspension events ranging from small-scale local, to small-scale far-field to hurricane-scale. One small-scale local resuspension event caused by inertial currents was identified based on local high current speeds (>10 cm s –1 ) and trap data. Low POC content combined with high lithogenic silica flux at 30 m above bottom (mab) compared to the flux at 120 mab, suggested local resuspension reaching 30 mab, but not 120 mab. Another similar event was detected by the changes in particle size distribution and settling speeds of particles in the benthic nepheloid layer. Flux data indicated two other small-scale events, which occurred at some distance, rather than locally. Inertia-driven resuspension of material in shallower areas surrounding the traps presumably transported this material downslope leaving a resuspension signal at 120 mab, but not at 30 mab. The passage of hurricane Isaac left a larger scale resuspension event that lasted a few days and was recorded in both traps. Although hurricanes cause large-scale events readily observable in sediment trap samples, resuspension events small in temporal and spatial scale are not easily recognizable in trapped material as they tend to provide less material and become part of the background signal in the long-term averaged trap samples. We suggest that these small-scale resuspension events, mostly unnoticed in conventional time-series sampling, play an important role in the redistribution and ultimate fate of sediment distribution on the seafloor.

Particles of all origins (biogenic, lithogenic, as well as anthropogenic) are fundamental components of the coastal ocean and are re-distributed by a wide variety of transport processes at both horizontal and vertical scales. Suspended particles can act as vehicles, as well as carbon and nutrient sources, for microorganisms and zooplankton before eventually settling onto the seafloor where they also provide food to benthic organisms. Different particle aggregation processes, driven by turbulence and particle stickiness, composition, abundance and size, impact the transport and sinking behavior of particles from the surface to the seafloor. In deep coastal waters, the deposition, resuspension, and accumulation of particles are driven by particle stickiness, composition and aggregate structure. In contrast, wave-driven and bottom current-driven processes in the nepheloid benthic boundary layer of shallow waters are of greater importance to the settling behavior of particles, while the retention capacity of benthic vegetation (e.g., seagrasses) further influences particle behavior. In this review, we consider the various processes by which particles are transported, as well as their sources and characteristics, in stratified coastal waters with a focus on Nordic seas. The role of particles in diminishing the quality of coastal waters is increasing in the Anthropocene, as particle loading by rivers and surface run-off includes not only natural particles, but also urban and agricultural particles with sorbed pollutants and contaminants of organic, inorganic and microplastic composition. Human activities such as trawling and dredging increase turbidity and further impact the transport of particles by resuspending particles and influencing their vertical and horizontal distribution patterns. An interdisciplinary approach combining physical, chemical and biological processes will allow us to better understand particle transport and its impact on coastal waters and estuaries at an ecosystem level. There is a need for development of novel analytical and characterization techniques, as well as new in situ sensors to improve our capacity to follow particle dynamics from nanometer to millimeter size scales.

Gas exchange reduction (GER) at the air-sea interface is positively related to the concentration of organic matter (OM) in the top centimetre of the ocean, as well as to phytoplankton abundance and primary production. The mechanisms relating OM to GER remain unclear, but may involve mechanical (rheological) damping of turbulence in the water immediately below the surface microlayer, damping of ripples and blocking of molecular diffusion by layers of OM, as well as electrical effects. To help guide future research in GER, particularly of CO 2 , we review published rheological properties of ocean water and cultures of phytoplankton and bacteria in both 3D and 2D deformation geometries, in water from both the surface layer and underlying water. Production of foam modulates air-sea exchange of many properties and substances, perhaps including climate-changing gases such as CO 2 . We thus also review biological modulation of production and decay of whitecaps and other sea foam. In the ocean literature on biological production of OM, particularly that which associates with the sea surface, the terms “surfactant” and “surface-active” have been given a variety of meanings that are sometimes vague, and may confuse. We therefore propose a more restricted definition of these terms in line with usage in surface science and organic chemistry. Finally, possible changes in OM-modulated GER are presented in relation to predicted global environmental changes.

