Chapter 19 Epiphytes of Seagrasses
453
boundary layer and redirecting spores over the top
of the canopy. van Keulen (1998) measured apparent increases in water velocity over the surface of
seagrass canopies, and postulated this was a form
of ‘skimming flow’. The structure of seagrass plants
has been shown to influence the water velocity profile through the canopy. Amphibolis plants have a
leafy, dense canopy with relatively bare stems; this
permits higher water velocities to penetrate into
the sub-canopy region than might be expected for
strap-leaved seagrasses, such as Posidonia spp. (van
Keulen and Borowitzka, 2000, 2002). While the
dense growths of epiphytes commonly observed on
stems of Amphibolis have usually been explained
by the long-lived nature of these stems, it is possible that the increased penetration of spores into
the sub-canopy layer of Amphibolis may also contribute. Ackerman (1986) showed that Z. marina
plants modified their flow environment to trap pollen
during flowering and similar observations have been
made for Amphibolis (Verduin, 1996). It is likely
that morphological roughness also enhances the recruitment of epiphyte spores into the canopy. Many
algal spores, by virtue of their small size, may be accommodated within the boundary layer surrounding
underwater surfaces (Charters et al., 1973) including seagrass leaves. While this would increase the
chances of spore adhesion once within close proximity of the substratum, by permitting the spore to
settle in the slower water flow, there would also be
some resistance to penetrating the boundary layer as
a result of ‘skimming flow’, whereby ambient water flow is redirected over the top of the boundary
layer. Consequently, surface roughness and smallscale turbulence may be important factors in determining final settlement.
Water motion also will influence the supply of
food for filter-feeding epiphytes; this may be of particular significance in genera such as Amphibolis
which have a sparse sub-canopy biomass that permits
enhanced penetration of water flow into the meadow.
Rates of nutrient uptake for algae and seagrasses
have been shown to be dependent on water velocity in several studies (e.g. Thomas et al., 2000) and
Cornelisen and Thomas (2002) have demonstrated
that ammonium uptake by the epiphytes of T. testudinum increased by an order of magnitude over
the range of water velocity (0.02–0.20 m s
−1 ) observed in the field. Water motion may play a role
in pumping nutrients out of the sediment, making
it available for epiphytes in the canopy (Koch and
Huettel, 2000; Koch et al., Chapter 8). Flushing of a
meadow will also enhance gas exchange, permitting
supply of inorganic carbon (CO 2 ), and flushing out
of waste products (O 2 ).
Physical damage is a clear result of water motion,
and is responsible for clearing old growth out of the
canopy at the end of summer. The high wave energies experienced during winter storms also remove
excess epiphyte growth that has accumulated during
the peak growth periods of summer (unpublished
results).
Recently, Schanz et al. (2002) highlighted
the possibility for complex physical–chemical–
biological interactions affecting epiphyte biomass.
They showed in both observational and experimental studies that differences in flow regime can affect
epiphyte biomass. Their data are consistent with a
‘cascading’ effect of water movement that is negatively correlated with grazer abundance and positively correlated with epiphyte abundance. At high
flow rates gastropod grazers are dislodged from seagrass leaves and so grazing pressure is reduced, leading, in turn, to higher epiphyte biomass and diversity.
The role of enhanced flow rates on nutrient provision
to the epiphytes was not explicitly tested, but there is
little doubt that a large amount of the response they
observed was related to the effect on grazers. Other
studies have also reported on the interaction between
hydrodynamics and leaf fouling (e.g. Fonseca et al.,
1982; Jacobs et al., 1983; Kendrick and Burt, 1997)
and together these studies emphasize the need to consider factors other than grazing, habitat complexity,
and nutrients as the prime determinants of epiphytic
algal assemblages.
