BSEP116B Biodiversity in the Baltic Sea - Helcom

of saline water from the North Sea through the
Danish Straits (Lass & Matthäus 2008). There is a
pronounced salinity gradient from south to north:
surface water salinity averages 20 psu in the southern
Kattegat, 8 psu in the Baltic Proper and 5 psu
in the Bothnian Sea. In the innermost parts of the
Bothnian Bay and the Gulf of Finland, the water is
practically limnic with a salinity 11 psu) that primarily occurs in the deep
basins of the Baltic Proper and in the Belt Sea and
Kattegat. Adult cod, however, migrate to all basins
except the Bothnian Bay.
Physiological stress is manifested in the limited body
size and slower growth rate of some marine species
that inhabit the Baltic Sea. Bladder wrack (Fucus
vesiculosus) is found down to a salinity of 4 psu.
The Baltic bladder wrack is, however, smaller and
displays a lower photosynthetic production potential
than bladder wrack in the Atlantic (Nygård &
Ekelund 2006). On the other hand, some freshwater
species such as pike and pike perch grow faster and
larger than in most rivers or lakes.
Oxygen conditions determine life in the deep
Vertical stratification of the water column is a
predominant feature in all Baltic basins (e.g., Lass
& Matthäus 2008). Stratification is caused by a
seasonal temperature gradient and by a more
permanent halocline. Salinity stratification is due
to layering of the dense high-salinity waters of
marine origin under the fresh water originating
from runoff from land and rivers, and it is particularly
pronounced in the open Baltic Proper and the
western Gulf of Finland.
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Figure 1.2. Map with distribution limits of Baltic Sea species either from
the marine or freshwater point of view. Source: Furman et al. 1998.
The halocline prevents vertical mixing of the
water column and consequent ventilation of the
deeper layers. Oxygen in the deeper layers is
consumed by microbial and chemical processes
mostly related to the degradation of organic