Radiometric validation of atmospheric correction for MERIS in the Baltic Sea based on continuous observations from ships and AERONET-OC

The Baltic Sea is a semi-enclosed sea that is optically dominated by coloured dissolved organic material (CDOM) and has relatively low sun elevation which makes accurate ocean colour remote sensing challenging in these waters. The high absorption, low scattering properties of the Baltic Sea are representative of other optically similar water bodies including the Arctic Ocean, Black Sea, coastal regions adjacent to the CDOM-rich estuaries such as the Amazon, and highly absorbing lakes where radiometric validation is essential in order to develop accurate remote sensing algorithms. Previous studies in this region mainly focused on the validation and improvementofstandardChlorophyll-a (Chla)andattenuation coefficient(kd)ocean colourproducts.Theprimary input to derive these is the water-leaving radiance (Lw) or remote sensing reflectance (Rrs) and it is therefore fundamental toobtainthemostaccurate Lw orRrs beforederivinghigherlevelproducts.Tothisend,theretrieval accuracy of Rrs from Medium Resolution Imaging Spectrometer (MERIS) imagery using six atmospheric correction processors was assessed through above-water measurements at two sites of the Aerosol Robotic Network for Ocean Colour (AERONET-OC; 363 measurements) and a shipborne autonomous platform from which the highest number of measurements were obtained (4986 measurements). The six processors tested were the CoastColour processor (CC), the Case 2 Regional processor for lakes (C2R-Lakes), the Case 2 Regional CoastColour processor (C2R-CC), the FUB/WeW water processor (FUB), the MERIS ground segment processor (MEGS) andPOLYMER. Allprocessorsexceptfor CChadsmallaverage absolutepercentage differences(ψ)inthe wavelength rangefrom 490 nmto 709 nm(ψ 60%. Compared to in situ values, the Rrs(709)/Rrs(665) band ratio had ψ 0.6. Using a score system based on all statistical tests, POLYMER scored highest, while C2R-CC, C2RLakesandFUBhadlowerscores.ThisstudyrepresentsthelargestdatabaseofinsituRrs,themostcomprehensive analysis of AC models for highly absorbing waters and for MERIS, conducted to date. The results have implications for the new generation of Copernicus Sentinel ocean colour satellites

Contrasting Wind Regimes Cause Differences in Primary Production in the Black Sea Eastern and Western Gyres.

12-year time series of SeaWiFS chlorophyll a (Chl-a), primary production (PP), sea surface temperature (SST), and meteorological wind speed were used to examine decadal changes in these parameters in the eastern and western Gyres of the Black Sea. In both Gyres, low wind speeds and SST led to higher PP. After 2004, there was a progressive decrease in PP and Chl-a, which co-varied with increasing SST. Chl-a and PP were significantly higher in the western Gyre compared to the eastern Gyre, especially from 1998 to 2004. Wind speed negatively correlated with PP in both Gyres, but the higher wind speed prior to relaxation in the western Gyre led to higher PP during spring and autumn. Variability in annual PP in both Gyres was coupled to fluctuations in the Multivariate ENSO Index (MEI), which affected the wind regime more in the eastern than in the western Gyre. The data suggest that localised wind regimes in the western gyre that are uncoupled from MEI, sustains higher PP in this area

Size Class Dependent Relationships between Temperature and Phytoplankton Photosynthesis-Irradiance Parameters in the Atlantic Ocean

Over the past decade, a number of methods have been developed to estimate size-class primary production from either in situ phytoplankton pigment data or remotely-sensed data. In this context, the first objective of this study was to compare two methods of estimating size class specific (micro-, nano-, and pico-phytoplankton) photosynthesis-irradiance (PE) parameters from pigment data. The second objective was to analyse the relationship between environmental variables (temperature, nitrate and PAR) and PE parameters in the different size-classes. A large dataset was used of simultaneous measurements of the PE parameters (n = 1,260) and phytoplankton pigment markers (n = 2,326), from 3 different institutes. There were no significant differences in mean PE parameters of the different size classes between the chemotaxonomic method of Uitz et al. (2008) and the pigment markers and carbon-to-Chl a ratios method of Sathyendranath et al. (2009). For both methods, mean maximum photosynthetic rates (PBm ) for micro-phytoplankton were significantly lower than those for pico-phytoplankton and nano-phytoplankton. The mean light limited slope (�B) for nano-phytoplankton were significantly higher than for the other size taxa. For micro-phytoplankton dominated samples identified using the Sathyendranath et al. (2009) method, both PBm and �B exhibited a significant, positive linear relationship with temperature, whereas for pico-phytoplankton the correlation with temperature was negative. Nano-phytoplankton dominated samples showed a positive correlation between PBm and temperature, whereas for �B and the light saturation parameter (Ek) the correlations were not significant. For the Uitz et al. (2008) method, only micro-phytoplankton PBm , pico-phytoplankton �B, nano- and pico-phytoplankton Ek exhibited significant relationships with temperature. The temperature ranges occupied by the size classes derived using these methods differed. The Uitz et al. (2008) method exhibited a wider temperature range compared to those derived from the Sathyendranath et al. (2009) method. The differences arise from the classification of mixed populations. Based on these patterns, we therefore recommend using the Sathyendranath et al. (2009) method to derive micro-phytoplankton PE parameters at sea water temperatures up to 8◦C during monospecific blooms and the Uitz et al. (2008) method to derive PE parameters of mixed populations over the temperature range from 8 to 18◦C. Both methods exhibited similar relationships between pico-phytoplankton PE parameters and temperatures >18◦C

