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Despite their role in modulating the marine ecosystem, variability and drivers of low-oxygen events in the offshore northern Benguela Upwelling System (BenUS) have been rarely investigated due to the events’ episodicity which is difficult to resolve using shipboard measurements. We address this issue using 4 months of high-resolution glider data collected between February and June 2018, 100 km offshore at 18 °S. We find that oxygen concentrations in the offshore northern Benguela are determined by the subsurface alternation of low-oxygen Angola-derived water and oxygenated water from the south at 100 - 500 m depth. We observe intermittent hypoxia (O2 < 60 µmol kg−1) which occurs on average for ∼30 % of the 4 month deployment and is driven by the time-varying subsurface pulses of Angola-derived tropical water. Hypoxic events are rather persistent at depths of 300 - 450 m, while they are more sporadic and have weekly duration at shallower depths (100 - 300 m). We find extreme values of hypoxia, with O2 minima of 16 µmol kg−1, associated with an anticyclonic eddy spinning from the undercurrent flowing on the BenUS shelf and showing no surface signature. Fine-scale patchiness and water mass mixing are associated with cross-frontal stirring by a large anticyclone recirculating tropical water into the northern BenUS. The dominance of physical drivers and their high variability on short time scales reveal a dynamic coupling between Angola and Benguela, calling for long-term and high-resolution measurements and studies focusing on future changes of both tropical oxygen minima and lateral fluxes in this region.

The northern Benguela Upwelling System is characterized by significant oxygen and particle anomalies due to the lateral influx of both oxygenated water from the south and low-oxygen water that flows south across the northern Angola-Benguela front (ABF). Mesoscale features developing at the front and in the shelf region of the upwelling system further modulate these anomalies. Here we present the results of a study based on high-resolution glider data collected from the surface to 1000 m depth in February – June 2018 at 100 km off the coast of the northern Benguela (18°S). These in-situ data are further interpreted and generalized using high-resolution model output from the physical Regional Ocean Modeling System (ROMS) coupled to the Biogeochemical Ecosystem Cycling (BEC) model. Using the glider data, we discuss the prevalence of low oxygen events characterized by O2 < 120 µmol (sub-lethal level), O2 < 60 µmol (hypoxia) and O2 < 30 µmol (severe hypoxia) as a function of depth and time. We present two different impacts of eddies on oxygen concentrations: extreme hypoxia associated to a subsurface anticyclone generated at the Benguela shelf, and mixing between high and low oxygen water at the rim of a large surface anticyclone generated at the ABF. Through a spike analysis of the glider data, we study the distribution of large and small particles as a function of depth and time, and relate them to the identified mesoscale structures. We find that the subsurface eddy corresponds to a local low in small particle concentrations, while the frontal anticyclone is associated to a deep export event of both small and large particles. Further, we find that the region is characterized by a deep particle layer between 300 m and 500 m. Using the model output, we identify the drivers of this deep particle layer and deep export events and discuss the relative role of physics and biology in the determination of the vertical distribution of particles in the region of study.

At high latitudes, the export of organic matter from the surface to the ocean interior, the biological carbon pump, has conventionally been attributed to the gravitational sinking of particulate organic carbon (POC). Conspicuous deficits in ocean carbon budgets have recently challenged this long-lived paradigm of a sole pathway. Multiple strands of evidence have demonstrated the importance of additional export pathways, including the particle injection pumps (PIPs). Recent model estimates revealed that PIPs have a comparable downward POC flux to the biological gravitational pump (BGP), but with potentially different seasonal signatures. To date, logistical constraints have prevented concomitant and extensive observations of these pumps, and little is known about the seasonality of their fluxes. Here, using year-round robotic observations and recent advances in optical signal analysis, we concurrently investigated the functioning of two PIPs - the mixed layer and eddy subduction pumps - and the BGP in Southern Ocean waters. By comparing three phytoplankton bloom cycles in contrasting environments, we show how physical forcing and phytoplankton phenology influence the magnitude and seasonality of these pumps, with implications for carbon sequestration efficiency.

