228
S.P. Baden and C. Bostrom
fauna and the infauna. Consequently, the composition of the leaf fauna is
partly determined by the infaunal assemblage.
On the Swedish west coast, detritivores (Microdeutopus gryllotalpa,
Erichtonius difformis and Corophium insidiosum) dominate the leaf fauna, but
are also abundant among the infauna. In the northern Baltic Sea, these species
are absent, but their function is partly maintained by detrivorous chironomid
larvae, which occur almost exclusively as leaf fauna (Bostrom and Bonsdorff
1997). On the west coast, chironomids make up the bulk of the infauna (S.P.
Baden, unpublished data), but are also represented in high numbers on the
leaves (3.5 x 10 3 individuals· m- 2 ; Baden et al. 2001). Here, the gastropod and
bivalve assemblages are mainly supported by the leaf canopy, while the sediment supports low numbers of molluscs. In the Baltic, mud snails (Hydrobia
spp.) are the most dominant group in both sediment and upon leaves (see
above), thus occupying a broader niche by vertical migrations. Adult blue
mussels (Mytilus edulis) are abundant on the sediment surface, while
juveniles are found in low or moderate numbers on the leaves. On the west
coast nematodes (mainly Southermia zosteraceae) are represented in high
numbers both in the sediment and on the leaves, while the Baltic Zostera beds
seem to have too poor a detrital settlement in order to support leaf canopy
nematodes, though this group is abundant in vegetated sediments (Bostrom
and Bonsdorff 1997).
Orth (1973) found that species abundant among or upon Zostera leaves
were found in lower numbers in the sediment, and vice versa. Our data
suggest that the west coast fauna is not restricted to a specific habitat, since at
least detrivorous amphipods, chironomids and nematodes dominate both the
sediment and the leaf canopy. In northern Baltic Zostera beds such patterns
are true only for mud snails (Hydrobia spp.), while other groups (e.g.
chironomids and nematodes) show more habitat-specific distributions.
10.4.4 Predators
The quantity and composition of leaf fauna and infauna are determined by
"bottom-up" as well as "top-down" processes of which the potential importance of "top-down" (predator) regulation is discussed here. Generally,
most seagrass meadows contain a richer fauna than the neighbouring unvegetated bottoms in terms of both abundance and biomass. Besides being a
three-dimensional habitat with sub-niches allowing more animals to exist, the
main hypothesis for the richer fauna is protection against predators. The
potential for exploitation of the rich fauna by both intermediate (small fishes
and invertebrates) and top (mainly piscivorous fishes) predators is thus a
complex balance between the relative sizes of predators and prey, as well as
the density and complexity of the leaf canopy and the characteristics of
S.P. Baden and C. Bostrom
fauna and the infauna. Consequently, the composition of the leaf fauna is
partly determined by the infaunal assemblage.
On the Swedish west coast, detritivores (Microdeutopus gryllotalpa,
Erichtonius difformis and Corophium insidiosum) dominate the leaf fauna, but
are also abundant among the infauna. In the northern Baltic Sea, these species
are absent, but their function is partly maintained by detrivorous chironomid
larvae, which occur almost exclusively as leaf fauna (Bostrom and Bonsdorff
1997). On the west coast, chironomids make up the bulk of the infauna (S.P.
Baden, unpublished data), but are also represented in high numbers on the
leaves (3.5 x 10 3 individuals· m- 2 ; Baden et al. 2001). Here, the gastropod and
bivalve assemblages are mainly supported by the leaf canopy, while the sediment supports low numbers of molluscs. In the Baltic, mud snails (Hydrobia
spp.) are the most dominant group in both sediment and upon leaves (see
above), thus occupying a broader niche by vertical migrations. Adult blue
mussels (Mytilus edulis) are abundant on the sediment surface, while
juveniles are found in low or moderate numbers on the leaves. On the west
coast nematodes (mainly Southermia zosteraceae) are represented in high
numbers both in the sediment and on the leaves, while the Baltic Zostera beds
seem to have too poor a detrital settlement in order to support leaf canopy
nematodes, though this group is abundant in vegetated sediments (Bostrom
and Bonsdorff 1997).
Orth (1973) found that species abundant among or upon Zostera leaves
were found in lower numbers in the sediment, and vice versa. Our data
suggest that the west coast fauna is not restricted to a specific habitat, since at
least detrivorous amphipods, chironomids and nematodes dominate both the
sediment and the leaf canopy. In northern Baltic Zostera beds such patterns
are true only for mud snails (Hydrobia spp.), while other groups (e.g.
chironomids and nematodes) show more habitat-specific distributions.
10.4.4 Predators
The quantity and composition of leaf fauna and infauna are determined by
"bottom-up" as well as "top-down" processes of which the potential importance of "top-down" (predator) regulation is discussed here. Generally,
most seagrass meadows contain a richer fauna than the neighbouring unvegetated bottoms in terms of both abundance and biomass. Besides being a
three-dimensional habitat with sub-niches allowing more animals to exist, the
main hypothesis for the richer fauna is protection against predators. The
potential for exploitation of the rich fauna by both intermediate (small fishes
and invertebrates) and top (mainly piscivorous fishes) predators is thus a
complex balance between the relative sizes of predators and prey, as well as
the density and complexity of the leaf canopy and the characteristics of