222
S.P. Baden and C. Bostrom
Kiel and high shoot density at bland. Biomasses of Zostera at the two
northernmost localities (Asko, Aland) are low and similar, i. e. around 40 g
DW . m- 2 • The leaf area of the Zostera follows the biomass pattern (Fig. 10.3)
but decreases by half in the transition zone (Kiel area) (3.9 and 4.9 m 2 • m- 2 at
Kiel and bland, respectively) and is further reduced in the northern Baltic
(1.4 m 2 • m- 2 at Asko and Aland). The complexity of the leaf canopy in terms of
shoot density is less than half at localities 6-9 compared to the west coast thus
ranging between 200 and 800 shoots· m 2 in the Baltic Sea. As the contribution
of limnetic phanerogams increases with decreasing salinity, co-occurrence of
Zostera with other flowering plants is common at the northernmost localities
(8-9). Potamogeton pectinatus, in particular, may form mixed stands with
Zostera at shallower depths. Association with brown algae (Fucus vesiculosus,
Chorda filum) is less common, but may increase the above-ground complexity
of Zostera meadows at sandy bottoms with gravel or boulders, while matforming, loose-lying algae constitute a serious threat to Zostera sites in the
northern Baltic Sea (Bostrom and Bonsdorff 1997) and elsewhere (KrukDowgiallo 1991; Schramm 1996).
10.4.2 Leaf Fauna
The Zostera leaf fauna is here defined as the sessile or motile fauna living on
the leaves, in contrast to the intermediate or large predators moving among
the leaves (see Sect. 10.4.5). The fauna is often associated with epiphytic
filamentous algae, detritus, microbial diatoms and bacteria, as described by
Jacobs (1982) and Baden et al. (2001), and consists mainly of detritivores,
grazers and suspensionlfilter feeders (Bronmark 1985; Baden 1990).
The abundance of leaf fauna decreases successively from being high
(250-80 10 3 ·m- 2 ) at localities 1-3, intermediate (6-40 10 3 ·m- 2 ) at localities 4-7
and low (1-l.5 10 3 m- 2 ) at localities 8 and 9 (Figs. 10.4, 10.7). At all localities,
the fauna is mainly composed of crustacean amphipods and isopods,
molluscs and copepods. At localities 1-5 crustaceans are dominated by
the detritivorous amphipods Erichtonius difformis, Corophium insidiosum
and Microdeutopus gryllotalpa. At localities 6-9 the crustacean assemblage is
dominated by Idotea baltica, while E. difformis (only locality 6), Jaera
albifrons and Gammarus spp. occur in smaller numbers. However, the
brackish northern Baltic Zostera meadows support only one species with an
analogous function to the tube building amphipods at other localities, namely
Leptocheirus pilosus. This amphipod is generally rare, but has been found on
Zostera leaves in the Tvarminne area at densities around 300 individuals· m- 2
(Lappalainen et al.1977; Bostrom 1996). In the fully marine regions (localities
1-6) the grazing gastropod fauna is dominated by Hydrobia spp., Rissoa spp.,
Littorina spp. and Gibbula cineraria. In brackish water (localities 7-9),
S.P. Baden and C. Bostrom
Kiel and high shoot density at bland. Biomasses of Zostera at the two
northernmost localities (Asko, Aland) are low and similar, i. e. around 40 g
DW . m- 2 • The leaf area of the Zostera follows the biomass pattern (Fig. 10.3)
but decreases by half in the transition zone (Kiel area) (3.9 and 4.9 m 2 • m- 2 at
Kiel and bland, respectively) and is further reduced in the northern Baltic
(1.4 m 2 • m- 2 at Asko and Aland). The complexity of the leaf canopy in terms of
shoot density is less than half at localities 6-9 compared to the west coast thus
ranging between 200 and 800 shoots· m 2 in the Baltic Sea. As the contribution
of limnetic phanerogams increases with decreasing salinity, co-occurrence of
Zostera with other flowering plants is common at the northernmost localities
(8-9). Potamogeton pectinatus, in particular, may form mixed stands with
Zostera at shallower depths. Association with brown algae (Fucus vesiculosus,
Chorda filum) is less common, but may increase the above-ground complexity
of Zostera meadows at sandy bottoms with gravel or boulders, while matforming, loose-lying algae constitute a serious threat to Zostera sites in the
northern Baltic Sea (Bostrom and Bonsdorff 1997) and elsewhere (KrukDowgiallo 1991; Schramm 1996).
10.4.2 Leaf Fauna
The Zostera leaf fauna is here defined as the sessile or motile fauna living on
the leaves, in contrast to the intermediate or large predators moving among
the leaves (see Sect. 10.4.5). The fauna is often associated with epiphytic
filamentous algae, detritus, microbial diatoms and bacteria, as described by
Jacobs (1982) and Baden et al. (2001), and consists mainly of detritivores,
grazers and suspensionlfilter feeders (Bronmark 1985; Baden 1990).
The abundance of leaf fauna decreases successively from being high
(250-80 10 3 ·m- 2 ) at localities 1-3, intermediate (6-40 10 3 ·m- 2 ) at localities 4-7
and low (1-l.5 10 3 m- 2 ) at localities 8 and 9 (Figs. 10.4, 10.7). At all localities,
the fauna is mainly composed of crustacean amphipods and isopods,
molluscs and copepods. At localities 1-5 crustaceans are dominated by
the detritivorous amphipods Erichtonius difformis, Corophium insidiosum
and Microdeutopus gryllotalpa. At localities 6-9 the crustacean assemblage is
dominated by Idotea baltica, while E. difformis (only locality 6), Jaera
albifrons and Gammarus spp. occur in smaller numbers. However, the
brackish northern Baltic Zostera meadows support only one species with an
analogous function to the tube building amphipods at other localities, namely
Leptocheirus pilosus. This amphipod is generally rare, but has been found on
Zostera leaves in the Tvarminne area at densities around 300 individuals· m- 2
(Lappalainen et al.1977; Bostrom 1996). In the fully marine regions (localities
1-6) the grazing gastropod fauna is dominated by Hydrobia spp., Rissoa spp.,
Littorina spp. and Gibbula cineraria. In brackish water (localities 7-9),