Measuring the underwater light field is a key mission of the international Biogeochemical-Argo program. Since 2012, 0–250 dbar profiles of downwelling irradiance at 380, 412 and 490 nm besides photosynthetically available radiation (PAR) have been acquired across the globe every 1 to 10 days. The resulting unprecedented amount of radiometric data has been previously quality-controlled for real-time distribution and ocean optics applications, yet some issues affecting the accuracy of measurements at depth have been identified such as changes in sensor dark responsiveness to ambient temperature, with time and according to the material used to build the instrument components. Here, we propose a quality-control procedure to solve these sensor issues to make Argo radiometry data available for delayed-mode distribution, with associated error estimation. The presented protocol requires the acquisition of ancillary radiometric measurements at the 1000 dbar parking depth and night-time profiles. A test on &gt, 10000 profiles from across the world revealed a quality-control success rate &gt, 90% for each band. The procedure shows similar performance in re-qualifying low radiometry values across diverse oceanic regions. We finally recommend, for future deployments, acquiring daily 1000 dbar measurements and one night profile per year, preferably during moonless nights and when the temperature range between the surface and 1000 dbar is the largest.

Three computational methods for modeling fracture are compared in the context of a class’ participation in the Third Sandia Fracture Challenge (SFC3). The SFC3 was issued to assess blind predictions of ductile fracture in a complex specimen geometry produced via additive manufacturing of stainless steel 316L powder. In this work, three finite-element-based methods are investigated: (1) adaptive remeshing, with or without material-state mapping; (2) element deletion; and (3) the extended finite element method. Each student team was tasked with learning about its respective method, calibrating model parameters, and performing blind prediction(s) of fracture/failure in the challenge-geometry specimen. Out of 21 teams who participated in the SFC3, three of the seven student teams from this class project ranked among the top five based on either global force-displacement or local strain predictions. Advantages and disadvantages of the three modeling approaches are identified in terms of mesh dependency, user-friendliness, and accuracy compared to experimental results. Recommendations regarding project management and organization are offered to facilitate future classroom participation in the Sandia Fracture Challenge or similar blind round-robin exercises.

The Greenland Ice Sheet (GrIS) is experiencing significant mass loss and freshwater discharge at glacier fronts. The freshwater input from Greenland will impact the physical properties of adjacent coastal seas, including important regions of deep water formation and contribute to global sea level rise. However, the biogeochemical impact of increasing freshwater discharge from the GrIS is less well constrained. Here, we demonstrate the use of bio-optical sensors on ocean gliders to track biogeochemical properties of meltwaters off southwest Greenland. Our results reveal that fresh, coastal waters, with an oxygen isotopic composition characteristic of glacial meltwater, are distinguished by a high optical backscatter and high levels of fluorescing dissolved organic matter (FDOM), representative of the overall coloured dissolved organic matter pool. Reconstructions of geostrophic velocities are used to show that these particle and FDOM-enriched coastal waters cross the strong boundary currents into the Labrador Sea. Meltwater input into the Labrador Sea is likely driven by mesoscale processes, such as eddy formation and local bathymetric steering, in addition to wind-driven Ekman transport. Ocean gliders housing bio-optical sensors can provide the high-resolution observations of both dissolved and particulate glacially-derived material that are needed to understand meltwater dispersal mechanisms and their sensitivity to future climatic change.

Biological processes in the subpolar Southern Ocean play a crucial role in the global carbon cycle, mediating CO2 exchange between the atmosphere and the densest waters of the global ocean. While historical perspectives have centred the importance of shelf-sea regions, recent reframing emphasises the role of the open ocean, and the cyclonic gyres. Here, we investigate the operation of the biological carbon pump (BCP) in the Weddell Gyre using satellite ocean colour and bio-Argo floats. We find first that a significant proportion (>54 %) of the inter-annual variability in NPP was explained by the area of open (ice-free) water. Spatial patterns suggest that peak productivity is associated with the ice edge. The seasonal decline in NPP occurs before ice cover returns, suggesting that other controls are limiting annual NPP (e.g. the exhaustion of iron). Comparing the shelf region to the open ocean, the shelf was seen to have higher rates of productivity, but NPP in the relatively less productive open ocean region accounted for ~95% of total carbon uptake each year. The total NPP in the Weddell Gyre (97-197 Tg C yr-1) is sufficient to supply the BCP-derived carbon that was previously observed to be exported from the region in Circumpolar Deep Water (~80 Tg C yr-1). NPP in the open ocean Weddell Gyre could thus provide the major source of carbon exported from the Weddell Gyre to the deep ocean via the horizontal circulation.

