216
S.P. Baden and C. Bostrom
Van der Maare11990; Wallentinus 1991) and are fairly stable both during the
growing season and from year to year. The Swedish west coast has a stratified
water mass consisting of Baltic water on top of the more saline water from the
North Sea, and generally surface salinity varies between 15 and 25 psu. However, after heavy rainfalls the surface water (down to 2 m) can show reduced
salinities for short periods and after upwelling situations high saline bottom
water can reach the surface. Thus, Zostera can experience salinity extremes
between 0 and 30 psu.
Changes in salinity along the gradient are expressed in both plants and
animals at the ecological, morphological and physiological levels. For macroalgae of marine origin the number of species decreases from about 300 to
about 100 going from the poly-saline environment in the Eastern Skagerrak
and Kattegat to the brackish Archipelago Sea, northern Baltic Sea. Among
these macro algae the number of green algal species shows a less dramatic
decline due to a complementary increase of freshwater species in the northern
Baltic Sea (Nielsen et al. 1995). The macro algal populations have, since their
recruitment into the Baltic Sea in ca. 7500 BP (Russell 1988), diverged
considerably from their marine origin, and adapted to brackish water
conditions by changes in physiology and morphology, sometimes creating
new ecotypes (Russell 1987). Salinity stress is expressed as altered cell volume,
ion content or changed concentration of organic solutes (Russell 1987). However, salinity cannot be directly linked to such differences, since reduced plant
sizes do not necessary show reduction in cell size, and plants with smaller
cells are not always smaller in thallus size (Russell 1985). The aquatic angiosperms show an opposite trend to macro algae. As all the angiosperms in the
study area, except Zostera, Zannichellia palustris and Ruppia spp., are of
lim netic origin, the number of flowering plants increases with decreasing
salinity, being 5 in the Kattegat and around 20 in the Baltic Archipelago Sea
area (Leppakoski et al. 1999; Snoeijs 1999). In fully saline water, carbonate is
the dominating form of carbon and both macro algae and angiosperms rely
on an uptake of carbon via carbonate instead of carbon dioxide. With decreasing salinity, the availability of carbon changes from carbonate to carbon
dioxide, causing carbon limitation, size reduction and lowered productivity in
Zostera (Pinnerup 1980; Stevenson 1988; Ohlsson and Andersson 1990;
Hellblom and Bjork 1999).
The fauna of the Baltic can be divided into three groups dependent on their
evolutionary background: marine immigrants, indigenous brackish water
species and limnetic immigrants. Most species have either marine or limnetic
background and live close to their physiological tolerance limit with regard to
salinity. The physiological strain of salinity affecting, for example, the
osmoregulation can result in increased energy expenditure and reduced size
as found for the marine bivalve Mytilus edulis (Remane 1934; Tedengren and
Kautsky 1986; Tedengren et al.1990), which recently has been divided into two
S.P. Baden and C. Bostrom
Van der Maare11990; Wallentinus 1991) and are fairly stable both during the
growing season and from year to year. The Swedish west coast has a stratified
water mass consisting of Baltic water on top of the more saline water from the
North Sea, and generally surface salinity varies between 15 and 25 psu. However, after heavy rainfalls the surface water (down to 2 m) can show reduced
salinities for short periods and after upwelling situations high saline bottom
water can reach the surface. Thus, Zostera can experience salinity extremes
between 0 and 30 psu.
Changes in salinity along the gradient are expressed in both plants and
animals at the ecological, morphological and physiological levels. For macroalgae of marine origin the number of species decreases from about 300 to
about 100 going from the poly-saline environment in the Eastern Skagerrak
and Kattegat to the brackish Archipelago Sea, northern Baltic Sea. Among
these macro algae the number of green algal species shows a less dramatic
decline due to a complementary increase of freshwater species in the northern
Baltic Sea (Nielsen et al. 1995). The macro algal populations have, since their
recruitment into the Baltic Sea in ca. 7500 BP (Russell 1988), diverged
considerably from their marine origin, and adapted to brackish water
conditions by changes in physiology and morphology, sometimes creating
new ecotypes (Russell 1987). Salinity stress is expressed as altered cell volume,
ion content or changed concentration of organic solutes (Russell 1987). However, salinity cannot be directly linked to such differences, since reduced plant
sizes do not necessary show reduction in cell size, and plants with smaller
cells are not always smaller in thallus size (Russell 1985). The aquatic angiosperms show an opposite trend to macro algae. As all the angiosperms in the
study area, except Zostera, Zannichellia palustris and Ruppia spp., are of
lim netic origin, the number of flowering plants increases with decreasing
salinity, being 5 in the Kattegat and around 20 in the Baltic Archipelago Sea
area (Leppakoski et al. 1999; Snoeijs 1999). In fully saline water, carbonate is
the dominating form of carbon and both macro algae and angiosperms rely
on an uptake of carbon via carbonate instead of carbon dioxide. With decreasing salinity, the availability of carbon changes from carbonate to carbon
dioxide, causing carbon limitation, size reduction and lowered productivity in
Zostera (Pinnerup 1980; Stevenson 1988; Ohlsson and Andersson 1990;
Hellblom and Bjork 1999).
The fauna of the Baltic can be divided into three groups dependent on their
evolutionary background: marine immigrants, indigenous brackish water
species and limnetic immigrants. Most species have either marine or limnetic
background and live close to their physiological tolerance limit with regard to
salinity. The physiological strain of salinity affecting, for example, the
osmoregulation can result in increased energy expenditure and reduced size
as found for the marine bivalve Mytilus edulis (Remane 1934; Tedengren and
Kautsky 1986; Tedengren et al.1990), which recently has been divided into two