3. Nutrients
The common wisdom on the effect of nutrients on
seagrasses is that increasing nutrient supply results
in the overgrowth of seagrasses by epiphytes and, in
extreme cases, the death of the seagrasses (Duarte,
1995; for further discussion see Ralph et al., Chapter 24, Section II A). Fundamental in developing this
paradigm have been some early studies (e.g. Orth and
Moore, 1983; Cambridge et al., 1986) which have
been cited regularly in the literature and repeatedly
‘confirmed’ with statements that nutrient enrichment
stimulated the growth of epiphytic algae (e.g. Wear
et al., 1999; Moore and Wetzel, 2000). Observational
and experimental studies have, indeed, reported
increases in epiphyte biomass in response to nutrient
453
boundary layer and redirecting spores over the top
of the canopy. van Keulen (1998) measured apparent increases in water velocity over the surface of
seagrass canopies, and postulated this was a form
of ‘skimming flow’. The structure of seagrass plants
has been shown to influence the water velocity profile through the canopy. Amphibolis plants have a
leafy, dense canopy with relatively bare stems; this
permits higher water velocities to penetrate into
the sub-canopy region than might be expected for
strap-leaved seagrasses, such as Posidonia spp. (van
Keulen and Borowitzka, 2000, 2002). While the
dense growths of epiphytes commonly observed on
stems of Amphibolis have usually been explained
by the long-lived nature of these stems, it is possible that the increased penetration of spores into
the sub-canopy layer of Amphibolis may also contribute. Ackerman (1986) showed that Z. marina
plants modified their flow environment to trap pollen
during flowering and similar observations have been
made for Amphibolis (Verduin, 1996). It is likely
that morphological roughness also enhances the recruitment of epiphyte spores into the canopy. Many
algal spores, by virtue of their small size, may be accommodated within the boundary layer surrounding
underwater surfaces (Charters et al., 1973) including seagrass leaves. While this would increase the
chances of spore adhesion once within close proximity of the substratum, by permitting the spore to
settle in the slower water flow, there would also be
some resistance to penetrating the boundary layer as
a result of ‘skimming flow’, whereby ambient water flow is redirected over the top of the boundary
layer. Consequently, surface roughness and smallscale turbulence may be important factors in determining final settlement.
Water motion also will influence the supply of
food for filter-feeding epiphytes; this may be of particular significance in genera such as Amphibolis
which have a sparse sub-canopy biomass that permits
enhanced penetration of water flow into the meadow.
Rates of nutrient uptake for algae and seagrasses
have been shown to be dependent on water velocity in several studies (e.g. Thomas et al., 2000) and
Cornelisen and Thomas (2002) have demonstrated
that ammonium uptake by the epiphytes of T. testudinum increased by an order of magnitude over
the range of water velocity (0.02–0.20 m s
−1 ) observed in the field. Water motion may play a role
in pumping nutrients out of the sediment, making
it available for epiphytes in the canopy (Koch and
Huettel, 2000; Koch et al., Chapter 8). Flushing of a
meadow will also enhance gas exchange, permitting
supply of inorganic carbon (CO 2 ), and flushing out
of waste products (O 2 ).
Physical damage is a clear result of water motion,
and is responsible for clearing old growth out of the
canopy at the end of summer. The high wave energies experienced during winter storms also remove
excess epiphyte growth that has accumulated during
the peak growth periods of summer (unpublished
results).
Recently, Schanz et al. (2002) highlighted
the possibility for complex physical–chemical–
biological interactions affecting epiphyte biomass.
They showed in both observational and experimental studies that differences in flow regime can affect
epiphyte biomass. Their data are consistent with a
‘cascading’ effect of water movement that is negatively correlated with grazer abundance and positively correlated with epiphyte abundance. At high
flow rates gastropod grazers are dislodged from seagrass leaves and so grazing pressure is reduced, leading, in turn, to higher epiphyte biomass and diversity.
The role of enhanced flow rates on nutrient provision
to the epiphytes was not explicitly tested, but there is
little doubt that a large amount of the response they
observed was related to the effect on grazers. Other
studies have also reported on the interaction between
hydrodynamics and leaf fouling (e.g. Fonseca et al.,
1982; Jacobs et al., 1983; Kendrick and Burt, 1997)
and together these studies emphasize the need to consider factors other than grazing, habitat complexity,
and nutrients as the prime determinants of epiphytic
algal assemblages.
3. Nutrients
The common wisdom on the effect of nutrients on
seagrasses is that increasing nutrient supply results
in the overgrowth of seagrasses by epiphytes and, in
extreme cases, the death of the seagrasses (Duarte,
1995; for further discussion see Ralph et al., Chapter 24, Section II A). Fundamental in developing this
paradigm have been some early studies (e.g. Orth and
Moore, 1983; Cambridge et al., 1986) which have
been cited regularly in the literature and repeatedly
‘confirmed’ with statements that nutrient enrichment
stimulated the growth of epiphytic algae (e.g. Wear
et al., 1999; Moore and Wetzel, 2000). Observational
and experimental studies have, indeed, reported
increases in epiphyte biomass in response to nutrient