Chapter 19 Epiphytes of Seagrasses
455
japonica but that shortly after the biomass returns
to control values (on a per m
2 basis) due to the increased sloughing of leaves, which were (presumed
to be) weakened by the epiphytic loads (Fong et al.,
2000). This demonstrates the complex interplay of
factors, including negative feedback loops that control epiphytic biomass.
Fish grazers have been implicated in direct control of epiphytic algal biomass. The pinfish Logodon
rhomboides and black mullet Mugil cephalus were
experimentally shown to reduce epiphyte biomass
through grazing, but only at times of the year when
the epiphytes were fleshy chlorophytes (Gacia et al.,
1999). At other times of the year, when red algae
dominate, the exclusion of fish from seagrass plots
makes no differences to epiphyte biomass. Elsewhere the same fish species (L. rhomboides) was
shown to exert a direct control on total epiphyte
biomass through grazing (Heck et al., 2000), though
this study did not specify the composition of the
epiphytic assemblage. Ruiz et al. (2001) also provide evidence that the reduced epiphyte load in P.
oceanica leaves near fish farms is due to increased
grazing.
Grazers can also influence epiphytic algal biomass
indirectly through the removal of the seagrass substratum. Urchins can consume 50–90% of T. testudinum production (Valentine et al., 1997) and
small vertebrate grazers have been shown to remove almost all seagrass production at other T. testudinum sites (Kirsch et al., 2002). It may well be that
this indirect control mechanism on epiphyte biomass
may not be consistent over time, since earlier studies showed that the effect of grazing by urchins on
seagrass was highly seasonal (Valentine and Heck,
1991). This indirect control can be so significant that
it overcomes nutrient-related effects on epiphytes
and can, conceivably, result in indirect control of
epiphytes through changes in their host substratum.
For example, the presence of grazers in seagrass
meadows produced higher densities of short shoots
(Valentine et al., 1997). The significance of these
sorts of morphological changes for epiphytic algal
abundance and distribution is unclear, and is worth
investigating as another mechanism by which grazing influences epiphytic assemblages.
Grazers can also influence epiphytic assemblage
structure (both species composition and abundance)
by selective feeding or by the removal of competitive dominants. Evidence for this has been reviewed
by Jernakoff et al. (1996), also van Montfrans et al.
(1982) demonstrated the role of selective grazing
by a gastropod grazer in the removal of loosely attached diatoms and the subsequent domination by
more strongly adhering diatom species. Two other
studies suggest that grazing cannot only control the
biomass and composition of epiphytic algae, but also
seasonality in composition. Fish grazing was shown
to control the biomass of algae on Z. japonica, but
only when the algal assemblage was dominated by
green algal epiphytes and not more grazing-resistant
red algae (Fong et al., 2000). Nielsen and Lethbridge
(1989) showed seasonal differences in the gut content of invertebrate grazers and they explained this in
terms of an active preference being demonstrated by
grazers rather than a seasonal pattern in food availability. Together, these results indicate a clear ability
for grazers to influence not only the composition of
epiphytic algal assemblages but also temporal patterns in composition.
A large body of work has clearly demonstrated
the potential for ‘top–down’ controls, in the form
of macroinvertebrate grazers, to over-ride ‘bottom–
up’ controls, such as nutrient enrichment effects,
on epiphytic biomass (Valentine and Duffy, Chapter 20; Heck and Orth, Chapter 22). Heck et al.
(2000) combined nutrient enrichment and top predator experiments. They noted few significant effects
of nutrient additions on epiphyte assemblages but
many effects of fish, including a reduction in epiphyte biomass. They attributed this to a combination of direct grazing of epiphytes by the fish and
the remaining mesograzers that had avoided predation by the fish. Peterson and Heck (2001) performed
a similar experiment, but added mussel surrogates
instead of fish. Again, the result was that nutrient
additions had little effect on epiphyte biomass but
the abundance was reduced in treatments containing
the mussel surrogates. This was explained in terms
of the surrogates providing structure that afforded
protection from predation for the mesograzers and
resulted in more intense grazing pressure. These and
other results (e.g. Moore and Wetzel, 2000) emphasize that any implied relationships between nutrient
enrichment and epiphytic biomass may well be oversimplifications if the role of top–down controls are
not also considered.
