BSEP116B Biodiversity in the Baltic Sea - Helcom

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Macoma baltica
where the influence of periodic salt-water inflows
is tangible. These patterns are more distinct closer
to the Danish Straits and the proximity to the
inflows. Since the largest recorded inflow in the
1950s, salinity has decreased in the entire Baltic
Proper and the dominance has changed from
marine species to typical brackish-water species
in the southeastern Gotland Basin (Figure 3.4.2).
Similarly, the ‘marine’ elements of the community
in the Gulf of Finland, such as the polychaete
Terebellides stroemi, have disappeared altogether.
The nature and magnitude of the salt-water
inflows have a direct influence on the strength of
the halocline and, hence, the oxygen dynamics
in the Baltic. Stratification of the water column in
combination with an increased organic loading
(which has been exacerbated by eutrophication)
has resulted in widespread hypoxia and anoxia.
This poses a severe threat to the biodiversity
of benthic invertebrate communities, which is
illustrated in the periodically very poor and low
diversity communities recorded in the southeastern
Gotland Basin and Gulf of Finland (Figure
3.4.2). During the mid-1990s, benthic communities
recovered owing to the stagnation period
that weakened stratification; however, with the
large salt-water intrusion in 1993, the halocline
strengthened and oxygen conditions deteriorated
(Laine et al. 2007, Norkko et al. 2007). In contrast,
changes in species composition are small
in the southern Bothnian Sea (SR5) and virtually
absent in the Bothnian Bay (BO3). This is due to
overall low species diversity. Nevertheless, invasive
species, such as the polychaete Marenzelleria
spp., are conspicuous elements of the system
and have established viable populations in many
areas of the Baltic Sea, as exemplified in southern
Bothnian Sea. In the Bothnian Sea, large natural
variations in abundance of the dominant species,
the amphipod Monoporeia affinis, are typical. In
this area, the influence of salt-water inflows is
minimal and there is not a strong halocline, which
would promote the formation of hypoxia. In the
Bothnian Sea, the Monoporeia populations have
been declining since the mid-1990s for reasons
that have not been clarified. However, recent
monitoring suggests that these populations may
be recovering.
When examining long-term trends from data collected
in 1965–2006, it becomes immediately
obvious that conditions were already disturbed
in the mid-1960s. Benthic invertebrate status in
the central parts of the Baltic Sea, in particular,
is more or less entirely controlled by the presence
or absence of hypoxia/anoxia. Already in
Hessle’s (1924) seminal work, hypoxia/anoxia
was reported in both coastal and open-sea areas.
However, he also reported on the presence of
species such as the polychaete Scoloplos armiger
at over 140 m depth in the eastern Gotland Basin,
which indicates that the spatial extent of hypoxic
bottom waters was limited at that time. Current
evidence suggests that the spatial and temporal
extent of oxygen deficiency has increased over
the past decades. In the light of historical work
(Hessle 1924), it is also likely that reference conditions
defined for open-sea areas in this assessment
are underestimates. Generally, Baltic benthic
macrofauna are characterized by small, shallowdwelling
species owing to low salinity and
transient hypoxia; historically, it was only in the
southern Baltic, where more mature communities
composed of deeper-dwelling larger species, e.g.,
some long-lived bivalves and large polychaetes,
could have developed (Tulkki 1965, Rumohr et al.
1996). However, currently macrobenthic communities
are severely degraded and abundances are
below a 40-year average in the entire Baltic Sea
(Norkko et al. 2007).
Functional aspects of biodiversity
The broad-scale patterns and changes in structural
biodiversity also translate into differences in functional
biodiversity. While the actual relationship