Search

Studying phytoplankton functional types (PFTs) from space is possible due to recent advances in remote sensing. Though a variety of products are available, the limited number of wavelengths available compared to the number of model parameters needed to be retrieved is still a major problem in using ocean-color data for PFT retrievals. Here, we investigated which band placement could improve retrievals of three particular PFTs (diatoms, coccolithophores and cyanobacteria). In addition to analyzing dominant spectral features in the absorption spectra of the target PFTs, two previously-developed methods using measured spectra were applied to simulated data. Such a synthetic dataset allowed for significantly increasing the number of scenarios and enabled a full control over parameters causing spectral changes. We evaluated the chosen band placement by applying an adapted ocean reflectance inversion, as utilized in the generalized inherent optical properties (GIOP) retrieval. Results show that the optimal band settings depend on the method applied to determine the bands placement, as well as on the internal variability of the dataset investigated. Therefore, continuous hyperspectral instruments would be most beneficial for discriminating multiple PFTs, though a small improvement in spectral sampling and resolution does not significantly modify the results. Bands, which could be added to future instruments (e.g., Ocean and Land Colour Instrument (OLCI) instrument on the upcoming Sentinel-3B,-3C,-3D, etc., and further satellites) in order to enhance PFT retrieval capabilities, were also determined. [ABSTRACT FROM AUTHOR]

Ascidians (Ascidiacea: Tunicata) are sessile suspension feeders that represent dominant epifaunal components of the Southern Ocean shelf benthos and play a significant role in the pelagic-benthic coupling. Here, we report the results of a first study on the relationship between the distribution patterns of eight common and/or abundant (putative) ascidian species, and environmental drivers in the waters off the northern Antarctic Peninsula. During RV Polarstern cruise XXIX/3 (PS81) in January-March 2013, we used seabed imaging surveys along 28 photographic transects of 2 km length each at water depths from 70 to 770 m in three regions (northwestern Weddell Sea, southern Bransfield Strait and southern Drake Passage), differing in their general environmental setting, primarily oceanographic characteristics and sea-ice dynamics, to comparatively analyze the spatial patterns in the abundance of the selected ascidians, reliably to be identified in the photographs, at three nested spatial scales. At a regional (100-km) scale, the ascidian assemblages of the Weddell Sea differed significantly from those of the other two regions, whereas at an intermediate 10-km scale no such differences were detected among habitat types (bank, upper slope, slope, deep/canyon) on the shelf and at the shelf break within each region. These spatial patterns were superimposed by a marked small-scale (10-m) patchiness of ascidian distribution within the 2-km-long transects. Among the environmental variables considered in our study, a combination of water-mass characteristics, sea-ice dynamics (approximated by 5-year averages in sea-ice cover in the region of or surrounding the photographic stations), as well as the seabed ruggedness, was identified as explaining best the distribution patterns of the ascidians. [ABSTRACT FROM AUTHOR]

The chlorophyll‐specific absorption spectrum of phytoplankton aPH∗(λ) $\left[{a}_{\mathrm{P}\mathrm{H}}^{\ast }(\lambda)\right]$ multiplied with phytoplankton chlorophyll provides the total absorption coefficient of phytoplankton [aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda)$], a fundamental quantity with significance in many marine biogeochemical (BGC) and environmental processes. Representing accurately the sources of variability of aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda)$ in BGC ocean models is a crucial task. The two main sources of variability in aPH∗(λ) ${a}_{\mathrm{P}\mathrm{H}}^{\ast }(\lambda)$ are changes in the pigment composition of the phytoplankton community and the size‐dependent constraints to pigment packaging. Therefore, changes in community structure and physiological state impact aPH∗(λ) ${a}_{\mathrm{P}\mathrm{H}}^{\ast }(\lambda)$ and consequently aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda)$. The objective of this work is to improve estimates of aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda)$ in a BGC model of the global ocean by portraying the variability of aPH∗(λ) ${a}_{\mathrm{P}\mathrm{H}}^{\ast }(\lambda)$ driven by the variable content in photoprotective carotenoids (PPCs) in the phytoplankton community. We used a three‐dimensional spectrally‐resolved BGC model to simulate the inherent and apparent optical properties of the global ocean based on its content on optically active constituents. The aPH∗(λ) ${a}_{\mathrm{P}\mathrm{H}}^{\ast }(\lambda)$ for each phytoplankton type represented in the model were made variable as a function of the type‐specific content in PPCs. By comparing model‐derived aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda)$ to satellite retrievals and an extensive field data set of optical and BGC observations, we concluded that photoprotective pigments content impacted significantly the contribution of the aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda)$ to the total non‐water absorption in the ocean. Pigment‐impacted aPH∗(λ) ${a}_{\mathrm{P}\mathrm{H}}^{\ast }(\lambda)$ contributed to reproduce the global variability of aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda)$ as well as the observed bio‐optical relationship between aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda)$ and chlorophyll. The improved representation of the aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda)$ of the phytoplankton community influenced model simulations in terms of water‐leaving radiances. Plain Language Summary: The multifaceted interactions between light and particles suspended in the ocean translate to complex light paths at larger scale, such as the color of the light that leaves the ocean, can be detected by satellites and is used to map the concentration of phytoplankton. In this work, we have developed an Earth‐system biogeochemical model that represents how different colors of light travel though the ocean and interact with phytoplankton and other suspended matter. As phytoplankton cells contain a variety of algal pigments and different pigments absorb different colors of light, the amount and diversity of pigments found in the phytoplankton community determines how much and which color of light they absorb. In our model, the pigments that phytoplankton accumulate to protect themselves from the harmful effects of light alter the amount of light that the cells absorb in the blue‐green region of the spectrum. Our model was able to better represent how much light phytoplankton absorbed when compared with many observations collected by expeditions and satellites in the global ocean. Our proposed model structure can have high potential implications in the future, from providing insight to the causes of changes in the color of the ocean to refining model estimates of phytoplankton production. Key Points: Ocean biogeochemical model represented the impact of photoprotective carotenoids (PPCs) in the light absorption coefficients of phytoplanktonProposed model characterized the global variability of phytoplankton light absorption when compared to in situ and satellite observationsThe impact of PPCs in phytoplankton light absorption reduced the dependence of reflectance on total chlorophyll [ABSTRACT FROM AUTHOR]

Abstract: Vertical profiles of O3 and NO2 abundances from the atmospheric instruments GOMOS (Global Ozone Monitoring by the Occultation of Stars), MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) and SCIAMACHY (Scanning Imaging Spectrometer for Atmospheric Chartography) all on-board the recently launched European Space Agency (ESA) Environmental Satellite (ENVISAT) are intercompared. These comparisons contribute to the validation of these data products by detecting systematic deviations, for example, wrong tangent height determinations, spectroscopic errors, and others. The cross comparison includes GOMOS data products retrieved by the GOMOS prototype processor from ACRI (Sophia Antipolis, France), the scientific SCIAMACHY data products from the Institute of Environmental Physics at University of Bremen (IUP) and the scientific MIPAS data products from the Institute for Meteorology and Climate Research in Karlsruhe (IMK) and Institute of Astrophysics in Andalusia (IAA). Coincident measurements were identified by limiting the time difference to 100min (duration of one orbit) and less than 500km between two observation points. When lower stratospheric ozone is strongly depleted during polar spring, a homogeneity condition was further imposed on the satellite measurements by requiring an upper limit on the potential vorticity difference at the 475K isentrope between both observations. Since geographically coincident NO2 measurements of the three instruments are performed during different times of the day and NO2 has a rather strong diurnal variability, matches of NO2 profiles were compared only where the solar zenith angle difference was below 5°. First results of the cross comparison show an agreement within 15% between 21 and 40km altitude for O3 profiles and an agreement within 20% between 27 and 40km altitude for NO2 profiles among the GOMOS, MIPAS and SCIAMACHY measurements. [Copyright &y& Elsevier]

