The Carbon Strategy Report of the Committee on Earth Observation Satellites (CEOS, 2014) identified a number of pools and fluxes of carbon in the ocean that are amenable to remote sensing. In ESA’s Biological Pump and Carbon Exchange Processes (BICEP) project, we have been investigating satellite methods to map marine primary production, phytoplankton carbon, particulate organic carbon, particulate inorganic matter and dissolved organic carbon. Time series of each of these products at 9 km, monthly resolution is being generated. The main input to the calculations is the ocean-colour fields generated by the Ocean Colour Climate Change Initiative (OC-CCI). These are supplemented by fields of photosynthetically available radiation at the surface of the ocean, sea-surface temperature, and sea-surface salinity (from CCI). For most of the products, the time series extends from 1998 to 2020, unless limited by availability of input data.
The primary production computations (Kulk et al. 2020, 2021) rely on an extensive in situ database of photosynthesis-irradiance parameters. The same parameter set is used, along with a photo-acclimation model, to compute phytoplankton carbon, ensuring that the allocation of resource (light) between production of carbon and chlorophyll is treated in an internally consistent manner. Various algorithms available for calculation of particulate organic carbon have been compared, before selecting one of the better-performing algorithms for generation of the time series. The algorithm for mapping dissolved organic matter is a novel one, that makes use of machine-learning tools.
In situ data bases have been created for validation and comparison of products, and for generation of uncertainty estimates. Algorithms for estimation of biological export production have also been implemented. The next major activity in the project is user consultation, to which end an online workshop on “Ocean Carbon from Space” is being organised on 14-18 February, 2022, with international collaboration and participation.


Reference:
CEOS (2014) CEOS Strategy for Carbon Observations from Space. The Committee on Earth Observation Satellites (CEOS) Response to the Group on Earth Observations (GEO) Carbon Strategy. Issue date: September 30 2014. Printed in Japan by JAXA and I&A Corporation

Kulk G, Platt T, Dingle J, Jackson T, Jönsson B, Bouman HA, Babin M, Doblin M, Estrada M, Figueiras FG, Furuya K, González N, Gudfinnsson HG, Gudmundsson K, Huang B, Isada T, Kovac Z, Lutz VA, Marañón E, Raman M, Richardson K, Rozema PD, Van de Poll WH, Segura V, Tilstone GH, Uitz J, van Dongen-Vogels V, Yoshikawa T, Sathyendranath S (2020). Primary production, an index of climate change in the ocean: Satellite-based estimates over two decades. Remote Sensing 12:826; doi:10.3390/rs12050826

Kulk G, Platt T, Dingle J, Jackson T, Jönsson B, Bouman HA, Babin M, Doblin M, Estrada M, Figueiras FG, Furuya K, González N, Gudfinnsson HG, Gudmundsson K, Huang B, Isada T, Kovac Z, Lutz VA, Marañón E, Raman M, Richardson K, Rozema PD, Van de Poll WH, Segura V, Tilstone GH, Uitz J, van Dongen-Vogels V, Yoshikawa T, Sathyendranath S (2021). Correction: Kulk et al. Primary Production, an Index of Climate Change in the Ocean: Satellite-Based Estimates over Two Decades. Remote Sensing 13:3462; doi:10.3390/rs13173462

While satellite remote sensing has revolutionized our understanding of global marine systems by providing synoptic and repeated global observations of phytoplankton stocks and rates of primary production, our present ability to quantify the export and fate of ocean net primary production (NPP) from satellite observations or to predict future fates using Earth system models is limited. To address this shortcoming, NASA, in partnership with the National Science Foundation, funded the Export Processes in the Ocean from RemoTe Sensing (EXPORTS) field campaign. By linking the state of the surface ecosystem, which is quantifiable through satellite remote sensing, with the fates of ocean NPP, EXPORTS has the objective to develop a predictive understanding of the export and fate of global ocean primary production and its implications for the Earth’s carbon cycle in present and future climates.

EXPORTS successfully completed two comprehensive field campaigns targeting two ecological end members: North Pacific Ocean (2018), which represented a low energy, biogeochemically homogeneous ecosystem, and the North Atlantic Ocean (2021), which represented a high energy system with high ecosystem complexity. Both field campaigns involved global and ocean class research vessels, and numerous autonomous platforms operating in a tightly coordinated fashion over a period of several months. Sampling of ecological and biogeochemical stocks, rates, and fluxes, was coupled with analyses of field and remote sensing data in near real time where possible to allow for adaptive sampling, targeting critical export processes, and unique opportunities.

