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The oceans play a fundamental role in the global carbon cycle, providing a sink for atmospheric carbon. Key to this role is the vertical transport of organic carbon from the surface to the deep ocean. This transport is a product of a diverse range of physical and biogeochemical processes that determine the formation and fate of this material, and in particular how much carbon is sequestered in the deep ocean. Models can be used to both diagnose biogeochemical processes and predict how the various processes will change in the future. Global biogeochemical models use simplified representations of food webs and processes but are converging on values for the export of organic carbon from the surface ocean. Other models concentrate on understanding specific processes and can be used to develop parameterizations for global models. Model development is continuing by adding representations and parameterizations of higher trophic levels and mesopelagic processes, and these are expected to improve model performance.

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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 [ABSTRACT FROM AUTHOR]

Abstract The lateral export of carbon from coastal marshes via tidal exchange is a key component of the marsh carbon budget and coastal carbon cycles. However, the magnitude of this export has been difficult to accurately quantify due to complex tidal dynamics and seasonal cycling of carbon. In this study, we use in situ , high-frequency measurements of dissolved inorganic carbon (DIC) and water fluxes to estimate lateral DIC fluxes from a U.S. northeastern salt marsh. DIC was measured by a CHANnelized Optical Sensor (CHANOS) that provided an in situ concentration measurement at 15-min intervals, during periods in summer (July – August) and late fall (December). Seasonal changes in the marsh had strong effects on DIC concentrations, while tidally-driven water fluxes were the fundamental vehicle of marsh carbon export. Episodic events, such as groundwater discharge and mean sea water level changes, can impact DIC flux through altered DIC concentrations and water flow. Variability between individual tides within each season was comparable to mean variability between the two seasons. Estimated mean DIC fluxes based on a multiple linear regression (MLR) model of DIC concentrations and high-frequency water fluxes agreed reasonably well with those derived from CHANOS DIC measurements for both study periods, indicating that high-frequency, modeled DIC concentrations, coupled with continuous water flux measurements and a hydrodynamic model, provide a robust estimate of DIC flux. Additionally, an analysis of sampling strategies revealed that DIC fluxes calculated using conventional sampling frequencies (hourly to two-hourly) of a single tidal cycle are unlikely to capture a representative mean DIC flux compared to longer-term measurements across multiple tidal cycles with sampling frequency on the order of tens of minutes. This results from a disproportionately large amount of the net DIC flux occurring over a small number of tidal cycles, while most tides have a near-zero DIC export. Thus, high-frequency measurements (on the order of tens of minutes or better) over the time period of interest are necessary to accurately quantify tidal exports of carbon species from salt marshes. Highlights • High-frequency, long-term measurements are needed to capture tidal salt marsh export of carbon. • Sporadic events can have a disproportionately large influence on mean carbon export during a given period. • Modeled concentrations and water fluxes based on high-frequency data can produce robust estimates of DIC export. [ABSTRACT FROM AUTHOR]

Abstract Predicting marine carbon sequestration in a changing climate requires mechanistic understanding of the processes controlling sinking particle flux under different climatic conditions. The recent occurrence of a warm anomaly (2014–2015) followed by an El Niño (2015–2016) in the southern sector of the California Current System presented an opportunity to analyze changes in the biological carbon pump in response to altered climate forcing. We compare primary production, mesozooplankton grazing, and carbon export from the euphotic zone during quasi-Lagrangian experiments conducted in contrasting conditions: two cruises during warm years - one during the warm anomaly in 2014 and one toward the end of El Niño 2016 – and three cruises during El Niño-neutral years. Results showed no substantial differences in the relationships between vertical carbon export and its presumed drivers (primary production, mesozooplankton grazing) between warm and neutral years. Mesozooplankton fecal pellet enumeration and phaeopigment measurements both showed that fecal pellets were the dominant contributor to export in productive upwelling regions. In more oligotrophic regions, fluxes were dominated by amorphous marine snow with negligible pigment content. We found no evidence for a significant shift in the relationship between mesozooplankton grazing rate and chlorophyll concentration. However, mass-specific grazing rates were lower at low-to-moderate chlorophyll concentrations during warm years relative to neutral years. We also detected a significant difference in the relationship between phytoplankton primary production and photosynthetically active radiation between years: at similar irradiance and nutrient concentrations, productivity decreased during the warm events. Whether these changes resulted from species composition changes remains to be determined. Overall, our results suggest that the processes driving export remain similar during different climate conditions, but that species compositional changes or other structural changes require further attention. Highlights • Productivity, grazing, and export were measured during El Niño-neutral and warm years. • Carbon export and controlling processes were similar during both climatic conditions. • Net primary production was lower at similar light and nutrients during warm years. • When export was high, it was associated with sinking mesozooplankton fecal pellets. • When export was low, sinking particles were predominantly unrecognizable POM. [ABSTRACT FROM AUTHOR]

The amount of shoot stem (i.e. woody part of the shoot) dry matter per unit shoot leaf dry matter (i.e. the shoot wood to leaf biomass ratio) has been reported to be lower in short shoots than in long ones, and this is related to the greater and earlier ability of short shoots to export carbon. This is important in fruit trees, since the greater and earlier carbon export ability of shoots with a lower wood to leaf biomass ratio improves fruit production. This ratio may vary with cultivars, training systems or plant age, but no study has previously investigated the possible effect of fruit production. In this study on two olive cultivars (i.e. Arbequina, with low growth rate, and Frantoio, with high growth rate) subject to different fruit production treatments, we found that at increasing fruit production, shoot length and shoot wood to leaf biomass ratio were proportionally reduced in the new shoots growing at the same time as the fruit. Specifically, fruit production proportionally reduced total new-shoot biomass, length, leaf area and average shoot length. With decreasing shoot length, shoot diameter, stem mass, internode length, individual leaf area and shoot wood to leaf biomass ratio also decreased. This may be viewed as a plant strategy to better support fruit growth in the current year, given the greater and earlier ability of short shoots to export carbon. Moreover, at the whole-tree level, the percentage of total tree biomass production invested in leaves was closely correlated with branching density, which differed significantly across cultivars. By branching more, Arbequina concentrates more shoots (thus leaves) per unit of wood (trunk, branches and root) mass, decreasing wood to leaf biomass ratio at the whole-tree level. Therefore, while, at the shoot level, shoot length determines shoot wood to leaf biomass ratio, at the canopy level branching density is also an important determinant of whole-tree wood to leaf biomass ratio. Whole-tree wood to leaf biomass ratio is likely to affect the canopy's ability to export carbon (i.e. towards fruits), as shoot wood to leaf biomass ratio affects the carbon export ability of the shoot. [ABSTRACT FROM AUTHOR]

