450
Michael A. Borowitzka, Paul Lavery, and Mike van Keulen
of the seagrass does appear to affect motile epifaunal abundance; for example, Edgar and Robertson
(1992) noted that more open stands of Amphibolis
spp. were relatively depauperate in motile epifauna
compared with dense stands.
The abundance and distribution of epiphytic
macroinvertebrate grazers is strongly influenced by
the abundance and distribution of the epiphytic algae or periphyton on which they graze (Bologna
and Heck, 1999; Fong et al., 2000; Valentine and
Duffy, Chapter 20, Sections V to VIII). The density of grazers is a function of both seagrass habitat
structure and the trophic attraction of the habitat.
The relative importance of these two factors appears
variable, with different studies weighting each factor differently. Artificial seagrass has been used to
examine the relative roles of these two factors and
showed that heterogeneity of a habitat was not, alone,
the major determinant of epiphytic grazer biomass;
trophic attractiveness of the habitat appears to be
more important (Bologna and Heck, 1999; Bostr¨ om
and Mattila, 1999). However, it seems that different
grazers respond to different factors and a comparison of fauna on A. griffithii and P. sinuosa found that
the abundance of amphipod and gastropod grazers
correlated with food availability on P. sinuosa and
with the biomass of leaves on A. griffithii, suggesting that in A. griffithii provision of cover was more
important (Jernakoff and Nielsen, 1998).
IV. Factors Affecting Distribution
and Abundance
The observed diversity, distribution, and abundance
of epiphytic organisms on seagrasses are the result of
the interaction of a number of factors and processes.
A fundamental determinant of potential epiphyte diversity is the availability of propagules to colonize
any available seagrass substrata. In order to try to
understand the settlement of epiphytic algae and invertebrates one also needs to consider some basic
aspects of the reproduction of these organisms. The
propagules of red and brown algae as well as diatoms
are non-motile and are wholly at the mercy of hydrodynamics. On the other hand, the propagules of
green algae such as Ulva and Enteromorpha and the
larvae of invertebrates are motile and therefore can
be expected to be able to show greater selectivity as
to the site of settlement. Actual settling and attachment to this substratum will be dependent mainly
on localized hydrodynamics (refer to Koch et al.,
Chapter 8). Successful recruitment and growth will
be a function of light, temperature, nutrients, grazing, and predation as well as other interactions of
the organisms with other organisms and their environment. The overall development of the epiphyte
community will be limited by the longevity of the
seagrass substratum.
A. Succession and Seasonality
The process of colonization of a new seagrass leaf is
easily seen by comparing differently aged leaves in
a leaf bundle on seagrasses such as Posidonia. The
earliest colonizers are bacteria and diatoms forming a biofilm, which can be observed on even the
youngest leaves (Novak, 1984; Sterrenburg et al.,
1995). This is soon followed by a range of algae,
especially crustose coralline algae, as well as hydrozoans and bryozoans. In general, organisms once
settled and established persist throughout the lifespan of their substratum, the leaf. Settlement is generally greatest at the edge of leaves and near the leaf
apex, probably because of the higher turbulence in
this region which favors the chance of propagules
coming in contact with the leaf surface so that they
can attach successfully (Jacobs et al., 1983; Trautman and Borowitzka, 1999). Early studies suggested
that certain algae such as Smithora naiadum and
M. stelliferum were host specific to particular seagrasses (e.g. Harlin, 1973; Ducker and Knox, 1978).
However, further field observations and studies using artificial seagrasses have shown this not to be
the case (Harlin, 1973; Lethbridge et al., 1988). The
biofilm which forms on the leaves may, however, be
an important factor for the settlement of other organisms, especially invertebrates (Wahl, 1989). The
prokaryotic biofilms that are a ubiquitous feature
of all surfaces in aquatic environments are complex
communities (Stoodley et al., 2002) and this complexity can be seen clearly in published micrographs
of seagrass microfilms (e.g. Novak, 1984; Mazzella,
1999). There are many non-seagrass examples showing that marine bacterial and diatom biofilms attract
or inhibit settlement and metamorphosis of invertebrate larvae (e.g. Wieczorek and Todd, 1998; Daume
et al., 1999; Harder et al., 2002; Steinberg et al.,
2002) and the role of such biofilms in structuring
Michael A. Borowitzka, Paul Lavery, and Mike van Keulen
of the seagrass does appear to affect motile epifaunal abundance; for example, Edgar and Robertson
(1992) noted that more open stands of Amphibolis
spp. were relatively depauperate in motile epifauna
compared with dense stands.
