Chlorophyll a concentrations from satellite remote sensing of ocean colour

	Authors: Wolfram Schrimpf, EC/JRC Jean-Noël Druon, EC/JRC Giuseppe Zibordi, EC/JRC Frederic Mélin, EC/JRC

Key Message

Chlorophyll-a concentrations derived from satellite remote sensing show a high spatial variability reaching relative high values in the Eastern and South Eastern part of the Baltic Sea. Although the absolute values have to be taken with care (see below), coastal areas with high chlorophyll-a concentrations may constitute potential problem areas for eutrophication. This phenomena can lead to severe oxygen depletion in the water column and as a result the complete benthic fauna and flora in the affected areas may temporarily disappear.
The chlorophyll-a concentrations show also significant inter-annual variability that is caused to a large extent by the variability of the meteorological conditions in the basin and its catchment (e.g. high/low precipitation in spring -> high/low river discharges -> high/low nutrient input and stratification -> higher/lower biological productivity).

Relevance of the indicator for describing developments in the environment
Chlorophyll concentration is an index of phytoplankton biomass and it is the most common property that characterizes marine productivity. Satellite remote-sensing images of ocean colour, calibrated as chlorophyll concentration provide a unique synoptic view of the marine ecosystem. This information is of high relevance as it improves our understanding in fields such as ocean bio-geochemistry, eutrophication, harmful algal blooms, fisheries, coastal zone management and mixed layer dynamics.
A major value of ocean colour lies in the long-term monitoring of the marine environment which will improve the understanding of the ecosystems functioning. It will also help to assess the response to anthropogenic pressures like agriculture, urban development and global change. Chlorophyll concentration, as the principle deliverable from ocean colour, has a dynamic range of at least four orders of magnitude over regions and seasons (from 0.01 to 100 mg.m-3).
In many regional sea ecosystems a considerable increase in the concentration of nutrients in coastal waters has been recorded in the last decades. A major source of these nutrients is agriculture and intensive livestock-farming which release these substances into the drainage basin. Nutrient enrichment of the waters stimulates the growth of phytoplankton, leading, in certain circumstances, to the phenomena of algal blooms and to anoxia in the lower part of the water column with destruction of the benthic fauna and flora. In addition, insufficient and selective sewage treatment can increase the input of nutrients into coastal marine waters and modify even more from the natural ratio between them (removal of phosphorous compared to nitrogen) that lead to the same effects as described above. Having this in mind, long-term monitoring capabilities of optical remote sensing satellites can help
1) to better understand the functioning of marine ecosystems and, as a consequence,
2) to verify the success of implementing environmental legislation like the Nitrates Directive and the Urban Water Treatment Directive.

Results and assessment

The maps given here are produced from SeaWiFS satellite images showing a) the July-August mean concentration of chlorophyll-like pigments in the Baltic Sea and b) the number of SeaWiFS valid observations for the two months. These ‘acquisition days’ images can help to evaluate the spatial and temporal significance of the satellite data. Relatively low numbers of data (one or two values – dark blue) in the main gulfs of the Baltic Sea and in the eastern North Sea, as well as in the most coastal near areas, archipelagos and the Wadden Sea, leads to interpret the mean chlorophyll values in these areas with more caution. Due to the lack of satellite data in the first 1-2 km from the coastline (cases of “land pollution”), chlorophyll concentrations inside most estuaries and fjords cannot be seen from the satellite. Temporal variations are also important with a much more cloud-free summer in 2001 (above sixteen observations – dark red) compared to the other years (especially 1998 and 2000 with a mean of five-six values – light blue/beige).

Click the images to enlarge!

Figure 1a. Chlorophyll-a map of the Baltic Sea from satellite remote sensing of Ocean Colour (July-August 1998).	Figure 1b. Spatial and temporal frequency of the satellite observation (July-August 1998).
Figure 2a. Chlorophyll-a map of the Baltic Sea from satellite remote sensing of Ocean Colour (July-August 1999).	Figure 2b. Spatial and temporal frequency of the satellite observation (July-August 1999).
Figure 3a. Chlorophyll-a map of the Baltic Sea from satellite remote sensing of Ocean Colour (July-August 2000).	Figure 3b. Spatial and temporal frequency of the satellite observation (July-August 2000).
Figure 4a.  Chlorophyll-a map of the Baltic Sea from satellite remote sensing of Ocean Colour (July-August 2001).	Figure 4b. Spatial and temporal frequency of the satellite observation (July-August 2001).

Validity of the chlorophyll-a product

The chlorophyll-a algorithm OC4-V4 is designed for oceanic waters and it can lead to large uncertainties in the Baltic Sea and in river influenced areas due to the presence of dissolved organic matter (DOM or yellow substance) and suspended particulate matter. Therefore the spatial gradients should be interpreted with caution. The temporal variability must also be analysed accounting for the number of valid observations.
A HELCOM Project Validation of Algorithms for Chlorophyll Retrieval from Satellite Data for the Baltic Sea area is currently evaluating algorithms for chlorophyll-a.

Characteristics of the chlorophyll-a maps:

- Data set: SeaWiFS (http://www.me.sai.jrc.it)
- Projection: Cylindrical
- Resolution: 2 km (at the center of the image)
- Atmospheric corrections: JRC/IES/IMW; ref 1,2,3
- Chlorophyll algorithm: Ocean Color 4 (OC4-V4); ref 4
- July-August mean from daily data
- Log color scale between 0.2 and 10 mg.m-3

References

1) B.Sturm and G.Zibordi, Atmospheric correction of SeaWiFS data by an approximate model and vicarious calibration. International Journal of Remote Sensing, 23:489-501, 2002.
2) B.Bulgarelli and G.Zibordi. Remote sensing of ocean color: assessment of an approximate atmospheric correction method. International Journal of Remote Sensing, 24:491-509, 2003.
3) F.Melin, G.Zibordi and J.F.Berthon. Assesment of atmospheric and marine SeaWiFS products for the North Adriatic Sea. IEEE Transactions in Geoscience and Remote Sensing, (in press), 2003.
4) J. E. O'Reilly, et al. Ocean Color Chlorophyll a Algorithms for SeaWiFS, OC2 and OC4: Version 4. In J. E. O'Reilly et al.: NASA Tech. Memo. 2000-206892, Vol. 11, S.B.Hooker and E.R. Firestone, Eds, NASA Goddard Space Flight Center, Greenbelt. Maryland, pp. 49, 2000.