This presentation will include early results from EXPORTS specifically as it seeks to answer its goals to characterize the oceanic carbon cycling processes and developing a predictive understanding of the fates of global NPP and their roles in the carbon cycle.

The flux of carbon dioxide (CO2) between the ocean and atmosphere is a crucial component of the carbon cycle and global climate. Satellite observations offer great potential for understanding and estimating air/sea CO2 fluxes over large spatial scales. The air/sea CO2 flux is often estimated from the concentration difference between air and sea (ΔC, strongly influenced by sea surface temperature and the cool skin effect), and the gas transfer velocity (K, typically parameterized with wind speed). Direct measurements of the in situ air/sea flux of CO2 by the eddy covariance technique can be coupled with measurements of ΔC to: i) estimate K and improve its parameterization with satellite-retrieved data products; and ii) provide an independent validation of satellite estimates of the air/sea CO2 flux.

Funding from the European Space Agency and the UK Natural Environment Research Council has enabled collection of an unprecedented dataset of CO2 flux observations from UK research ships. Measurements have been made in the Southern Ocean, the Arctic Ocean and along extensive North-South transects in the Atlantic Ocean. A comprehensive assessment of the uncertainties in the eddy covariance flux observations has demonstrated that the technique is accurate in the mean and has identified that precise flux estimates (signal:noise ratio >3) can be obtained when data are averaged for between 1 and 3 hours. The optimal averaging time is a function of the ΔC, with shorter averaging times needed when the ΔC is larger.

The database of CO2 fluxes and air-sea concentration differences have been used to investigate the CO2-specific gas transfer velocity in a range of environmental conditions. The CO2 fluxes and gas transfer velocities have helped to illuminate the potential controls on air/sea gas transfer, including the possible role of surfactants in affecting gas transfer across the air-sea interface in the Southern Ocean. Data from the Atlantic Ocean suggest that radar scattering by waves may present an improvement compared to wind speed-based parameterizations of gas transfer. The eddy covariance flux data have also been used to identify the potential for bias in flux estimates due to near surface stratification in the Arctic, which is not captured when seawater concentrations are measured ~6 m below the sea surface. Near surface stratification is also potentially important in tropical environments, which often experience strong solar insolation and light winds.

Finally, the potential for direct eddy covariance observations being used to validate satellite-based air/sea CO2 flux estimates will be discussed, along with plans to develop future air/sea CO2 flux systems and Fiducial Reference Material (FRM).

The North Brazil Current (NBC) flows northward across the Equator, passes the mouth of the Amazon River, and forms in its retroflection near 8°N large oceanic eddies, called North Brazil Current rings. In this work, we take advantage of an unprecedented set of in-situ observations in combination with satellite-based measurements to investigate the processes that drive the variability of the air-sea CO2 fluxes in the western tropical Atlantic Ocean. The in-situ data originates from three research ships operating in winter 2020 during the EUREC4A-OA/ATOMIC campaign and the Tara sailing vessel operating in summer 2021 during the Microbiome campaign. Through multivariable regression, we determine predictors of fugacity of CO2 (fCO2) from salinity, temperature, and chlorophyll-a. Applying the predictors to satellite-based maps of salinity, chlorophyll-a and temperature we create high resolution fCO2 maps that nicely highlight contrasting properties in the region and seasons.

In February 2020, the area is a CO2 sink (-1.7 TgC.month-1), previously underestimated by a factor 10. The NBC rings transport saline and high fCO2 water indicative of their equatorial origins and are a small source of CO2 at regional scale. Their main impact on the variability of biogeochemical parameters is through the filaments they entrain into the open ocean. During the winter campaign, a nutrient rich freshwater plume from the Amazon River is entrained from the shelf up to 12°N and caused a phytoplankton bloom leading to a significant carbon drawdown (~20 % of the total sink). On the other hand, saltier filaments of shelf water rich in detrital material act as strong local sources of CO2. Spatial distribution of fCO2 is therefore strongly influenced by ocean dynamics south of 12°N. The less variable North Atlantic subtropical water extends from Barbados northward. They represent ~60 % of the total sink due to their lower temperature associated with winter cooling and strong winds.

