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 thumbnails below to enlarge the
images
 | 
|
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.
Helsinki Commission