Response management and damage assessment during and after environmental disasters such as the Deepwater Horizon (DWH) oil spill require an ecological baseline and a solid understanding of the main drivers of the ecosystem. During the DWH event, a large fraction of the spilled oil was transported to depth via sinking marine snow, a routing of spilled oil unexpected to emergency response planners. Because baseline knowledge of particle export in the Northern Gulf of Mexico and how it varies spatially and temporally was limited, we conducted a detailed assessment of the potential drivers of deep (~1400 m depth) particle fluxes during 2012–2016 using sediment traps at three contrasting sites in the Northern Gulf of Mexico: near the DWH site, at an active natural oil seep site, and at a site considered typical for background conditions. The DWH site, located ~70 km from the Mississippi River Delta, showed flux patterns that were strongly linked to the Mississippi nitrogen discharge and an annual subsequent surface bloom. Fluxes carried clear signals of combustion products, which likely originated from pyrogenic sources that were transported offshore via the Mississippi plume. The seep and reference sites were more strongly influenced by the open Gulf of Mexico, did not show a clear seasonal flux pattern, and their overall sedimentation rates were lower than those at the DWH site. At the seep site, based on polycyclic aromatic hydrocarbon data, we observed indications of three different pathways for “natural” oiled-snow sedimentation: scavenging by sinking particles at depth, weathering at the surface before incorporation into sinking particles, and entry into the food web and subsequent sinking in form of detritus. Overall, sedimentation rates at the three sites were markedly different in quality and quantity owing to varying degrees of riverine and oceanic influences, including natural seepage and contamination by combustion products.

The 2010 Deep Water Horizon oil well failure released billions of gallons of crude oil into the deep Gulf of Mexico, and, combined with chemical dispersants, this oil caused significant coral mortality. However, the mechanisms by which oil and dispersed oil impact deep marine fauna are not well understood. Here, we investigate the effects of oil and dispersed oil on a black coral common in the deep Gulf of Mexico, Leiopathes glaberrima . This coral occurs in several color morphs that show ecological and genetic differences. We hypothesized that dispersed oil would be more detrimental to coral health than oil alone and that this difference would be detectable in the gene expression response of the colonies even at sub-lethal concentrations. In two experiments, four and six colonies of red and white color morphs were exposed to oil, dispersant, and dispersed oil for a minimum of 96 hours. Visual assessment indicated that indeed dispersant and dispersed oil treatments were more damaging than oil alone, for target concentrations of 25 mg L –1 . Decline in health was observed for all treatments, independently of color morphotype, but the decline was faster in the white colonies exposed to dispersant. The responses to the treatments were also investigated by monitoring gene expression after 24 hours of sub-lethal chemical exposure. Coral gene expression differed by chemical stressor. Interestingly, the polycyclic aromatic hydrocarbon biomarker gene, cytochrome P450, was only up-regulated in dispersed oil but not oil alone, suggesting that the dispersant increased the availability of such hydrocarbons in the tissue. The gene expression response was apparent at 24 hours when visual impacts were not (yet) detectable. The use of chemical dispersants in oil-spill remediation may cause health declines in deep-water corals and deserves further study.

