Monitoring the Bio-optical State of the Baltic Sea Ecosystem with Remote Sensing and Autonomous In Situ Techniques - The Baltic Sea Basin - page 409

Bay the water is visibly coloured brown due to the high runoff of humic substances
originating from bogs, lakes, and rivers (Kirk 2010 ) .
Besides the CDOM content, the phytoplankton biomass, given as the concen-
tration of the main phytoplankton pigment chlorophyll- a , and the load of total
suspended matter (TSM) characterize a water body optically . 1 In coastal areas, ter-
restrial and river runoff in addition to wind-driven re-suspension of sediments leads
to high TSM loads.

20.1.2 Seasonal Variations in Optical Properties

Major seasons of high runoff are the thawing period in spring and when the annual
precipitation reaches its maximum in summer (Voipio 1981 ) . Along with terrestrial
runoff, precipitation increases the input of nutrients and dissolved and particulate
matter, but decreases the salinity (Meier and Kauker 2003 ) .
According to Voipio ( 1981 ) , the spring bloom in the southern and central parts
of the Baltic Proper occurs generally in the second half of April, whereas in the
northern Baltic Sea it occurs in early May, somewhat later in the Bothnian Sea, and
in the Northern Bothnian Bay not until June, which is partially related to a later
development of the seasonal thermocline. However, the spring bloom has shifted
forward since the 1980s and may be already observed between March and April in
the southern Baltic, in early April in the Northern Baltic, in mid-April in the Gulf of
Finland (Fleming and Kaitala 2007 ) , and in the central Gotland Sea in May (Siegel
and Gerth 2008 ) . From the end of June or early July onwards plankton blooms of
nitrogen-fixing, filamentous cyanobacteria start to occur in the Baltic Proper con-
sisting mostly of Nodularia spumigena, Aphanizomenon sp., and - in low-salinity
areas - of several Anabaena species. This annual summer phytoplankton bloom is
exceptional in its intensity, extent, and duration. Until early autumn, the extensive
blooms rise to the surface during calm and stable weather conditions. Scientific
reports of filamentous cyanobacteria in the Baltic Sea date back to the middle of the
nineteenth century. Paleo-oceanographic studies indicate their occurrence as long as
the Baltic has been a brackish sea (Bianchi et al. 2000 ) .

20.1.3 Eutrophication in the Baltic Sea

Eutrophication has been identified as the main environmental problem of the Baltic
Sea ecosystem (HELCOM 2007 ) . It is caused by a combination of increased nutri-
ent input from land and atmosphere, which increases phytoplankton biomass. The
increased organic production leads to organic matter enrichment in the bottom
sediments after the spring bloom, which in turn leads to a higher consumption of

1 The CDOM content is measured in terms of absorption at 440 nm; unit: [m -1 ]), chlorophyll- a
concentration is measured in units of [ g/l]) and TSM load is measured in units of [g/m 3 ]). μ