Monitoring the Bio-optical State of the Baltic Sea Ecosystem with Remote Sensing and Autonomous In Situ Techniques - The Baltic Sea Basin - page 414

Fig. 20.3 Secchi depth measurement (Photo: Susanne Kratzer, Irish Sea 1997)

been explained by the increase in phytoplankton biomass (Sandén and Håkansson
1996 ) and therefore Secchi depth has been used as an indicator for eutrophication. In
the coastal areas and the Stockholm archipelago, a recovery of the Secchi depth has
been observed over the last decade, whereas the decreasing trend seems to continue
for the open Baltic Sea (Bernes 2006 ) .
Another way to measure water transparency is by measuring the rate of decrease
of light with depth. Light energy which enters the water from above and is transmit-
ted downwards is known as downwelling irradiance, E d . In the case of monochro-
matic light with uniform angular distribution E d diminishes in an approximately
exponential manner with depth:

E d (0) e − K d Z (Beer's Law) E d ( z ) =

where E d (0) and E d ( z ) are the values of downward irradiance just below the surface
and at depth z , respectively (Kirk 2010 ) . K d is the average value of the diffuse atten-
uation coefficient for the downwelling light field over any defined depth interval;
note that K d , the rate of light decrease, is wavelength dependent. The diffuse attenu-
ation coefficient and Secchi depth inversely correlated (Kratzer et al. 2003 , Kratzer
and Tett 2009 ) . They are influenced not only by phytoplankton or Chlorophyll- a
concentration but also by CDOM and TSM load. Secchi depth can therefore only
be used as an indicator for eutrophication where phytoplankton clearly dominates
water attenuation (Wasmund et al. 2001 ) or where there is little variability in any of
the two other optical components.
Chemical parameters such as the nutrient concentrations cannot be retrieved opti-
cally as they do not sufficiently interact with light. However, they may be retrieved
indirectly, e.g. by relating Secchi depth to the total nitrogen concentration (Tett et al.
2003 ) .