The Angola-Benguela frontal region represents an extremely dynamic portion of the ocean located along the south-western African coast, at the northern edge of the South Atlantic gyre. At this boundary, the northern warm and saline waters of the Angola Basin mix with the southern colder and fresher waters carried by the Benguela current through a combination of processes that span a wide range of spatio-temporal scales. This study combines the use of underwater glider data collected between February and June 2018 with a high resolution 3D physical-biogeochemical model to investigate how these lateral exchanges impact the oxygen and organic carbon distributions in the proximity of the front. From the glider data, we identify a set of salinity, oxygen and organic carbon anomalies impacting the first 500 m of the water column during February-June 2018. Using satellite images of physical and biological data and an eddy identification algorithm, we discuss these anomalies in the context of the surrounding physical and biological setting at the time of measurement and identify key processes that may be responsible for the observed tracer patterns. We employ the Regional Ocean Modeling System (ROMS) coupled with the Biogeochemistry Ecosystem Circulation model (BEC) to further explain and upscale our findings. We study the dynamics of cross-frontal exchanges of oxygen and organic carbon in the first 500 m depth. We show how the coupling between long filaments and intense anticyclonic eddies forming at the front generates a complex pattern of recirculation of Angola Basin-derived saline and low-oxygen waters into the oxygenated Benguela region. Finally, we quantify the oxygen lateral transport coupled with these dynamics, and discuss the implications for the biological activity in the region.

The high-latitude regions are experiencing some of the most rapid environmental changes observed anywhere on Earth, especially in recent years. The Greenland Ice Sheet, for example, is experiencing significant mass loss largely through surface melting, but also via ice discharge at glacier fronts. As well as changing freshwater budgets and ocean stratification and mixing, there has been increasing focus on the role of glaciers and ice sheets in supplying particulate and dissolved organic material and inorganic nutrients to marine systems. Here, we explore how a combination of ship-board and high-resolution ocean glider observations in shelf waters off SW Greenland inform on how these nutrients reach the coastal oceans and, eventually, mix off the shelf and into the open ocean. We find that the proportion of meltwater calculated using salinity and oxygen isotope mass balance agrees well with estimates from glider sensors. These meltwaters contain low dissolved macronutrients, but are characterised by high particulate and high dissolved organic content. Bio-optic sensors on the gliders reveal strong meltwater signals in fluorescing dissolved organic matter (FDOM), and a detectable signal in optical backscatter; these signals can be now observed extending further out into the open ocean in compiled biogeochemical (BGC) argo float data. The mixing of both dissolved and particulate macronutrients and organic matter off the shelf is likely driven by advection in geostrophic currents, tidal and buoyancy forcing, and is also impacted by storm events via wind-driven changes in mixed layer depth and resuspension.

