The Leaf Canopy of Seagrass Beds: Faunal Community in a Salinity Gradient
227
in the following mean proportions: 14.7,3.2 and 9.6 g DW . m- 2 (Baden et al.
2001). In a RDA analysis, the amphipod fauna was mostly related to the
amount of detritus on the leaves, as shown in Fig. 10.6. In the Baltic Sea, the
Zostera leaves were clean from fouling during the study (August) but tended
to accumulate epiphytic algae on the leaves after the growing season (November-May).
A leaf canopy dominated by detritivores is unusual and the potential role of
a detrivorous leaf fauna in seagrass has not been investigated. Mobile detritivores on seagrass are gained by increased inputs of organic matter and may
through tube building further enhance the effects of eutrophication by
binding more detritus to the leaves, increasing the negative effects of shading
(Philipp art 1995). Most investigations on the fouling communities deal with a
leaf fauna community dominated by grazers. One focus in seagrass research
has been on the relationship between epiphytes and grazers, thoroughly
reviewed by Van Montfrans et al. (1984) and Jernakoff et al. (1996). Epiphytic
fouling of seagrass leaves, with subsequent shading effects, increase with
increasing amounts of nutrients (bottom-up regulation processes) (Borum
1985; Lapointe et al. 1994; Duarte 1995). An important role for grazers is to
reduce the effects of nutrient surplus (top-down regulation processes)
(Williams and Ruckelshaus 1993; Hauxwell et al. 1998). Such processes might
in turn be affected by the influence of intermediate and top predators, as
hypothesized by Heck et al. (2000).
At the exposed localities in the Baltic, fouling does not seem to be a
problem to Zostera during the growing period. This might be due to physical
stress by water movements preventing epiphytes and detritus from growing
or accumulating, or because epiphytes (and Zostera) are efficiently grazed
upon, as observed in the Kiel Bight. Differences in leaf growth provide
conflicting evidence, as the leaf turnover rate in the Kattegat area is 14 days
(Denmark: Sand Jensen 1977) compared to about 60 days in the Baltic (Roos
2000). This rules out the possibility that the difference in fouling between the
Swedish west coast and the Baltic Sea is due to a difference in turnover rate of
the leaves.
10.4.3 Couplings Between Leaf Fauna and Infauna
Even though the leaf canopy fauna and the sedimentary in fauna may be
treated as separate systems, the fauna in the basal part of the leaf canopy and
the animals living at the sediment surface most likely interact or migrate
vertically. The proportion of animals found among leaves or on the sediment
may also vary with season, leaf density, leaf growth, or depend on mobility, life
stage or reproductive status of the animals (Orth 1973). Further, sampling
precision does not usually allow for a clear-cut distinction between the leaf
227
in the following mean proportions: 14.7,3.2 and 9.6 g DW . m- 2 (Baden et al.
2001). In a RDA analysis, the amphipod fauna was mostly related to the
amount of detritus on the leaves, as shown in Fig. 10.6. In the Baltic Sea, the
Zostera leaves were clean from fouling during the study (August) but tended
to accumulate epiphytic algae on the leaves after the growing season (November-May).
A leaf canopy dominated by detritivores is unusual and the potential role of
a detrivorous leaf fauna in seagrass has not been investigated. Mobile detritivores on seagrass are gained by increased inputs of organic matter and may
through tube building further enhance the effects of eutrophication by
binding more detritus to the leaves, increasing the negative effects of shading
(Philipp art 1995). Most investigations on the fouling communities deal with a
leaf fauna community dominated by grazers. One focus in seagrass research
has been on the relationship between epiphytes and grazers, thoroughly
reviewed by Van Montfrans et al. (1984) and Jernakoff et al. (1996). Epiphytic
fouling of seagrass leaves, with subsequent shading effects, increase with
increasing amounts of nutrients (bottom-up regulation processes) (Borum
1985; Lapointe et al. 1994; Duarte 1995). An important role for grazers is to
reduce the effects of nutrient surplus (top-down regulation processes)
(Williams and Ruckelshaus 1993; Hauxwell et al. 1998). Such processes might
in turn be affected by the influence of intermediate and top predators, as
hypothesized by Heck et al. (2000).
At the exposed localities in the Baltic, fouling does not seem to be a
problem to Zostera during the growing period. This might be due to physical
stress by water movements preventing epiphytes and detritus from growing
or accumulating, or because epiphytes (and Zostera) are efficiently grazed
upon, as observed in the Kiel Bight. Differences in leaf growth provide
conflicting evidence, as the leaf turnover rate in the Kattegat area is 14 days
(Denmark: Sand Jensen 1977) compared to about 60 days in the Baltic (Roos
2000). This rules out the possibility that the difference in fouling between the
Swedish west coast and the Baltic Sea is due to a difference in turnover rate of
the leaves.
10.4.3 Couplings Between Leaf Fauna and Infauna
Even though the leaf canopy fauna and the sedimentary in fauna may be
treated as separate systems, the fauna in the basal part of the leaf canopy and
the animals living at the sediment surface most likely interact or migrate
vertically. The proportion of animals found among leaves or on the sediment
may also vary with season, leaf density, leaf growth, or depend on mobility, life
stage or reproductive status of the animals (Orth 1973). Further, sampling
precision does not usually allow for a clear-cut distinction between the leaf