A global in situ data set for validation of ocean colour products from the ESA Ocean Colour Climate Change Initiative (OC-CCI) is presented. This version of the compilation, starting in 1997, now extends to 2021, which is important for the validation of the most recent satellite optical sensors such as Sentinel 3B OLCI and NOAA-20 VIIRS. The data set comprises in situ observations of the following variables: spectral remote-sensing reflectance, concentration of chlorophyll- a , spectral inherent optical properties, spectral diffuse attenuation coefficient, and total suspended matter. Data were obtained from multi-project archives acquired via open internet services or from individual projects acquired directly from data providers. Methodologies were implemented for homogenization, quality control, and merging of all data. Minimal changes were made on the original data, other than conversion to a standard format, elimination of some points, after quality control and averaging of observations that were close in time and space. The result is a merged table available in text format. Overall, the size of the data set grew with 148 432 rows, with each row representing a unique station in space and time (cf. 136 250 rows in previous version; Valente et al., 2019). Observations of remote-sensing reflectance increased to 68 641 (cf. 59 781 in previous version; Valente et al., 2019). There was also a near tenfold increase in chlorophyll data since 2016. Metadata of each in situ measurement (original source, cruise or experiment, principal investigator) are included in the final table. By making the metadata available, provenance is better documented and it is also possible to analyse each set of data separately. The compiled data are available at 10.1594/PANGAEA.941318 (Valente et al., 2022). [ABSTRACT FROM AUTHOR]

A global compilation of in situ data is useful to evaluate the quality of ocean-colour satellite data records. Here we describe the data compiled for the validation of the ocean-colour products from the ESA Ocean Colour Climate Change Initiative (OC-CCI). The data were acquired from several sources (including, inter alia, MOBY, BOUSSOLE, AERONET-OC, SeaBASS, NOMAD, MERMAID, AMT, ICES, HOT and GeP&CO) and span the period from 1997 to 2018. Observations of the following variables were compiled: spectral remote-sensing reflectances, concentrations of chlorophyll a, spectral inherent optical properties, spectral diffuse attenuation coefficients and total suspended matter. The data were from multi-project archives acquired via open internet services or from individual projects, acquired directly from data providers. Methodologies were implemented for homogenization, quality control and merging of all data. No changes were made to the original data, other than averaging of observations that were close in time and space, elimination of some points after quality control and conversion to a standard format. The final result is a merged table designed for validation of satellite-derived ocean-colour products and available in text format. Metadata of each in situ measurement (original source, cruise or experiment, principal investigator) was propagated throughout the work and made available in the final table. By making the metadata available, provenance is better documented, and it is also possible to analyse each set of data separately. This paper also describes the changes that were made to the compilation in relation to the previous version (Valente et al., 2016). [ABSTRACT FROM AUTHOR]

Summary: In the marine realm, microorganisms are responsible for the bulk of primary production, thereby sustaining marine life across all trophic levels. Longhurst provinces have distinct microbial fingerprints; however, little is known about how microbial diversity and primary productivity change at finer spatial scales. Here, we sampled the Atlantic Ocean from south to north (~50°S–50°N), every ~0.5° latitude. We conducted measurements of primary productivity, chlorophyll‐a and relative abundance of 16S and 18S rRNA genes, alongside analyses of the physicochemical and hydrographic environment. We analysed the diversity of autotrophs, mixotrophs and heterotrophs, and noted distinct patterns among these guilds across provinces with high and low chlorophyll‐a conditions. Eukaryotic autotrophs and prokaryotic heterotrophs showed a shared inter‐province diversity pattern, distinct from the diversity pattern shared by mixotrophs, cyanobacteria and eukaryotic heterotrophs. Additionally, we calculated samplewise productivity‐specific length scales, the potential horizontal displacement of microbial communities by surface currents to an intrinsic biological rate (here, specific primary productivity). This scale provides key context for our trophically disaggregated diversity analysis that we could relate to underlying oceanographic features. We integrate this element to provide more nuanced insights into the mosaic‐like nature of microbial provincialism, linking diversity patterns to oceanographic transport through primary production. [ABSTRACT FROM AUTHOR]

Abstract: The low surface nitrate concentration and high atmospheric iron input in the tropical eastern North Atlantic provide beneficial conditions for N2 fixation. Varying abundances of diazotrophs have been observed and an Fe- and P-colimitation of N2 fixation was reported in this ocean region. It is however unclear, how different limiting factors control the temporal variability of N2 fixation and what the role of Fe-limitation is in a region with high fluxes of dust deposition. To study the environmental controls on N2 fixation, an one-dimensional ecosystem model is coupled with a physical model for the Tropical Eastern North Atlantic Times-series Station (TENATSO), north of the Cape Verde Islands. The model describes diazotrophy according to the physiology of Trichodesmium, taking into account a growth dependence on light, temperature, iron, dissolved inorganic (DIP) and organic phosphorus (DOP). The modelled total Chl a is compared with satellite-derived total Chl a and modelled Trichodesmium (Tri) compared with satellite-derived cyanobacterial Chl a as well as with High Performance Liquid Chromatography data. Model results show a complex pattern of competitive as well as mutually beneficial interactions between diazotrophs and non-diazotrophic phytoplankton. High DOP availability after spring blooms of non-diazotrophic phytoplankton and the ability of Trichodesmium to take up DOP are crucial for allowing a maximal abundance of Tri in autumn. Part of the reactive nitrogen newly fixed by diazotrophs is directly excreted or released through mortality, significantly fuelling the growth of non-diazotrophic phytoplankton in autumn and winter. Fe consumption by non-diazotrophic phytoplankton earlier in the year makes Fe limitation of Tri in late summer more acute, whereas Tri growth in surface waters reduces phytoplankton abundance deeper in the water column by light limitation. Overall, the atmospheric iron input at the TENATSO site is required to enable diazotrophic growth and to support the observed abundance of non-diazotrophic phytoplankton. [Copyright &y& Elsevier]

Investigates the mesoscale distribution of primary production at the Antarctic Polar Front. Measurements of global solar radiation from the survey; Variations of the percent light depths; Variation of atmospheric carbon dioxide concentration between glacial and interglacial periods.

Optically active water constituents attenuate solar radiation and hence affect the vertical distribution of energy in the upper ocean. To understand their implications, we operate an ocean biogeochemical model coupled to a general circulation model with sea ice. Incorporating the effect of phytoplankton and colored dissolved organic matter (CDOM) on light attenuation in the model increases the sea surface temperature in summer and decreases sea ice concentration in the Arctic Ocean. Locally, the sea ice season is reduced by up to one month. CDOM drives a significant part of these changes, suggesting that an increase of this material will amplify the observed Arctic surface warming through its direct thermal effect. Indirectly, changing advective processes in the Nordic Seas may further intensify this effect. Our results emphasize the phytoplankton and CDOM feedbacks on the Arctic ocean and sea ice system and underline the need to consider these effects in future modeling studies to enhance their plausibility. Plain Language Summary: The amount of microalgae and colored dissolved organic material in the ocean determines how much light is absorbed in the surface waters and how much can reach greater depths. The vertical distribution of energy affects the upper ocean temperature and general circulation. Here, we use a numerical ocean model with biogeochemistry and sea ice, in which the individual effects of microalgae and colored dissolved organic matter can be turned on and off separately. When both effects are turned on, the summertime surface temperatures in the Arctic are larger and consequently more sea ice melts, so that the sea ice season is shorter by up to one month. We find that, to a large extent, the colored dissolved material is responsible for these changes. An increase of this material due to climate change will amplify the observed Arctic surface warming. For better projections of climate change, new models should account for the effect of these light‐absorbing water constituents. Key Points: Colored dissolved material is responsible for a significant part of the induced surface warming and sea ice loss in the Arctic OceanThe combined effect of optical constituents reduces the sea ice season by up to one monthConsidering the properties of optical constituents and their variability will enhance the plausibility of future modeling studies [ABSTRACT FROM AUTHOR]