Submesoscale dynamics O(1–100 km) are associated with enhanced vertical velocities and evolve on a time scale similar to that of biological production (hours to days). Here we consider an annual cycle of submesoscale dynamics and their relation to productivity and export in a small (20 × 20 km) region of the northeast Atlantic Ocean. In this region, a springtime bloom is initiated by restratification of the mixed layer in June, although intermittent shoaling of the mixed layer maintains phytoplankton populations throughout the year. An optical community index suggests a dominance of large species (e.g., diatoms) during spring and picophytoplankton during the winter. We review three types of submesoscale instabilities—mixed layer (baroclinic), gravitational, and symmetric—and consider the impact of each on export of fixed carbon out of the surface layer. Mixed layer instabilities can potentially export material out of the mixed layer during winter, although the vertical velocity across the base of the mixed layer is sensitive to the parameterization scheme. Symmetric instabilities, in contrast, provide a clear mechanism for rapid export out of the mixed layer. A crucial factor determining export potential is the strength of the pycnocline at the base of the mixed layer. Export production is sensitive to the degree of overlap that exists between intense submesoscale activity associated with deep mixed layers in the winter and high productivity associated with the spring restratification, meaning that physically driven export of fixed carbon will likely happen over a short time window during spring. [ABSTRACT FROM AUTHOR]

Copepod fecal pellets are ubiquitous throughout the oceans. Their production and export can represent a highly efficient pathway of carbon export. However, the role these fecal pellets play in carbon export in the Changjiang (Yangtze) River estuary is not well known. Two cruises were carried out in the Changjiang estuary in the spring and summer of 2013, during which time carbon biomass, production, and export of copepod fecal pellets were studied. Spring and summer fecal pellet carbon biomass ranged 0.30–1.01 mg C/m3 (mean=0.56±0.20 mg C/m3) and 0.31–1.18 mg C/m3 (mean=0.64±0.24 mg C/m3), respectively, significantly lower than phytoplankton. At most stations, fecal pellet carbon biomass was higher in surface or subsurface layers than deeper layers. Production rates ranged 0.65–1.49 pellets/(ind.∙h) (mean=1.02±0.27 pellets/(ind.∙h)) in spring and 0.62–1.34 pellets/(ind.∙h) (mean=0.98±0.22 pellets/(ind.∙h)) in summer, within the range reported in previous studies. Higher production rates of fecal pellets occurred at stations with higher chlorophyll a concentrations, and production rates of copepods of size 500–1 000 μm greater than copepods >1 000 μm during both cruises. The potential export flux of fecal pellets was slightly higher in summer (mean=68.95±14.37 mg C/(m2 ∙d)) than spring (mean=52.08±11.33 mg C/(m2 ∙d)) owing to higher summer copepod abundances. To our knowledge, this study is the first of its kind in the Changjiang estuary, and it confirms the significant role of copepod fecal pellets in local carbon export. [ABSTRACT FROM AUTHOR]

New production (New P, the rate of net primary production (NPP) supported by exogenously supplied limiting nutrients) and net community production (NCP, gross primary production not consumed by community respiration) are closely related but mechanistically distinct processes. They set the carbon balance in the upper ocean and define an upper limit for export from the system. The relationships, relative magnitudes and variability of New P (from 15NO3-uptake), O2: argon-based NCP and sinking particle export (based on the 238U: 234Th disequilibrium) are increasingly well documented but still not clearly understood. This is especially true in remote regions such as polar marginal ice zones. Here we present a 3-year dataset of simultaneous measurements made at approximately 50 stations along the Western Antarctic Peninsula (WAP) continental shelf in midsummer (January) 2012-2014. Net seasonal-scale changes in water column inventories (0-150 m) of nitrate and iodide were also estimated at the same stations. The average daily rates based on inventory changes exceeded the shorter-term rate measurements. A major uncertainty in the relative magnitude of the inventory estimates is specifying the start of the growing season following sea-ice retreat. New P and NCP(O2) did not differ significantly. New P and NCP(O2) were significantly greater than sinking particle export from thorium-234. We suggest this is a persistent and systematic imbalance and that other processes such as vertical mixing and advection of suspended particles are important export pathways. This article is part of the theme issue 'The marine system of the west Antarctic Peninsula: status and strategy for progress in a region of rapid change'. [ABSTRACT FROM AUTHOR]

Translocation of assimilates is a fundamental process involving carbon and water balance affecting source/sink relationships. Diurnal patterns of CO2 exchange, translocation (carbon export), and transpiration of an intact tomato source leaf were determined during 14CO2 steady-state labeling under different wavelengths at three preset photosynthetic rates. Daily patterns showed that photosynthesis and export were supported by all wavelengths of light tested including orange and green. Export in the light, under all wavelengths was always higher than that at night. Export in the light varied from 65-83% of the total daily carbon fixed, depending on light intensity. Photosynthesis and export were highly correlated under all wavelengths (r = 0.90-0.96). Export as a percentage of photosynthesis (relative export) decreased as photosynthesis increased by increasing light intensity under all wavelengths. These data indicate an upper limit for export under all spectral conditions. Interestingly, only at the medium photosynthetic rate, relative export under the blue and the orange light-emitting diodes (LEDs) were higher than under white and red-white LEDs. Stomatal conductance, transpiration rates, and water-use-efficiency showed similar daily patterns under all wavelengths. Illuminating tomato leaves with different spectral quality resulted in similar carbon export rates, but stomatal conductance and transpiration rates varied due to wavelength specific control of stomatal function. Thus, we caution that the link between transpiration and C-export may be more complex than previously thought. In summary, these data indicate that orange and green LEDs, not simply the traditionally used red and blue LEDs, should be considered and tested when designing lighting systems for optimizing source leaf strength during plant production in controlled environment systems. In addition, knowledge related to the interplay between water and C-movement within a plant and how they are affected by environmental stimuli, is needed to develop a better understanding of source/sink relationships. [ABSTRACT FROM AUTHOR]