Accuracy assessment of primary production models with and without photoinhibition using Ocean Colour Climate Change Initiative data in the North East Atlantic Ocean.

The accuracy of three satellite models of primary production (PP) of varying complexity was assessed against 95 in situ 14C uptake measurements from the North East Atlantic Ocean (NEA). The models were run using the European Space Agency (ESA), Ocean Colour Climate Change Initiative (OC-CCI) version 3.0 data. The objectives of the study were to determine which is the most accurate PP model for the region in different provinces and seasons, what is the accuracy of the models using both high (daily) and low (eight day) temporal resolution OC-CCI data, and whether the performance of the models is improved by implementing a photoinhibition function? The Platt-Sathyendranath primary production model (PPPSM) was the most accurate over all NEA provinces and, specifically, in the Atlantic Arctic province (ARCT) and North Atlantic Drift (NADR) provinces. The implementation of a photoinhibition function in the PPPSM reduced its accuracy, especially at lower range PP. The Vertical Generalized Production Model-VGPM (PPVGPM) tended to over-estimate PP, especially in summer and in the NADR. The accuracy of PPVGPM improved with the implementation of a photoinhibition function in summer. The absorption model of primary production (PPAph), with and without photoinhibition, was the least accurate model for the NEA. Mapped images of each model showed that the PPVGPM was 150% higher in the NADR compared to PPPSM. In the North Atlantic Subtropical Gyre (NAST) province, PPAph was 355% higher than PPPSM, whereas PPVGPM was 215% higher. A sensitivity analysis indicated that chlorophyll-a (Chl a), or the absorption of phytoplankton, at 443 nm (aph (443)) caused the largest error in the estimation of PP, followed by the photosynthetic rate terms and then the irradiance functions used for each model

Effect of CO2 enrichment on phytoplankton photosynthesis in the North Atlantic sub-tropical gyre.

The effects of changes in CO2 concentration in seawater on phytoplankton community structure and photosynthesis were studied in the North Atlantic sub-tropical gyre. Three shipboard incubations were conducted for 48 h at ∼760 ppm CO2 and control (360 ppm CO2) from 49°N to 7°N during October and November 2010. Elevated CO2 caused a decrease in pH to ∼7.94 compared to ∼8.27 in the control. During one experiment, the biomass of nano- and picoeukaryotes increased under CO2 enrichment, but primary production decreased relative to the control. In two of the experiments the biomass was dominated by dinoflagellates, and there was a significant increase in the maximum photosynthetic rate (PmB) and light-limited slope of photosynthesis (αB) at CO2 concentrations of 760 ppm relative to the controls. 77 K emission spectroscopy showed that the higher photosynthetic rates measured under CO2 enrichment increased the connection of reversible photosystem antennae, which resulted in an increase in light harvesting efficiency and carbon fixation

Micro-phytoplankton photosynthesis, primary production and potential export production in the Atlantic Ocean

