444
Michael A. Borowitzka, Paul Lavery, and Mike van Keulen
significantly lower biomass on the leaves (Borowitzka et al., 1990; Lavery and Vanderklift, 2002).
Interestingly, an apico-basal pattern of epiphyte distribution is not observed on some seaweeds such as
Cystoseira (Belegratis et al., 1999). The rhizomes
of seagrasses generally carry a low epiphyte load
compared with the leaves, with the exception of P.
oceanica where greater species richness and higher
biomass on the exposed and long-lived rhizomes has
been reported (Piazzi and Cinelli, 2000; Mateo et al.,
Chapter 7). The low abundance of epiphytes on rhizomes is probably due to the fact that the rhizomes
of almost all seagrasses are buried most of the time.
There are few truly comparative studies of the epiphyte assemblages on different seagrasses (but see
Pinckney and Micheli, 1998; Trautman and Borowitzka, 1999; Wear et al., 1999; Vanderklift and Lavery, 2000; Lavery and Vanderklift, 2002). In contrast,
there are a large number of studies on the biomass
and composition of epiphyte assemblages on single
seagrass species, usually related to questions about
the effect of environmental factors on these assemblages. Comparisons across species are often, therefore, gleaned from the literature. However, this is
fraught with difficulty as the studies are performed
over different time scales and, as this chapter will
show, time scale is a crucial source of variability in
epiphyte assemblages.
While we might expect differences in patterns of
distribution between seagrasses with markedly different morphologies, subtle differences in morphology can also have significant effects on epiphyte assemblages. For example, P. sinuosa and P. australis
are morphologically similar seagrasses with straplike leaves; the leaf of P. sinuosa is narrower and
is concave in transverse section unlike P. australis,
which is wider and flat. Trautman and Borowitzka
(1999) found that the epiphyte assemblage on P. sinuosa leaves differed between the two leaf sides, with
greater species richness and biomass on the convex
surface, including some species restricted to only
that side; no differences in species richness or abundance of epiphytes were noted between adaxial and
abaxial sides of leaves of P. australis. Similar observations were made in respect to epiphytic invertebrates (see below).
Seagrasses with distinct morphological differentiation tend to provide several distinct microhabitats for epiphytes and there are reports of epiphytic
algae that are confined to particular plant parts. In
Amphibolis, for example, few species of epiphyte
are common to both stem and leaf, with most on
the stem and some showing specificity to a particular part of the stem (Lethbridge et al., 1988;
Borowitzka et al., 1990). This within-plant spatial
specificity of epiphytes is not confined to seagrass
species with long-lived stems. Cullinane et al. (1985)
found a similar pattern on Z. marina, except that
some taxa were confined to different parts of the
leaf/sheath structure. In the Mediterranean, Piazzi
and Cinelli (2000) noted distinct leaf and rhizome
assemblages on P. oceanica, with 28 leaf taxa (dominated by encrusting coralline algae and brown algae species) but 56 species on the rhizome (dominated by filamentous Rhodophyta—Acrothamnion,
Womersleyella, and the crustose Peyssonnelia); only
13 taxa were common to both niches. Borowitzka
et al. (1990) also observed distinct fine-scale patterns in the distribution of epiphytic algae; on A.
griffithii leaves, for example, plants of the green
alga Bryopsis plumosa were usually found only
on the leaf tips whereas the rhodophyte Ceramium
puberulum was most common at the base of the
leaves near the leaf junctions. These observations
suggest that even small differences in turnover
rates of plant parts as well as localized variation
in hydrodynamics can provide sufficient habitat
differentiation to result in distinct epiphytic algal
assemblages.
The composition of the assemblages of epiphytic
algae includes taxa from all the major algal phyla.
Generally, red algae dominate the composition and
biomass and diatoms are almost ubiquitous. This
dominance by rhodophytes reflects the dominance
of this phylum in the surrounding environment.
Where other phyla dominate the seagrass epiphyte
flora, there is an implication of unusual environmental conditions, such as nutrient enrichment resulting in an abundance of green algae or cyanobacteria (Coleman and Burkholder, 1994; Ierodiaconou
and Laurenson, 2002) or changes in energy levels
and depth resulting in shifts between diatoms and
cyanobacteria (Pinckney and Micheli, 1998). Consequently, most reports of cyanobacterial or green
algal domination suggest these are either transient,
or regular but seasonal, events coinciding with seasonal nutrient inputs. Alternatively, they are from
estuarine seagrass habitats.
The number of species of epiphytic algae found
on different seagrass species varies considerably.
