214
S.P. Baden and C. Bostrom
Zostera populations living near their temperature and salinity limits in the
Baltic Sea, and thus under what normally could be considered as environmental stress, represent an interesting example of the opposite. Here, sexual
reproduction at exposed sandy bottoms has minor importance for recruitment and population maintenance, whereas vegetative propagation (clonal
growth) and dispersal by drifting plants are more common. At the Aland
Islands (northern Baltic Sea) almost complete monoclonality has been found
(Bostrom 1996; Bostrom and Bonsdorff 1997; Reusch et al. 1999a); however,
similar results have been obtained in fully saline cold-temperate Zostera
meadows (Nova Scotia, Canada: Reusch et al.1999b).
Seagrasses are three-dimensional structures rich in microhabitats and
niches hosting abundant and diverse animal assemblages. One focus within
seagrass research has been the "bottom-up" (Sand-Jensen 1977; Borum 1985;
Tomasko and Lapointe 199I) versus "top-down" (Nelson 1979, 1981; Robertson 1984; Summerson and Peterson 1984; Bronmark 1985; Heck et al. 2000)
regulating processes, investigating the interrelationship and energy flow between nutrients, primary producers, herbivores, detritivores, filter feeders and
finally primary and secondary consumers (predators). However, very few of
these investigations have aimed at a holistic view on food webs in the seagrass
meadows (Moller et al.1985; Heck et al. 2000) as most studies involve only one
or two trophic levels (see references above, Heck and Crowder 199I).
10.1.2 Aims of the Study
In this study we demonstrate features of the flora and fauna associated with
the Zostera leaf canopy along a > 1500 km salinity gradient reaching from the
fully saline (30 psu) Swedish/Norwegian border in the Skagerrak, through the
polyhaline Kattegat and the Danish straits and Kiel Bight (10-15psu), to the
Aland Islands (Finland) in the brackish (6 psu) northern Baltic Sea (Fig.l0.I).
The investigated area encompasses five large marine and estuarine sea areas
(Kattegat and Skagerrak, Gulf of Kiel, Baltic Proper, Aland Sea; Nielsen et al.
1995) thus covering both a latitudinal (60-54°N) and a longitudinal (12-200E)
gradient (Fig. 10.I). Localities 1-5 are hereafter referred to as the Swedish
west coast, while the Swedish east coast refers to localities 7-9. Kiel is referred
to as locality 6. Here we first describe the structural features of the leaf canopy
i. e., shoot density, biomass, leaf area and co-occurrence with other
phanerogams. Then we compare species composition, abundance, biomass,
diversity and functional aspects (mobility and feeding groups) of the leaf
faunal assemblage in different regions. Possible couplings between the leaf
fauna and the sediment sub-systems are discussed. In many areas, predation
is considered as an important structuring factor for seagrass leaf fauna
(Nelson 1979, 1981; Heck and Orth 1980; Heck and Crowder 1991, review;
S.P. Baden and C. Bostrom
Zostera populations living near their temperature and salinity limits in the
Baltic Sea, and thus under what normally could be considered as environmental stress, represent an interesting example of the opposite. Here, sexual
reproduction at exposed sandy bottoms has minor importance for recruitment and population maintenance, whereas vegetative propagation (clonal
growth) and dispersal by drifting plants are more common. At the Aland
Islands (northern Baltic Sea) almost complete monoclonality has been found
(Bostrom 1996; Bostrom and Bonsdorff 1997; Reusch et al. 1999a); however,
similar results have been obtained in fully saline cold-temperate Zostera
meadows (Nova Scotia, Canada: Reusch et al.1999b).
Seagrasses are three-dimensional structures rich in microhabitats and
niches hosting abundant and diverse animal assemblages. One focus within
seagrass research has been the "bottom-up" (Sand-Jensen 1977; Borum 1985;
Tomasko and Lapointe 199I) versus "top-down" (Nelson 1979, 1981; Robertson 1984; Summerson and Peterson 1984; Bronmark 1985; Heck et al. 2000)
regulating processes, investigating the interrelationship and energy flow between nutrients, primary producers, herbivores, detritivores, filter feeders and
finally primary and secondary consumers (predators). However, very few of
these investigations have aimed at a holistic view on food webs in the seagrass
meadows (Moller et al.1985; Heck et al. 2000) as most studies involve only one
or two trophic levels (see references above, Heck and Crowder 199I).
10.1.2 Aims of the Study
In this study we demonstrate features of the flora and fauna associated with
the Zostera leaf canopy along a > 1500 km salinity gradient reaching from the
fully saline (30 psu) Swedish/Norwegian border in the Skagerrak, through the
polyhaline Kattegat and the Danish straits and Kiel Bight (10-15psu), to the
Aland Islands (Finland) in the brackish (6 psu) northern Baltic Sea (Fig.l0.I).
The investigated area encompasses five large marine and estuarine sea areas
(Kattegat and Skagerrak, Gulf of Kiel, Baltic Proper, Aland Sea; Nielsen et al.
1995) thus covering both a latitudinal (60-54°N) and a longitudinal (12-200E)
gradient (Fig. 10.I). Localities 1-5 are hereafter referred to as the Swedish
west coast, while the Swedish east coast refers to localities 7-9. Kiel is referred
to as locality 6. Here we first describe the structural features of the leaf canopy
i. e., shoot density, biomass, leaf area and co-occurrence with other
phanerogams. Then we compare species composition, abundance, biomass,
diversity and functional aspects (mobility and feeding groups) of the leaf
faunal assemblage in different regions. Possible couplings between the leaf
fauna and the sediment sub-systems are discussed. In many areas, predation
is considered as an important structuring factor for seagrass leaf fauna
(Nelson 1979, 1981; Heck and Orth 1980; Heck and Crowder 1991, review;