Estuaries are important subcomponents of the coastal ocean, but knowledge about the temporal and spatial variability of their carbonate chemistry, as well as their contribution to coastal and global carbon fluxes, are limited. In the present study, we measured the temporal and spatial variability of biogeochemical parameters in a saltmarsh estuary in Southern California, the San Dieguito Lagoon (SDL). We also estimated the flux of dissolved inorganic carbon (DIC) and total organic carbon (TOC) to the adjacent coastal ocean over diel and seasonal timescales. The combined net flux of DIC and TOC (FDIC + TOC) to the ocean during outgoing tides ranged from − 1.8±0.5 × 103 to 9.5±0.7 × 103 mol C h−1 during baseline conditions. Based on these fluxes, a rough estimate of the net annual export of DIC and TOC totaled 10±4 × 106 mol C year−1. Following a major rain event (36 mm rain in 3 days), FDIC + TOC increased and reached values as high as 29.0 ± 0.7 × 103 mol C h−1. Assuming a hypothetical scenario of three similar storm events in a year, our annual net flux estimate more than doubled to 25 ± 4 × 106 mol C year−1. These findings highlight the importance of assessing coastal carbon fluxes on different timescales and incorporating event scale variations in these assessments. Furthermore, for most of the observations elevated levels of total alkalinity (TA) and pH were observed at the estuary mouth relative to the coastal ocean. This suggests that SDL partly buffers against acidification of adjacent coastal surface waters, although the spatial extent of this buffering is likely small. [ABSTRACT FROM AUTHOR]

We describe the basis of a theory for interpreting measurements of two key biogeochemical fluxes—primary production by phytoplankton (p, μg C · L−1 · day−1) and biological carbon export from the surface ocean by sinking particles (f, mg C · m−2 · day−1)—in terms of their probability distributions. Given that p and f are mechanistically linked but variable and effectively measured on different scales, we hypothesize that a quantitative relationship emerges between collections of the two measurements. Motivated by the many subprocesses driving production and export, we take as a null model that large‐scale distributions of p and f are lognormal. We then show that compilations of p and f measurements are consistent with this hypothesis. The compilation of p measurements is extensive enough to subregion by biome, basin, depth, or season; these subsets are also well described by lognormals, whose log‐moments sort predictably. Informed by the lognormality of both p and f we infer a statistical scaling relationship between the two quantities and derive a linear relationship between the log‐moments of their distributions. We find agreement between two independent estimates of the slope and intercept of this line and show that the distribution of f measurements is consistent with predictions made from the moments of the p distribution. These results illustrate the utility of a distributional approach to biogeochemical fluxes. We close by describing potential uses and challenges for the further development of such an approach. [ABSTRACT FROM AUTHOR]

Carbon export from the surface to the deep ocean is a primary control on global carbon budgets and is mediated by plankton that are sensitive to physical forcing. Earth system models generally do not resolve ocean mesoscale circulation ( O(10–100) km), scales that strongly affect transport of nutrients and plankton. The role of mesoscale circulation in modulating export is evaluated by comparing global ocean simulations conducted at 1° and 0.1° horizontal resolution. Mesoscale resolution produces a small reduction in globally integrated export production (<2%); however, the impact on local export production can be large (±50%), with compensating effects in different ocean basins. With mesoscale resolution, improved representation of coastal jets block off‐shelf transport, leading to lower export in regions where shelf‐derived nutrients fuel production. Export is further reduced in these regions by resolution of mesoscale turbulence, which restricts the spatial area of production. Maximum mixed layer depths are narrower and deeper across the Subantarctic at higher resolution, driving locally stronger nutrient entrainment and enhanced summer export production. In energetic regions with seasonal blooms, such as the Subantarctic and North Pacific, internally generated mesoscale variability drives substantial interannual variation in local export production. These results suggest that biogeochemical tracer dynamics show different sensitivities to transport biases than temperature and salinity, which should be considered in the formulation and validation of physical parameterizations. Efforts to compare estimates of export production from observations and models should account for large variability in space and time expected for regions strongly affected by mesoscale circulation. [ABSTRACT FROM AUTHOR]

Abstract: Carbon export efficiency (e‐ratio) is defined as the fraction of organic carbon fixed through net primary production (NPP) that is exported out of the surface productive layer of the ocean. Recent observations for the Southern Ocean suggest a negative e‐ratio versus NPP relationship, and a reduced dependency of export efficiency on temperature, different than in the global domain. In this study, we complement information from a passive satellite sensor with novel space‐based lidar observations of ocean particulate backscattering to infer NPP over the entire annual cycle, and estimate Southern Ocean export rates from five different empirical models of export efficiency. Inferred Southern Ocean NPP falls within the range of previous studies, with a mean estimate of 15.8 (± 3.9) Pg C yr−1 for the region south of 30 °S during the 2005–2016 period. We find that an export efficiency model that accounts for silica(Si)‐ballasting, which is constrained by observations with a negative e‐ratio versus NPP relationship, shows the best agreement with in situ‐based estimates of annual net community production (annual export of 2.7 ± 0.6 Pg C yr−1 south of 30 °S). By contrast, models based on the analysis of global observations with a positive e‐ratio versus NPP relationship predict annually integrated export rates that are ∼ 33% higher than the Si‐dependent model. Our results suggest that accounting for Si‐induced ballasting is important for the estimation of carbon export in the Southern Ocean. [ABSTRACT FROM AUTHOR]

Temperate Highland Peat Swamps on Sandstone (THPSS) are upland wetlands, similar to fens in the Northern Hemisphere and are found at the headwaters of low-order streams on the plateaus of Eastern Australia. They are classified as endangered ecological communities under State and National legislation. Previous works have identified particular geomorphic characteristics that are important to carbon storage in these low energy sediment accumulation zones. Changes in the geomorphic structure of THPSS, such as channelisation, may have profound implications for carbon storage. To assess the effect of channelisation on carbon budgets in these ecosystems it is essential to identify and quantify differences in carbon export, emissions and stocks of carbon of intact swamps and those that have become channelised. We undertook seasonal sampling of the perched swamp aquifers and surface waters of two intact swamps and two channelised fills in the Blue Mountains of New South Wales, Australia, to investigate differences in carbon exports and emissions between the two swamp types. We found that channelised fills' mean CO 2 emissions were almost four times higher than intact swamps with mean CH 4 emissions up to five times higher. Annual fluvial carbon exports for channelised fills were up to 18 times that of intact swamps. Channelised fill exports and emissions can represent up to 2% of the total swamp carbon stocks per annum which is 40 times higher than the intact swamps. This work clearly demonstrates that changes in geomorphic structure brought about by incision and channelisation results in profound changes to the carbon storage function of THPSS. [ABSTRACT FROM AUTHOR]