The biological carbon pump exports carbon fixed by photosynthesis out of the surface ocean and transfers it to the deep, mostly in the form of sinking particles. Despite the importance of the pump in regulating the air‐sea CO2 balance, the magnitude of global carbon export remains unclear, as do its controlling mechanisms. A possible sinking flux of carbon to the mesopelagic zone may be via the mixed‐layer pump: a seasonal net detrainment of particulate organic carbon (POC)‐rich surface waters, caused by sequential deepening and shoaling of the mixed layer. In this study, we present a full year of daily small‐particle POC concentrations derived from glider optical backscatter data, to study export variability at the Porcupine Abyssal Plain (PAP) sustained observatory in the Northeast Atlantic. We observe a strong seasonality in small‐particle transfer efficiency, with a maximum in winter and early spring. By calculating daily POC export driven by mixed‐layer variations, we find that the mixed‐layer pump supplies an annual flux of at least 3.0 ± 0.9 g POC·m−2·year−1 to the mesopelagic zone, contributing between 5% and 25% of the total annual export flux and likely contributing to closing a gap in the mesopelagic carbon budget found by other studies. These are, to our best knowledge, the first high‐frequency observations of export variability over the course of a full year. Our results support the deployment of bio‐optical sensors on gliders to improve our understanding of the ocean carbon cycle on temporal scales from daily to annual., Key Points Gliders captured a full annual cycle of high‐frequency POC variability over the upper mesopelagic zoneSmall‐particle transfer efficiency displayed a strong seasonality, peaking in winter and early springThe data indicate a significant annual carbon supply to the mesopelagic zone by small particles through the mixed‐layer pump

Fixation of organic carbon by phytoplankton is the foundation of nearly all open-ocean ecosystems and a critical part of the global carbon cycle. But the quantification and validation of ocean primary productivity at large scale remains a major challenge due to limited coverage of ship-based measurements and the difficulty of validating diverse measurement techniques. Accurate primary productivity measurements from autonomous platforms would be highly desirable due to much greater potential coverage. In pursuit of this goal we estimate gross primary productivity over 2 months in the springtime North Atlantic from an autonomous Lagrangian float using diel cycles of particulate organic carbon derived from optical beam attenuation. We test method precision and accuracy by comparison against entirely independent estimates from a locally parameterized model based on chlorophyll a and light measurements from the same float. During nutrient-replete conditions (80 % of the study period), we obtain strong relative agreement between the independent methods across an order of magnitude of productivities (r2 = 0.97), with slight underestimation by the diel cycle method (−19 ± 5 %). At the end of the diatom bloom, this relative difference increases to −58 % for a 6-day period, likely a response to SiO4 limitation, which is not included in the model. In addition, we estimate gross oxygen productivity from O2 diel cycles and find strong correlation with diel-cycle-based gross primary productivity over the entire deployment, providing further qualitative support for both methods. Finally, simultaneous estimates of net community productivity, carbon export, and particle size suggest that bloom growth is halted by a combination of reduced productivity due to SiO4 limitation and increased export efficiency due to rapid aggregation. After the diatom bloom, high Chl a-normalized productivity indicates that low net growth during this period is due to increased heterotrophic respiration and not nutrient limitation. These findings represent a significant advance in the accuracy and completeness of upper-ocean carbon cycle measurements from an autonomous platform.

International audience; The Southern Ocean (SO) hosts plankton communities that impact the biogeochemical cycles of the global ocean. However, weather conditions in the SO restrict mainly in situ observations of plankton communities to spring and summer, preventing the description of biological successions at an annual scale. Here, we use shipboard observations collected in the Indian sector of the SO to develop a multivariate relationship between physical and bio-optical data, and, the composition and carbon content of the plankton community. Then we apply this multivariate relationship to five biogeochemical Argo (BGC-Argo) floats deployed within the same bio-geographical zone as the ship-board observations to describe spatial and seasonal changes in plankton assemblage. The floats reveal a high contribution of bacteria below the mixed layer, an overall low abundance of picoplankton and a seasonal succession from nano-to microplankton during the spring bloom. Both naturally iron-fertilized waters downstream of the Crozet and Kerguelen Plateaus show elevated phytoplankton biomass in spring and summer but they differ by a nano-or microplankton dominance at Crozet and Kerguelen, respectively. The estimated plankton group successions appear consistent with independent estimations of particle diameter based on the optical signals. Furthermore, the comparison of the plankton community composition in the surface layer with the presence of large mesopelagic particles diagnosed by spikes of optical signals provides insight into the nature and temporal changes of ecological vectors that drive particle export. This study emphasizes the power of BGC-Argo floats for investigating important biogeochemical processes at high temporal and spatial resolution.

