456
Michael A. Borowitzka, Paul Lavery, and Mike van Keulen
seagrass leaves. This was attributed to the increased
growth rate of the leaves resulting from increased nutrient availability and/or the reduction in the availability of epiphyte propagules due to the filtering
of the mussels. Seagrass beds support a high density of filter feeding organisms and preliminary estimates by Lemmens et al. (1996) indicated that the
filter-feeders, especially the epiphytic species, associated with Posidonia and Amphibolis meadows are
potentially able to filter the overlying water column
daily.
2. Interactions Between Other Biota and
Seagrass Epiphytes
The potential exists for interactions between biota
other than the epiphyte and host to affect seagrass
epiphytes. This has been clearly demonstrated in
terms of grazers affecting epiphyte biomass and
composition (see above). However, other forms of
interaction have been reported including competition between algae and indirect interactions between
fauna and epiphytes. Piazzi and Cinelli (2000) described different algal assemblages on the leaves
and rhizomes of Posidonia oceanica in the western Mediterranean. They found the leaves to support
28 species of macroalgae, compared with 51 associated with the rhizomes with the rhizomes dominated
by the turf-forming red algal species Acrothamnion
preissii and Womersleyella setacea, both introduced
species. In a later study, Piazzi et al. (2002) looked at
the rhizome assemblage in more detail and found that
the functional diversity of the rhizome assemblage
was low (mainly filamentous species) at sites where
these introduced species were present and diverse
when they were absent. The authors also attributed
the absence of seasonal changes in the rhizome
assemblages to the presence of these introduced
species. They conclude that a competition between
the introduced and native species plays an important
role in structuring the rhizome algal assemblages
and their spatial and temporal patterns. Peterson
and Heck (2001) reported quite a different interaction. They introduced mussel mimics into a seagrass
meadow and observed an increase in grazer density
and decrease in epiphyte biomass. They suggest that
the mussel shells provide shelter for epibenthic grazers, thereby creating greater grazing pressure on the
epiphytes.
V. Conclusions
The seagrass epiphytes are an integral, complex, and
dynamic component of seagrass ecosystems. They
can make up a significant proportion of the total primary production of these ecosystems and are generally the primary food source for the associated fauna
as well as contributing to detrital food webs. Epiphytes also provide shelter for many species. Normally, the seagrass epiphytes do not seem to have a
detrimental effect on their seagrass host; however,
under certain conditions that lead to a ‘bloom’ in the
epiphytic algae they may contribute to the decline of
the seagrass.
A lack of knowledge about the physiology of epiphytic algae also limits our ability to predict how
they will respond to changes in nutrient availability
and changes in the light environment. For example,
and existing models of seagrass ecosystems need to
make some major assumptions about the responses
of epiphytic algae to changes in the environment (e.g.
Plus et al., 2003). Furthermore, our understanding
of the factors controlling seagrass epiphyte diversity
and abundance is still limited by the nature of the
complex interactions between physical factors (e.g.
light, temperature, water movement, nutrients) and
biological interactions (competition for space, grazing, and predation). Variability in the growth rate and
longevity of the seagrass substratum further complicates experimental studies and data interpretation.
The use of artificial seagrasses is one approach to
reduce some of this variability and combined with
well-designed natural and manipulative field experiments will lead to greater understanding of these
communities.
References
Ackerman JD (1986) Mechanistic implications for pollination in
the marine angiosperm Zostera marina. Aquat Bot 24: 343–
353
Aladro-Lubel MA and Mart´ ınez-Murillo ME (1999) Epibiotic protozoa (Ciliophora) on a community of Thalassia testudinum Banks ex K¨ onig in a coral reef in Veracruz, Mexico.
