BSEP116B Biodiversity in the Baltic Sea - Helcom

Large nutrient inputs in combination with long
residence times mean that nutrients discharged
to the sea remain in the sea for a long time, even
decades, before being flushed out of the Baltic Sea
into the Skagerrak and North Sea surface waters or
being buried into the sediments.
6.5.2 Eutrophication status of the
Baltic Sea
The eutrophication status of the Baltic Sea in
2001–2006 was extensively assessed, analysed
and evaluated in the HELCOM integrated thematic
assessment of eutrophication (HELCOM 2009a).
According to the report, the overall eutrophication
status of the Baltic Sea is unacceptable. Only
13 of the areas assessed in the report were classified
as being ’eutrophication non-problem areas’,
while 176 areas were classified as ‘eutrophication
problem areas’. The non-problem areas were
found in the Gulf of Bothnia and in the Kattegat
(Figure 6.5.1). The overall view, however, is
not completely dark because nutrient inputs to
the Baltic seem to have decreased slightly from
1995–2000 to 2001–2006. In most sub-basins,
the highest surface concentrations of nutrients
were observed in the 1980s and during the past
two decades there have been encouraging signs
of decreasing surface nutrient concentrations in
many of the sub-basins.
a major problem in the Baltic Sea. Harmful algal
blooms represent periods of reduced biodiversity
and the toxins produced by algae are a threat to
other organisms.
Extensive seagrass meadows and perennial macroalgal
communities harbour the highest biodiversity
found in coastal shallow-water areas. In the
HELCOM Red List of Marine and Coastal Biotopes
and Biotope Complexes, eutrophication was considered
to be the main threat, along with general
pollution, to the marine and coastal biotopes and
biotope complexes of the Baltic Sea (HELCOM
1998).
Eutrophication has complex effects on the state of
submerged aquatic vegetation: (1) reduced light
penetration through the water column, caused by
increased pelagic production, limits the depth penetration
of submerged species such as eelgrass and
bladder wrack; (2) increased sedimentation can
prevent the settlement of new specimens on the
seafloor and reduces the amount of suitable substrate
to be colonized by perennial species on all
types of substrates; and (3) the excess of nutrients
during the whole vegetation period often favours
opportunistic species with short life cycles and
rapid development over the perennial species with
lower productivity, causing a shift in community
composition.
6.5.3 Effects of eutrophication on
biodiversity
Eutrophication has direct as well as indirect negative
impacts on biodiversity. The manifestations
of the large-scale eutrophication problem are
well known in most parts of the Baltic Sea; these
include turbid water caused by high quantities of
planktonic algae and other planktonic organisms,
mats of macroalgae stranded on shores, reduced
distribution of benthic habitats such as eelgrass
meadows, or oxygen depletion resulting in the
death of benthic animals and fish.
The abundance of phytoplankton reflects the
productivity of the planktonic ecosystem. Phytoplankton
blooms in spring and summer are
periods of naturally high production supplying
energy to the ecosystem. However, excessive algal
blooms and especially blooms of harmful algae,
such as cyanobacteria or certain haptophytes, are
The composition of animal communities living on
the seafloor of the Baltic Sea reflects the conditions
of the environment. In the eutrophication
process, broad-scale changes in the composition
of the communities usually accompany the
increasing organic enrichment of the sediments.
At advanced stages of eutrophication, oxygen
depletion becomes common. In many areas of
the Baltic, the seafloor animals are exposed to
widespread oxygen depletion or even complete
anoxia. As a result, the biodiversity on the seafloor
is reduced or animal communities are completely
destroyed if anoxia is long lasting (see also
Chapter 3.4, Benthic invertebrate communities).
Permanent anoxia is common in deep, permanently
stratified basins of the Baltic Sea, such
as the Gotland Basin. In shallow areas, oxygen
depletion mainly occurs seasonally.
The effects of eutrophication are also manifested
in fish communities. In principle, eutrophication
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