452
Michael A. Borowitzka, Paul Lavery, and Mike van Keulen
B. The Physical Environment
1. Light and Temperature
Light clearly has a key influence on the distribution
and abundance of algal epiphytes. The widely observed apico-basal distribution of epiphytes on seagrasses largely appears to be determined by the light
regime: algal epiphytes can out-compete animal epiphytes near the plant apex where there is more light,
and vice versa, where there is less light. On a more
subtle scale, the distribution on different sides of a
leaf as observed in P. sinuosa (Trautman and Borowitzka, 1999) may also be determined in part by the
light regime, with higher abundances of algal epiphytes recorded on the side that receives more light.
On a larger scale, the position of a plant within a
meadow or patch can influence the light penetrating
the canopy, and hence the epiphyte load observed
on it. Edge effects on irradiance penetration into
patches of seagrass may play a role in determining epiphyte load and distribution. Carruthers (1994)
showed that epiphyte biomass in Amphibolis meadows was significantly greater at 50% canopy density than at 100%, suggesting that additional light
may play a significant role in determining epiphyte
load. However, no edge effects in epiphyte biomass
were observed by Saunders et al. (2003) in Z. marina
beds in Plymouth Sound and this may be a function
of the much smaller size and lesser structural complexity of Zostera compared to Amphibolis as well
as the lack of a true, well developed ‘canopy’, which
means that there is no marked light gradient. The
effect of light availability on epiphyte abundance can
also be seen in studies of seagrass epiphytes over a
depth gradient (e.g. Buia et al., 1992; Jagtap, 1998;
Lepoint et al., 1999).
Not surprisingly, light intensity affects the growth
rate of epiphytic algae (e.g. Lewis et al., 2002);
however, epiphyte cover also affects seagrass photosynthesis. Dixon (1999) examined the role of
epiphytic growth on attenuating light available to
leaves of T. testudinum and concluded that epiphyte
growth attenuated some 33% of PAR at depth. Drake
et al. (2003) also observed that epiphytes did not
act merely as neutral density filters, but preferentially absorbed light in the blue and red regions thus
competing for photons with the underlying leaves.
Gallegos et al. (1991) distinguished PUR (photosynthetically useable radiation) from PAR (photosynthetically available radiation), to emphasize the
role of epiphytic growth and phytoplankton in the
water column in attenuating particular wavelengths
of light available to seagrass leaves (see also Zimmerman, Chapter 13, Section XI). Neckles (1993)
examined the spectral influence of the epiphyte matrix on attenuated light and concluded that epiphytic
growth attenuated light across a broad spectrum. A
recent study by Brush and Nixon (2002) showed that
epiphytic algae rapidly attenuated light, but that at
higher epiphyte densities this attenuation leveled off
to a relatively constant value as the epiphytes floated
out from the edges of the seagrass blade. They also
observed that red algal epiphytes (e.g. Polysiphonia
sp.) attenuated light faster than green algal epiphytes
(e.g. Cladophora sp.) highlighting the importance of
the morphology of the epiphytic algae. Encrusting
algae such as the coralline algae also reduce light
more than erect filamentous species (Bulthuis and
Woelkerling, 1983a; Cebri´ an et al., 1999).
Temperature will also exert a significant effect
on the growth of epiphytic organisms and for algal temperature effects would be expected to interact strongly with light availability. There have been
no explicit studies of the effect of temperature on
seagrass epiphytes; however, there is indirect evidence from seasonal studies which show that epiphyte biomass increases with increasing light and
temperature (e.g. Alcoverro et al., 1997; Lepoint
et al., 1999; Toyohara et al., 1999). Other factors,
such as nutrient availability, however, will interact
with these effects. For example, Jacobs et al. (1983)
and Borum (1985) demonstrated that epiphytic algal abundance could be either maximal or minimal
in summer depending on the degree of nutrient enrichment in the water body.
2. Water Motion
Water motion influences almost every aspect of epiphyte growth (see also Koch et al., Chapter 8). Unfortunately, the study of hydrodynamics as it applies
to seagrass epiphyte communities is in a primitive
state, and requires considerably more attention. The
following outlines some of the major issues.
