Maritime Monitoring

SENTINEL-3 OLCI Maritime Monitoring

OLCI Marine

Measuring ocean colour from space allows information to be gathered about marine biological constituents. This measurement relates to water colouration (visible spectrum), which is affected by elements present in the water and especially by population with phytoplankton biomass (as indexed by Chl-a) which constitutes the first element of the trophic chain, and associated detrital material. For coastal and shallow waters, colouration of waters can also be the result of the release of terrestrial waters loaded with suspended sediment and organic matter, as well as re-suspension due to wave agitation. Initially designed for research studies in marine biology and carbon cycle, this observation technique has spawned a number of applications oriented toward marine area management and coastal zone management. Known reliable applications making use of ocean colour that benefit from OLCI, are summarised in the following sections.

Figure 1: GlobColour Chlorophyll Monthly Product from OLCI-A (image courtesy of ACRI-ST)

Net Primary Production Estimates

One of the important and unique applications of basin to global-scale Chl-a maps is to calculate global ocean primary production. Using algorithms that incorporate satellite-based Chl-a to calculate regional to global-scale estimates of annual Net Primary Production [1] (NPP) provides important insights into the function of ocean ecosystems and biogeochemical processes. A key finding from NPP calculation based on satellite data is that ocean and terrestrial NPP contributes more or less equally to global productivity [2]. As a result of the continuity over time of international ocean colour missions since 1998, NPP is monitored on a global scale. The essential function of carbon sink in the ocean is therefore regularly estimated. Climatic trends in such carbon uptake will be calculated once the time series of ocean colour observations is long enough, reinforcing the crucial need to maintain continuity in ocean colour missions over the next decades.

Figure 2: Map of the primary production
Figure 2: Map of the primary production for February 2019 in mgC.m-2.d-1.(image courtesy of CMEMS)

Algal Bloom and Water Quality Monitoring

Algal bloom detection has been the subject of a number of intensive research works during the last decade. The results of these works have been transformed into an operational capacity to trigger alerts for some invasive micro-algae. Within the framework of the GMES Service Element (e.g. Coastwatch and Marcoast) operational services have been set up and are still operating. The next scientific challenge is, whenever possible, to identify the type of algae and the harmfulness of the detected species together with the bloom strength and extent.

Figure 3: Algal bloom in the North Sea, S3-A/OLCI, 27/05/2017 (image courtesy of ESA)

Mesoscales Process

As for SST, ocean colour provides spatial repartition of front and eddies at the ocean surface. The detection and identification of surface patterns can be achieved either using one of the available water-leaving reflectances at a given wavelength, or by a combination of them. Depending on the purpose, it can also be done with high level products such as water transparency, giving an integrated estimate of vertical visibility through the ocean upper layer. When oriented toward mesoscale circulations analysis or biological productivity (e.g. of large marine ecosystems, such as up-welling zones), these analyses are often performed by associating other Earth Observation measurements, such as SST and Sea Surface Height (SSH) (both available and collocated with ocean colour observations on Sentinel-3). In this case, marine habitats have been analysed for some species as a function of these three components (Chl-a, SST, SSH) and indications can be provided in NRT (less than 3 hours after measurement by satellite) to optimise fish catches and/or to avoid fishing near protected species (e.g. turtles). Although still experimental, fish stocks and their evolution can then be assessed in productive zones (Large Marine Ecosystem).

Figure 4: GlobColour Monthly Chlorophyll Regional Product (July 2019 - from a high concentration of 30 mg/m3 in red to 0.01 mg in purple)(image courtesy of ACRI-ST)

Extension to 3D Marine Biology

Although limited to surface observation (the upper sea layer of one optical depth), several techniques, ranging from statistical to numerical, allow expansion of this information to greater depths and provide a better 3D description of the biological field. Besides this vertical extension, phytoplankton size class and types (e.g. diatoms) can also be determined from ocean colour through robust algorithms [3] [4].

The assimilation of Chl-a measurements from space, or of IOP in biogeochemical modelling, is under way.

Sedimentary Processes

Providing reliable atmospheric correction, ocean colour gives access to geographical extent and composition of turbid plumes, allowing monitoring of flood expansion and its impact at sea. This also provides a means of establishing the fluxes of terrestrial material at sea and of monitoring their fluctuations in space and time. This is particularly useful for the study of large estuaries.

Figure 5: River discharge off Korean peninsula, S3B/OLCI, 21/05/2019 (image courtesy of ESA)

[1] Yoder, J.A., S.C. Doney, D.A. Siegel and C. Wilson. 2010. Study of marine ecosystems and biogeochemistry now and in the future: Examples of the unique contributions from space. Oceanography 23(4):104-117, doi:10.5670/oceanog.2010.09.

[2] Field, C.B., M.J. Behrenfeld, J.T. Randerson and P. Falkowski. 1998. Primary Production of the biosphere: Integrating terrestrial and oceanic components. Science 281:237-240

[3] Uitz, J., D. Stramski, B. Gentili, F. D'Ortenzio, and H. Claustre (2012), Estimates of phytoplankton class-specific and total primary production in the Mediterranean Sea from satellite ocean color observations, Global Biogeochemical Cycles, 26, GB2024, doi:10.1029/2011GB004055.

[4] Alvain S., Moulin C., Dandonneau Y., and H. Loisel (2008) Seasonal distribution and succession of fominant phytoplankton groups in the global ocean : A satellite view., Global Biogeochem. Cycles, 22.

For further information about marine applications and services available, see: Copernicus website.


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