Dissolvedorganic carbon (DOC) was determined at the Gerlache Strait, a coastal area off the Northern Antarctic Peninsula, in the austral summers of 2015 and 2016 during NAUTILUS I and II cruises, respectively. Hydrographic conditions in the area were very different between the study years. Despite that, the surface mixed layer displayed higher DOC concentrations compared to intermediate and deeper waters, as a result of the strong water column stratification, for both years. DOC accumulation on the surface is significantly related to the net community production (NCP), and stronger when the nitrate surface deficit (ΔNO3 - ) is higher (2016). The presence of a surface/subsurface thermal front at the central Gerlache Strait likely promotes the export of surface-produced organic carbon to deeper layers. A weak correlation between DOC and apparent oxygen utilization (AOU) suggests that the degradationof organic carbon in the deeper water column has a small role in oxygen consumption (less than 10%). This “recalcitrant” DOC characteristic below the surface layers observed in this study is similar to that found twenty years ago in the same area (FRUELA campaign, early Summer 1995). The Gerlache Strait allows, as an adequate regional scale study, assessments on the efficiency of the export and distribution of organic carbon in deep water masses in view of the observed trends of freshening, cooling, and lightening in the Antarctic Peninsula shelf waters. [ABSTRACT FROM AUTHOR]

The Peruvian upwelling zone is one of the most productive marine ecosystems in the world with a spectacular, pronounced oxygen minimum zone (OMZ). Globally OMZs are increasing in size and intensity with far-reaching consequences for the marine biological carbon pump and carbon export; thus, these zones need to be carefully monitored to be able to understand future climate change impacts. The current study was carried out in 2013 and 2017 to quantify the vertical flux of organic matter exported out of the productive surface layer by measuring 234Th 238U disequilibria in the water column. Samples were collected in January 2013 and May 2017 along an identical transect located at 12°S off the Peruvian coast near Lima, Peru. Th-234 fluxes ranged from 0 to 2088 ± 95 dpm m−2 d−1 in 2013 and 698 ± 63 to 3648 ± 113 dpm m−2 d−1 in 2017. The corresponding POC fluxes varied between 0 and 164.2 ± 7.9 mg C m−2 d−1 in 2013 and 22.7 ± 2.7 to 133.1 ± 15.2 mg C m−2 d−1 in 2017, with POC fluxes gradually decreasing with distance from the coast. Despite higher POC fluxes, the export efficiencies were found to be extremely low due to high particle remineralization rates observed within the euphotic zone. • Particulate Organic Carbon fluxes vary between 22.7 ± 2.7 and 164.2 ± 7.9 mg C m-2 d-1. • The export efficiency (e-ratio) ranged from 0.01 to 0.03, suggesting an extremely low export efficiency. • Efficiency of BCP is low in Peruvian Coast likely due to high remineralization in the water column and zooplankton grazing. [ABSTRACT FROM AUTHOR]

Marine snow aggregates often dominate carbon export from the surface layer to the deep ocean. Therefore, understanding the formation and properties of aggregates is essential to the study of the biological pump. Previous studies have observed a relationship between phytoplankton growth phase and the production of transparent exopolymer particles (TEP), the sticky particles secreted by phytoplankton that act as the glue during aggregate formation. In this experimental study, we aim to determine the effect of phytoplankton growth phase on properties related to aggregate settling. Cultures of the diatom Thalassiosira weissflogii were grown to four different growth phases and incubated in rotating cylindrical tanks to form aggregates. Aggregate excess density and delayed settling time through a sharp density gradient were quantified for the aggregates that were formed, and relative TEP concentration was measured for cultures before aggregate formation. Compared to the first growth phase, later phytoplankton growth phases were found to have higher relative TEP concentration and aggregates with lower excess densities and longer delayed settling times. These findings may suggest that, although particle concentrations are higher at later stages of phytoplankton blooms, aggregates may be less dense and sink slower, thus affecting carbon export.

Abstract: The vertical transport of surface water and carbon into ocean's interior, known as subduction, is one of the main mechanisms through which the ocean influences Earth's climate. New instrumental approaches have shown the occurrence of localized and intermittent subduction episodes associated with small‐scale ocean circulation features. These studies also revealed the importance of such events for the export of organic matter, the so‐called eddy‐pump. However, the transient and localized nature of episodic subduction hindered its large‐scale evaluation to date. In this work, we present an approach to detect subduction events at the scale of the Southern Ocean using measurements collected by biogeochemical autonomous floats (BGCArgo). We show how subduction events can be automatically identified as anomalies of spiciness and Apparent Oxygen Utilization (AOU) below the mixed layer. Using this methodology over more than 4,000 profiles, we detected 40 subduction events unevenly distributed across the Sothern Ocean. Events were more likely found in hot spots of eddy kinetic energy (EKE), downstream major bathymetric features. Moreover, the bio‐optical measurements provided by BGCArgo allowed measuring the amount of Particulate Organic Carbon (POC) being subducted and assessing the contribution of these events to the total downward carbon flux at 100 m (EP100). We estimated that the eddy‐pump represents less than 19% to the EP100 in the Southern Ocean, although we observed particularly strong events able to locally duplicate the EP100. This approach provides a novel perspective on where episodic subduction occurs that will be naturally improved as BGCArgo observations continue to increase. [ABSTRACT FROM AUTHOR]

Climate changes are mainly characterized by an increase in air temperature and a decrease in rainfalls. Potassium (K) nutrition is generally considered to alleviate plants tolerance to water deficit, especially by improving photosynthesis and phloem transport of carbohydrates from leaves to roots. The main objective of this study was to measure the effect of K on sugar transport and allocation under water-stressed conditions on maize ( Zea mays L.). Maize plants were grown in pots under different water and K treatments. We used CO pulse-labelling to determine carbon exportation from leaves with δC analysis, within 1 week. The diurnal sugar content in leaves was measured, and net carbon assimilation accessed. Water deficit strongly reduced plant growth, while K nutrition appeared to be efficient in attenuating these effects. K deficiency significantly decreased starch content in leaves under well-watered but not under water-stressed treatment. A leaf carbon mass balance showed that K increased sugar export on a daily time scale, while instantaneous δC measurements did not show any significant effect, partly because of the very rapid δC decline after labelling. Our home-made labelling chamber proved to be successful in monitoring diurnal changes in δC for a C4 plant with high photosynthetic rates and fast carbon export, and also in determining the effect of a K deficiency on sugar export. Our results highlight a need for research into carbon export on leaves of different ages in fast-growing crops under the combined effect of water and nutrient stress. [ABSTRACT FROM AUTHOR]