The abundance and distribution of epiphytic
macroinvertebrate grazers is strongly influenced by
the abundance and distribution of the epiphytic algae or periphyton on which they graze (Bologna
and Heck, 1999; Fong et al., 2000; Valentine and
Duffy, Chapter 20, Sections V to VIII). The density of grazers is a function of both seagrass habitat
structure and the trophic attraction of the habitat.
The relative importance of these two factors appears
variable, with different studies weighting each factor differently. Artificial seagrass has been used to
examine the relative roles of these two factors and
showed that heterogeneity of a habitat was not, alone,
the major determinant of epiphytic grazer biomass;
trophic attractiveness of the habitat appears to be
more important (Bologna and Heck, 1999; Bostr¨ om
and Mattila, 1999). However, it seems that different
grazers respond to different factors and a comparison of fauna on A. griffithii and P. sinuosa found that
the abundance of amphipod and gastropod grazers
correlated with food availability on P. sinuosa and
with the biomass of leaves on A. griffithii, suggesting that in A. griffithii provision of cover was more
important (Jernakoff and Nielsen, 1998).
IV. Factors Affecting Distribution
and Abundance
The observed diversity, distribution, and abundance
of epiphytic organisms on seagrasses are the result of
the interaction of a number of factors and processes.
A fundamental determinant of potential epiphyte diversity is the availability of propagules to colonize
any available seagrass substrata. In order to try to
understand the settlement of epiphytic algae and invertebrates one also needs to consider some basic
aspects of the reproduction of these organisms. The
propagules of red and brown algae as well as diatoms
are non-motile and are wholly at the mercy of hydrodynamics. On the other hand, the propagules of
green algae such as Ulva and Enteromorpha and the
larvae of invertebrates are motile and therefore can
be expected to be able to show greater selectivity as
to the site of settlement. Actual settling and attachment to this substratum will be dependent mainly
on localized hydrodynamics (refer to Koch et al.,
Chapter 8). Successful recruitment and growth will
be a function of light, temperature, nutrients, grazing, and predation as well as other interactions of
the organisms with other organisms and their environment. The overall development of the epiphyte
community will be limited by the longevity of the
seagrass substratum.
A. Succession and Seasonality
The process of colonization of a new seagrass leaf is
easily seen by comparing differently aged leaves in
a leaf bundle on seagrasses such as Posidonia. The
earliest colonizers are bacteria and diatoms forming a biofilm, which can be observed on even the
youngest leaves (Novak, 1984; Sterrenburg et al.,
1995). This is soon followed by a range of algae,
especially crustose coralline algae, as well as hydrozoans and bryozoans. In general, organisms once
settled and established persist throughout the lifespan of their substratum, the leaf. Settlement is generally greatest at the edge of leaves and near the leaf
apex, probably because of the higher turbulence in
this region which favors the chance of propagules
coming in contact with the leaf surface so that they
can attach successfully (Jacobs et al., 1983; Trautman and Borowitzka, 1999). Early studies suggested
that certain algae such as Smithora naiadum and
M. stelliferum were host specific to particular seagrasses (e.g. Harlin, 1973; Ducker and Knox, 1978).
However, further field observations and studies using artificial seagrasses have shown this not to be
the case (Harlin, 1973; Lethbridge et al., 1988). The
biofilm which forms on the leaves may, however, be
an important factor for the settlement of other organisms, especially invertebrates (Wahl, 1989). The
prokaryotic biofilms that are a ubiquitous feature
of all surfaces in aquatic environments are complex
communities (Stoodley et al., 2002) and this complexity can be seen clearly in published micrographs
of seagrass microfilms (e.g. Novak, 1984; Mazzella,
1999). There are many non-seagrass examples showing that marine bacterial and diatom biofilms attract
or inhibit settlement and metamorphosis of invertebrate larvae (e.g. Wieczorek and Todd, 1998; Daume
et al., 1999; Harder et al., 2002; Steinberg et al.,
2002) and the role of such biofilms in structuring