Emissions of black carbon (BC), a product of incomplete combustion of fossil fuels, biofuels and biomass, are high in the Asia-Pacific region, yet input pathways and rates to the ocean are not well constrained. Atmospheric and riverine inputs of BC in Halong Bay (Vietnam), a hotspot of atmospheric BC, were studied at monthly intervals during one year. Climate in Halong Bay is governed by the monsoon regime, characterized by a northeast winter monsoon (dry season) and southeast summer monsoon (wet season). During the dry season, atmospheric BC concentrations averaged twice those observed during the wet season. In the sea surface microlayer (SML) and underlying water (ULW), concentrations of particulate BC (PBC) averaged 539 and 11 µmol C L –1 , respectively. Dissolved BC (DBC) concentrations averaged 2.6 µmol C L –1 in both the SML and ULW. Seasonal variations indicated that PBC concentration in the SML was controlled by atmospheric deposition during the dry season, while riverine inputs controlled both PBC and DBC concentrations in ULW during the wet season. Spatiotemporal variations of PBC and DBC during the wet season suggest that river runoff was efficient in transporting PBC that had accumulated on land during the dry season, and in mobilizing and transporting DBC to the ocean. The annual river flux of PBC was about 3.8 times higher than that of DBC. The monsoon regime controls BC input to Halong Bay by favoring dry deposition of BC originating from the north during the dry season, and wet deposition and river runoff during the wet season. High PBC concentrations seem to enhance the transfer of organic carbon from dissolved to particulate phase by adsorbing dissolved organic carbon and stimulating aggregation. Such processes may impact the availability and biogeochemical cycling of other dissolved substances, including nutrients, for the coastal marine ecosystem.

The export of organic matter from ocean to atmosphere represents a substantial carbon flux in the Earth system, yet the impact of environmental drivers on this transfer is not fully understood. This work presents dissolved and particulate organic carbon (DOC, POC) concentrations, their enrichment factors in the sea surface microlayer (SML), and equivalent measurements in marine aerosol particles across the Atlantic Ocean. DOC concentrations averaged 161 ± 139 µmol L –1 (n = 78) in bulk seawater and 225 ± 175 µmol L –1 (n = 79) in the SML; POC concentrations averaged 13 ± 11 µmol L –1 (n = 80) and 17 ± 10 µmol L –1 (n = 80), respectively. High DOC and POC enrichment factors were observed when samples had low concentrations, and lower enrichments when concentrations were high. The impacts of wind speed and chlorophyll-a levels on concentrations and enrichment of DOC and POC in seawater were insignificant. In ambient submicron marine aerosol particles the concentration of water-soluble organic carbon was approximately 0.2 µg m –3 . Water-insoluble organic carbon concentrations varied between 0.01 and 0.9 µg m –3 , with highest concentrations observed when chlorophyll-a concentrations were high. Concerted measurements of bulk seawater, the SML and aerosol particles enabled calculation of enrichment factors of organic carbon in submicron marine ambient aerosols, which ranged from 10 3 to 10 4 during periods of low chlorophyll-a concentrations and up to 10 5 when chlorophyll-a levels were high. The results suggest that elevated local biological activity enhances the enrichment of marine-sourced organic carbon on aerosol particles. However, implementation of the results in source functions based on wind speed and chlorophyll-a concentrations underestimated the organic fraction at low biological activity by about 30%. There may be additional atmospheric and oceanic parameters to consider for accurately predicting organic fractions on aerosol particles.

Weathered crude oil sank to the seafloor following the Deepwater Horizon disaster in 2010, removing this oil from further physical and photo-chemical degradation processes and leaving benthic processes as the mechanisms for altering and remediating this hydrocarbon source. To quantify potential microbial oil degradation rates at the seafloor, and associated changes in sediment microbial community structure and pore fluid composition, we used a benthic lander system to deploy novel sediment flow-through chambers at a natural hydrocarbon seep in the Gulf of Mexico (at a depth of 1226 m in lease block GC600) roughly 265 km southwest of the Deepwater Horizon wellhead (at 1500 m depth). Sediment amended with 20% unweathered crude oil had elevated rates of sulfate reduction over the course of the 5-month-long experiment as compared to an unamended control, yielding potential rates of sulfate reduction (600–800 mmol m –2 d –1 ) among the highest measured in hydrocarbon-influenced seafloor sediment. Oil amendment also stimulated methane production towards the end of the experiment, and led to slightly higher cell densities without significant changes in microbial community structure, based on 16S rRNA gene sequence libraries and fatty acid profiles. Assuming a link between sulfate reduction and hydrocarbon degradation, these results suggest that electron acceptor availability may become limiting in heavily oiled deep-sea environments, resulting in minimal degradation of deposited oil. This study provides unique data on seafloor sediment responses to oil deposition, and reveals the value of using observatories to fill the gap in understanding deep-sea microbial processes, especially for ephemeral and stochastic events such as oil spills.