In contemporary oceans diatoms are an important group of eukaryotic phytoplankton that typically dominate in upwelling regions and at high latitudes. They also make significant contributions to sporadic blooms that often occur in springtime. Recent surveys have revealed global information about their abundance and diversity, as well as their contributions to biogeochemical cycles, both as primary producers of organic material and as conduits facilitating the export of carbon and silicon to the ocean interior. Sequencing of diatom genomes is revealing the evolutionary underpinnings of their ecological success by examination of their gene repertoires and the mechanisms they use to adapt to environmental changes. The rise of the diatoms over the last hundred million years is similarly being explored through analysis of microfossils and biomarkers that can be traced through geological time, as well as their contributions to seafloor sediments and fossil fuel reserves. The current review aims to synthesize current information about the evolution and biogeochemical functions of diatoms as they rose to prominence in the global ocean. [ABSTRACT FROM AUTHOR]

The biological carbon pump (BCP) transports organic carbon from the surface to the ocean's interior via sinking particles, vertically migrating organisms, and passive transport of organic matter by advection and diffusion. While many studies have quantified sinking particles, the magnitude of passive transport remains poorly constrained. In the Southern Ocean weak thermal stratification, strong vertical gradients in particulate organic matter, and weak vertical nitrate gradients suggest that passive transport from the euphotic zone may be particularly important. We compile data from seasonal time series at a coastal site near Palmer Station, annual regional cruises in the Western Antarctic Peninsula (WAP), cruises throughout the broader Southern Ocean, and SOCCOM (Southern Ocean Carbon and Climate Observations and Modeling) autonomous profiling floats to estimate spatial and temporal patterns in vertical gradients of nitrate, particulate nitrogen (PN), and dissolved organic carbon. Under a steady state approximation, the ratio of ∂PN/∂ z to ∂NO3−/∂ z suggests that passive transport of PN may be responsible for removing 46% (37%-58%) of the nitrate introduced into the surface ocean of the WAP (with dissolved organic matter contributing an additional 3-6%) and for 23% (19%-28%) of the BCP in the broader Southern Ocean. A simple model parameterized with in situ nitrate, PN, and primary production data suggested that passive transport was responsible for 54% of the magnitude of the BCP in the WAP. Our results highlight the potential importance of passive transport (by advection and diffusion) of organic matter in the Southern Ocean but should only be considered indicative of high passive transport (rather than conclusive evidence) due to our steady state assumptions. [ABSTRACT FROM AUTHOR]

The phytoplankton bloom in the Ross Sea is the largest in spatial extent and one of the most productive in Antarctica, yet the fate of the summer bloom remains poorly understood. Here we present carbon system data from the first biogeochemical process cruise to be conducted in both the western and central Ross Sea during late summer (February-March 2013). Using one-dimensional carbon budgets, we found evidence for substantial positive net community production (425 ± 204 mmol C m−2 d−1) during the late summer in Terra Nova Bay (TNB) of the western Ross Sea, which was rapidly exported to below 200 m. In addition, seasonally integrated carbon export was higher in diatom-dominated TNB (7.3 ± 0.9 mol C m−2) compared to the Phaeocystis antarctica-dominated central Ross Sea (3.4 ± 0.8 mol C m−2). Substantial late summer productivity and export may be a widespread phenomenon in Antarctic coastal regions that is not accounted for in regional carbon models. [ABSTRACT FROM AUTHOR]

This study involved a baseline evaluation of fluvial carbon export and degas rates in three nested rural catchments (1 to 80 km2) in Taboão, a representative experimental catchment of the Upper Uruguay River Basin. Analyses of the carbon content in stream waters and the catchment carbon yield were based on 4-year monthly in situ data and statistical modeling using the United States Geological Survey load estimator model. We also estimated pCO2 and degas fluxes using carbonate equilibrium and gas-exchange formulas. Our results indicated that the water was consistently pCO2 saturated (~90% of the cases) and that the steep terrain favors high gas evasion rates. The mean calculated fluvial export was 5.4 tC·km−2·year−1 with inorganic carbon dominating (dissolved inorganic carbon:dissolved organic carbon ratio >4), and degas rates (~40 tC km−2·year−1) were nearly sevenfold higher than the downstream export. The homogeneous land use in this nested catchment system results in similar water-quality characteristics, and therefore, export rates are expected to be closely related to the rainfall-runoff relationships at each scale. Although the sampling campaigns did not fully reproduce storm-event conditions and related effects such as flushing or dilution of in-stream carbon, our results indicated a potential link between dissolved inorganic carbon and slower hydrological pathways related to subsurface water storage and movement. [ABSTRACT FROM AUTHOR]

Atmospheric carbon dioxide levels are strongly controlled by the depth at which the organic matter that sinks out of the surface ocean is remineralized. This depth is generally estimated from particle flux profiles measured using sediment traps. Inherent in this analysis is a steady state assumption that export from the surface does not significantly change in the time it takes material to reach the deepest trap. However, recent observations suggest that a significant fraction of material in the mesopelagic zone sinks slowly enough to bring this into doubt. We use data from a study in the North Atlantic during July/August 2009 to challenge the steady state assumption. An increase in biogenic silica flux with depth was observed which we interpret, based on vertical profiles of diatom taxonomy, as representing the remnants of the spring diatom bloom sinking slowly (<40 m d−1). We were able to reproduce this behavior using a simple model using satellite-derived export rates and literature-derived remineralization rates. We further provide a simple equation to estimate 'additional' (or 'excess') particulate organic carbon supply to the dark ocean during nonsteady state conditions, which is not captured by traditional sediment trap deployments. In seasonal systems, mesopelagic net organic carbon supply could be wrong by as much as 25% when assuming steady state. We conclude that the steady state assumption leads to misinterpretation of particle flux profiles when input fluxes from the upper ocean vary on the order of weeks, such as in temperate and polar regions with strong seasonal cycles in export. [ABSTRACT FROM AUTHOR]