Optical particle measurements are emerging as an important technique for understanding the ocean carbon cycle, including contributions to estimates of their downward flux, which sequesters carbon dioxide (CO2) in the deep sea. Optical instruments can be used from ships or installed on autonomous platforms, delivering much greater spatial and temporal coverage of particles in the mesopelagic zone of the ocean than traditional techniques, such as sediment traps. Technologies to image particles have advanced greatly over the last two decades, but the quantitative translation of these immense datasets into biogeochemical properties remains a challenge. In particular, advances are needed to enable the optimal translation of imaged objects into carbon content and sinking velocities. In addition, different devices often measure different optical properties, leading to difficulties in comparing results. Here we provide a practical overview of the challenges and potential of using these instruments, as a step toward improvement and expansion of their applications.

Breaking up is easy to do Sinking particles transport carbon to the seafloor, where they are buried in sediments and either provide food for benthic organisms or sequester the carbon they contain. However, only ∼30% of the maximum flux reaches depths of a kilometer. This loss cannot be fully accounted for by current measurements. Briggs et al. used data collected by robotic Biogeochemical-Argo floats to quantify total mesopelagic fragmentation and found that this process accounts for roughly half of the observed flux loss (see the Perspective by Nayak and Twardowski). Fragmentation is thus perhaps the most important process controlling the remineralization of sinking organic carbon. Science , this issue p. 791 ; see also p. 738

During the North Atlantic Aerosols and Marine Ecosystems Study in the western North Atlantic, float‐based profiles of fluorescent dissolved organic matter and backscattering exhibited distinct spike layers at ∼ 300 m. The locations of the spikes were at depths similar or shallower to where a ship‐based scientific echo sounder identified layers of acoustic backscatter, an Underwater Vision Profiler detected elevated concentration of zooplankton, and mesopelagic fish were sampled by a mesopelagic net tow. The collocation of spike layers in bio‐optical properties with mesopelagic organisms suggests that some can be detected with float‐based bio‐optical sensors. This opens the door to the investigation of such aggregations/layers in observations collected by the global biogeochemical‐Argo array allowing the detection of mesopelagic organisms in remote locations of the open ocean under‐sampled by traditional methods., Key Points We present concurrent observations of mesopelagic organisms with echo sounders, a camera, a net tow, and profiling floatsLayers of mesopelagic organisms coincide with distinct spike layers in fluorescence and backscattering profilesNovel application of biogeochemical‐Argo float enables the detection of mesopelagic organisms and their migration

A critical driver of the ocean carbon cycle is the downward flux of sinking organic particles, which acts to lower the atmospheric carbon dioxide concentration. This downward flux is reduced by more than 70% in the mesopelagic zone (100 to 1000 meters of depth), but this loss cannot be fully accounted for by current measurements. For decades, it has been hypothesized that the missing loss could be explained by the fragmentation of large aggregates into small particles, although data to test this hypothesis have been lacking. In this work, using robotic observations, we quantified total mesopelagic fragmentation during 34 high-flux events across multiple ocean regions and found that fragmentation accounted for 49 ± 22% of the observed flux loss. Therefore, fragmentation may be the primary process controlling the sequestration of sinking organic carbon.

In situ chlorophyll fluorometers have been used to quantify the distribution of chlorophyll concentration in natural waters for decades. However, chlorophyll fluorescence is depressed during daylight hours due to non-photochemical quenching (NPQ). Corrections attempted to date have provided improvement but still remain unsatisfactory, often over-estimating the expected value. In this study, we examine the relationship between NPQ and instantaneous Photosynthetically Active Radiation (iPAR) using field data from BGC-Argo floats equipped with Chlorophyll-a fluorometers and radiometers. This analysis leads to an improved NPQ correction that incorporates both iPAR and mixed layer depth (MLD) and is validated against data collected at sunrise or sunset. The optimal NPQ light threshold is found to be iPAR = 15 μmol quanta m−2 s−1, and the proposed methods based on such a light threshold correct the NPQ effect more accurately than others, except in “shallow-mixing” waters (NPQ light threshold depth deeper than MLD). For these waters, an empirical-relationship-based method is proposed for improvement of NPQ correction using an iPAR profile. It is therefore recommended that, for optimal NPQ corrections, profiling floats measuring chlorophyll fluorescence in daytime be equipped with iPAR radiometers.