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this recordMicro-phytoplankton is the >20 μm component of the phytoplankton community and plays a major role in the global ocean carbon pump, through the sequestering of anthropogenic CO2 and export of organic carbon to the deep ocean. To evaluate the global impact of the marine carbon cycle, quantification of micro-phytoplankton primary production is paramount. In this paper we use both in situ data and a satellite model to estimate the contribution of micro-phytoplankton to total primary production (PP) in the Atlantic Ocean. From 1995 to 2013, 940 measurements of primary production were made at 258 sites on 23 Atlantic Meridional Transect Cruises from the United Kingdom to the South African or Patagonian Shelf. Micro-phytoplankton primary production was highest in the South Subtropical Convergence (SSTC ∼ 409 ± 720 mg C m−2 d−1), where it contributed between 38 % of the total PP, and was lowest in the North Atlantic Gyre province (NATL ∼ 37 ± 27 mg C m−2 d−1), where it represented 18 % of the total PP. Size-fractionated photosynthesis-irradiance (PE) parameters measured on AMT22 and 23 showed that micro-phytoplankton had the highest maximum photosynthetic rate (PmB) (∼5 mg C (mg Chl a)−1 h−1) followed by nano- (∼4 mg C (mg Chl a)−1 h−1) and pico- (∼2 mg C (mg Chl a)−1 h−1). The highest PmB was recorded in the NATL and lowest in the North Atlantic Drift Region (NADR) and South Atlantic Gyre (SATL). The PE parameters were used to parameterise a remote sensing model of size-fractionated PP, which explained 84 % of the micro-phytoplankton in situ PP variability with a regression slope close to 1. The model was applied to the SeaWiFS time series from 1998–2010, which illustrated that micro-phytoplankton PP remained constant in the NADR, NATL, Canary Current Coastal upwelling (CNRY), Eastern Tropical Atlantic (ETRA), Western Tropical Atlantic (WTRA) and SATL, but showed a gradual increase in the Benguela Upwelling zone (BENG) and South Subtropical Convergence (SSTC). The mean annual carbon fixation of micro-phytoplankton was highest in the CNRY (∼140 g C m−2 yr−1), and lowest in the SATL (27 g C m−2 yr−1). A Thorium-234 based export production (ThExP) algorithm was applied to estimates of total PP in each province. There was a strong coupling between micro-phytoplankton PP and ThExP in the NADR and SSTC where between 23 and 39 % of micro-phytoplankton PP contributed to ThExP. The lowest contribution by micro-phytoplankton to ThExP was in the ETRA and WTRA which were 15 and 21 % respectively. The results suggest that micro-phytoplankton PP in the SSTC is the most efficient export system and the ETRA is the least efficient in the Atlantic Ocean.UK Natural Environment Research Council National CapabilityPOGOEU FP7 project GreenSeasNCE

High photosynthetic rates associated with pico and nanophytoplankton communities and high stratification index in the North West Atlantic

The biological dynamics of pelagic marine ecosystems are strongly influenced by the size structure and ecological succession of phytoplankton, which in turn modifies photosynthetic efficiency. Variability in photosynthetic rates is closely coupled with changes in community structure, but it is difficult to obtain coincident data at high enough resolution to characterise these changes. In this study, we employ hierarchical cluster analysis on chlorophyll-normalised high performance liquid chromatography (HPLC) pigment concentrations from the North West Atlantic, to identify seasonal successional trends amongst phytoplankton populations. Changes in phytoplankton community were also analysed as a function of mean equivalent spherical diameter (MESD) derived from absorption measurements, photosynthetic rates, water-column stratification and temperature. Well-mixed conditions in spring to early summer were associated with populations of large cells containing high concentrations of fucoxanthin, chlorophyll-c1 and chlorophyll-c2 relative to chlorophyll-a (Chl a). As stratification increased over the course of the summer, these cells were replaced by populations dominated by chlorophyll-b, 19'-hexanoyloxyfucoxanthin, 19'-butanoyloxyfucoxanthin and divinyl chlorophyll-a, indicative of small picophytoplankton. As stratification decreased in autumn, MESD and alloxanthin increased, suggesting the presence of cryptophytes. Positive relationships were found between MESD and the quantum yield of photosynthesis (φm) for 7 out of the 8 phytoplankton clusters identified, while negative relationships between mean mixed layer photosynthetically active radiation and φm and the light limited slope of photosynthesis (αB) were observed for 4 clusters, as a result of nutrient limitation and photo-protection. The highest photosynthetic rates were associated with a pico & nanophytoplankton communities, which increased from spring to late summer as stratification intensified. By contrast, diatom communities had the lowest photosynthetic rates throughout the year. These successional patterns in the dominant phytoplankton size-class and phenology support Margalef's mandala in terms of the relationship between turbulence and community structure. The study sheds new light on assemblages dominated by smaller cells, under warm, stratified conditions, having higher photosynthetic efficiencies, which has implications for the carbon flux in the NW Atlantic

Derivation of seawater <i>p</i>CO<sub>2</sub> from net community production identifies the South Atlantic Ocean as a CO<sub>2</sub> source