The total daily phytoplankton loss from the mixed layer can be estimated as the difference between gross primary production (GPP) and the realized change in phytoplankton carbon biomass. Here a modeling approach is used to estimate the total loss rates for five hydrographic domains on the eastern Bering Sea Shelf during a warm (2000–2006) and a cold (2007–2010) period. Model results indicate that the average daily rate of GPP in the mixed layer for all domains is on average slightly higher than in warm years (950±726 mg C m −2 d −1 ) than in cold years (859±640 mg C m −2 d −1 ), but is not significantly different. Similarly, the daily phytoplankton total loss rate from the mixed layer in all domains is on average slightly higher in warm years (961±747 mg C m −2 d −1 ) than in cold years (888±691 mg C m −2 d −1 ), but the difference is not significant. That total loss rates show the same warm vs. cold year pattern as GPP, suggests similar seasonal and latitudinal variations and magnitudes of change for both processes. The annual total loss is compared with the sum of individual process losses (e.g., mixing, grazing, sinking, etc.), with the discrepancy being generally larger than ~15% of the total loss both in warm and cold years. The model results also show that annual respiration is generally greater than losses due to zooplankton grazing and sinking both in warm and cold years. Compared among domains, significant differences ( t -test, P <0.05) between northern and southern domains (defined as North and South of 60°N) are observed for GPP rate, total daily loss rate and each of the individual loss terms in cold years, while values for southern domains are higher than those of northern domains. In warm years there were no significant differences between domains. Furthermore, these results indicate that total loss rates reflect patterns in GPP rate implying a similar metabolic balance within the ecosystem in both warm and cold years. [ABSTRACT FROM AUTHOR]

During the SESAME EU FP6 project, all available particulate organic carbon (POC) data collected from drifting sediment trap and Underwater Vision Profiler deployments (INSU PROOF database, 1991–2011) were gathered in order to assess carbon export at the scale of the Mediterranean Sea. In this study, we observed that particle size, POC export, and the contribution of microphytoplankton to the phytoplankton community structure, all decreased following the west to east net primary production gradient. One the other hand, no clear longitudinal gradient was found regarding particle composition (C/N ratio or lipid content). The above longitudinal patterns were also observed at the seasonal scale from spring to summer in the northwestern sub-basin. These observations suggest that particle size rather than organic matter composition controls fluxes of POC in the Mediterranean Sea. The comparison between POC and dissolved organic carbon (DOC) fluxes highlights the different time-scale of physicals vertical mechanisms and suggests that DOC flux can play an underestimated role in the supply of fresh carbon to the deep waters Mediterranean Sea. Indeed, DOC supply to deeper layers can be one order of magnitude larger than particle carbon flux but occurs in pulses when stratification breaks due to (i) deep-water formation, or (ii) winter mixing. In contrast, the vertical export of POC occurs throughout the year bringing weak, but almost continuous, energy to meso- and bathypelagic organisms. [ABSTRACT FROM AUTHOR]

Biogeochemical and diatom export fluxes are presented from two bathypelagic sediment trap deployments in the Antarctic Zone of the Southern Ocean. One of the sediment traps was deployed in very productive, naturally iron-fertilized waters downstream of South Georgia (P3, 2000 m) and compared to a deployment in moderately productive waters upstream of the island system (P2, 1500 m). At both sites significant diatom export events occurred in spring (November) and contained mostly empty cells that were associated with low particulate organic carbon (POC) fluxes. A summer export pulse occurred one month later at P2 (end February/March) compared to P3 (end January). Diatom fluxes at P3 were one order of magnitude higher than at P2, a difference mainly attributed to the short and intense export of resting spores from Chaetoceros Hyalochaete and Thalassiosira antarctica species. Aside from these resting spores, diatom export assemblages at both sites were dominated by empty Fragilariopsis kerguelensis frustules. The fraction of diatoms exported as empty frustules was considerably lower at P3 (52%) than P2 (91%). This difference was related to the flux of intact diatom resting spores at P3 and may partially explain the lower Si:C export stoichiometry observed at P3 (1.1) compared to P2 (1.5). Through the enumeration of full diatom frustules and subsequent biomass calculations we estimate that diatom resting spores account for 42% of annual POC flux in the productive waters downstream of South Georgia. At both sites the contribution of diatom vegetative stages to POC fluxes was considerably lower (<5%). From these analyses we conclude that resting spore export contributes towards the slightly higher bathypelagic (POC) flux at P3 (40.6 mmol m −2 y −1 ) compared to P2 (26.4 mmol m −2 y −1 ). We compared our sediment trap records with previously published diatom assemblage data from the mixed layer and surface sediments (3760 m) around South Georgia. The relative proportion of diatom resting spores within diatom assemblages increases as a function of depth and is explained by selective preservation of their robust frustules. Our study highlights the significance of diatom resting spore export as a carbon vector out of the mixed layer. Furthermore, the contribution or resting spores to POC flux in the bathypelagic ocean and sediments suggests they play a particularly important role in sequestering biologically fixed CO 2 over climatically relevant timescales. [ABSTRACT FROM AUTHOR]

The northern Gulf of Mexico (NGOM) is one of the well-studied areas of global ocean, yet direct estimates of upper ocean particulate organic carbon (POC) fluxes from this region are limited. The present work reports vertical fluxes of POC from the oligotrophic region of NGOM utilizing short-lived radionuclide pairs 234 Th/ 238 U and 210 Po/ 210 Pb. In spite of the difference in time scale both 210 Po and 234 Th based estimates are in reasonable agreement with sinking POC fluxes, caught in sediment traps. POC flux estimates ranged between 22–41 mg C m − 2 day − 1 at 150 m and 6–40 mg C m − 2 day − 1 at 250 m. The average export efficiency at base of euphotic zone (E z ) was found to be 0.07 ± 0.03 while the export ratio (T 100 ) at 100 m below euphotic zone was found to be 0.66 ± 0.18 indicating that most of the attenuation of NPP in this region is set in the surface layer (low E z ratio) and there is relatively little flux attenuation in the subsurface (high T 100 ) which is typical for oligotrophic settings. Satellite based export efficiencies predicted by the Laws and Dunne models are on average found to be two times higher than the in situ observations while estimates from the Henson model were found to be similar or lower. This is consistent with the observation on a global scale where we find export estimates from in situ data to be consistently lower than those predicted by the Laws export model for the temperature range of 20–25 °C. The discrepancy between modelled estimates and in situ measurements of POC fluxes highlights the fact that global empirical models of satellite based POC fluxes that only consider temperature are overly simple and may need further refinement for ocean biome specific scaling to accurately predict export fluxes. [ABSTRACT FROM AUTHOR]

