Geoscience Reference
In-Depth Information
offshore (Gidhagen 1987 , Lehmann and Myrberg 2008 ) . Figure 20.2 illustrates the
pattern of sea surface temperature in the northwestern Baltic Sea. The sequence
of binned images (i.e. 10-daily averages) shows the development and the break-
up of the seasonal summer thermocline from July to September 2001 (Kratzer and
Tet t 2009 ) . The colder filaments shown on the second plate of Fig. 20.2 are typical
features caused by coastal upwelling. Because of the permanent salinity stratifica-
tion with a brackish, lighter layer at the top, upwelling plays an important role in
keeping a cyanobacteria bloom alive. Upwelling may mix phosphorus-rich water
from below the thermocline with the nutrient-depleted upper layers, thus sustain-
ing cyanobacterial growth (Leppänen et al. 1988 ) . Up-welling seems to stimulate
mostly the production of Aphanizomenon sp. However, there is a lag of 2-3 weeks
before this effect takes place (Vahtera et al. 2005 , Lehmann and Myrberg 2008 ) , and
the initial effect is a decrease of cyanobacteria biomass due to dilution and decrease
in water temperature, which has a strong negative effect on the growth of N. spumi-
gena as this species has a growth optimum at somewhat higher temperatures than
Aphanizomenon sp. (Kononen and Leppänen 1997 ) .
20.1.5 Bio-optical Properties of Natural Waters
In 1961, Preisendorfer introduced a system which separated optical properties into
two categories - inherent and apparent (Kirk 2010 ) . Apparent optical properties
(AOPs) of a water body are all the properties depending on the geometry of the
light field, e.g. the radiant quantities radiance (I) and irradiance (E). Inherent optical
properties (IOPs) are independent of changes in the radiance distribution and depend
only on the optical substances within the aquatic medium. Examples for IOPs are the
absorption coefficient, the scattering coefficient, or the backscattering coefficient.
Changes in ocean colour are changes in the spectral variation of the sea surface
reflectance, R . R is strongly correlated to the ratio of backscattering to absorption
coefficient:
b b ) 1 ,
+
R
fb b ( a
where f
0.33 (Morel and Prieur 1977 ) . This means that both the scattering and the
absorption properties of the in-water optical constituents, as well of the water deter-
mine the spectral reflectance, and therefore the colour emerging from the sea. The
main scatterers in a natural water body are pure seawater and TSM. Total absorp-
tion is made up of the sum of absorption by water, phytoplankton pigments, TSM,
as well as CDOM.
Clear ocean waters belong to so-called optical Case-1 waters. In these waters, the
optical signal is dominated only by the sea water itself, by chlorophyll- a , and co-
varying CDOM (Morel and Prieur 1977 ) . Chlorophyll- a has two absorption peaks, a
major peak in the blue part of the electromagnetic spectrum around 440 nm and one
in the red part around 670 nm. In the green part of the spectrum, there is the lowest
=
Search WWH ::




Custom Search