Multiple environmental factors control benthic community patterns, and their relative importance varies with spatial scale. Since this variation is difficult to evaluate quantitatively, extensive sampling across a broad range of spatial scales is required. Here, we present a first case study on Southern Ocean shelf benthos, in which mega‐epibenthic communities and biota‐environment relationships have been explored at multiple spatial scales. The analyses encompassed 20 seafloor, water‐column, and sea‐ice parameters, as well as abundances of 18 mega‐epibenthic taxa in a total of 2799 high‐resolution seabed images taken at 28 stations at 32–786 m depth off the northern Antarctic Peninsula. Based on a priori nesting of sampling stations into ecoregions, subregions, and habitats, analyses indicated most pronounced patchiness levels at finest (within transects among adjacent seabed photos) and largest (among ecoregions) spatial scale considered. Using an alternative approach, explicitly involving the spatial distances between the geo‐referenced data, Moran's Eigenvector mapping (MEM) classified the continuum of spatial scales into four categories: broad (> 60 km), meso (10–60 km), small (2–10 km), and fine (< 2 km). MEM analyses generally indicated an increase in mega‐epibenthic community complexity with increasing spatial scale. Moreover, strong relationships between biota and environmental drivers were found at scales of > 2 km. In contrast, few environmental variables contributed to explaining biotic structures at finer scales. These are likely rather determined by nonmeasured environmental variables, as well as biological traits and interactions that are assumed to be most effective at small spatial scales. [ABSTRACT FROM AUTHOR]

A global compilation of in situ data is useful to evaluate the quality of ocean-colour satellite data records. Here we describe the data compiled for the validation of the ocean-colour products from the ESA Ocean Colour Climate Change Initiative (OC-CCI). The data were acquired from several sources (including, inter alia, MOBY, BOUSSOLE, AERONET-OC, SeaBASS, NOMAD, MERMAID, AMT, ICES, HOT and GeP&CO) and span the period from 1997 to 2018. Observations of the following variables were compiled: spectral remote-sensing reflectances, concentrations of chlorophyll a , spectral inherent optical properties, spectral diffuse attenuation coefficients and total suspended matter. The data were from multi-project archives acquired via open internet services or from individual projects, acquired directly from data providers. Methodologies were implemented for homogenization, quality control and merging of all data. No changes were made to the original data, other than averaging of observations that were close in time and space, elimination of some points after quality control and conversion to a standard format. The final result is a merged table designed for validation of satellite-derived ocean-colour products and available in text format. Metadata of each in situ measurement (original source, cruise or experiment, principal investigator) was propagated throughout the work and made available in the final table. By making the metadata available, provenance is better documented, and it is also possible to analyse each set of data separately. This paper also describes the changes that were made to the compilation in relation to the previous version (Valente et al., 2016). The compiled data are available at 10.1594/PANGAEA.898188 (Valente et al., 2019). [ABSTRACT FROM AUTHOR]

A global compilation of in situ data is useful to evaluate the quality of ocean-colour satellite data records. Here we describe the data compiled for the validation of the ocean-colour products from the ESA Ocean Colour Climate Change Initiative (OC-CCI). The data were acquired from several sources (including, inter alia, MOBY, BOUSSOLE, AERONET-OC, SeaBASS, NOMAD, MERMAID, AMT, ICES, HOT, GeP&CO) and span the period from 1997 to 2018. Observations of the following variables were compiled: spectral remote-sensing reflectances, concentrations of chlorophyll-a, spectral inherent optical properties, spectral diffuse attenuation coefficients and total suspended matter. The data were from multi-project archives acquired via open internet services or from individual projects, acquired directly from data providers. Methodologies were implemented for homogenisation, quality control and merging of all data. No changes were made to the original data, other than averaging of observations that were close in time and space, elimination of some points after quality control and conversion to a standard format. The final result is a merged table designed for validation of satellite-derived ocean-colour products and available in text format. Metadata of each in situ measurement (original source, cruise or experiment, principal investigator) were propagated throughout the work and made available in the final table. By making the metadata available, provenance is better documented, and it is also possible to analyse each set of data separately. This paper also describes the changes that were made to the compilation in relation to the previous version (Valente et al., 2016). [ABSTRACT FROM AUTHOR]

[Display omitted] The ocean plays a central role in modulating the Earth's carbon cycle. Monitoring how the ocean carbon cycle is changing is fundamental to managing climate change. Satellite remote sensing is currently our best tool for viewing the ocean surface globally and systematically, at high spatial and temporal resolutions, and the past few decades have seen an exponential growth in studies utilising satellite data for ocean carbon research. Satellite-based observations must be combined with in-situ observations and models, to obtain a comprehensive view of ocean carbon pools and fluxes. To help prioritise future research in this area, a workshop was organised that assembled leading experts working on the topic, from around the world, including remote-sensing scientists, field scientists and modellers, with the goal to articulate a collective view of the current status of ocean carbon research, identify gaps in knowledge, and formulate a scientific roadmap for the next decade, with an emphasis on evaluating where satellite remote sensing may contribute. A total of 449 scientists and stakeholders participated (with balanced gender representation), from North and South America, Europe, Asia, Africa, and Oceania. Sessions targeted both inorganic and organic pools of carbon in the ocean, in both dissolved and particulate form, as well as major fluxes of carbon between reservoirs (e.g., primary production) and at interfaces (e.g., air-sea and land–ocean). Extreme events, blue carbon and carbon budgeting were also key topics discussed. Emerging priorities identified include: expanding the networks and quality of in-situ observations; improved satellite retrievals; improved uncertainty quantification; improved understanding of vertical distributions; integration with models; improved techniques to bridge spatial and temporal scales of the different data sources; and improved fundamental understanding of the ocean carbon cycle, and of the interactions among pools of carbon and light. We also report on priorities for the specific pools and fluxes studied, and highlight issues and concerns that arose during discussions, such as the need to consider the environmental impact of satellites or space activities; the role satellites can play in monitoring ocean carbon dioxide removal approaches; economic valuation of the satellite based information; to consider how satellites can contribute to monitoring cycles of other important climatically-relevant compounds and elements; to promote diversity and inclusivity in ocean carbon research; to bring together communities working on different aspects of planetary carbon; maximising use of international bodies; to follow an open science approach; to explore new and innovative ways to remotely monitor ocean carbon; and to harness quantum computing. Overall, this paper provides a comprehensive scientific roadmap for the next decade on how satellite remote sensing could help monitor the ocean carbon cycle, and its links to the other domains, such as terrestrial and atmosphere. [ABSTRACT FROM AUTHOR]

Abstract: Iron (Fe), cobalt (Co), and vitamin B12 addition experiments were performed in the eastern Equatorial Pacific/Peruvian upwelling zone during the 2015 El Niño event. Near the Peruvian coastline, apparent photosystem II photochemical efficiencies (Fv/Fm) were unchanged by nutrient addition and chlorophyll a tripled in untreated controls over 2 days, indicating nutrient replete conditions. Conversely, Fe amendment further away from the coastline in the high nitrate, low Fe zone significantly increased Fv/Fm and chlorophyll a concentrations. Mean chlorophyll a was further enhanced following supply of Fe + Co and Fe + B12 relative to Fe alone, but this was not statistically significant; further offshore, reported Co depletion relative to Fe could enhance responses. The persistence of Fe limitation in this system under a developing El Niño, as previously demonstrated under non‐El Niño conditions, suggests that diminished upwelled Fe is likely an important factor driving reductions in offshore phytoplankton productivity during these events. [ABSTRACT FROM AUTHOR]

