576
P. J. Ralph, D. Tomasko, K. Moore, S. Seddon, and C. M. O. Macinnis-Ng
deposited through time (Koch, 1999). This makes it
difficult to reverse the effects of elevated sedimentation, especially in a relatively closed bay or estuary as
there is very little opportunity for the sediments to be
flushed out of the system. The relationships between
wave and current induced sediment re-suspension
in shallow water areas and light attenuation are not
as yet well quantified and are important areas for
further work. Ward et al. (1984) found that the influence of waves on sediment re-suspension in an
estuarine embayment of a system long influenced
by human activity was affected by the relationship
between the seagrass canopy height relative to the
water depth. Suspended particulate matter concentrations were significantly lower within areas of the
embayment colonized by seagrass beds. However,
when water levels were elevated by spring tides or
storm surges, the plants were less effective at attenuating wave energy and suspended sediment concentrations increased inside the beds. Moore (1996)
observed over a range of water levels that the capacity of eelgrass beds to reduce suspended sediment
concentrations within the beds decreased with decreasing plant cover in the bed. At approximately
25–50% cover there was little difference between
vegetated and un-vegetated areas.
3. Turbidity
The primary detrimental effect of sediment turbidity, as with eutrophication-linked phytoplankton
blooms, is the increased attenuation of light. Reduced light over prolonged periods can deplete seagrass carbon reserves or, in cases of extreme light
deprivation, the lack of photosynthetically-produced
oxygen can lead to sediment anoxia and more rapid
mortality. Turbidity has been responsible for some
very significant seagrass losses in the past, such as
the massive 100,000 ha loss of seagrass during a
cyclone in Hervey Bay (Queensland, Australia) and
its aftermath causing widespread flooding and prolonged turbidity, where seagrasses in shallow water were uprooted in heavy seas and those below
10 m depth were thought to have died from light
deprivation due to increased river runoff associated
with land clearing and erosion (Preen et al., 1995).
Similarly, an extensive decline of 17,890 ha of seagrass and macro-algae occurred in Western Port Bay
(Victoria, Australia) during the 1970s and 1980s
(Bulthuis et al., 1984a). Although the causes are
not known with certainty, losses have been correlated with large volumes of terrestrial runoff, originating from extensive land clearing and soil erosion within the catchment and waterways, as well
as activities such as regular dredging of shipping
channels. The pattern and nature of the Western Port
decline suggests an increasing exposure of the intertidal seagrass beds to desiccation due to the accretion of sediments on mudflats causing higher bed
elevations, as well as directly smothering seagrasses
with sediments and reducing light through persistent turbidity (Bulthuis et al., 1984a; Shepherd et al.,
1989; Stephens, 1995). It is also likely that high turbidity and nutrients, associated with sediment resuspension over denuded areas, in combination with
erosion from waves and tidal currents played a role in
accelerating and perpetuating further seagrass losses
(Bulthuis et al., 1984b; Clarke and Kirkman, 1989;
Walker and McComb, 1992). Onuf (1994) found
that the timing and spacing of dredging activity
in Laguna Madre, Texas correlated closely with
decreases in seagrass distribution. These impacts,
which were long-lasting over wide areas, were not
related to the sediment plumes of the initial dredging
operations themselves, which were short lived but to
the re-working and re-suspension of the dredged material that had been deposited in shallow areas.
