Other Space Activities

Ocean Colour

Feb 2, 2024

Measurement Types

Ocean Colour is the apparent hue of water produced by backscattered sunlight after interaction with the water and its microscopic column composition. These interactions lead to the absorption or scattering of photons at different wavelengths, giving the ocean its distinctive colours. Satellite ocean colour radiometry involves detecting variations in spectrally-resolved water-leaving radiances, which is used to investigate the quality and quantity of materials that compose bodies of water from local to global scales. 1) 2)

The different hues of the open ocean are mainly governed by phytoplankton, which are microscopic plants containing chlorophyll that give them their green hue. Spectral variations reveal presences of phytoplankton, sediments, suspended particulates, and dissolved compounds - both detrital (e.g. sand, silicates, salts) and organic (e.g. algae, dissolved organic matter). Varying intensities of these spectral variations provide information on the concentrations of these substances. High concentrations of phytoplankton (high productivity) gives the ocean a blue-green to green colour. Coastal waters are more optically complex due to the influence of re-suspended particles, river discharges, or coloured dissolved organic matter (CDOM), independent of phytoplankton content. 1) 2) 3)

Figure 1: Phytoplankton bloom in the Baltic Sea imaged by ESA’s EnviSat (Image credit: ESA)

Ocean colour data is studied by using algorithms that relate characteristics of the water reflectance signal to the property of interest. Due to the scattering of light by the atmosphere in the satellite’s line of sight, the ocean colour signal represents less than ten percent of the total radiance measured, hence corrections are imperative to retrieve the water signal 4). Sun glint also affects radiance measurements, particularly in subtropical latitudes where observations are contaminated by specular reflections. 3) 5)

Radiometers and imaging spectrometers measure water-leaving radiances in the visible and near-infrared (VNIR) spectrum, ranging from 400-800 nm. Fine spectral details are detected by instruments like Hyperspectral imagers that feature narrow and contiguous spectral bands around 10 nm wide, spatial resolutions of typically ≤1 km, and very high radiometric resolutions. 5)

Measurements of ocean colour are imperative for not only understanding ocean productivity and biogeochemical cycling, but also for studying the transport of oceanic carbon dioxide (CO2) from the surface to the deep ocean. Phytoplankton content inferred by ocean colour can be used as an indirect measurement of ocean biomass and productivity. These parameters associated with ocean colour are of considerable oceanographic and climatological significance, as oceanic productivity leads the air-to-sea exchange of greenhouse gases like CO2. 3) 5)

Figure 2: The global carbon budget for 2023 represents the sum of all emissions and removals of anthropogenic (human-made) CO2. Half of all CO2 emitted into the atmosphere is removed by sinks, through vegetation uptake by photosynthesis in land sinks and diffusion in ocean sinks. The monitoring of carbon sinks like the ocean is therefore crucial for understanding and predicting climate change. 6) (Image credit: Global Carbon Budget)

On a local-scale, ocean colour paired with sea-surface temperature measurements can be used to indicate presence of fish stocks, as well as monitor water quality and pollution levels derived from algal blooms. Coastal ocean colour measurements are particularly important where they can be used to spot signs of coastal erosion and sediment transfer. 3)

More applications of ocean colour data include the monitoring of ocean acidification, marine biodiversity and its adaptation to climate change; validating and improving ocean models and data assimilation; and studying bio-feedback mechanisms and marine pollution. 7)

Since 1997, a global effort has enabled the contiguous measurement of ocean colour, made primarily by one-off satellite missions. This has since been superseded by long term ocean-colour constellations like the CEOS Ocean Colour Radiometry Virtual Constellation (OCR-VC), established by the International Ocean Colour Coordinating Group (IOCCG). OCR-VC has the goal of providing sustained data records of well-calibrated and validated satellite ocean colour datasets from multiple-satellite measurements. 3) 8) 9)

The ocean colour element of ESA’s Climate Change Initiative (OC-CCI) is dedicated to fulfilling the contiguous record of ocean colour data, providing a coherent and stable record of error-characterised data streams from multi-sensor archives. The project’s data archives contain satellite observations from ESA, NASA and the National Oceanic and Atmospheric Administration (NOAA). 10) 11)

Example Products

Ocean colour is an essential climate variable (ECV) under the Global Climate Observing System (GCOS), with spectrally-resolved water-leaving radiances and chlorophyll-a concentrations identified as required ECV products. 3) 12)

Table 1: GCOS ocean colour data product requirements 3)
Data product	Acquisition frequency	Spatial resolution (km)	Measurement uncertainty	Stability/decade
Water-leaving radiance	Daily	4	5% in blue and green wavelengths	0.5%
Chl-a concentration	Weekly averages	4	30%	3%

Water-leaving radiance

Figure 3: OC-CCI Web GIS Data Portal (Image credit: PML)

Figure 3 shows CCI’s interactive global ocean colour map tool that allows for in-depth analysis of biological and chlorophyll indicators, optical properties and water classes from various satellite instruments.