As consequences of global warming sea-ice shrinking, permafrost thawing and changes in fresh water and terrestrial material export have already been reported in the Arctic environment. These processes impact light penetration and primary production. To reach a better understanding of the current status and to provide accurate forecasts Arctic biogeochemical and physical parameters need to be extensively monitored. In this sense, bio-optical properties are useful to be measured due to the applicability of optical instrumentation to autonomous platforms, including satellites. This study characterizes the non-water absorbers and their coupling to hydrographic conditions in the poorly sampled surface waters of the central and eastern Arctic Ocean. Over the entire sampled area colored dissolved organic matter (CDOM) dominates the light absorption in surface waters. The distribution of CDOM, phytoplankton and non-algal particles absorption reproduces the hydrographic variability in this region of the Arctic Ocean which suggests a subdivision into five major bio-optical provinces: Laptev Sea Shelf, Laptev Sea, Central Arctic/Transpolar Drift, Beaufort Gyre and Eurasian/Nansen Basin. Evaluating ocean color algorithms commonly applied in the Arctic Ocean shows that global and regionally tuned empirical algorithms provide poor chlorophyll-a (Chl-a) estimates. The semi-analytical algorithms Generalized Inherent Optical Property model (GIOP) and Garver-Siegel-Maritorena (GSM), on the other hand, provide robust estimates of Chl-a and absorption of colored matter. Applying GSM with modifications proposed for the western Arctic Ocean produced reliable information on the absorption by colored matter, and specifically by CDOM. These findings highlight that only semi-analytical ocean color algorithms are able to identify with low uncertainty the distribution of the different optical water constituents in these high CDOM absorbing waters. In addition, a clustering of the Arctic Ocean into bio-optical provinces will help to develop and then select province-specific ocean color algorithms. [ABSTRACT FROM AUTHOR]

234 Th-derived carbon export fluxes were measured in the Atlantic Ocean under the GEOTRACES framework to evaluate basin-scale export variability. Here, we present the results from the northern half of the GA02 transect, spanning from the equator to 64°N. As a result of limited site-specific C/ 234 Th ratio measurements, we further combined our data with previous work to develop a basin wide C/ 234 Th ratio depth curve. While the magnitude of organic carbon fluxes varied depending on the C/ 234 Th ratio used, latitudinal trends were similar, with sizeable and variable organic carbon export fluxes occurring at high latitudes and low to negligible fluxes occurring in oligotrophic waters. Our results agree with previous studies, except at the boundaries between domains, where fluxes were relatively enhanced. Three different models were used to obtain satellite-derived net primary production (NPP). In general, NPP estimates had similar trends along the transect, but there were significant differences in the absolute magnitude depending on the model used. Nevertheless, organic carbon export efficiencies were generally < 25%, with the exception of a few stations located in the transition area between the riverine and the oligotrophic domains and between the oligotrophic and the temperate domains. Satellite-derived organic carbon export models from Dunne et al. (2005) (D05), Laws et al. (2011) (L11) and Henson et al. (2011) (H11) were also compared to our 234 Th-derived carbon exports fluxes. D05 and L11 provided estimates closest to values obtained with the 234 Th approach (within a 3-fold difference), but with no clear trends. The H11 model, on the other hand, consistently provided lower export estimates. The large increase in export data in the Atlantic Ocean derived from the GEOTRACES Program, combined with satellite observations and modeling efforts continue to improve the estimates of carbon export in this ocean basin and therefore reduce uncertainty in the global carbon budget. However, our results also suggest that tuning export models and including biological parameters at a regional scale is necessary for improving satellite-modeling efforts and providing export estimates that are more representative of in situ observations. [ABSTRACT FROM AUTHOR]

Phytoplankton play a central role in the planetary cycling of important elements and compounds. Understanding how phytoplankton are responding to climate change is consequently a major question in Earth Sciences. Monitoring phytoplankton is key to answering this question. Satellite remote sensing of ocean colour is our only means of monitoring phytoplankton in the entire surface ocean at high temporal and large spatial scales, and the continuous ocean-colour data record is now approaching a length suitable for addressing questions around climate change, at least in some regions. Yet, developing ocean-colour algorithms for climate change studies requires addressing issues of ambiguity in the ocean-colour signal. For example, for the same chlorophyll-a concentration (Chl-a) of phytoplankton, the colour of the ocean can be different depending on the type of phytoplankton present. One route to tackle the issue of ambiguity is by enriching the ocean-colour data with information on sea surface temperature (SST), a good proxy of changes in three phytoplankton size classes (PSCs) independent of changes in total Chl-a, a measure of phytoplankton biomass. Using a global surface in-situ dataset of HPLC (high performance liquid chromatography) pigments, size-fractionated filtration data, and concurrent satellite SST spanning from 1991 to 2021, we re-tuned, validated and advanced an SST-dependent three-component model that quantifies the relationship between total Chl-a and Chl-a associated with the three PSCs (pico-, nano- and microplankton). Similar to previous studies, striking dependencies between model parameters and SST were captured, which were found to improve model performance significantly. These relationships were applied to 40 years of monthly composites of satellite SST, and significant trends in model parameters were observed globally, in response to climate warming. Changes in these parameters highlight issues in estimating long-term trends in phytoplankton biomass (Chl-a) from ocean colour using standard empirical algorithms, which implicitly assume a fixed relationship between total Chl-a and Chl-a of the three size classes. The proposed ecological model will be at the centre of a new ocean-colour modelling framework, designed for investigating the response of phytoplankton to climate change, described in subsequent parts of this series of papers. [Display omitted] • Compile the largest global in-situ dataset of total and size-fractionated Chl-a. • Advance SST-dependent model of phytoplankton size structure for global application. • Model parameters have changed over the last 40 years globally. • Concerns raised about satellite-derived Chl-a trends using standard algorithms. [ABSTRACT FROM AUTHOR]

Carbon fixation by phytoplankton plays a key role in the uptake of atmospheric CO 2 in the Southern Ocean. Yet, it still remains unclear how efficiently the particulate organic carbon (POC) is exported and transferred from ocean surface waters to depth during phytoplankton blooms. In addition, little is known about the processes that control the flux attenuation within the upper twilight zone. Here, we present results of downward POC and particulate organic nitrogen fluxes during the decline of a vast diatom bloom in the Atlantic sector of the Southern Ocean in summer 2012. We used thorium-234 ( 234 Th) as a particle tracer in combination with drifting sediment traps (ST). Their simultaneous use evidenced a sustained high export rate of 234 Th at 100 m depth in the weeks prior to and during the sampling period. The entire study area, of approximately 8000 km 2 , showed similar vertical export fluxes in spite of the heterogeneity in phytoplankton standing stocks and productivity, indicating a decoupling between production and export. The POC fluxes at 100 m were high, averaging 26±15 mmol C m −2 d −1 , although the strength of the biological pump was generally low. Only <20% of the daily primary production reached 100 m, presumably due to an active recycling of carbon and nutrients. Pigment analyses indicated that direct sinking of diatoms likely caused the high POC transfer efficiencies (~60%) observed between 100 and 300 m, although faecal pellets and transport of POC linked to zooplankton vertical migration might have also contributed to downward fluxes. [ABSTRACT FROM AUTHOR]

The vertical distribution of 234 Th was measured along the 10°E meridian between 44°S and 53°S in the Antarctic Circumpolar Current (ACC) during the austral summer of 2012. The overarching goal of this work was to estimate particulate organic carbon (POC) export across three fronts: the Sub-Antarctic Front (SAF), the Antarctic Polar Front (APF) and the Southern Polar Front (SPF). Steady state export fluxes of 234 Th in the upper 100 m ranged from 1600 to 2600 dpm m −2 d −1 , decreasing with increasing latitude. Using large particle (>53 μm) C/ 234 Th ratios, the 234 Th-derived POC fluxes at 100 m ranged from 25 to 41 mmol C m −2 d −1 . Observed C/ 234 Th ratios decreased with increasing depth north of the APF while south of the APF, ratios remained similar or even increased with depth. These changes in C/ 234 Th ratios are likely due to differences in the food web. Indeed, satellite images, together with macronutrients and dissolved iron concentrations suggest two different planktonic community structures north and south of the APF. Our results indicate that higher ratios of POC flux at 100 m to primary production occurred in nanophytoplankton dominated surface waters, where primary production rates were lower. Satellite images prior to the expedition suggest that the higher export efficiencies obtained in the northern half of the transect may be the result of the decoupling between production and export (Buesseler 1998). Transfer efficiencies to 400 m, i.e . the fraction of exported POC that reached 400 m, were found to be higher south of the APF, where diatoms were dominant and salps largely abundant. This suggests different remineralization pathways of sinking particles, influencing the transfer efficiency of exported POC to depth. [ABSTRACT FROM AUTHOR]

