BSEP116B Biodiversity in the Baltic Sea - Helcom

3 COMMUNITIES
Communities are assemblages of species within
an ecosystem. The composition of species within a
community influences fundamental processes such
as the productivity, stability and trophic interactions
within the food web and thereby also the
overall functioning of the ecosystem. The communities
specifically addressed in this chapter are
Baltic phytoplankton, habitat-forming species,
zooplankton, benthic invertebrates, and fish. The
different communities form an intricate web with
predatory, competitive, synergistic and commensal
interactions. Thus, changes in one community
inevitably affect other components of the Baltic
biodiversity (Figure 3.1).
The Baltic Sea Action Plan includes the ecological
objective ’Thriving and balanced communities of
plants and animals’. Owing to their fundamental
role in the ecosystem, assessment of the composition
of the communities as well as of their key
species provides a central component for determining
the conservation status of the Baltic Sea.
3.1 Phytoplankton
communities
The Baltic Sea phytoplankton community is a
diverse mixture of microscopic algae representing
several taxonomic groups, with more than 1 700
species recorded (Hällfors 2004). The phytoplankton
composition in different sub-basins of the
Baltic Sea depends, among other things, on the
salinity (Carstensen et al. 2004, Wasmund & Siegel
2008). Diatoms and dinoflagellates are characteristic
in the saline waters of the southern Baltic
Sea, the Belt Sea and the Kattegat, whereas phytoplankton
groups preferring less saline water, such
as cyanobacteria and chlorophytes, are commonly
found in the northern Baltic Sea. The geographic
distribution pattern complicates the use of certain
phytoplankton groups as Baltic-wide indicators
of the ecological state (Carstensen et al. 2004,
Gasiūnaitė et al. 2005).
Phytoplankton are dominant primary producers in
both the coastal and open Baltic Sea, and serve
as the energy source for the higher components
of the food web. The socio-economic importance
of phytoplankton is largely associated with the
negative impact of algal blooms and their potential
toxicity (HELCOM 2006a). Algal blooms decrease
water transparency and light availability, thus
affecting submerged vegetation in the coastal
areas. In addition, blooms increase the sedimentation
of organic material, which, in turn, increases
oxygen consumption in near-bottom waters and
induces internal nutrient loading (HELCOM 2002).
Dense and potentially toxic blooms reduce the
recreational use of the water, pose a health risk,
and have economic implications, e.g., for fisheries.
Blooms of toxic phytoplankton species may
also inhibit the growth and reproduction of other
aquatic organisms (e.g., Uronen 2007).
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Figure 3.1. Food web illustration depicting the links among Baltic Sea communities (Hermanni Backer).