In this study, the role of texture on the quasi-static and dynamic response of pure magnesium and magnesium alloy AZ31B is investigated. Texture is imparted through thermo-mechanical processes of hot-rolling or equal channel angular extrusion. Constant strain-rate, both dynamic and quasi-static, and strain-rate jump experiments, dynamic-quasi-static, quasi-static-dynamic and dynamic-dynamic, are used to examine plastic flow anisotropy and strain hardening response. Observations of the macroscopic material behavior, specifically the stress-trajectory, are supported by electron back-scatter diffraction analysis to gain insights of texture evolution and predominant deformation processes taking place during deformation increments.

The ratio of two in situ optical measurements – chlorophyll fluorescence (Chl F) and optical particulate backscattering (bbp) – varied with changes in phytoplankton community composition during the North Atlantic Bloom Experiment in the Iceland Basin in 2008. Using ship-based measurements of Chl F, bbp, chlorophyll a (Chl), high-performance liquid chromatography (HPLC) pigments, phytoplankton composition and carbon biomass, we found that oscillations in the ratio varied with changes in plankton community composition; hence we refer to Chl F/bbp as an "optical community index". The index varied by more than a factor of 2, with low values associated with pico- and nanophytoplankton and high values associated with diatom-dominated phytoplankton communities. Observed changes in the optical index were driven by taxa-specific chlorophyll-to-autotrophic carbon ratios and by physiological changes in Chl F associated with the silica limitation. A Lagrangian mixed-layer float and four Seagliders, operating continuously for 2 months, made similar measurements of the optical community index and followed the evolution and later demise of the diatom spring bloom. Temporal changes in optical community index and, by implication, the transition in community composition from diatom to post-diatom bloom communities were not simultaneous over the spatial domain surveyed by the ship, float and gliders. The ratio of simple optical properties measured from autonomous platforms, when carefully validated, provides a unique tool for studying phytoplankton patchiness on extended temporal scales and ecologically relevant spatial scales and should offer new insights into the processes regulating patchiness.

International audience; In the last few decades, phytoplankton biomass has been commonly studied from space. However, satellite analysis of non-algal particles (NAPs), including heterotrophic bacteria and viruses, is relatively recent. In this work, we estimate the backscattering coefficient associated with the NAP fraction that does not covary with chlorophyll based on satellite particulate backscattering coefficient and chlorophyll (b(bp)NAP). b(bp)NAP is computed at 100-km resolution using 19 years of monthly satellite data. We find clear differences in b(bp)NAP between northern and southern oceans. High b(bp)NAP values are found in the Arctic and Southern Oceans, the North Atlantic area influenced by the Gulf Stream current, as well as shelf regions (i.e., Patagonian shelf) affected by upwelling regimes. Low correlation between chlorophyll and backscattering prevents precise b(bp)NAP estimations in oligotrophic areas (e.g., subtropical gyres). These b(bp)NAP estimations lead to a reduction to half in satellite-based phytoplankton biomass estimates respect to previously published results. Plain Language Summary In the ocean, there are different seawater constituents that contribute to the inherent optical properties: Phytoplankton is a major constituent and one of the most studied in the last decade. Other important constituents are the colored dissolved organic matter and non-algal particles (NAPs). NAP includes (i) heterotrophic organisms such as bacteria, micrograzers, and viruses; (ii) detrital organic particles such as fecal pellets and cell debris; and (iii) mineral particles of both biogenic (e.g., calcite liths and shells) and terrestrial origin (e.g., clays and sand). This study is the first attempt to define the NAP from space, and its spatial variability, and how it contributes to refine the phytoplankton carbon biomass estimation. We estimate the backscattering coefficient associated with the NAP fraction that does not covary with chlorophyll from the satellite particulate backscattering coefficient and chlorophyll (bbpNAP). Our main results evidence a geographical variability of bbpNAP from northern to southern oceans with two distinct regimes: one associated with the productive areas in which bbpNAP and biomass are anticorrelated and another in which bbpNAP high values are in regions dominated by inorganic origin. We demonstrate that the spatial variability of bbpNAP should not be ignored.