The physical and biological carbon pumps in the different hydrographic and biogeochemical regimes of the Atlantic Sector of the Southern Ocean are controlled by a series of coupled physical, chemical and biological processes and a project named Eddy-Pump was designed to study them. The Eddy Pump field campaign was carried out during RV Polarstern Cruise ANT-XXVIII/3 between January and March 2012. Particular emphasis was laid on the differences which occur along the axis of the Antarctic Circumpolar Current (ACC) with its associated mesoscale eddy field. The study sites were selected in order to represent (1) the central ACC with its regular separation in different frontal jets, investigated by a meridional transect along 10°E; (2) a large-scale bloom west of the Mid-Atlantic Ridge which lasted several months with conspicuous chlorophyll-poor waters to its immediate east studied by a three-dimensional mesoscale survey centred at 12°40′W; and (3) the Georgia Basin north of the island of South Georgia, which regularly features an extended and dense phytoplankton bloom, was investigated by a mesoscale survey centred at 38°12′W. While Eddy-Pump represents an interdisciplinary project by design, we here focus on describing the variable physical environment within which the different biogeochemical regimes developed. For describing the physical environment we use measurements of temperature, salinity and density, of mixed-layer turbulence parameters, of dynamic heights and horizontal current vectors, and of flow trajectories obtained from surface drifters and submerged floats. This serves as background information for the analyses of biological and chemical processes and of biogeochemical fluxes addressed by other papers in this issue. The section along 10°E between 44°S and 53°S showed a classical ACC structure with well-known hydrographic fronts, the Subantarctic Front (SAF) at 46.5°S, the Antarctic Polar Front (APF) split in two, at 49.25°S and 50.5°S, and the Southern Polar Front (SPF) at 52.5°S. Each front was associated with strong eastward flows. The West Mid-Atlantic Ridge Survey showed a weak and poorly resolved meander structure between the APF and the SPF. During the first eight days of the survey the oceanographic conditions at the Central Station at 12°40′W remained reasonably constant. However after that, conditions became more variable in the thermocline with conspicuous temperature inversions and interleavings and also a decrease in temperature in the surface layer. At the very end of the period of observation the conditions in the thermocline returned to being similar to those observed during the early part of the period with however the mixed layer temperature raised. The period of enhanced thermohaline variability was accompanied by increased currents. The Georgia Basin Survey showed a very strong zonal jet at its northern edge which connects to a large cyclonic meander that itself joins an anticyclonic eddy in the southeastern quadrant. The water mass contrasts in this survey were stronger than in the West Mid-Atlantic Ridge Survey, but similar to those met along 10°E with the exception that the warm and saline surface water typical of the northern side of the SAF was not covered by the Georgia Basin Survey. Mixed layers found during Eddy-Pump were typically deep, but varied between the three survey areas; the mean depths and standard variations of the mixed layer along the 10°E were 77.2±24.7 m, at the West Mid-Atlantic Ridge 66.7±17.7 m, and in the Georgia Basin 36.8±10.7 m. [ABSTRACT FROM AUTHOR]

Phytoplankton community structure and their physiological response in the vicinity of the Antarctic Polar Front (APF; 44°S to 53°S, centred at 10°E) were investigated as part of the ANT-XXVIII/3 Eddy-Pump cruise conducted in austral summer 2012. Our results show that under iron-limited ( < 0.3 μ mol m − 3 ) conditions, high total chlorophyll-a (TChl-a) concentrations ( > 0.6 mg m − 3 ) can be observed at stations with deep mixed layer ( > 60 m ) across the APF. In contrast, light was excessive at stations with shallower mixed layer and phytoplankton were producing higher amounts of photoprotective pigments, diadinoxanthin (DD) and diatoxanthin (DT), at the expense of TChl-a, resulting in higher ratios of (DD+DT)/TChl-a. North of the APF, significantly lower silicic acid (Si(OH) 4 ) concentrations ( < 2 mmol m − 3 ) lead to the domination of nanophytoplankton consisting mostly of haptophytes, which produced higher ratios of (DD+DT)/TChl-a under relatively low irradiance conditions. The Si(OH) 4 replete ( > 5 mmol m − 3 ) region south of the APF, on the contrary, was dominated by microphytoplankton (diatoms and dinoflagellates) with lower ratios of (DD+DT)/TChl-a, despite having been exposed to higher levels of irradiance. The significant correlation between nanophytoplankton and (DD+DT)/TChl-a indicates that differences in taxon-specific response to light are also influencing TChl-a concentration in the APF during summer. Our results reveal that provided mixing is deep and Si(OH) 4 is replete, TChl-a concentrations higher than 0.6 mg m − 3 are achievable in the iron-limited APF waters during summer. [ABSTRACT FROM AUTHOR]

Ocean color remote sensing of chlorophyll concentration has revolutionized our understanding of the biology of the oceans. However, a comprehensive understanding of the structure and function of oceanic ecosystems requires the characterization of the spatio-temporal variability of various phytoplankton functional types (PFTs), which have differing biogeochemical roles. Thus, recent bio-optical algorithm developments have focused on retrieval of various PFTs. It is important to validate and inter-compare the existing PFT algorithms; however direct comparison of retrieved variables is non-trivial because in those algorithms PFTs are defined differently. Thus, it is more plausible and potentially more informative to focus on emergent properties of PFTs, such as phenology. Furthermore, ocean color satellite PFT data sets can play a pivotal role in informing and/or validating the biogeochemical routines of Earth System Models. Here, the phenological characteristics of 10 PFT satellite algorithms and 7 latest-generation climate models from the Coupled Model Inter-comparison Project (CMIP5) are inter-compared as part of the International Satellite PFT Algorithm Inter-comparison Project. The comparison is based on monthly satellite data (mostly SeaWiFS) for the 2003–2007 period. The phenological analysis is based on the fraction of microplankton or a similar variable for the satellite algorithms and on the carbon biomass due to diatoms for the climate models. The seasonal cycle is estimated on a per-pixel basis as a sum of sinusoidal harmonics, derived from the Discrete Fourier Transform of the variable time series. Peak analysis is then applied to the estimated seasonal signal and the following phenological parameters are quantified for each satellite algorithm and climate model: seasonal amplitude, percent seasonal variance, month of maximum, and bloom duration. Secondary/double blooms occur in many areas and are also quantified. The algorithms and the models are quantitatively compared based on these emergent phenological parameters. Results indicate that while algorithms agree to a first order on a global scale, large differences among them exist; differences are analyzed in detail for two Longhurst regions in the North Atlantic: North Atlantic Drift Region (NADR) and North Atlantic Subtropical Gyre West (NASW). Seasonal cycles explain the most variance in zonal bands in the seasonally-stratified subtropics at about 30° latitude in the satellite PFT data. The CMIP5 models do not reproduce this pattern, exhibiting higher seasonality in mid and high-latitudes and generally much more spatially homogeneous patterns in phenological indices compared to satellite data. Satellite data indicate a complex structure of double blooms in the Equatorial region and mid-latitudes, and single blooms on the poleward edges of the subtropical gyres. In contrast, the CMIP5 models show single annual blooms over most of the ocean except for the Equatorial band and Arabian Sea. [ABSTRACT FROM AUTHOR]