The particulate organic carbon (POC) to 234 Th ratio, or POC/ 234 Th, is crucial to constrain the 234 Th-derived downward export flux. Marine particles for 234 Th based export studies are typically collected via two sampling modes: filtration of Niskin or GO-FLO bottle water samples and filtration of in situ pumped samples. Large discrepancies have been frequently observed between these bottle and pump filtration methods and attributed to the difference in adsorption of dissolved material. We noted however, that simultaneous measurements of carbon and 234 Th have not been made to evaluate such a putative effect on the POC/ 234 Th ratio. In this study, we adopted a “two filters in-line” approach method with one stacked on another and we measured both carbon and 234 Th to compare their adsorption behavior in bottle and pump sampling modes. We proposed that the 234 Th and carbon on the second filter (filter-B) might be attributable to direct adsorption from the dissolved phase (including the partial capture from the submicron particles) and/or breakdown of particles initially collected on the first filter (filter-A). The relative importance of these two processes differed between the two filtration modes. For bottle sampling, we observed significantly positive correlation (R 2 = 0.54) between the 234 Th recorded on the second filter and dissolved 234 Th activities, suggesting that adsorption was the dominant contributor to the second filter. The overestimation due to adsorption (filter-B/filter-A) is estimated to be 22 ± 7% for POC and 25 ± 17% for particulate 234 Th. The effect of adsorption on the POC/ 234 Th, on the other hand, would be negligible since the adsorption-corrected and -uncorrected POC/ 234 Th on filter-A were similar. For the large volume pumping mode, the absorbed 234 Th on the filters appeared to reach the saturation state as evidenced by the fact that the amount of particulate 234 Th (dpm cm − 2 ) on filter-B was independent of the filtered sample volume. We justified that the saturation amount should be > 0.21 dpm cm − 2 which was the highest unsaturated value from the bottle sample, but < 0.31 dpm cm − 2 which was the upper value on filter-B taken from pump samples. The calculated adsorption still dominated for 234 Th on filter-B and particle breakdown might be important in the euphotic zone. The collected POC on filter-B tended to increase with the filtration volume which implied that DOC adsorption remained under-saturated. Similar to bottle filtration, the corrected POC/ 234 Th was comparable to the uncorrected POC/ 234 Th on filter-A. We thus conclude that adsorption did not appear to influence the POC/ 234 Th ratio in either sampling mode. We suggested that the positive offset in POC/ 234 Th between bottle and pump sampling, if any, cannot be caused by adsorption. Other mechanisms such as zooplankton invasion and/or artificial particle formation need to be examined in order to resolve the discrepancy. [ABSTRACT FROM AUTHOR]

Summary Coarse particulate organic matter (CPOM) provides a food source for benthic organisms, and the fluvial transport of CPOM is one of the forms in which carbon is exported from a forested basin. However, little is known about transport dynamics of CPOM, its relation to discharge, and its annual exports from mountain streams. Much of this knowledge gap is due to sampling difficulties. In this study, CPOM was sampled over one-month snowmelt high flow seasons in two high-elevation, subalpine, streams in the Rocky Mountains. Bedload traps developed for sampling gravel bedload were found to be suitable samplers for CPOM transport. CPOM transport rates were well related to flow in consecutive samples but showed pronounced hysteresis over the diurnal fluctuations of flow, between consecutive days, and over the rising and falling limbs of the high-flow season. In order to compute annual CPOM load, hysteresis effects require intensive sampling and establishing separate rating curves for all rising and falling limbs. Hysteresis patterns of CPOM transport relations identified in the well-sampled study streams may aid with estimates of CPOM transport and export in less well-sampled Rocky Mountain streams. Transport relations for CPOM were similar among three high elevation mountain stream with mainly coniferous watersheds. Differences among streams can be qualitatively attributed to differences in CPOM contributions from litter fall, from the presence of large woody debris, its grinding into CPOM sized particles by gravel–cobble bedload transport, hillslope connectivity, drainage density, and biological consumption. CPOM loads were 3.6 and 3.2 t/yr for the two Rocky Mountain streams. Adjusted to reflect decadal averages, values increased to 11.3 and 10.2 t/yr. CPOM yields related to the entire watershed were 2.7 and 4 kg/ha/yr for the years studied, but both streams exported similar amounts of 6.5 and 6.6 kg/ha/yr when taking the forested portion of the watershed into account. To reflect decadal averages, CPOM yields per basin area were adjusted to 8.6 and 12.6 kg/ha/yr and to 21 kg/ha/yr for the forested watershed parts. CPOM yield may be more meaningfully characterized if annual CPOM loads are normalized by the area of a seam along the stream banks together with the stream surface area rather than by the forested or total watershed area. [ABSTRACT FROM AUTHOR]

We construct mass-balance based estimates of carbon (C) export fractions from the water column across the Chukchi Sea shelf. Export is calculated as the difference between phytoplankton drawdown of dissolved inorganic C (DIC) and the accumulation of autochthonous particulate and dissolved organic C in the water column. Organic carbon (C org ) exports of >50% of DIC drawdown are ubiquitous across the shelf, even during, or shortly after, phytoplankton blooms, suggesting widespread and strong pelagic-benthic coupling. Export fractions on the shelf were generally greater in the less-productive Alaska Coastal Water than in the more productive Bering Shelf-Anadyr Water. Additionally, export fractions were greater in 2011 than in 2010, highlighting the significant spatial and inter-annual variability of the fate of C org in this ecologically and biogeochemically important, and rapidly changing, ecosystem. [ABSTRACT FROM AUTHOR]

We developed a one-dimensional model to estimate salp contributions to vertical carbon flux at the Bermuda Atlantic Time-series Study (BATS) site in the North Atlantic subtropical gyre for a 17-yr period (April 1994 to December 2011). We based the model parameters on published rates of salp physiology and experimentally determined sinking and decomposition rates of salp carcasses. Salp grazing was low during non-bloom conditions, but routinely exceeded 100% of chlorophyll standing stock and primary production during blooms. Fecal pellet production was the largest source of salp carbon flux (78% of total), followed by respiration below 200 m (19%), sinking of carcasses (3%), and DOC excretion below 200 m (<0.1%). Thalia democratica , Salpa fusiformis , Salpa aspera , Wheelia cylindrica , and Iasis zonaria were the five highest contributors, accounting for 95% of total salp-mediated carbon flux. Seasonally, salp flux was higher during spring-summer than fall-winter, due to seasonal changes in species composition and abundance. Salp carbon export to 200 m was on average 2.3 mg C m −2 d −1 across the entire time series. This is equivalent to 11% of the mean 200 m POC flux measured by sediment traps in the region. During years with significant salp blooms, however, annually-averaged salp carbon export was the equivalent of up to 60% of trap POC flux at 200 m. Salp carbon flux attenuated slowly, and at 3200 m the average modeled carbon from salps was 109% of the POC flux measured in sediment traps at that depth. Migratory and carcass carbon export pathways should also be considered (alongside fecal pellet flux) as facilitating carbon export to sequestration depths in future studies. [ABSTRACT FROM AUTHOR]