Figure 4: Map depicting the change in ocean colour between 2002 and 2022. Darker green colours indicate the largest change in colour. (Image credit: NASA, 13)

The study visualised above is the product of over 20 years worth of NASA ocean colour data from the MODIS instrument onboard the Aqua satellite. Changes in chlorophyll levels were indicated by the ocean’s colour, used to assess the productivity of phytoplankton. Northern latitudes were not analysed due to high noise levels and darker ocean colours. The study found that 56% of the global oceans surface had changed colour during the 20 year timeframe, particularly around the equator. To investigate whether this ocean colour change could be linked to climate change, the researchers created a model that simulated the ocean’s ecosystem in response to high levels of greenhouse gases. The model was run between the years 2000 and 2015, and its results compared it to real imagery from Aqua from 2002 and 2022. The model suggested that a change in the ocean colour due to climate change would be significant by 2020 in 46% of the global ocean, a value comparable to the Aqua-derived 56%. 14) 15)

Chlorophyll-a (Chl-a) Concentration

Figure 5: Mean Chl-a concentration in 2018 from the Copernicus Climate Change Services (C3S) Ocean Colour (Image credit: Copernicus Service Catalogue, 16))

The C3S Ocean Colour dataset provides daily estimates of chlorophyll-a density (mg mg-3) in near-surface water, as well as the fraction of downwelling solar irradiance reflected by the ocean surface at a given wavelength (sr-1). Satellite data is obtained from various sensors, including SeaWiFS (SeaStar), MERIS (Envisat), MODIS (Aqua), VIIRS (JPSS), and OLCI (Sentinel-3), with global coverage from 1997 - present. 16)

Ocean Colour satellite measurements directly support the United Nations (UN) Sustainable Development Goals (SDGs) SDG 6 and 14. SDG 14 aims “to conserve and sustainably use the oceans, seas and marine resources for sustainable development”. Chl-a satellite measurements can be used to monitor changes in eutrophication (the loading of excess nutrients into coastal environments due to anthropogenic activities), which is a detrimental process for the environment and coastal populations. Increased eutrophication has led to an increase in global dead zones - water bodies with too little oxygen to support marine life - from approximately 400 in 2008 to 700 in 2019. 17) 18)

Figure 6: Global chlorophyll concentration map in April 2004 from merged MERIS and MODIS data (Image credit: GlobColour)

More data products available through OC-CCI include: 2)

Related Missions

Oceanography-dedicated

Copernicus Sentinel 3

Sentinel-3 is a constellation of two identical radar imaging satellites of ESA’s Copernicus programme, supported by the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT). Sentinel-3A, launched in February 2016, and Sentinel-3B, launched in April 2018 both carry the Ocean and Land Colour Instrument (OLCI) which observes the Earth’s surface in 21 spectral bands, detecting water-leaving radiances that can imply varying levels of algal pigment concentrations, nutrification, and coastal erosion. S3's global ocean colour monitoring reveals the implications of climate change and human activities on the environment and the carbon cycle. 19)

PACE (Plankton, Aerosol, Cloud, ocean Ecosystem) Mission

PACE is a NASA mission planned for launch in February 2024 that will make global ocean colour measurements to provide extended data records on ocean ecology and global biogeochemistry. The satellite carries the Ocean Color Instrument (OCI) that will measure water-leaving radiances across the spectrum from ultraviolet (UV) to short-wave infrared (SWIR). This Hyperspectral imager will monitor how phytoplankton communities evolve over time globally.

Oceansat

Oceansat is a constellation of three oceanography satellites of the Indian Space Research Organisation (ISRO), all of which carry Ocean Colour Monitor (OCM) instruments operating since 1999 with the launch of Oceansat-2.

Read more: Oceansat-1 | Oceansat-2 | Oceansat-3

GCOM-W (Global Change Observation Mission - Water)

SeaHawk

Haiyang/Ocean-1A| Haiyang/Ocean-1B

Climate monitoring with Ocean Colour auxiliary

Aqua / Terra

Aqua (EOS/PM-1) and Terra (EOS/AM-1) are NASA Earth Observing System (EOS) missions with collaboration between the US, Japan (JAXA), Canada (CSA), and Brazil (INPE), launched in May 2002 and December 1999 respectively. Both missions have a wide range of global climate applications and support ocean colour measurements with their Moderate-Resolution Imaging Spectroradiometer (MODIS), which studies ocean colour and surface temperature, phytoplankton and biogeochemistry. Aqua and Terra’s ocean colour measurements made in tandem with Greenhouse Gas measurements provide global climate monitoring and help scientists understand the carbon cycle through the ocean and atmosphere.