The paper describes the first exact results in optimal design of three-phase elastic structures. Two isotropic materials, the "strong" and the "weak" one, are laid out with void in a given two-dimensional domain so that the compliance plus weight of a structure is minimized. As in the classical two-phase problem, the optimal layout of three phases is also determined on two levels: macro- and microscopic. On the macrolevel, the design domain is divided into several subdomains. Some are filled with pure phases, and others with their mixtures (composites). The main aim of the paper is to discuss the non-uniqueness of the optimal macroscopic multiphase distribution. This phenomenon does not occur in the two-phase problem, and in the three-phase design it arises only when the moduli of material isotropy of "strong" and "weak" phases are in certain relation., 8 pages, 4 figures

Budgets of nitrate, dissolved oxygen, and particulate organic carbon (POC) were constructed from data collected on-board a Lagrangian, profiling float deployed between April 4 and May 25, 2008, as part of the North Atlantic Bloom Experiment. These measurements were used to estimate net community production (NCP) and apparent export of POC along the float trajectory. A storm resulting in deep mixing and temporary suspension of net production separated the bloom into early (April 23–27) and main (May 6–13) periods over which ∼264 and ∼805 mmol C m −2 were produced, respectively. Subtraction of the total POC production from the NCP yielded maximum estimates of apparent POC export amounting to ∼92 and 574 mmol C m −2 during the early and main blooms, respectively. The bloom terminated the following day and ∼282 mmol C m −2 were lost due to net respiration (70%) and apparent export (30%). Thus, the majority of the apparent export of POC occurred continuously during the main bloom and a large respiration event occurred during bloom Termination. A comparison of the POC flux during the main bloom period with independent estimates at greater depth suggest a rapid rate of remineralization between 60 and 100 m. We suggest the high rates of remineralization in the upper layers could explain the apparent lack of carbon overconsumption (C:N>6.6) in the North Atlantic during the spring bloom.

An aggregate flux event was observed by ship and by four underwater gliders during the 2008 sub-polar North Atlantic spring bloom experiment (NAB08). At the height of the diatom bloom, aggregates were observed as spikes in measurements of both particulate backscattering coefficient ( b bp ) and chlorophyll a fluorescence. Optical sensors on the ship and gliders were cross-calibrated through a series of simultaneous profiles, and b bp was converted to particulate organic carbon. The aggregates sank as a discrete pulse, with an average sinking rate of ∼75 m d −1 ; 65% of aggregate backscattering and 90% of chlorophyll fluorescence content was lost between 100 m and 900 m. Mean aggregate organic carbon flux at 100 m in mid-May was estimated at 514 mg C m −2 d −1 , consistent with independent flux estimates. The use of optical spikes observed from gliders provides unprecedented coupled vertical and temporal resolution measurements of an aggregate flux event.

Lower bounds are obtained on the maximum field strength in one or both phases in a body containing two-phases. These bounds only incorporate boundary data that can be obtained from measurements at the surface of the body, and thus may be useful for determining if breakdown has necessarily occurred in one of the phases, or that some other nonlinearities have occurred. It is assumed the response of the phases is linear up to the point of electric, dielectric, or elastic breakdown, or up to the point of the onset of nonlinearities. These bounds are calculated for conductivity, with one or two sets of boundary conditions, for complex conductivity (as appropriate at fixed frequency when the wavelength is much larger than the body, i.e., for quasistatics), and for two-dimensional elasticity. Sometimes the bounds are optimal when the field is constant in one of the phases, and using the algorithm of Kang, Kim, and Milton (2012) a wide variety of inclusion shapes having this property, for appropriately chosen bodies and appropriate boundary conditions, are numerically constructed. Such inclusions are known as $E_\Omega$-inclusions., Comment: 30 pages, 7 figures

