BSEP116B Biodiversity in the Baltic Sea - Helcom

the total BT identified for each main group of traits
(Table 3.4.2), and the total number of biological
traits expressed in each main group during 2000–
2006 is also presented for the selected stations
(Figure 3.4.3). Because one species may express
several traits, the number of traits often exceeds
the total number of species at a station.
The results from this analysis highlight the increase
in the number of expressed traits from the Bothnian
Bay in the north towards the southern parts of the
Baltic (Figure 3.4.3). The gradient is particularly clear
for traits such as adult longevity and size, which
reflect the successional stages described by Rumohr
et al. (1996). Only at the comparatively more marine
areas of the Arkona Basin does the functional diversity
increase substantially. Importantly, the analysis
highlights the limited functional diversity of benthic
communities in the Baltic main basins.
The loss of entire functional groups and traits
owing to widespread hypoxia has obvious implications
for the functioning of Baltic Sea ecosystems.
While no permanent species extinctions in the
benthos have necessarily occurred in the Baltic, it
is clear that abundances over great expanses have
been reduced to such a level that they may be
viewed as functionally extinct, i.e., with severely
reduced or no functional importance.
decades owing to eutrophication (Karlson et al.
2002, Diaz & Rosenberg 2008).
It seems unrealistic that BSAP targets can be
reached within the given time frame. Recovery is
likely to take decades even though initial recovery
may be rapid in areas where nutrients are dramatically
reduced and oxygen conditions improve.
3.4.3 Recommendations
Benthic invertebrates form well-defined and structured
communities that are an important component
of the Baltic Sea biodiversity. The measures
that are needed to improve the living conditions of
these organisms include reduction of eutrophication
through decreasing nutrient loads to the sea
to alleviate oxygen depletion, as well as restrictions
to physical disturbance of the environment (dredging
and trawling).
Existing fairly long time-series of data on zoobenthic
communities will be invaluable for future
assessments of the environmental impacts of
human activities and climate change research.
Thus, it is important to continue broad-scale monitoring
of benthic macrofaunal communities in the
Baltic and to maintain the frequency and intensity
of current activities.
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3.4.2 Conclusions
Soft-sediment macrofaunal communities in the
open-sea areas of the Baltic Sea are naturally
constrained by the strong horizontal and vertical
gradients in salinity. These conditions result in
strong gradients in species and functional diversity
throughout the Baltic Sea.
Obviously multiple stressors affect benthic communities
in the Baltic. However, eutrophication
has emerged as the major stressor, with symptoms
of disturbance apparent at all trophic levels in the
ecosystem. The increased prevalence of oxygendepleted
deep water is perhaps the single most
important factor influencing the structural and
functional biodiversity of benthic communities in
the open-sea areas of the Baltic Sea (Andersin et
al. 1978, Karlson et al. 2002). While hypoxia is to
some degree a natural phenomenon in the Baltic,
it is also clear that the spatial and temporal extent
of oxygen deficiency has increased over the past
3.5 Fish communities
Baltic Sea fish communities consist of representatives
of various origins: marine species, freshwater
species, migratory species, glacial relicts and alien
species. Representatives of these categories have
different preferences for environmental conditions
and the composition of fish communities therefore
varies in different regions of the Baltic Sea, primarily
depending on salinity, water temperature,
oxygen content and nutrient concentrations.
Fish perform several important roles in the ecosystem:
they act as essential consumers of planktonic
and benthic invertebrates, thus structuring the lower
food web; they serve as a food for marine top predators
(for mammals, other fish species and fish-eating
birds); they substantially facilitate pelagic-benthic
coupling; and they may also serve as transmitters of
parasites. While commercial fish species receive high
attention owing to their role as human food and