We employ profiling floats with dissolved oxygen sensors to observe in situ temporal oxygen evolution below the mixed layer, allowing us to characterize net respiration of organic carbon in eight distinct regions over the globe. Export and export efficiency are generally high in locations with strong seasonal variability and low in locations of weak seasonality. Vertically integrated respiration is weakly, yet significantly, correlated with remote observations of chlorophyll, net primary production, and planktonic community size structure. These correlations suggest that regimes of high net primary production and large phytoplankton fuel elevated respiration at depth. Several regions of float-based observations intersect with sites of other detailed observations (e.g., Hawaii and Sargasso Sea), which allows us to compare our results to independent studies. We find that there is good agreement among export production estimates at highly seasonal locations, and that float-based observations may be biased low at weakly seasonal locations. We posit that the reason for the low-latitude discrepancy is the relative steady state of oxygen concentration caused by weak seasonality and shallow wintertime mixed layer depths. [ABSTRACT FROM AUTHOR]

Understanding the influence of anthropogenic forcing on the marine biosphere is a high priority. Climate change-driven trends need to be accurately assessed and detected in a timely manner. As part of the effort towards detection of long-term trends, a network of ocean observatories and time series stations provide high quality data for a number of key parameters, such as pH, oxygen concentration or primary production (PP). Here, we use an ensemble of global coupled climate models to assess the temporal and spatial scales over which observations of eight biogeochemically relevant variables must be made to robustly detect a long-term trend. We find that, as a global average, continuous time series are required for between 14 ( pH) and 32 (PP) years to distinguish a climate change trend from natural variability. Regional differences are extensive, with low latitudes and the Arctic generally needing shorter time series (<~30 years) to detect trends than other areas. In addition, we quantify the 'footprint' of existing and planned time series stations, that is the area over which a station is representative of a broader region. Footprints are generally largest for pH and sea surface temperature, but nevertheless the existing network of observatories only represents 9-15% of the global ocean surface. Our results present a quantitative framework for assessing the adequacy of current and future ocean observing networks for detection and monitoring of climate change-driven responses in the marine ecosystem. [ABSTRACT FROM AUTHOR]

Abstract This contribution provides an introduction to a sequence of five papers (CCE I- CCE V) that describe the impact of the Warm Anomaly of 2014–15 and El Niño 2015–16 on the pelagic food web of the southern California Current Ecosystem. These contributions analyze the influence of these two warm water perturbations on satellite-based measures of ocean fronts, export efficiency out of the euphotic zone, copepod egg production, mesozooplankton community structure, and a synthesis of primary production, mesozooplankton grazing, and gravitational fluxes of organic carbon. [ABSTRACT FROM AUTHOR]

Estimates of dissolved organic carbon (DOC) release by marine macrophyte communities (seagrass meadows and macroalgal beds) based on in situ benthic chambers from published and unpublished are compiled in this study. The effect of temperature and light availability on DOC release by macrophyte communities was examined. Almost 85 % of the seagrass communities and all of macroalgal communities examined acted as net sources of DOC. Net DOC fluxes in seagrass communities increase positively with water temperature. In macroalgal communities net DOC fluxes under light exceeded those under dark condition, however, this trend was weaker in seagrass communities. Shading of a mixed seagrass meadow in The Philippines led to a significant reduction on the net DOC release when shading was maintained for 6 days compared to only 2 days of shading. Net DOC fluxes increased with increasing community respiration, but were independent of primary production or net community production. The estimated global net DOC flux, and hence export, from marine macrophytes is about 0.158 ± 0.055 Pg C yr-1 or 0.175 ± 0.056 Pg C yr-1 depending on the global extent of seagrass meadows considered.

Sinking rates of phytoplankton community with variable taxonomic composition in the offshore Changjiang (Yangtze River) estuary were measured during two cruises in spring and summer, 2011. A homogenous-sample method SETCOL was used to determine the sinking rates. Phytoplankton community was dominated by dinoflagellates in spring and diatoms in summer, and two species Prorocentrum dentatum and Skeletonema dorhnii formed algal blooms in the survey area during the two cruises, respectively. Phytoplankton sinking rates ranged from 0.13 to 1.04 m day − 1 (average = 0.61 ± 0.24 m day − 1 ) in spring and 0.28 to 1.71 m day − 1 (average = 0.80 ± 0.34 m day − 1 ) in summer. In the surface layer, phytoplankton sinking rates at the P. dentatum bloom stations in spring were lower than that at the S. dorhnii bloom stations in summer. No significant correlation was found between phytoplankton sinking rates and most of the environmental parameters during the two cruises, except for temperature and nitrite concentration in summer. A significant correlation was observed between phytoplankton sinking rates and phytoplankton community structure in the surface layers: the higher dominance of diatom in the phytoplankton community corresponded to higher phytoplankton sinking rate. Therefore, the phytoplankton community structure other than the environmental parameters, is the important factor to affect the sinking rates greatly. The consequent carbon flux caused by phytoplankton sinking was estimated, and results suggested that the carbon flux to bottom water during the S. dorhnii bloom (average = 63.13 ± 48.16 mg C m − 2 day − 1 ) in summer was about 2.4 fold of that during the P. dentatum bloom (average = 26.10 ± 26.25 mg C m − 2 day − 1 ) in spring. These findings provide us some insight in understanding the carbon export contributed by marine phytoplankton in the coastal sea, where frequent phytoplankton blooms and high following carbon export occur. [ABSTRACT FROM AUTHOR]

The advantages of remote sensing using Unmanned Aerial Vehicles (UAVs) are a high spatial resolution of images, temporal flexibility and narrow-band spectral data from different wavelengths domains. This enables the detection of spatio-temporal dynamics of environmental variables, like plant-related carbon dynamics in agricultural landscapes. In this paper, we quantify spatial patterns of fresh phytomass and related carbon (C) export using imagery captured by a 12-band multispectral camera mounted on the fixed wing UAV Carolo P360. The study was performed in 2014 at the experimental area CarboZALF-D in NE Germany. From radiometrically corrected and calibrated images of lucerne (Medicago sativa), the performance of four commonly used vegetation indices (VIs) was tested using band combinations of six near-infrared bands. The highest correlation between ground-based measurements of fresh phytomass of lucerne and VIs was obtained for the Enhanced Vegetation Index (EVI) using near-infrared band b899. The resulting map was transformed into dry phytomass and finally upscaled to total C export by harvest. The observed spatial variability at field- and plot-scale could be attributed to small-scale soil heterogeneity in part. [ABSTRACT FROM AUTHOR]