It should be noted, however, that most studies of
the algal epiphytes of seagrasses focus only on the
Michael A. Borowitzka, Paul Lavery, and Mike van Keulen
significantly lower biomass on the leaves (Borowitzka et al., 1990; Lavery and Vanderklift, 2002).
Interestingly, an apico-basal pattern of epiphyte distribution is not observed on some seaweeds such as
Cystoseira (Belegratis et al., 1999). The rhizomes
of seagrasses generally carry a low epiphyte load
compared with the leaves, with the exception of P.
oceanica where greater species richness and higher
biomass on the exposed and long-lived rhizomes has
been reported (Piazzi and Cinelli, 2000; Mateo et al.,
Chapter 7). The low abundance of epiphytes on rhizomes is probably due to the fact that the rhizomes
of almost all seagrasses are buried most of the time.
There are few truly comparative studies of the epiphyte assemblages on different seagrasses (but see
Pinckney and Micheli, 1998; Trautman and Borowitzka, 1999; Wear et al., 1999; Vanderklift and Lavery, 2000; Lavery and Vanderklift, 2002). In contrast,
there are a large number of studies on the biomass
and composition of epiphyte assemblages on single
seagrass species, usually related to questions about
the effect of environmental factors on these assemblages. Comparisons across species are often, therefore, gleaned from the literature. However, this is
fraught with difficulty as the studies are performed
over different time scales and, as this chapter will
show, time scale is a crucial source of variability in
epiphyte assemblages.
While we might expect differences in patterns of
distribution between seagrasses with markedly different morphologies, subtle differences in morphology can also have significant effects on epiphyte assemblages. For example, P. sinuosa and P. australis
are morphologically similar seagrasses with straplike leaves; the leaf of P. sinuosa is narrower and
is concave in transverse section unlike P. australis,
which is wider and flat. Trautman and Borowitzka
(1999) found that the epiphyte assemblage on P. sinuosa leaves differed between the two leaf sides, with
greater species richness and biomass on the convex
surface, including some species restricted to only
that side; no differences in species richness or abundance of epiphytes were noted between adaxial and
abaxial sides of leaves of P. australis. Similar observations were made in respect to epiphytic invertebrates (see below).
Seagrasses with distinct morphological differentiation tend to provide several distinct microhabitats for epiphytes and there are reports of epiphytic
algae that are confined to particular plant parts. In
Amphibolis, for example, few species of epiphyte
are common to both stem and leaf, with most on
the stem and some showing specificity to a particular part of the stem (Lethbridge et al., 1988;
Borowitzka et al., 1990). This within-plant spatial
specificity of epiphytes is not confined to seagrass
species with long-lived stems. Cullinane et al. (1985)
found a similar pattern on Z. marina, except that
some taxa were confined to different parts of the
leaf/sheath structure. In the Mediterranean, Piazzi
and Cinelli (2000) noted distinct leaf and rhizome
assemblages on P. oceanica, with 28 leaf taxa (dominated by encrusting coralline algae and brown algae species) but 56 species on the rhizome (dominated by filamentous Rhodophyta—Acrothamnion,
Womersleyella, and the crustose Peyssonnelia); only
13 taxa were common to both niches. Borowitzka
et al. (1990) also observed distinct fine-scale patterns in the distribution of epiphytic algae; on A.
griffithii leaves, for example, plants of the green
alga Bryopsis plumosa were usually found only
on the leaf tips whereas the rhodophyte Ceramium
puberulum was most common at the base of the
leaves near the leaf junctions. These observations
suggest that even small differences in turnover
rates of plant parts as well as localized variation
in hydrodynamics can provide sufficient habitat
differentiation to result in distinct epiphytic algal
assemblages.
The composition of the assemblages of epiphytic
algae includes taxa from all the major algal phyla.
Generally, red algae dominate the composition and
biomass and diatoms are almost ubiquitous. This
dominance by rhodophytes reflects the dominance
of this phylum in the surrounding environment.
Where other phyla dominate the seagrass epiphyte
flora, there is an implication of unusual environmental conditions, such as nutrient enrichment resulting in an abundance of green algae or cyanobacteria (Coleman and Burkholder, 1994; Ierodiaconou
and Laurenson, 2002) or changes in energy levels
and depth resulting in shifts between diatoms and
cyanobacteria (Pinckney and Micheli, 1998). Consequently, most reports of cyanobacterial or green
algal domination suggest these are either transient,
or regular but seasonal, events coinciding with seasonal nutrient inputs. Alternatively, they are from
estuarine seagrass habitats.
The number of species of epiphytic algae found
on different seagrass species varies considerably.
It should be noted, however, that most studies of
the algal epiphytes of seagrasses focus only on the