BSEP116B Biodiversity in the Baltic Sea - Helcom

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Depth limit (m)
the Gulf of Bothnia in the north and the Gulf of
Finland in the east, where it lives at its salinity tolerance
limit. This limit is around a salinity of 4 psu,
with occasional reports of isolated and sparse
populations (individuals) at salinities down to
2 psu, although these are special cases in sites with
heavy nutrient loads and probably a higher local
salinity (Waern 1952, Pekkari 1956). Although the
habitat requirements of F. vesiculosus with respect
to salinity are fulfilled almost everywhere in the
Baltic Sea, the alga also requires a firm substrate
and low to moderate exposure to ice and waves in
order to form stable and healthy communities. The
distribution of the species has a strong biological
control through competition for space with other
perennial macroalgae in the southwestern parts
of the Baltic Sea where salinity is higher, while
this control disappears in areas with lower salinity
where other perennial macroalgae are not able
to compete for the substrate. Thus, in the inner
Baltic Sea, the distribution of the species is mainly
controlled by environmental variables such as light
availability (Figure 3.2.2).
14
12
Baltic entrance
10
8
6
4
2
0
0 2 4 6 8 10 12
Secchi depth (m)
Depth limit (m)
14
12
Central Baltic
10
8
6
4
2
0
0 2 4 6 8 10 12
Secchi depth (m)
Figure 3.2.2. Relation between depth distribution of Fucus
vesiculosus and water transparency in the Baltic Sea entrance area
(including data from Kattegat, the Danish Belts Øresund and the
Baltic between Zealand, Denmark and Scania, Sweden) and the
central Baltic (including data from the Kiel Bight, Hanö Bay, the
Arkona Sea, the Baltic Proper, the Gulf of Riga and the Gulf of
Finland). Modified from Torn et al. 2006.
Several studies indicate that F. vesiculosus in the
Baltic Sea is threatened by pollution both locally
close to point sources and on a larger scale owing
to a general eutrophication of the Baltic Sea (e.g.
Kautsky et al. 1992, Schramm 1996). Decreases in
distribution and the disappearance of F. vesiculosus
have been reported from locally polluted areas
such as the inner Stockholm Archipelago, Tallinn
Bay, the Gulf of Gdańsk, the Helsinki Archipelago,
and the Gulf of Riga. Decreases have also been
reported from areas with little local pollution, e.g.,
the Lagskär skerries, the Tvärminne area and the
Öregrund Archipelago, and it has been suggested
that large-scale hydrographic changes may have
contributed to the negative development in Fucus
communities (Torn et al. 2006). However, in recent
years bladder wrack has increased its depth extension
in the northern Baltic Proper and Åland Sea
(Gräsö area). It has also re-established on several
sites where it was absent in the early 1990s (H.
Kautsky, pers. comm., in prep.).
Zostera marina (Eelgrass)
Seagrasses are marine angiosperms providing
important ecological components of coastal ecosystems
worldwide. Out of 66 known large seagrass
species, only two inhabit the Baltic Sea and
only one, eelgrass Zostera marina L., is found to
the northern limit of the Baltic Proper. This species
provides a structure for the benthic environment
and associated communities on soft, sandy
bottoms.
Zostera marina inhabits mixed and sandy substrates
with moderate wave exposure. Wave and ice
prevent this species from inhabiting the shallowest
parts (< 2 m) of the coastal slope, while light availability
limits its depth distribution.
The depth distribution of eelgrass is not dependent
on the salinity in the Baltic Sea (Baden & Boström
2001). This is not surprising because eelgrass can
tolerate a broad range in salinity (5–35 psu, den
Hartog 1970). A study in the Kattegat/Belt region
on eelgrass maximum depth limit also found no
correlation with salinity (Greve & Krause-Jensen
2003). Nonetheless, it takes longer to re-establish
deep eelgrass communities at the lower salinity
limit in the inner Baltic where eelgrass mainly
grows vegetatively (Reusch et al. 1999) than at the
entrance of the Baltic where seeds play an important
role in its dispersal (Nielsen & Olesen 1994).
In the Baltic Sea, there have been considerable
changes in the distribution characteristics of eelgrass
in the past. The wasting diseases almost eliminated
the species from the southern Baltic Sea in
the 1930s. In most of the areas on the Danish and
German coasts, the former distribution range has
not recovered or the conditions have deteriorated
during recent decades. At present, there is a nearcomplete
lack of the species on the Polish, Lithuanian,
and Latvian coasts. The area and abundance