Aquat Bot 65: 239–254
Alcoverro T, Duarte CM and Romero J (1997) The influence of
herbivores on Posidonia oceanica epiphytes. Aquat Bot 56:
93–104
Alongi G, Cormaci M and Pizzuto F (1993) La microflora epifita
delle foglie di Halophila stipulacea (Forssk.) Aschers. Del
Porto di Catania. Biol Mar Medit 1: 287–288
Michael A. Borowitzka, Paul Lavery, and Mike van Keulen
seagrass leaves. This was attributed to the increased
growth rate of the leaves resulting from increased nutrient availability and/or the reduction in the availability of epiphyte propagules due to the filtering
of the mussels. Seagrass beds support a high density of filter feeding organisms and preliminary estimates by Lemmens et al. (1996) indicated that the
filter-feeders, especially the epiphytic species, associated with Posidonia and Amphibolis meadows are
potentially able to filter the overlying water column
daily.
2. Interactions Between Other Biota and
Seagrass Epiphytes
The potential exists for interactions between biota
other than the epiphyte and host to affect seagrass
epiphytes. This has been clearly demonstrated in
terms of grazers affecting epiphyte biomass and
composition (see above). However, other forms of
interaction have been reported including competition between algae and indirect interactions between
fauna and epiphytes. Piazzi and Cinelli (2000) described different algal assemblages on the leaves
and rhizomes of Posidonia oceanica in the western Mediterranean. They found the leaves to support
28 species of macroalgae, compared with 51 associated with the rhizomes with the rhizomes dominated
by the turf-forming red algal species Acrothamnion
preissii and Womersleyella setacea, both introduced
species. In a later study, Piazzi et al. (2002) looked at
the rhizome assemblage in more detail and found that
the functional diversity of the rhizome assemblage
was low (mainly filamentous species) at sites where
these introduced species were present and diverse
when they were absent. The authors also attributed
the absence of seasonal changes in the rhizome
assemblages to the presence of these introduced
species. They conclude that a competition between
the introduced and native species plays an important
role in structuring the rhizome algal assemblages
and their spatial and temporal patterns. Peterson
and Heck (2001) reported quite a different interaction. They introduced mussel mimics into a seagrass
meadow and observed an increase in grazer density
and decrease in epiphyte biomass. They suggest that
the mussel shells provide shelter for epibenthic grazers, thereby creating greater grazing pressure on the
epiphytes.
V. Conclusions
The seagrass epiphytes are an integral, complex, and
dynamic component of seagrass ecosystems. They
can make up a significant proportion of the total primary production of these ecosystems and are generally the primary food source for the associated fauna
as well as contributing to detrital food webs. Epiphytes also provide shelter for many species. Normally, the seagrass epiphytes do not seem to have a
detrimental effect on their seagrass host; however,
under certain conditions that lead to a ‘bloom’ in the
epiphytic algae they may contribute to the decline of
the seagrass.
A lack of knowledge about the physiology of epiphytic algae also limits our ability to predict how
they will respond to changes in nutrient availability
and changes in the light environment. For example,
and existing models of seagrass ecosystems need to
make some major assumptions about the responses
of epiphytic algae to changes in the environment (e.g.
Plus et al., 2003). Furthermore, our understanding
of the factors controlling seagrass epiphyte diversity
and abundance is still limited by the nature of the
complex interactions between physical factors (e.g.
light, temperature, water movement, nutrients) and
biological interactions (competition for space, grazing, and predation). Variability in the growth rate and
longevity of the seagrass substratum further complicates experimental studies and data interpretation.
The use of artificial seagrasses is one approach to
reduce some of this variability and combined with
well-designed natural and manipulative field experiments will lead to greater understanding of these
communities.
References
Ackerman JD (1986) Mechanistic implications for pollination in
the marine angiosperm Zostera marina. Aquat Bot 24: 343–
353
Aladro-Lubel MA and Mart´ ınez-Murillo ME (1999) Epibiotic protozoa (Ciliophora) on a community of Thalassia testudinum Banks ex K¨ onig in a coral reef in Veracruz, Mexico.
Aquat Bot 65: 239–254
Alcoverro T, Duarte CM and Romero J (1997) The influence of
herbivores on Posidonia oceanica epiphytes. Aquat Bot 56:
93–104
Alongi G, Cormaci M and Pizzuto F (1993) La microflora epifita
delle foglie di Halophila stipulacea (Forssk.) Aschers. Del
Porto di Catania. Biol Mar Medit 1: 287–288