Chapter 24 Human Impacts on Seagrasses
577
dams above the tidal influence of the system that
had filled with sediment over many decades. Not
only did the freshet of water from the storm erode
sediment from the catchment basin, but also huge
quantities of sediments stored behind the dams were
re-suspended, removed, and deposited in the estuary
(Grant-Gross, et al., 1978).
Once the integrity of the meadow has been
damaged (whether chemical or physical) it will
be exposed to increased damage by sediment resuspension and siltation, as seen in Western Port Bay,
finally leading to auto-catalytic decline (Larkum and
West, 1982). More recent work has shown that once
the bottom cover of seagrass has decreased to 25–
50% then re-suspension increases markedly (Moore,
1996).
5. Ecophysiology of Sediment Impacts
Both sedimentation and turbidity reduce the light
available for photosynthesis and hence production
of carbohydrate reserves. In this respect, the physiological effects of poor water clarity on seagrasses are
the same whether the cause is increased suspended
particulate matter, or eutrophication induced phytoplankton blooms, except that sedimentation and
smothering have the added stress of inhibiting gas
exchange. The minimum light requirements for seagrasses vary between 5 and 20% surface irradiance
(Dennison et al., 1993), although it seems that the
duration of time exposed to low light conditions is
also important. For example, while Halophila ovalis
appears to be able to survive for 1 month under low
light conditions (Longstaff and Dennison, 1999),
species with larger rhizomes and hence greater carbon storage, have managed to survive more than
5 months at minimum light requirements (Gordon
et al., 1994; Lee and Dunton, 1997). Other species
are much less tolerant, for example, Z. marina has
been observed to decline after as little as 20 days
of reduced light availability during summer (Moore
et al., 1997). However, this could be an interactive
effect of reduced light with elevated temperature.
Duarte et al. (1997) predicted that mortality from
burial should increase with decreasing seagrass size,
but that capacity to recover should also increase with
decreasing size. They tested this on a number of
species in the Philippines and found that the largest
species, Enhalus acoroides, had significant reductions in shoot density only after 10 months, while
Thalassia hemprichii mortality increased with burial
depth and most of the smaller fast-growing species
showed an initial decline in deeper sediments, with
some species (e.g. Halodule uninervis, Syringodium
isoetifolium, and Cymodocea serrulata) able to recover after approximately 1 year.
Not surprisingly, the effects of turbidity on the
ecophysiology of seagrasses have often been investigated indirectly through shading experiments. There
have been numerous light reduction experiments
done in laboratories and in situ on a wide range of
seagrass species at one time or another (e.g. Dennison and Alberte, 1982; Abal et al., 1994; Short et al.,
1995; Moore et al., 1997; Longstaff and Dennison,
1999; Moore and Wetzel, 2000). The most consistent
conclusion regarding the effects of reduced light, and
therefore implied effects of turbidity, is reduction
of photosynthetic capacity, productivity and consequently the depletion of storage reserves, resulting
in increasing shoot mortality and ultimately the decline of whole meadows. Burke et al. (1996) found
that experimental shading of eelgrass in the spring
for only 3 weeks reduced sugar concentration in the
leaves, rhizomes and roots by 40–51%. Potential
non-structural carbohydrate reserve (NSC) storage
during shading was reduced by approximately 66%.
Moore et al. (1997) found that as little as 20 days of
light reduction by high springtime turbidities at eelgrass transplant sites in the same region resulted in
complete loss of transplant during the summer compared to little loss in areas that did not experience the
springtime light reductions but had similar summertime conditions. Such studies suggest that certain
species may have critical periods for both growth
and storage of NSC that enable plants to maintain
positive carbon balances during subsequent stressful periods during the year.
Sediment, especially silt, can also have an impact
on gas exchange, in addition to causing anoxic conditions and producing toxic sediment sulfide levels, as discussed earlier. While sulfide has been
shown to negatively affect seagrass photosynthesis, metabolism, and growth (Goodman et al., 1995;
Erskine and Koch, 2000; Holmer and Bondgaard,
2001), research is needed to determine whether sediment has any impact on gas exchange. If the sediment
were to increase the diffusion boundary layer for gas
exchange across the leaf surface, this could severely
inhibit photosynthesis. This gas exchange problem
would occur concurrently with reduced light penetration to the leaf surface.