A key step in assessing the global carbon budget is the determination of the partial pressure of CO2 in seawater (pCO2 (sw)). Spatially complete observational fields of pCO2 (sw) are routinely produced for regional and global ocean carbon budget assessments by extrapolating sparse in situ measurements of pCO2 (sw) using satellite observations. As part of this process, satellite chlorophyll a (Chl a) is often used as a proxy for the biological drawdown or release of CO2. Chl a does not, however, quantify carbon fixed through photosynthesis and then respired, which is determined by net community production (NCP). In this study, pCO2 (sw) over the South Atlantic Ocean is estimated using a feed forward neural network (FNN) scheme and either satellite-derived NCP, net primary production (NPP) or Chl a to compare which biological proxy produces the most accurate fields of pCO2 (sw) . Estimates of pCO2 (sw) using NCP, NPP or Chl a were similar, but NCP was more accurate for the Amazon Plume and upwelling regions, which were not fully reproduced when using Chl a or NPP. A perturbation analysis assessed the potential maximum reduction in pCO2 (sw) uncertainties that could be achieved by reducing the uncertainties in the satellite biological parameters. This illustrated further improvement using NCP compared to NPP or Chl a. Using NCP to estimate pCO2 (sw) showed that the South Atlantic Ocean is a CO2 source, whereas if no biological parameters are used in the FNN (following existing annual carbon assessments), this region appears to be a sink for CO2. These results highlight that using NCP improved the accuracy of estimating pCO2 (sw) and changes the South Atlantic Ocean from a CO2 sink to a source. Reducing the uncertainties in NCP derived from satellite parameters will ultimately improve our understanding and confidence in quantification of the global ocean as a CO2 sink

Modelling size-fractionated primary production in the Atlantic Ocean from remote sensing

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this recordMarine primary production influences the transfer of carbon dioxide between the ocean and atmosphere, and the availability of energy for the pelagic food web. Both the rate and the fate of organic carbon from primary production are dependent on phytoplankton size. A key aim of the Atlantic Meridional Transect (AMT) programme has been to quantify biological carbon cycling in the Atlantic Ocean and measurements of total primary production have been routinely made on AMT cruises, as well as additional measurements of size-fractionated primary production on some cruises. Measurements of total primary production collected on the AMT have been used to evaluate remote-sensing techniques capable of producing basin-scale estimates of primary production. Though models exist to estimate size-fractionated primary production from satellite data, these have not been well validated in the Atlantic Ocean, and have been parameterised using measurements of phytoplankton pigments rather than direct measurements of phytoplankton size structure. Here, we re-tune a remote-sensing primary production model to estimate production in three size fractions of phytoplankton (10 μm) in the Atlantic Ocean, using measurements of size-fractionated chlorophyll and size-fractionated photosynthesis-irradiance experiments conducted on AMT 22 and 23 using sequential filtration-based methods. The performance of the remote-sensing technique was evaluated using: (i) independent estimates of size-fractionated primary production collected on a number of AMT cruises using 14C on-deck incubation experiments and (ii) Monte Carlo simulations. Considering uncertainty in the satellite inputs and model parameters, we estimate an average model error of between 0.27 and 0.63 for log10-transformed size-fractionated production, with lower errors for the small size class (10 μm), and errors generally higher in oligotrophic waters. Application to satellite data in 2007 suggests the contribution of cells 2 μm to total primary production is approximately equal in the Atlantic Ocean.UK National Centre for Earth ObservationNatural Environment Research Council (NERC)Plymouth Marine Laborator

Comparison of Above-Water Seabird and TriOS Radiometers along an Atlantic Meridional Transect

The Fiducial Reference Measurements for Satellite Ocean Color (FRM4SOC) project has carried out a range of activities to evaluate and improve the state-of-the-art in ocean color radiometry. This paper described the results from a ship-based intercomparison conducted on the Atlantic Meridional Transect 27 from 23rd September to 5th November 2017. Two different radiometric systems, TriOS-Radiation Measurement Sensor with Enhanced Spectral resolution (RAMSES) and Seabird-Hyperspectral Surface Acquisition System (HyperSAS), were compared and operated side-by-side over a wide range of Atlantic provinces and environmental conditions. Both systems were calibrated for traceability to SI (Système international) units at the same optical laboratory under uniform conditions before and after the field campaign. The in situ results and their accompanying uncertainties were evaluated using the same data handling protocols. The field data revealed variability in the responsivity between TRiOS and Seabird sensors, which is dependent on the ambient environmental and illumination conditions. The straylight effects for individual sensors were mostly within ±3%. A near infra-red (NIR) similarity correction changed the water-leaving reflectance (ρw) and water-leaving radiance (Lw) spectra significantly, bringing also a convergence in outliers. For improving the estimates of in situ uncertainty, it is recommended that additional characterization of radiometers and environmental ancillary measurements are undertaken. In general, the comparison of radiometric systems showed agreement within the evaluated uncertainty limits. Consistency of in situ results with the available Sentinel-3A Ocean and Land Color Instrument (OLCI) data in the range from (400…560) nm was also satisfactory (-8% < Mean Percentage Difference (MPD) < 15%) and showed good agreement in terms of the shape of the spectra and absolute values