Monitoring the Bio-optical State of the Baltic Sea Ecosystem with Remote Sensing and Autonomous In Situ Techniques - The Baltic Sea Basin - page 411

to improve access to light, subsequently building up as surface accumulations.
Furthermore, the gas vacuoles of the cyanobacteria reflect strongly, which makes
them visible in satellite imagery. The image also demonstrates how cyanobacte-
ria blooms act as visible tracers of Baltic Sea dynamics. The horizontal surface
current field in the Baltic Sea has a weak cyclonic pattern with anticlockwise rota-
tion (Kullenberg 1981 , Stigebrandt 2001 ) . Residual currents from the north occur
along the Swedish coast and from the south along the Finnish coast (Kahru et al.
1995 , Victorov 1996 ) . The meso-scale features of the cyanobacterial bloom seen in
Fig. 20.1 indicate horizontal eddies and fronts.
Kahru ( 1997 ) discussed a potential increase of cyanobacteria blooms in the 1990s
based on sea surface temperature (SST) satellite data from 1982 to 1994. Siegel et al.
( 2006 ) showed an increase in summer temperature in the Baltic Sea during the 1990s
and early this century and could connect it to the mass occurrence of cyanobacte-
ria (Siegel and Gerth 2008 ) . Foremost the toxic cyanobacteria species Nodularia
spumigena is favoured by higher water temperatures in its growth (Kononen and
Leppänen 1997 ) .
In summer, strong thermal stratification can be studied in the Baltic Sea, along
with local wind-driven coastal upwelling of colder, subsurface water at 5-20 km
Fig. 20.2 Sea Surface Temperature (SST) in the northwestern Baltic Sea, derived from
NOAA/AVHRR data of the period 7 July-4 September 2001, binned into composite images of
10 days each. The 10-day composites reveal temperature differences close to the coast, most prob-
ably caused by coastal upwelling. Station BY 31 is Landsort Deep, the deepest part of the Baltic
Sea with 459 m depth (from: Kratzer and Tett 2009 )