The detection and prediction of changes in coastal ecosystems require a better understanding of the complex physical, chemical and biological interactions, which involves that observations should be performed continuously. For this reason, there is an increasing demand for small, simple and cost-effective in situ sensors to analyze complex coastal waters at a broad range of scales. In this context, this study seeks to explore the potential of beam attenuation spectra, c(λ), measured in situ with an advanced-technology optical transmissometer, for assessing temporal and spatial patterns in the complex estuarine waters of Alfacs Bay (NW Mediterranean) as a test site. In particular, the information contained in the spectral beam attenuation coefficient was assessed and linked with different biogeochemical variables. The attenuation at λ = 710 nm was used as a proxy for particle concentration, TSM, whereas a novel parameter was adopted as an optical indicator for chlorophyll a (Chl-a) concentration, based on the local maximum of c(λ) observed at the long-wavelength side of the red band Chl-a absorption peak. In addition, since coloured dissolved organic matter (CDOM) has an important influence on the beam attenuation spectral shape and complementary measurements of particle size distribution were available, the beam attenuation spectral slope was used to analyze the CDOM content. Results were successfully compared with optical and biogeochemical variables from laboratory analysis of collocated water samples, and statistically significant correlations were found between the attenuation proxies and the biogeochemical variables TSM, Chl-a and CDOM. This outcome depicted the potential of high-frequency beam attenuation measurements as a simple, continuous and cost-effective approach for rapid detection of changes and patterns in biogeochemical properties in complex coastal environments. [ABSTRACT FROM AUTHOR]

Time-series studies of arctic marine ecosystems are rare. This is not surprising since polar regions are largely only accessible by means of expensive modern infrastructure and instrumentation. In 1999, the Alfred Wegener Institute, Helmholtz-Centre for Polar and Marine Research (AWI) established the LTER (Long-Term Ecological Research) observatory HAUSGARTEN crossing the Fram Strait at about 79° N. Multidisciplinary investigations covering all parts of the open-ocean ecosystem are carried out at a total of 21 permanent sampling sites in water depths ranging between 250 and 5500 m. From the outset, repeated sampling in the water column and at the deep seafloor during regular expeditions in summer months was complemented by continuous year-round sampling and sensing using autonomous instruments in anchored devices (i.e., moorings and free-falling systems). The central HAUSGARTEN station at 2500 m water depth in the eastern Fram Strait serves as an experimental area for unique biological in situ experiments at the seafloor, simulating various scenarios in changing environmental settings. Long-term ecological research at the HAUSGARTEN observatory revealed a number of interesting temporal trends in numerous biological variables from the pelagic system to the deep seafloor. Contrary to common intuition, the entire ecosystem responded exceptionally fast to environmental changes in the upper water column. Major variations were associated with a Warm-Water-Anomaly evident in surface waters in eastern parts of the Fram Strait between 2005 and 2008. However, even after 15 years of intense time-series work at HAUSGARTEN, we cannot yet predict with complete certainty whether these trends indicate lasting alterations due to anthropologically-induced global environmental changes of the system, or whether they reflect natural variability on multiyear time-scales, for example, in relation to decadal oscillatory atmospheric processes. [ABSTRACT FROM AUTHOR]

A dedicated aerial cetacean survey was conducted concurrently to a standardised net trawl survey for krill in order to investigate distribution patterns of large whales and different krill species and to investigate relationships of these. Distance sampling data were used to produce density surface models for humpback ( Megaptera novaeangliae) and fin whales ( Balaenoptera physalus) around the West Antarctic Peninsula (WAP). Abundance for both species was estimated over two strata in the Bransfield Strait and Drake Passage. Distinct distribution patterns suggest horizontal niche partitioning of the two whale species around the WAP, with fin whales aggregating at the shelf edge of the South Shetland Islands in the Drake Passage and humpback whales in the Bransfield Strait. Krill biomass estimated from the concurrent krill survey was used along with CTD data from the same expedition, bathymetric parameters and satellite data on chlorophyll- a and ice concentration to model krill distribution. Comparisons of the predicted distributions of both whale species with the predicted distributions of Euphausia superba, Euphausia crystallorophias and Thysanoessa macrura suggest a complex relationship rather than a straightforward correlation between krill and whales. However, results indicate that fin whales were feeding in an area dominated by T. macrura, while humpback whales were found in areas of higher E. superba biomass. Our results provide abundance estimates for humpback whales and, for the first time, fin whales in the WAP and contribute important information on feeding ecology and habitat use of these two species in the Southern Ocean. [ABSTRACT FROM AUTHOR]

Photophysiological processes as well as uptake characteristics of iron and inorganic carbon were studied in inshore phytoplankton assemblages of the Western Antarctic Peninsula (WAP) and offshore assemblages of the Drake Passage. Chlorophyll a concentrations and primary productivity decreased from in- to offshore waters. The inverse relationship between low maximum quantum yields of photochemistry in PSII ( F v / F m ) and large sizes of functional absorption cross sections ( σ PSII ) in offshore communities indicated iron-limitation. Congruently, the negative correlation between F v / F m values and iron uptake rates across our sampling locations suggest an overall better iron uptake capacity in iron-limited pelagic phytoplankton communities. Highest iron uptake capacities could be related to relative abundances of the haptophyte Phaeocystis antarctica . As chlorophyll a -specific concentrations of humic-like substances were similarly high in offshore and inshore stations, we suggest humic-like substances may play an important role in iron chemistry in both coastal and pelagic phytoplankton assemblages. Regarding inorganic carbon uptake kinetics, the measured maximum short-term uptake rates ( V max(CO2) ) and apparent half-saturation constants ( K 1/2(CO2) ) did not differ between offshore and inshore phytoplankton. Moreover, V max(CO2) and K 1/2(CO2) did not exhibit any CO 2 -dependent trend over the natural p CO 2 range from 237 to 507 µatm. K 1/2(CO2) strongly varied among the sampled phytoplankton communities, ranging between 3.5 and 35.3 µmol L −1 CO 2 . While in many of the sampled phytoplankton communities, the operation of carbon-concentrating mechanisms (CCMs) was indicated by low K 1/2(CO2) values relative to ambient CO 2 concentrations, some coastal sites exhibited higher values, suggesting down-regulated CCMs. Overall, our results demonstrate a complex interplay between photophysiological processes, iron and carbon uptake of phytoplankton communities of the WAP and the Drake Passage. [ABSTRACT FROM AUTHOR]

The authors discuss the role of biological pump in the global carbon cycle. The authors mention that the oceanic biological pump is a complex suite of processes that causes the transfer of particulates and dissolved organic carbon from the surface to the bottom of the ocean thus removing carbon from the atmosphere. They also discuss the temporal and spatial variation of the biological pump.

The extent of light absorption in the near-infrared spectral region (NIR; 700-900 nm) of natural suspended particles was investigated by determining the absorption and mass-specific absorption coefficients of samples from different environments: river, coastal waters, tropical lagoon, and oceanic waters. Large amounts of sample were collected onto glass-fiber filters and measured inside the integrating sphere of a spectrophotometer. The absorption coefficient of particle suspension was also determined for visible wavelengths with a point-source integrating cavity absorption meter. Measurable nonzero particulate absorption in the NIR was determined in all samples, even in algal cultures. It was highest in the river samples (e.g., 1.7 m-1 at 850 nm), reaching values similar to the NIR absorption of pure water--a strong NIR absorber. Lowest values were in oligotrophic waters and in algal cultures. Ratios of absorption at 750 nm to absorption at 442 and 672 nm varied between 2% to 30% and 3% to 80%, respectively. Mass-specific absorption in the NIR at 850 nm was also highest in the river (0.012 m² g-1) and lowest in oligotrophic waters (0.002-0.003 m² g-1). The observed NIR absorption can partly be explained by absorption of minerogenic particles, whereas the contribution of organic detritus to the NIR absorption is still mostly unknown. [ABSTRACT FROM AUTHOR]