577
dams above the tidal influence of the system that
had filled with sediment over many decades. Not
only did the freshet of water from the storm erode
sediment from the catchment basin, but also huge
quantities of sediments stored behind the dams were
re-suspended, removed, and deposited in the estuary
(Grant-Gross, et al., 1978).
Once the integrity of the meadow has been
damaged (whether chemical or physical) it will
be exposed to increased damage by sediment resuspension and siltation, as seen in Western Port Bay,
finally leading to auto-catalytic decline (Larkum and
West, 1982). More recent work has shown that once
the bottom cover of seagrass has decreased to 25–
50% then re-suspension increases markedly (Moore,
1996).
5. Ecophysiology of Sediment Impacts
Both sedimentation and turbidity reduce the light
available for photosynthesis and hence production
of carbohydrate reserves. In this respect, the physiological effects of poor water clarity on seagrasses are
the same whether the cause is increased suspended
particulate matter, or eutrophication induced phytoplankton blooms, except that sedimentation and
smothering have the added stress of inhibiting gas
exchange. The minimum light requirements for seagrasses vary between 5 and 20% surface irradiance
(Dennison et al., 1993), although it seems that the
duration of time exposed to low light conditions is
also important. For example, while Halophila ovalis
appears to be able to survive for 1 month under low
light conditions (Longstaff and Dennison, 1999),
species with larger rhizomes and hence greater carbon storage, have managed to survive more than
5 months at minimum light requirements (Gordon
et al., 1994; Lee and Dunton, 1997). Other species
are much less tolerant, for example, Z. marina has
been observed to decline after as little as 20 days
of reduced light availability during summer (Moore
et al., 1997). However, this could be an interactive
effect of reduced light with elevated temperature.
Duarte et al. (1997) predicted that mortality from
burial should increase with decreasing seagrass size,
but that capacity to recover should also increase with
decreasing size. They tested this on a number of
species in the Philippines and found that the largest
species, Enhalus acoroides, had significant reductions in shoot density only after 10 months, while
Thalassia hemprichii mortality increased with burial
depth and most of the smaller fast-growing species
showed an initial decline in deeper sediments, with
some species (e.g. Halodule uninervis, Syringodium
isoetifolium, and Cymodocea serrulata) able to recover after approximately 1 year.
Not surprisingly, the effects of turbidity on the
ecophysiology of seagrasses have often been investigated indirectly through shading experiments. There
have been numerous light reduction experiments
done in laboratories and in situ on a wide range of
seagrass species at one time or another (e.g. Dennison and Alberte, 1982; Abal et al., 1994; Short et al.,
1995; Moore et al., 1997; Longstaff and Dennison,
1999; Moore and Wetzel, 2000). The most consistent
conclusion regarding the effects of reduced light, and
therefore implied effects of turbidity, is reduction
of photosynthetic capacity, productivity and consequently the depletion of storage reserves, resulting
in increasing shoot mortality and ultimately the decline of whole meadows. Burke et al. (1996) found
that experimental shading of eelgrass in the spring
for only 3 weeks reduced sugar concentration in the
leaves, rhizomes and roots by 40–51%. Potential
non-structural carbohydrate reserve (NSC) storage
during shading was reduced by approximately 66%.
Moore et al. (1997) found that as little as 20 days of
light reduction by high springtime turbidities at eelgrass transplant sites in the same region resulted in
complete loss of transplant during the summer compared to little loss in areas that did not experience the
springtime light reductions but had similar summertime conditions. Such studies suggest that certain
species may have critical periods for both growth
and storage of NSC that enable plants to maintain
positive carbon balances during subsequent stressful periods during the year.
Sediment, especially silt, can also have an impact
on gas exchange, in addition to causing anoxic conditions and producing toxic sediment sulfide levels, as discussed earlier. While sulfide has been
shown to negatively affect seagrass photosynthesis, metabolism, and growth (Goodman et al., 1995;
Erskine and Koch, 2000; Holmer and Bondgaard,
2001), research is needed to determine whether sediment has any impact on gas exchange. If the sediment
were to increase the diffusion boundary layer for gas
exchange across the leaf surface, this could severely
inhibit photosynthesis. This gas exchange problem
would occur concurrently with reduced light penetration to the leaf surface.