Filter feeders in seagrass beds can also influence
the epiphyte population. For example, Peterson and
Heck (2001) found that the presence of the mussel Modiolus americanus increased the growth of
T. testudinum and reduced the epiphyte load on the
455
japonica but that shortly after the biomass returns
to control values (on a per m
2 basis) due to the increased sloughing of leaves, which were (presumed
to be) weakened by the epiphytic loads (Fong et al.,
2000). This demonstrates the complex interplay of
factors, including negative feedback loops that control epiphytic biomass.
Fish grazers have been implicated in direct control of epiphytic algal biomass. The pinfish Logodon
rhomboides and black mullet Mugil cephalus were
experimentally shown to reduce epiphyte biomass
through grazing, but only at times of the year when
the epiphytes were fleshy chlorophytes (Gacia et al.,
1999). At other times of the year, when red algae
dominate, the exclusion of fish from seagrass plots
makes no differences to epiphyte biomass. Elsewhere the same fish species (L. rhomboides) was
shown to exert a direct control on total epiphyte
biomass through grazing (Heck et al., 2000), though
this study did not specify the composition of the
epiphytic assemblage. Ruiz et al. (2001) also provide evidence that the reduced epiphyte load in P.
oceanica leaves near fish farms is due to increased
grazing.
Grazers can also influence epiphytic algal biomass
indirectly through the removal of the seagrass substratum. Urchins can consume 50–90% of T. testudinum production (Valentine et al., 1997) and
small vertebrate grazers have been shown to remove almost all seagrass production at other T. testudinum sites (Kirsch et al., 2002). It may well be that
this indirect control mechanism on epiphyte biomass
may not be consistent over time, since earlier studies showed that the effect of grazing by urchins on
seagrass was highly seasonal (Valentine and Heck,
1991). This indirect control can be so significant that
it overcomes nutrient-related effects on epiphytes
and can, conceivably, result in indirect control of
epiphytes through changes in their host substratum.
For example, the presence of grazers in seagrass
meadows produced higher densities of short shoots
(Valentine et al., 1997). The significance of these
sorts of morphological changes for epiphytic algal
abundance and distribution is unclear, and is worth
investigating as another mechanism by which grazing influences epiphytic assemblages.
Grazers can also influence epiphytic assemblage
structure (both species composition and abundance)
by selective feeding or by the removal of competitive dominants. Evidence for this has been reviewed
by Jernakoff et al. (1996), also van Montfrans et al.
(1982) demonstrated the role of selective grazing
by a gastropod grazer in the removal of loosely attached diatoms and the subsequent domination by
more strongly adhering diatom species. Two other
studies suggest that grazing cannot only control the
biomass and composition of epiphytic algae, but also
seasonality in composition. Fish grazing was shown
to control the biomass of algae on Z. japonica, but
only when the algal assemblage was dominated by
green algal epiphytes and not more grazing-resistant
red algae (Fong et al., 2000). Nielsen and Lethbridge
(1989) showed seasonal differences in the gut content of invertebrate grazers and they explained this in
terms of an active preference being demonstrated by
grazers rather than a seasonal pattern in food availability. Together, these results indicate a clear ability
for grazers to influence not only the composition of
epiphytic algal assemblages but also temporal patterns in composition.
A large body of work has clearly demonstrated
the potential for ‘top–down’ controls, in the form
of macroinvertebrate grazers, to over-ride ‘bottom–
up’ controls, such as nutrient enrichment effects,
on epiphytic biomass (Valentine and Duffy, Chapter 20; Heck and Orth, Chapter 22). Heck et al.
(2000) combined nutrient enrichment and top predator experiments. They noted few significant effects
of nutrient additions on epiphyte assemblages but
many effects of fish, including a reduction in epiphyte biomass. They attributed this to a combination of direct grazing of epiphytes by the fish and
the remaining mesograzers that had avoided predation by the fish. Peterson and Heck (2001) performed
a similar experiment, but added mussel surrogates
instead of fish. Again, the result was that nutrient
additions had little effect on epiphyte biomass but
the abundance was reduced in treatments containing
the mussel surrogates. This was explained in terms
of the surrogates providing structure that afforded
protection from predation for the mesograzers and
resulted in more intense grazing pressure. These and
other results (e.g. Moore and Wetzel, 2000) emphasize that any implied relationships between nutrient
enrichment and epiphytic biomass may well be oversimplifications if the role of top–down controls are
not also considered.
Filter feeders in seagrass beds can also influence
the epiphyte population. For example, Peterson and
Heck (2001) found that the presence of the mussel Modiolus americanus increased the growth of
T. testudinum and reduced the epiphyte load on the