Ocean acidification is intensifying and hypoxia is projected to expand in the California Current large marine ecosystem as a result of processes associated with the global emission of CO 2 . Observed changes in the California Current outpace those in many other areas of the ocean, underscoring the pressing need to adopt management approaches that can accommodate uncertainty and the complicated dynamics forced by accelerating change. We argue that changes occurring in the California Current large marine ecosystem provide opportunities and incentives to adopt an integrated, systems-level approach to resource management to preserve existing ecosystem services and forestall abrupt change. Practical options already exist to maximize the benefits of management actions and ameliorate impending change in the California Current, for instance, adding ocean acidification and hypoxia to design criteria for marine protected areas, including consideration of ocean acidification and hypoxia in fisheries management decisions, and fully enforcing existing laws and regulations that govern water quality and land use and development.

The deep sea encompasses the largest ecosystems on Earth. Although poorly known, deep seafloor ecosystems provide services that are vitally important to the entire ocean and biosphere. Rising atmospheric greenhouse gases are bringing about significant changes in the environmental properties of the ocean realm in terms of water column oxygenation, temperature, pH and food supply, with concomitant impacts on deep-sea ecosystems. Projections suggest that abyssal (3000–6000 m) ocean temperatures could increase by 1°C over the next 84 years, while abyssal seafloor habitats under areas of deep-water formation may experience reductions in water column oxygen concentrations by as much as 0.03 mL L –1 by 2100. Bathyal depths (200–3000 m) worldwide will undergo the most significant reductions in pH in all oceans by the year 2100 (0.29 to 0.37 pH units). O 2 concentrations will also decline in the bathyal NE Pacific and Southern Oceans, with losses up to 3.7% or more, especially at intermediate depths. Another important environmental parameter, the flux of particulate organic matter to the seafloor, is likely to decline significantly in most oceans, most notably in the abyssal and bathyal Indian Ocean where it is predicted to decrease by 40–55% by the end of the century. Unfortunately, how these major changes will affect deep-seafloor ecosystems is, in some cases, very poorly understood. In this paper, we provide a detailed overview of the impacts of these changing environmental parameters on deep-seafloor ecosystems that will most likely be seen by 2100 in continental margin, abyssal and polar settings. We also consider how these changes may combine with other anthropogenic stressors (e.g., fishing, mineral mining, oil and gas extraction) to further impact deep-seafloor ecosystems and discuss the possible societal implications.

The Northeast Atlantic mackerel ( Scomber scombrus ) stock has increased and expanded its summer feeding migration west- and northwards since 2006, entailing large geopolitical challenges for the countries harvesting this species. A common perspective is that climatic warming opens up new regions for biota in the north. It has also been suggested that the presently large pelagic fish stocks deplete prey resources in the eastern North Atlantic during their summer feeding phase, forcing the stocks west towards the Irminger Sea in their search for food. Here, we suggest that the declining nutrient (silicate) concentrations observed along the northern European continental slope reduce primary and thus secondary production, exacerbating food scarceness in the east and adding to the incentive to migrate westward. The new westward feeding route requires that the fish cross the Iceland Basin, which during the summer season quickly becomes nutrient-depleted and thus might act as a barrier to migration after the spring bloom. Using mackerel and zooplankton abundance data from the International Ecosystem Summer Surveys in the Nordic Seas, we suggest that the oligotrophic waters in the central Iceland Basin force the fish to migrate through a narrow ‘corridor’ along the south Iceland shelf, where nutrients are replenished and both primary and secondary production are higher.