In August-September 2021, the Amazon River plume influences most of the north-western tropical Atlantic Ocean. The dynamics of the plume are complex, driven by the NBC retroflection and ring formation. In response, a large variability of surface salinity and of the CO2 air-sea flux is observed. In the core of the plume, a phytoplankton bloom drives an important CO2 sink, while old remnants of the plume located to the north west present a dampened signal, influenced by both surface salinity and temperature.

Coloured dissolved organic matter (CDOM) in marine environments impacts primary production due to its absorption effect on the photosynthetically active radiation. In coastal seas, CDOM originates from terrestrial sources predominantly and causes spatial and temporal changing patterns of light absorption which should be considered in marine biogeochemical models. We propose a model approach in which Earth Observation (EO) products are used to define boundary conditions of CDOM concentrations in an ecosystem model of the Baltic Sea. CDOM concentrations in riverine water derived from EO products serve as forcing for the ecosystem model. For this reason, we introduced an explicit CDOM state variable in the ecosystem model.

Deriving a good quantitative estimate of the CDOM absorption from satellite measurements, under high CDOM concentration conditions as in Baltic Sea is a challenging task. The water is almost completely absorbing in the visible, short-wavelength bands, and overall, the signal level is very low. Atmospheric correction requires a good model of the water leaving reflectance as lower boundary condition. Thus, we carefully characterised the water using historic and own in-situ measurements and derived a new bio-optical model for the Baltic Sea water. This was used in an innovative approach combining a polymer-like approach of inverting the atmospheric signal with a C2RCC forward model for the water reflectance. This approach was applied to Sentinel 3 and Sentinel 2 data and validated with in-situ reflectance and water constituent concentration measurements.

We show that the light absorption by CDOM in the ecosystem model can be improved considerably in comparison to approaches where CDOM is estimated from salinity. The model performance increases especially with respect to spatial CDOM patterns due to the consideration of single river properties. The introduction of high-quality CDOM data with sufficiently high spatial resolution provided by the new generation of ESA satellite sensor systems (Sentinel 2 MSI and Sentinel 3 OLCI) has proven to improve the results substantially. Such data are essential, especially when local differences in riverine CDOM concentrations exist.
This work was carried out under the ESA EO Science for Society study “BALTIC+ Sea-Land biogeochemical linkages (SeaLaBio).

Oceanic subtropical gyres have a critical role in the global carbon budget due to their immense size either if characterized by oligotrophic waters. In the last decades, the North Atlantic Subtropical Gyre (NASTG) has experienced the fastest enlargement of oligotrophic waters in response to ocean warming, worldwide. Here, we study the trophic regime changes in the NASTG by using 21 years (1998-2018) of satellite chlorophyll-a (CHL) data, complemented with other variables such as sea surface temperature (SST), optical backscatter coefficient, Secchi disk and mixed layer depth (MLD). To this aim, we describe the spatial/temporal variability of the least productive waters, coupled with an inter-annual variability analysis of key environmental variables. In the last 21 years, the ultra-oligotrophic waters (settled as CHL ≤ 0.04 mg-3, differing from previous literature limits, 0.07 mg-3 or 0.1 mg-3) have expanded in space and increased in time, accounting for an area growth rate of around 96.34% and an increase of average months per year of 53.76% with respect to the beginning of the time series. This expansion and prevalence is found to be concurrent with a continuous increase of SST of more than 0.5°C in the area thus detected, but also associated with a deepening of the MLD. These observations thus point out driving factors that are more complex than the local stratification increase and vertical nutrient flux reduction, generally hypothesized. Future works in this pathway may include: (i) combined observations by satellite data and robotic autonomous platforms (e.g. Biogeochemical-Argo floats) to better understand how ocean warming impacts the trophic regime and vertical distribution of phytoplankton (e.g. biomass, physiology) using both environmental (e.g. temperature, salinity) and physical variables (e.g. horizontal advection); (ii) study the inter-annual variability of net primary production and carbon fluxes to quantify the impact of the observed desertification on the ocean biogeochemistry (i.e., biological carbon pump).