References

1) Sathyendranath S, Brewin RJW, Brockmann C, Brotas V, Calton B, Chuprin A, Cipollini P, Couto AB, Dingle J, Doerffer R, et al. “An Ocean-Colour Time Series for Use in Climate Studies: The Experience of the Ocean-Colour Climate Change Initiative (OC-CCI),” Sensors. 2019; 19(19):4285. https://doi.org/10.3390/s19194285

2) “Ocean Color,” NASA Goddard Space Flight Centre, Ocean Biology Distributed Active Archive Center, URL: https://oceancolor.gsfc.nasa.gov/

3) “Ocean Colour/Biology,” CEOS Earth Observation Handbook, 2014, URL: https://www.eohandbook.com/eohb2014/earth_observation_plans_ocean.html

4) Wang M, Son S, Shi W. Evaluation of MODIS SWIR and NIR-SWIR atmospheric correction algorithms using SeaBASS data. Remote Sensing of Environment. 2009 Mar 16;113(3):635-44.

5) “Ocean Colour Instruments,” CEOS Earth Observation Handbook, 2014, URL: https://www.eohandbook.com/eohb2014/sat_earth_obs_ocean_colour.html

6) “Global Carbon Budget,” global Carbon Atlas, URL: https://globalcarbonatlas.org/budgets/carbon-budget/

7) Groom S, Sathyendranath S, Ban Y, Stewart B, Brewin R, Brotas V, et al. “Satellite Ocean Colour: Current Status and Future Perspective,” Frontiers in Marine Science, 2019; Vol. 6, URL: https://www.frontiersin.org/articles/10.3389/fmars.2019.00485.

8) “Ocean Colour Radiometry,” Committee on Earth Observation Satellites, URL: https://ceos.org/ourwork/virtual-constellations/ocr/

9) “International Ocean Colour Coordinating Group,” URL: https://ioccg.org/

10) “Ocean Colour,” ESA Climate Office, URL: https://climate.esa.int/en/projects/ocean-colour/

11) “OceanColour-CCI,” Plymouth Marine Laboratory, 2023, URL: https://www.oceancolour.org/

12) “CEOS Strategy for Carbon Observations from Space,” CEOS, September 2014, URL: https://ceos.org/document_management/Publications/WGClimate_CEOS-Strategy-for-Carbon-Observations-from-Space_Apr2014.pdf

13) Cael BB, Bisson K, Boss E, Erickson ZK. "How many independent quantities can be extracted from ocean color?," Limnology and Oceanography Letters. 2023 Mar 14.

14) Caitlin Dempsey, “CLIMATE CHANGE IS AFFECTING THE COLOR OF THE OCEAN,” GeographyRealm, 23 October 2023, URL: https://www.geographyrealm.com/climate-change-color-ocean/

15) Cael BB, Bisson K, Boss E, Dutkiewicz S, Henson S. Global climate-change trends detected in indicators of ocean ecology. Vol. 619, Nature. Springer Science and Business Media LLC; 2023. p. 551–4. URL: http://dx.doi.org/10.1038/s41586-023-06321-z

16) “Copernicus Service Catalogue,” Copernicus EU, URL: https://www.copernicus.eu/en/access-data/copernicus-services-catalogue/satellite-ocean-colour

17) “United Nations Environment Programme (2023). Measuring Progress: Water-related ecosystems and the SDGs. Nairobi,” URL: https://wesr.unep.org/measuring-progress/water-related-ecosystems-and-sdgs/sdgs/pdf/DEWA_Measuring_Progress_2023.pdf

18) “United Nations Sustainable Development Goals 2021 Report. SDG 14: Life Below Water,” URL: https://unstats.un.org/sdgs/report/2021/goal-14

19) Ewa Kwiatkowska, “Sentinel-3: Ocean Colour,” EUMETSAT, URL: https://www.youtube.com/watch?v=Klsd5ZpAVts

20) Bezy JL, Delwart S, Rast M. MERIS-A new generation of ocean-colour sensor onboard Envisat. ESA bulletin. 2000 Aug 1;103:48-56.

21) “Sentinel-5P+ Innovation Ocean Colour (S5P+-I-OC),” ESA eo science for society, 2019, URL: https://eo4society.esa.int/projects/sentinel-5p-innovation-expro-theme-7-ocean-colour-s5p-i-oc/

The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (eoportal@symbios.space).