Abstract: As part of the GEOTRACES Polarstern expedition ANTXXIV/3 (ZERO and DRAKE) we have measured the vertical distribution of 234Th on sections through the Antarctic Circumpolar Current along the zero meridian and in Drake Passage and on an EW section through the Weddell Sea. Steady state export fluxes of 234Th from the upper 100m, derived from the depletion of 234Th with respect to its parent 238U, ranged from 621±105 to 1773±90dpmm−2 d−1. This 234Th flux was converted into an export flux of organic carbon ranging from 3.1 to 13.2mmolCm−2 d−1 (2.1–9.0mmolCm−2 d−1) using POC/234Th ratio of bulk (respectively >50μm) suspended particles at the export depth (100m). Non-steady state fluxes assuming zero flux under ice cover were up to 23% higher. In addition, particulate and dissolved 234Th were measured underway in high resolution in the surface water with a semi-automated procedure. Particulate 234Th in surface waters is inversely correlated with light transmission and pCO2 and positively with fluorescence and optical backscatter and is interpreted as a proxy for algal biomass. High resolution underway mapping of particulate and dissolved 234Th in surface water shows clearly where trace elements are absorbed by plankton and where they are exported to depth. Quantitative determination of the export flux requires the full 234Th profile since surface depletion and export flux become decoupled through changes in wind mixed layer depth and in contribution to export from subsurface layers. In a zone of very low algal abundance (54–58°S at the zero meridian), confirmed by satellite Chl-a data, the lowest carbon export of the ACC was observed, allowing Fe and Mn to maintain their highest surface concentrations. An ice-edge bloom that had developed in December/January in the zone 60–65°S as studied during the previous leg had caused a high export flux at 64.5°S when we visited the area 2 months later (February/March). The ice-edge bloom had then shifted south to 65–69°S evident from uptake of CO2 and dissolved Fe, Mn and 234Th, without causing export yet. In this way, the parallel analysis of 234Th can help to explain the scavenging behavior of other trace elements. [Copyright &y& Elsevier]

Spaceborne imaging spectroscopy, also called hyperspectral remote sensing, has shown huge potential to improve current water colour retrievals and, thereby, the monitoring of inland and coastal water ecosystems. However, the quality of water colour retrievals strongly depends on successful removal of the atmospheric/surface contributions to the radiance measured by satellite sensors. Atmospheric correction (AC) algorithms are specially designed to handle these effects, but are challenged by the hundreds of narrow spectral bands obtained by hyperspectral sensors. In this paper, we investigate the performance of Polymer AC for hyperspectral remote sensing over coastal waters. Polymer is, in nature, a hyperspectral algorithm that has been mostly applied to multispectral satellite data to date. Polymer was applied to data from the Hyperspectral Imager for the Coastal Ocean (HICO), validated against in situ multispectral (AERONET-OC) and hyperspectral radiometric measurements, and its performance was compared against that of the hyperspectral version of NASA's standard AC algorithm, L2gen. The match-up analysis demonstrated very good performance of Polymer in the green spectral region. The mean absolute percentage difference across all the visible bands varied between 16% (green spectral region) and 66% (red spectral region). Compared with L2gen, Polymer remote sensing reflectances presented lower uncertainties, greater data coverage, and higher spectral similarity to in situ measurements. These results demonstrate the potential of Polymer to perform AC on hyperspectral satellite data over coastal waters, thus supporting its application in current and future hyperspectral satellite missions. [ABSTRACT FROM AUTHOR]

A field intercomparison was conducted at the Acqua Alta Oceanographic Tower (AAOT) in the northern Adriatic Sea, from 9 to 19 July 2018 to assess differences in the accuracy of in- and above-water radiometer measurements used for the validation of ocean colour products. Ten measurement systems were compared. Prior to the intercomparison, the absolute radiometric calibration of all sensors was carried out using the same standards and methods at the same reference laboratory. Measurements were performed under clear sky conditions, relatively low sun zenith angles, moderately low sea state and on the same deployment platform and frame (except in-water systems). The weighted average of five above-water measurements was used as baseline reference for comparisons. For downwelling irradiance ( E d ), there was generally good agreement between sensors with differences of <6% for most of the sensors over the spectral range 400 nm–665 nm. One sensor exhibited a systematic bias, of up to 11%, due to poor cosine response. For sky radiance ( L s k y ) the spectrally averaged difference between optical systems was <2.5% with a root mean square error (RMS) <0.01 mWm−2 nm−1 sr−1. For total above-water upwelling radiance ( L t ), the difference was <3.5% with an RMS <0.009 mWm−2 nm−1 sr−1. For remote-sensing reflectance ( R r s ), the differences between above-water TriOS RAMSES were <3.5% and <2.5% at 443 and 560 nm, respectively, and were <7.5% for some systems at 665 nm. Seabird-Hyperspectral Surface Acquisition System (HyperSAS) sensors were on average within 3.5% at 443 nm, 1% at 560 nm, and 3% at 665 nm. The differences between the weighted mean of the above-water and in-water systems was <15.8% across visible bands. A sensitivity analysis showed that E d accounted for the largest fraction of the variance in R r s , which suggests that minimizing the errors arising from this measurement is the most important variable in reducing the inter-group differences in R r s . The differences may also be due, in part, to using five of the above-water systems as a reference. To avoid this, in situ normalized water-leaving radiance ( L w n ) was therefore compared to AERONET-OC SeaPRiSM L w n as an alternative reference measurement. For the TriOS-RAMSES and Seabird-HyperSAS sensors the differences were similar across the visible spectra with 4.7% and 4.9%, respectively. The difference between SeaPRiSM L w n and two in-water systems at blue, green and red bands was 11.8%. This was partly due to temporal and spatial differences in sampling between the in-water and above-water systems and possibly due to uncertainties in instrument self-shading for one of the in-water measurements. [ABSTRACT FROM AUTHOR]

This study presents an algorithm for globally retrieving chlorophyll a (Chl- a) concentrations of phytoplankton functional types (PFTs) from multi-sensor merged ocean color (OC) products or Sentinel-3A (S3) Ocean and Land Color Instrument (OLCI) data from the GlobColour archive in the frame of the Copernicus Marine Environmental Monitoring Service (CMEMS). The retrieved PFTs include diatoms, haptophytes, dinoflagellates, green algae and prokaryotic phytoplankton. A previously proposed method to retrieve various phytoplankton pigments, based on empirical orthogonal functions (EOF), is investigated and adapted to retrieve Chl-a concentrations of multiple PFTs using extensive global data sets of in situ pigment measurements and matchups with satellite OC products. The performance of the EOF-based approach is assessed and cross-validated statistically. The retrieved PFTs are compared with those derived from diagnostic pigment analysis (DPA) based on in situ pigment measurements. Results show that the approach predicts well Chl- a concentrations of most of the mentioned PFTs. The performance of the approach is, however, less accurate for prokaryotes, possibly due to their general low variability and small concentration range resulting in a weak signal which is extracted from the reflectance data and corresponding EOF modes. As a demonstration of the approach utilization, the EOF-based fitted models based on satellite reflectance products at nine bands are applied to the monthly GlobColour merged products. Climatological characteristics of the PFTs are also evaluated based on ten years of merged products (2002−2012) through inter-comparisons with other existing satellite derived products on phytoplankton composition including phytoplankton size class (PSC), SynSenPFT, OC-PFT and PHYSAT. Inter-comparisons indicate that most PFTs retrieved by our study agree well with previous corresponding PFT/PSC products, except that prokaryotes show higher Chl- a concentration in low latitudes. PFT dominance derived from our products is in general well consistent with the PHYSAT product. A preliminary experiment of the retrieval algorithm using eleven OLCI bands is applied to monthly OLCI products, showing comparable PFT distributions with those from the merged products, though the matchup data for OLCI are limited both in number and coverage. This study is to ultimately deliver satellite global PFT products for long-term continuous observation, which will be updated timely with upcoming OC data, for a comprehensive understanding of the variability of phytoplankton composition structure at a global or regional scale. • Multi-sensor ocean color remote sensing data are used to estimate six PFTs. • Chlorophyll a concentrations of multiple PFTs are well retrieved globally. • The retrieved PFTs are very comparable to other equivalent products. • Plausible PFT dominance information can be derived on a global scale. • First application to OLCI shows high potential in a continuous long-term observation. [ABSTRACT FROM AUTHOR]