To enhance the accuracy of carbon cycling models as applied to sea ice in the changing Arctic, we analyzed a large data set of particulate organic carbon (POC) and nitrogen (PON) measurements in first-year bottom sea ice (n = 257) from two Arctic shelves, the Canadian Arctic Archipelago and Beaufort Sea shelf, including dark winter and spring seasonal measurements. Wide ranges of sea-ice POC:PON ratios were observed during both the dark winter (12–46 mol:mol) and spring (3–24 mol:mol) periods. Sea-ice POC:PON ratios and chlorophyll a concentrations were significantly higher in the Archipelago versus the Beaufort Sea shelf (p < 0.01), yet there was a highly significant relationship between sea-ice POC and PON during spring for both shelves (r 2 = 0.94). POC:PON ratios were not consistent over the range of measured POC and PON concentrations, justifying the use of a power function model to best describe the relationship between POC and PON. Distinct relationships between POC:PON ratios and chlorophyll-based biomass were observed for the dark winter and the spring: dark winter sea-ice POC:PON ratios decreased with increasing sea-ice biomass whereas spring POC:PON ratios increased with increasing sea-ice biomass. The transition from the dark period to the spring growth period in first-year sea ice represented a distinct stoichiometric shift in POC:PON ratios. Our results demonstrate that the Redfield ratio has limited applicability over the four-order of magnitude range of biomass concentrations observed in first-year sea ice on Arctic shelves. This study emphasizes the need for variable POC:PON stoichiometry in sea-ice biogeochemical models and budget estimates, in particular at high biomass concentrations and when considering seasonality outside of the spring period in first year ice. The use of a power function model for POC:PON relationships in sea ice is also recommended to better constrain carbon estimates in biogeochemical sea-ice models.

Thermal diffusivity (TD) is a measure of the temperature response of a material to external thermal forcing. In this study, TD values for marine sediments were determined in situ at two locations on the Cascadia Margin using an instrumented sediment probe deployed by a remotely operated vehicle. TD measurements in this area of the NE Pacific Ocean are important for characterizing the upslope edge of the methane hydrate stability zone, which is the climate-sensitive boundary of a global-scale carbon reservoir. The probe was deployed on the Cascadia Margin at water depths of 552 and 1049 m for a total of 6 days at each site. The instrumented probe consisted of four thermistors aligned vertically, one sensor exposed to the bottom water and one each at 5, 10, and 15 cm within the sediment. Results from each deployment were analyzed using a thermal conduction model applying a range of TD values to obtain the best fit with the experimental data. TD values corresponding to the lowest standard deviations from the numerical model runs were selected as the best approximations. Overall TDs of Cascadia Margin sediments of 4.33 and 1.15 × 10 –7 m 2 s –1 were calculated for the two deployments. These values, the first of their kind to be determined from in situ measurements on a methane hydrate-rich continental margin, are expected to be useful in the development of models of bottom-water temperature increases and their implications on a global scale.

Morphological and phylogenetic analyses suggest that the ability to precipitate carbonates evolved several times in marine invertebrates in the past 600 million years. Over the past decade, there has been a profusion of genomic, transcriptomic, and proteomic analyses of calcifying representatives from three metazoan phyla: Cnidaria, Echinodermata, and Mollusca. Based on this information, we compared proteins intimately associated with precipitated calcium carbonate in these three phyla. Specifically, we used a cluster analysis and gene ontology approach to compare ∼1500 proteins, from over 100 studies, extracted from calcium carbonates in stony corals, in bivalve and gastropod mollusks, and in adult and larval sea urchins to identify common motifs and differences. Our analysis suggests that there are few sequence similarities across all three phyla, supporting the independent evolution of biomineralization. However, there are core sets of conserved motifs in all three phyla we examined. These motifs include acidic proteins that appear to be responsible for the nucleation reaction as well as inhibition; structural and adhesion proteins that determine spatial patterning; and signaling proteins that modify enzymatic activities. Based on this analysis and the fossil record, we propose that biomineralization is an extremely robust and highly controlled process in metazoans that can withstand extremes in pH predicted for the coming century, similar to their persistence through the Paleocene-Eocene Thermal Maximum (∼55 Mya).