4. Smothering
Very little work has been done to date investigating
the effects of smothering and burial by sediments;
however, it appears that tolerance is linked to sediment condition and seagrass species. For example,
Posidonia spp. in Holdfast Bay (South Australia)
survived for approximately 4 months buried under
aerobic sediments compared with less than 2 weeks
in anoxic sediments (Clarke, 1987) and less than
a week for the more fragile Halophila australis
(Clarke and Kirkman, 1989). However, the mechanisms of mortality were not investigated in this
study, so the relative contributions of total light reduction vs. lack of gas exchange during sediment
burial are not clear. Orth and Moore (1983) reported considerable dieback in submerged aquatic
vegetation following a tropical storm in 1972 that
deposited tremendous amounts of sediment from
the catchment into the upper Chesapeake Bay burying aquatic vegetation in that region. Much of this
acute impact could be related to the construction of
P. J. Ralph, D. Tomasko, K. Moore, S. Seddon, and C. M. O. Macinnis-Ng
deposited through time (Koch, 1999). This makes it
difficult to reverse the effects of elevated sedimentation, especially in a relatively closed bay or estuary as
there is very little opportunity for the sediments to be
flushed out of the system. The relationships between
wave and current induced sediment re-suspension
in shallow water areas and light attenuation are not
as yet well quantified and are important areas for
further work. Ward et al. (1984) found that the influence of waves on sediment re-suspension in an
estuarine embayment of a system long influenced
by human activity was affected by the relationship
between the seagrass canopy height relative to the
water depth. Suspended particulate matter concentrations were significantly lower within areas of the
embayment colonized by seagrass beds. However,
when water levels were elevated by spring tides or
storm surges, the plants were less effective at attenuating wave energy and suspended sediment concentrations increased inside the beds. Moore (1996)
observed over a range of water levels that the capacity of eelgrass beds to reduce suspended sediment
concentrations within the beds decreased with decreasing plant cover in the bed. At approximately
25–50% cover there was little difference between
vegetated and un-vegetated areas.
3. Turbidity
The primary detrimental effect of sediment turbidity, as with eutrophication-linked phytoplankton
blooms, is the increased attenuation of light. Reduced light over prolonged periods can deplete seagrass carbon reserves or, in cases of extreme light
deprivation, the lack of photosynthetically-produced
oxygen can lead to sediment anoxia and more rapid
mortality. Turbidity has been responsible for some
very significant seagrass losses in the past, such as
the massive 100,000 ha loss of seagrass during a
cyclone in Hervey Bay (Queensland, Australia) and
its aftermath causing widespread flooding and prolonged turbidity, where seagrasses in shallow water were uprooted in heavy seas and those below
10 m depth were thought to have died from light
deprivation due to increased river runoff associated
with land clearing and erosion (Preen et al., 1995).
Similarly, an extensive decline of 17,890 ha of seagrass and macro-algae occurred in Western Port Bay
(Victoria, Australia) during the 1970s and 1980s
(Bulthuis et al., 1984a). Although the causes are
not known with certainty, losses have been correlated with large volumes of terrestrial runoff, originating from extensive land clearing and soil erosion within the catchment and waterways, as well
as activities such as regular dredging of shipping
channels. The pattern and nature of the Western Port
decline suggests an increasing exposure of the intertidal seagrass beds to desiccation due to the accretion of sediments on mudflats causing higher bed
elevations, as well as directly smothering seagrasses
with sediments and reducing light through persistent turbidity (Bulthuis et al., 1984a; Shepherd et al.,
1989; Stephens, 1995). It is also likely that high turbidity and nutrients, associated with sediment resuspension over denuded areas, in combination with
erosion from waves and tidal currents played a role in
accelerating and perpetuating further seagrass losses
(Bulthuis et al., 1984b; Clarke and Kirkman, 1989;
Walker and McComb, 1992). Onuf (1994) found
that the timing and spacing of dredging activity
in Laguna Madre, Texas correlated closely with
decreases in seagrass distribution. These impacts,
which were long-lasting over wide areas, were not
related to the sediment plumes of the initial dredging
operations themselves, which were short lived but to
the re-working and re-suspension of the dredged material that had been deposited in shallow areas.
4. Smothering
Very little work has been done to date investigating
the effects of smothering and burial by sediments;
however, it appears that tolerance is linked to sediment condition and seagrass species. For example,
Posidonia spp. in Holdfast Bay (South Australia)
survived for approximately 4 months buried under
aerobic sediments compared with less than 2 weeks
in anoxic sediments (Clarke, 1987) and less than
a week for the more fragile Halophila australis
(Clarke and Kirkman, 1989). However, the mechanisms of mortality were not investigated in this
study, so the relative contributions of total light reduction vs. lack of gas exchange during sediment
burial are not clear. Orth and Moore (1983) reported considerable dieback in submerged aquatic
vegetation following a tropical storm in 1972 that
deposited tremendous amounts of sediment from
the catchment into the upper Chesapeake Bay burying aquatic vegetation in that region. Much of this
acute impact could be related to the construction of