The Leaf Canopy of Seagrass Beds: Faunal Community in a Salinity Gradient
219
may contribute to the enrichment of shallow coastal sediments (Pihl et al.
1999).
The growing conditions of Zostera along the salinity gradient are also
reflected in the morphology and bed structure of the plants (Fig. 10.2). On the
Swedish west coast, Zostera forms dense and continuous meadows and has
generally long, wide leaves as described for plants living under sheltered conditions (Den Hartog 1970). The local physical settings determine the coverage
pattern of Baltic Zostera beds, which are far more patchy in character
compared to the Swedish. west coast, and support shorter shoots in lower
densities. Similarly, strong water movements in exposed areas create small
sized rocky shore algae with low biomass compared to algal belts in sheltered
areas (Snoeijs 1999). Two growth forms of eelgrass are common both on the
west coast and in the Baltic: a shorter dense shallow water form and a sparse
taller form at the maximum depth of distribution. Such within-site size
differences in morphology may be explained by compensation in vertical
growth in response to reduced light levels, or by differing sediment characteristics, i. e. higher stability, organic content and increased nutrient levels at
the lower distribution limit in contrast to the less-stable and organic-poor
sediment at the upper limit of distribution (Den Hartog 1970).
10.3 Methods
10.3.1 Vegetation and Leaf Canopy Fauna
Along the Swedish west coast (1997) and in the Baltic (1998) the leaf fauna
was sampled semi-randomly by a diver using a net bag on a frame, covering
the depth range of the eelgrass bed. Similar bag-sampling techniques have
also been used for collection of Fucus fauna in the Baltic (Haahtela 1969;
Fagerholm 1978). The sampling (bag size and number of samples) was
optimized according to the height and density of the leaf canopy. Thus, at
localities 1-5 plants and animals were sampled with a bag covering an area of
0.123 m 2 (height: 75 cm, mesh size: 200/lm, n=6) and at localities 6-9 with a
bag covering 0.031 m 2 (height: 50 cm, mesh size 250 /lm, n=10). Samples were
obtained by placing the net bag over the leaf canopy, and cutting the leaves
above the sediment surface. At localities 1-5 the percentage coverage of
Zostera was estimated in a randomly thrown frame of 0.25 m 2 (n=15), while a
measure of coverage at localities 6-9 was performed by counting shoot
densities within a randomly thrown frame covering 0.0625 m 2 (n=40). Visual
observations of mobile epifauna (shrimps, crabs, benthivorous fish) were
carried out during all sampling. In the laboratory the leaves were washed with
219
may contribute to the enrichment of shallow coastal sediments (Pihl et al.
1999).
The growing conditions of Zostera along the salinity gradient are also
reflected in the morphology and bed structure of the plants (Fig. 10.2). On the
Swedish west coast, Zostera forms dense and continuous meadows and has
generally long, wide leaves as described for plants living under sheltered conditions (Den Hartog 1970). The local physical settings determine the coverage
pattern of Baltic Zostera beds, which are far more patchy in character
compared to the Swedish. west coast, and support shorter shoots in lower
densities. Similarly, strong water movements in exposed areas create small
sized rocky shore algae with low biomass compared to algal belts in sheltered
areas (Snoeijs 1999). Two growth forms of eelgrass are common both on the
west coast and in the Baltic: a shorter dense shallow water form and a sparse
taller form at the maximum depth of distribution. Such within-site size
differences in morphology may be explained by compensation in vertical
growth in response to reduced light levels, or by differing sediment characteristics, i. e. higher stability, organic content and increased nutrient levels at
the lower distribution limit in contrast to the less-stable and organic-poor
sediment at the upper limit of distribution (Den Hartog 1970).
10.3 Methods
10.3.1 Vegetation and Leaf Canopy Fauna
Along the Swedish west coast (1997) and in the Baltic (1998) the leaf fauna
was sampled semi-randomly by a diver using a net bag on a frame, covering
the depth range of the eelgrass bed. Similar bag-sampling techniques have
also been used for collection of Fucus fauna in the Baltic (Haahtela 1969;
Fagerholm 1978). The sampling (bag size and number of samples) was
optimized according to the height and density of the leaf canopy. Thus, at
localities 1-5 plants and animals were sampled with a bag covering an area of
0.123 m 2 (height: 75 cm, mesh size: 200/lm, n=6) and at localities 6-9 with a
bag covering 0.031 m 2 (height: 50 cm, mesh size 250 /lm, n=10). Samples were
obtained by placing the net bag over the leaf canopy, and cutting the leaves
above the sediment surface. At localities 1-5 the percentage coverage of
Zostera was estimated in a randomly thrown frame of 0.25 m 2 (n=15), while a
measure of coverage at localities 6-9 was performed by counting shoot
densities within a randomly thrown frame covering 0.0625 m 2 (n=40). Visual
observations of mobile epifauna (shrimps, crabs, benthivorous fish) were
carried out during all sampling. In the laboratory the leaves were washed with