Monitoring the Bio-optical State of the Baltic Sea Ecosystem with Remote Sensing and Autonomous In Situ Techniques - The Baltic Sea Basin - page 413

chlorophyll absorption. Remote sensing algorithms for retrieval of chlorophyll- a are
based on empirical relationships between changes in phytoplankton pigment con-
centration and ratios of water-leaving radiance at different wavelengths, e.g. the ratio
of blue to green radiances, or reflectances (Gordon and Morel 1983 , Sathyendranath
et al. 1994 , Aiken et al. 1995 ) . For these waters, chlorophyll can be determined from
space with high accuracy.
In coastal, optically complex waters, such as the Baltic Sea, the optical prop-
erties are influenced not only by water itself and by phytoplankton but also by
varying concentrations of CDOM and total suspended matter (TSM) . 2 These waters
are referred to as optical Case-2 waters (Morel and Prieur 1977 ) . CDOM absorp-
tion in the Baltic Sea is high compared to other seas (Siegel et al. 1999 , Darecki
et al. 2003 ) and CDOM is normally the dominant optical signal in the Baltic Sea
(Kowalczuk et al. 2006 , Kratzer 2000 , Kratzer and Tett 2009 ) . It absorbs strongly in
the blue spectral region and the absorption decreases exponentially with increasing
wavelength. The exponent (slope factor) for CDOM in the Baltic differs from other
seas. Schwarz et al. ( 2002 ) showed that the mean exponent measured in the Baltic
Sea is relatively high: 0.0193 ( ± 0.0024), whereas in other marine areas it is 0.0165

( 0.0035). CDOM absorption is still significant in the green spectral region around

550 nm; hence standard band ratio algorithms based on the blue to green band ratio
tend to overestimate the concentration of chlorophyll- a in the Baltic Sea (Jorgensen
1999 ) . Coastal waters with high concentrations of suspended sediments, e.g. waters
highly influenced by tidal action, have a relatively high backscatter because inor-
ganic sediments increase the backscatter of light from the water body, and therefore
the reflection. However, the waters of the open Baltic Sea are dominated by CDOM
absorption and therefore reflect relatively little because of the high CDOM absorp-
tion. The open Baltic Sea appears therefore much darker from space than, e.g., the
North Sea.

±

20.1.6 Historical Trends in Water Quality Assessment

Secchi depth is one of the oldest methods used in oceanography. It originated with
Angelo Secchi (1818-1878), who was requested to measure the transparency in the
Mediterranean Sea. Awhite circular disk of 30 cm diameter is lowered into the water
(Fig. 20.3 ) until the observer loses sight of it (Preisendorfer 1986 ) . The observer
notes the depth at which the disk vanishes; the deeper the Secchi depth, the clearer
the water. In the Baltic Sea it is common to use Secchi depth as an indicator for
eutrophication (Kautsky et al. 1986 , Sandén and Håkansson 1996 , HELCOM 2007 ) .
In the open Baltic the Secchi depth varies from a couple of meters during strong
blooms to almost 20 during winter in the southern Baltic. Long time series indicate
that the Secchi depth has decreased about 0.05 m/year since 1910. This decrease has

2 TSM is also referred to as suspended particulate matter (SPM) and consists of an organic and an
inorganic fraction (organic and inorganic SPM).