Geoscience Reference
In-Depth Information
long-term monitoring of water quality from space. The wide aerial coverage, the
frequent repetition and continuity of the satellite observations, the consistency of
the measured data, and a relative cost-effectiveness clearly respond to the demands
of a modern operational monitoring system, and the requirements of effective Baltic
Sea management. An overview of existing monitoring approaches is given, and
operational online systems that combine remote sensing and autonomous in situ
measurements are discussed.
Keywords
Baltic Sea
·
Optically complex waters
·
Bio-optical monitoring
·
Remote
Sensing
·
MERIS
·
validation
20.1 Introduction
20.1.1 The Baltic Sea from an Optical Perspective
The Baltic Sea basin may be regarded as an extended fjord of the Atlantic Ocean or
as a large estuary with relatively weak tides of less than 5 cm and with broad shallow
margins. The geology of Scandinavia and the northern Baltic Sea is characterized by
rifts of old rigid rocks, whereas the shoreline of the southern Baltic consists mostly
of younger, more easily eroding rocks leading to sandy beaches and sand flats along
The Baltic Sea water is brackish in nature due to its restricted water exchange with
column is characterized by a permanent density stratification with a brackish surface
layer and heavier bottom water of higher salinity originating from the North Sea.
The permanent halocline ranges between 40 and 70 m depth. A seasonal thermocline
develops during spring and summer at depths between 15 and 20 m in most parts of
the Baltic Sea, providing another density barrier for vertical exchange. Apart from
vertical density stratification, the high fluvial input from the north and the saline
input of water from the North Sea produce a horizontal salinity gradient across the
whole Baltic Sea basin. The surface salinity decreases progressively from 8-6 in
the Baltic Sea Proper, to 6-5 in the Bothnian Sea, down to 3-2 in the Bothnian
Bay. The salinity in the surface mixed layer is hence very low compared to other
semi-enclosed seas such as, e.g., the Mediterranean Sea with 38.
The dominance of freshwater from river discharge is associated with a high con-
tent of humic substances, consisting of humic and fulvic acids. The main part of
humic substances can be measured optically and is also termed coloured dissolved
organic matter (CDOM). Another common term for CDOM is yellow substance, as
it is yellow in colour because of its high absorption in the blue part of the visible
spectrum.
Salinity is inversely related to CDOM: the higher the freshwater influence, the
both, surface salinity and CDOM, across the whole Baltic Sea basin. In the Bothnian
