The Leaf Canopy of Seagrass Beds: Faunal Community in a Salinity Gradient
217
species, M. edulis (Swedish west coast) and M. trossulus (Baltic Sea) (Vainola
and Hvilsom 1991; Seed 1992). As only a restricted number of marine and
limnetic animals have adapted to the brackish environment the total faunal
assemblage decreases from> 1500 to :::::50 in the study area (Kautsky and van
der Maarel 1990; Bonsdorff and Pearson 1999). This reduction in species
diversity allows for development of generalistic behaviour and expanded
niche breadth of marine species in brackish water (Dahl 1974). Such structural and functional changes in biodiveristy reduce ecosystem complexity
and lead to fewer alternative energy flow pathways (Elmgren 1984; Tedengren
1990).
10.2.2 Physical Settings and Substrate Characteristics
On the Swedish west coast (localities 1-5), the main habitats for Zostera are
sheltered muddy bottoms; however, it may also grow on more exposed
localities. The vertical distribution of Zostera beds in this area is mainly between 0.7 and 3 m (Baden and Pihl 1984) but low-density patches can grow
down to 8 m. In the Baltic Sea (localities 7-9) Zostera is found from 1 to 7 m
depth and, in contrast, the habitat requirements are exclusively exposed, or
moderately exposed shores with sandy sediments. The Kiel Bight (site 6) lies
in a transition zone between marine (North Sea) and brackish (Baltic Proper)
water and show fluctuations (1O-18psu) in salinity. In this region Zostera is
found both at exposed sandy shores and in sheltered muddy embayments
(Reusch 1994; Lotze 1998). Such enclosed bays north of this region are
generally dominated by marsh plants (Scirpus spp.) in the upper littoral,
succeeded by reed belts (Phragmites australis) and fresh water angiosperms
(Potamogeton spp., Myriophyllum spp, Ranunculus spp.) or charophytes
(Chara spp.). For all localities the openness of the bay, measured as effective
fetch (Lf in km: Hakansson and Jansson 1983), shows that localities 1-3 are
sheltered having Lf of 0.2-1.6 km, localities 4-5 are exposed having Lf close to
3 km and localities 6-9 are all very exposed with Lf more than 4 km. The
effective fetch of localities 4-5 is similar to the exposure of Baltic localities.
The temperature range at all localities is similar being between 0 and 25°C
(Pihl and Rosenberg 1982; Baden and Pih11984) with about 0-3 months of ice
cover on the West coast and 1-4 months of ice cover in the Baltic (Leppakoski
and Bonsdorff 1989).
Sediment and particle trapping are characteristic features of seagrass beds
(Ward et al.I984).As a consequence of exposure, the localities on the Swedish
west coast have higher sedimentation rates compared with the Baltic localities, building up a muddy substrate with an organic content of2-25 % (Baden
and Pihl 1984) compared with <2 % (Bostrom and Bonsdorff 1997) at the
Baltic localities (Fig. 10.2). On the west coast, the C/N ratio of sedimenting
217
species, M. edulis (Swedish west coast) and M. trossulus (Baltic Sea) (Vainola
and Hvilsom 1991; Seed 1992). As only a restricted number of marine and
limnetic animals have adapted to the brackish environment the total faunal
assemblage decreases from> 1500 to :::::50 in the study area (Kautsky and van
der Maarel 1990; Bonsdorff and Pearson 1999). This reduction in species
diversity allows for development of generalistic behaviour and expanded
niche breadth of marine species in brackish water (Dahl 1974). Such structural and functional changes in biodiveristy reduce ecosystem complexity
and lead to fewer alternative energy flow pathways (Elmgren 1984; Tedengren
1990).
10.2.2 Physical Settings and Substrate Characteristics
On the Swedish west coast (localities 1-5), the main habitats for Zostera are
sheltered muddy bottoms; however, it may also grow on more exposed
localities. The vertical distribution of Zostera beds in this area is mainly between 0.7 and 3 m (Baden and Pihl 1984) but low-density patches can grow
down to 8 m. In the Baltic Sea (localities 7-9) Zostera is found from 1 to 7 m
depth and, in contrast, the habitat requirements are exclusively exposed, or
moderately exposed shores with sandy sediments. The Kiel Bight (site 6) lies
in a transition zone between marine (North Sea) and brackish (Baltic Proper)
water and show fluctuations (1O-18psu) in salinity. In this region Zostera is
found both at exposed sandy shores and in sheltered muddy embayments
(Reusch 1994; Lotze 1998). Such enclosed bays north of this region are
generally dominated by marsh plants (Scirpus spp.) in the upper littoral,
succeeded by reed belts (Phragmites australis) and fresh water angiosperms
(Potamogeton spp., Myriophyllum spp, Ranunculus spp.) or charophytes
(Chara spp.). For all localities the openness of the bay, measured as effective
fetch (Lf in km: Hakansson and Jansson 1983), shows that localities 1-3 are
sheltered having Lf of 0.2-1.6 km, localities 4-5 are exposed having Lf close to
3 km and localities 6-9 are all very exposed with Lf more than 4 km. The
effective fetch of localities 4-5 is similar to the exposure of Baltic localities.
The temperature range at all localities is similar being between 0 and 25°C
(Pihl and Rosenberg 1982; Baden and Pih11984) with about 0-3 months of ice
cover on the West coast and 1-4 months of ice cover in the Baltic (Leppakoski
and Bonsdorff 1989).
Sediment and particle trapping are characteristic features of seagrass beds
(Ward et al.I984).As a consequence of exposure, the localities on the Swedish
west coast have higher sedimentation rates compared with the Baltic localities, building up a muddy substrate with an organic content of2-25 % (Baden
and Pihl 1984) compared with <2 % (Bostrom and Bonsdorff 1997) at the
Baltic localities (Fig. 10.2). On the west coast, the C/N ratio of sedimenting