Phytoplankton in the ocean are extremely diverse. The abundance of various intracellular pigments are often used to study phytoplankton physiology and ecology, and identify and quantify different phytoplankton groups. In this study, phytoplankton absorption spectra ( a p h (λ) ) derived from underway flow-through AC-S measurements in the Fram Strait are combined with phytoplankton pigment measurements analyzed by high-performance liquid chromatography (HPLC) to evaluate the retrieval of various pigment concentrations at high spatial resolution. The performances of two approaches, Gaussian decomposition and the matrix inversion technique are investigated and compared. Our study is the first to apply the matrix inversion technique to underway spectrophotometry data. We find that Gaussian decomposition provides good estimates (median absolute percentage error, MPE 21–34%) of total chlorophyll-a (TChl-a), total chlorophyll-b (TChl-b), the combination of chlorophyll-c1 and -c2 (Chl-c1/2), photoprotective (PPC) and photosynthetic carotenoids (PSC). This method outperformed one of the matrix inversion algorithms, i.e., singular value decomposition combined with non-negative least squares (SVD-NNLS), in retrieving TChl-b, Chl-c1/2, PSC, and PPC. However, SVD-NNLS enables robust retrievals of specific carotenoids (MPE 37–65%), i.e., fucoxanthin, diadinoxanthin and 19 ′ -hexanoyloxyfucoxanthin, which is currently not accomplished by Gaussian decomposition. More robust predictions are obtained using the Gaussian decomposition method when the observed a p h (λ) is normalized by the package effect index at 675 nm. The latter is determined as a function of "packaged" a p h (675) and TChl-a concentration, which shows potential for improving pigment retrieval accuracy by the combined use of a p h (λ) and TChl-a concentration data. To generate robust estimation statistics for the matrix inversion technique, we combine leave-one-out cross-validation with data perturbations. We find that both approaches provide useful information on pigment distributions, and hence, phytoplankton community composition indicators, at a spatial resolution much finer than that can be achieved with discrete samples. [ABSTRACT FROM AUTHOR]

Optically active water constituents can strongly attenuate in-water penetrative radiation and affect the upper ocean heat content. Arctic rivers supply the Arctic Ocean with a considerable amount of highly-absorbing organic material which is expected to increase, as a result of thawing permafrost in Siberia. Here, we investigate the effect of the variability of optically active water constituents on the heat budget of the Arctic Ocean. As a first step, we simulate locally the radiative heating by means of coupled atmosphere-ocean radiative transfer modelling (RTM SCIATRAN). By using satellite remote sensing retrievals of Coloured Dissolved Organic Matter (CDOM), Total Suspended Matter (TSM), Chlorophyll-a (Chl-a) and sea surface temperature data as input to the RTM simulations, we present the spatial distribution of potential radiative heating in the Laptev Sea. For upscaling, we use an ocean biogeochemical model coupled to a general circulation model (Darwin-MITgcm) to simulate the dynamics of the different constituents in response to Arctic amplification. We further set up the general circulation model to take into account the biogeochemical processes so that their feedback on Arctic Ocean's surface heating, stratification and sea ice melting can be assessed. Results show that high concentration of CDOM, TSM and Chl-a in Arctic waters increase the heating rate at the surface of the ocean and affect the heat fluxes to the atmosphere. The induced surface heating result in higher ice melting rates with potential implications to upper ocean stratification and primary production. [ABSTRACT FROM AUTHOR]

The seasonal cycle of sea ice and its snow cover dominates the amount of light reaching the upper layers of the polar oceans, with various important implications for biological processes taking place within the water column. At the same time, the scarcity of observations in space and time, particularly in the remotest and most inaccessible regions of the central Arctic Ocean, limits our understanding of how the physical and biological realms are intertwined.In this study, we combine year-round observations of the atmosphere, sea ice, snow and the upper ocean by an Ice-Tethered Profiler and other complementary buoys in the Transpolar Drift in 2015/16, to establish linkages between the timing of the physical processes at the ice surface, and its effects on the biosphere associated with the upper ocean layers. Our results for example confirm that a strong summer surface snow and ice melt, which subsequently increases light transmittance through the ice, triggers a strong phytoplankton bloom, also suggesting that water column processes reflect the seasonal cycle of surface properies. This study highlights the importance of multi-disciplinary, year-round observations in the polar oceans to better understand global climate- and ecsystem properties. [ABSTRACT FROM AUTHOR]

The sea surface microlayer (SML) is the uppermost layer of the oceanic water column. As the direct interface between the ocean and the atmosphere, it may serve as an important source of organic substances in marine aerosol particles and of effective ice nucleating particles (INP) for the formation of ice crystals in clouds. As the most abundant free monosaccharide, dissolved free glucose was quantified in Arctic SML and bulk water samples by applying high performance anionic exchange chromatography (HPAEC) with prior desalination via electrodialysis. A positive correlation could be observed between the concentration of free glucose and the ice nucleating (IN) activity in several Arctic water samples collected in the ice-free ocean, in open leads within the ice pack, in the marginal ice zone, and in melt ponds. This finding supports the assumption that the IN activity in Arctic seawater is strongly coupled to microbiological processes. Clustering water samples using hierarchical cluster analysis with the phytoplankton pigment composition as input data showed very strong correlations between the free glucose concentration and IN activity within the individual clusters. This indicates that the correlation between the IN activity and free glucose is linked to the specific composition and the physiological state of a phytoplankton community. Since free glucose did not show significant potential to nucleate ice itself, we propose that free glucose may serve as an "easy-to-measure" tracer for IN activity in Arctic water samples.This work was supported by the DFG-funded TR 172 (Arctic Amplification). [ABSTRACT FROM AUTHOR]

The Arctic is warming twice as fast as other parts of the globe. The loss of Arctic ice and changes in the Arctic environment are dramatic – but year-round in situ observations of this harsh, remote, and hardly accessible environment, are still scarce.To overcome this lack of data and improve scientific insight, the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI) started to build up the Arctic Observatory FRAM (FRontiers in Arctic Marine Monitoring). Scientifically, FRAM grounds on more than fifteen years of AWI time-series observations in the Fram Strait and the central Arctic, including the LTER Observatory HAUSGARTEN. FRAM provides various sensor platforms to improve multidisciplinary year-round observations of the Arctic Ocean system in Fram Strait and the central Arctic, covering the entire water column, the sea ice, and the sea floor: Moorings, ice-tethered and mobile sensor platforms, fixed-point benthic installations as well as ship-based instrumentation and regular sampling campaigns allow a wide range of physical, biogeochemical, and biological measurements of Essential Ocean Variables (EOV). Field data are being cross validated by satellite observations and used to improve model simulations. FRAM is planned as a sustained observatory infrastructure for long-term operation to sustain multidisciplinary time-series. Large parts of FRAM are now operational, with ongoing and future extensions and further technical developments. The full set of physical, biogeochemical, and biological observations will be made freely available via the AWI data portal (https://data.awi.de).This talk provides a) background information on FRAM infrastructure and b) present new mooring strategies in combination with FRAM's latest technical developments on sensors, samplers, observation platforms for autonomous year-round deployments and ship-based observations carried out during annual expeditions. Novel technologies include profiling winches, ice-tethered platforms, automated filtration systems, imaging systems for marine snow aggregates and zooplankton, as well as benthic crawlers for biogeochemical observations. These new technologies allow improved spatiotemporal coverage of observations in these remote areas and provide a comprehensive insight into dynamics in nutrient availability and primary productivity, particulate organic matter export, and its fate below the mixed surface layer and at the deep seafloor. [ABSTRACT FROM AUTHOR]