Previous studies have demonstrated that songbirds often use exotic plants as nesting substrates and may suffer elevated predation rates relative to nests placed in native plants. Veeries (Catharus fuscescens) frequently build nests in an exotic shrub, Japanese barberry (Berberis thunbergii), in forests of southeastern New York State, USA. We monitored Veery nesting success over a six-year period and supplemented these observations with experimental nests to determine whether predation rates differed between nests placed on the ground, in barberry shrubs, or in alternative native substrates. Using experimental nests, we found that nests raised above the ground in either barberry or native plants survived significantly longer than ground nests, but predation did not differ between the two former sites. Similarly, Veery nests on the ground suffered higher predation rates than nests in either raised native substrates or barberry; however, despite a substantially lower daily mortality rate in barberry, the dif...

The ratio of two in situ optical measurements, chlorophyll fluorescence (Chl F) and optical particulate backscattering (bbp), varied with changes in phytoplankton community composition during the North Atlantic Bloom experiment in the Iceland Basin in 2008. Using ship-based measurements of Chl F, bp, chlorophyll a (Chl), HPLC pigments, phytoplankton composition and carbon biomass, we found that oscillations in the ratio varied with changes in plankton community composition; hence we refer to Chl F/bp as an "optical community index". The index varied by more than a factor of two, with low values associated with pico- and nanophytoplankton and high values associated with diatom dominated phytoplankton communities. A Lagrangian mixed-layer float and four Seagliders, operating continuously for two months, made similar measurements of the optical community index and followed the evolution and later demise of the diatom spring bloom. Temporal changes in optical community index and, by implication the transition in community composition from diatom to post-diatom bloom communities, were not simultaneous over the spatial domain surveyed by the ship, float and gliders. Not only phytoplankton biomass, but also community composition was patchy at the submesoscale. The ratio of simple optical properties measured from autonomous platforms, when carefully validated, provides a tool for studying phytoplankton patchiness on extended temporal scales and ecological relevant spatial scales, and should offer new insights into the processes regulating patchiness.

The ability to estimate mean particle size using simple, low-power optical instruments promises to greatly expand coverage of particle size measurements in the ocean and advance understanding of myriad processes from sediment transport to biological carbon sequestration. Here we present a method for estimating the mean diameter of particles in suspension from high-resolution time series of simple optical measurements, such as beam attenuation or optical backscattering. Validation results from a laboratory clay aggregation experiment show a good fit with independent mean particle diameter estimates in the 10–80 μm diameter range, with relative biases of 17%–38% and relative root mean square errors of 10%–24%. In the 80–200 μm range, quantitative validation data were not available, but our mean diameter estimates correlated strongly with particle settling rates.

[1] The co-variability of particulate backscattering (bbp) and attenuation (cp) coefficients and particulate organic carbon (POC) provides a basis for estimating POC on spatial and temporal scales that are impossible to obtain with traditional sampling and chemical analysis methods. However, the use of optical proxies for POC in the open ocean is complicated by variable relationships reported in the literature between POC and cp or bbp. During the 2008 North Atlantic Bloom experiment, we accrued a large data set consisting of >300 POC samples and simultaneously measured cp and bbp. Attention to sampling detail, use of multiple types of POC blanks, cross-calibration of optical instruments, and parallel measurements of other biogeochemical parameters facilitated distinction between natural and methodological-based variability. The POC versuscp slope varied with plankton community composition but not depth; slopes were 11% lower for the diatom versus the recycling community. Analysis of literature POC versus cp slopes indicates that plankton composition is responsible for a large component of that variability. The POC versus bbp slope decreased below the pycnocline by 20%, likely due to changing particle composition associated with remineralization and fewer organic rich particles. The higher bbp/cp ratios below the mixed layer are also indicative of particles of lower organic density. We also observed a peculiar platform effect that resulted in ∼27% higher values for downcast versus upcast bbp measurements. Reduction in uncertainties and improvement of accuracies of POC retrieved from optical measurements is important for autonomous sampling, and requires community consensus for standard protocols for optics and POC.