The supply of propagules of algae and invertebrates is controlled by water motion. The reduction
of water flow by seagrass canopies might be expected to assist in the trapping of spores within the
seagrass bed; however, very dense canopies may in
fact act more as a solid surface, forming an artificial
Michael A. Borowitzka, Paul Lavery, and Mike van Keulen
B. The Physical Environment
1. Light and Temperature
Light clearly has a key influence on the distribution
and abundance of algal epiphytes. The widely observed apico-basal distribution of epiphytes on seagrasses largely appears to be determined by the light
regime: algal epiphytes can out-compete animal epiphytes near the plant apex where there is more light,
and vice versa, where there is less light. On a more
subtle scale, the distribution on different sides of a
leaf as observed in P. sinuosa (Trautman and Borowitzka, 1999) may also be determined in part by the
light regime, with higher abundances of algal epiphytes recorded on the side that receives more light.
On a larger scale, the position of a plant within a
meadow or patch can influence the light penetrating
the canopy, and hence the epiphyte load observed
on it. Edge effects on irradiance penetration into
patches of seagrass may play a role in determining epiphyte load and distribution. Carruthers (1994)
showed that epiphyte biomass in Amphibolis meadows was significantly greater at 50% canopy density than at 100%, suggesting that additional light
may play a significant role in determining epiphyte
load. However, no edge effects in epiphyte biomass
were observed by Saunders et al. (2003) in Z. marina
beds in Plymouth Sound and this may be a function
of the much smaller size and lesser structural complexity of Zostera compared to Amphibolis as well
as the lack of a true, well developed ‘canopy’, which
means that there is no marked light gradient. The
effect of light availability on epiphyte abundance can
also be seen in studies of seagrass epiphytes over a
depth gradient (e.g. Buia et al., 1992; Jagtap, 1998;
Lepoint et al., 1999).
Not surprisingly, light intensity affects the growth
rate of epiphytic algae (e.g. Lewis et al., 2002);
however, epiphyte cover also affects seagrass photosynthesis. Dixon (1999) examined the role of
epiphytic growth on attenuating light available to
leaves of T. testudinum and concluded that epiphyte
growth attenuated some 33% of PAR at depth. Drake
et al. (2003) also observed that epiphytes did not
act merely as neutral density filters, but preferentially absorbed light in the blue and red regions thus
competing for photons with the underlying leaves.
Gallegos et al. (1991) distinguished PUR (photosynthetically useable radiation) from PAR (photosynthetically available radiation), to emphasize the
role of epiphytic growth and phytoplankton in the
water column in attenuating particular wavelengths
of light available to seagrass leaves (see also Zimmerman, Chapter 13, Section XI). Neckles (1993)
examined the spectral influence of the epiphyte matrix on attenuated light and concluded that epiphytic
growth attenuated light across a broad spectrum. A
recent study by Brush and Nixon (2002) showed that
epiphytic algae rapidly attenuated light, but that at
higher epiphyte densities this attenuation leveled off
to a relatively constant value as the epiphytes floated
out from the edges of the seagrass blade. They also
observed that red algal epiphytes (e.g. Polysiphonia
sp.) attenuated light faster than green algal epiphytes
(e.g. Cladophora sp.) highlighting the importance of
the morphology of the epiphytic algae. Encrusting
algae such as the coralline algae also reduce light
more than erect filamentous species (Bulthuis and
Woelkerling, 1983a; Cebri´ an et al., 1999).
Temperature will also exert a significant effect
on the growth of epiphytic organisms and for algal temperature effects would be expected to interact strongly with light availability. There have been
no explicit studies of the effect of temperature on
seagrass epiphytes; however, there is indirect evidence from seasonal studies which show that epiphyte biomass increases with increasing light and
temperature (e.g. Alcoverro et al., 1997; Lepoint
et al., 1999; Toyohara et al., 1999). Other factors,
such as nutrient availability, however, will interact
with these effects. For example, Jacobs et al. (1983)
and Borum (1985) demonstrated that epiphytic algal abundance could be either maximal or minimal
in summer depending on the degree of nutrient enrichment in the water body.
2. Water Motion
Water motion influences almost every aspect of epiphyte growth (see also Koch et al., Chapter 8). Unfortunately, the study of hydrodynamics as it applies
to seagrass epiphyte communities is in a primitive
state, and requires considerably more attention. The
following outlines some of the major issues.
The supply of propagules of algae and invertebrates is controlled by water motion. The reduction
of water flow by seagrass canopies might be expected to assist in the trapping of spores within the
seagrass bed; however, very dense canopies may in
fact act more as a solid surface, forming an artificial