578
P. J. Ralph, D. Tomasko, K. Moore, S. Seddon, and C. M. O. Macinnis-Ng
6. Recovery
Very few studies have investigated recovery after
a sediment-induced seagrass decline, but as mentioned earlier, once sediments are resuspended they
can create a cycle of continued turbidity and sedimentation. In the case of Western Port Bay, the denuded tidal flats, where the abundance of fine silt
and a regime of strong tidal currents ensured that
fine sediments were continually resuspended, which
were not conditions conducive to seagrass recolonization, growth and survival. Of the 12,970 ha
of Heterozostera tasmanica, Zostera muelleri, and
Amphibolis antarctica that were lost between 1973
and 1984 (Bulthuis and Woelkerling, 1981; Bulthuis
et al., 1984a), a follow-up survey by Stephens (1995)
revealed that only 3,400 ha of the affected area had
recovered (less that 2% recovery per year). Most of
this was recovery by Z. muelleri colonizing intertidal mud banks that were previously dominated by
H. tasmanica, indicating that these beds are possibly too shallow now for H. tasmanica to survive increased periods of exposure to desiccation
(S. Seddon, personal observation). In Hervey Bay,
2 years after the cyclone and floods, limited recovery had occurred from surviving seed stock, but only
in deeper water (>10 m), whereas the lack of recovery in shallow water was attributed to the lack of
viable seed surviving after the cyclone (Preen et al.,
1995).
C. Herbicides
1. Sources of Herbicides
Herbicides are introduced to estuaries mainly as
runoff from agricultural and municipal applications, whilst several herbicides are directly introduced as biocides on ships hulls. Recent studies
of herbicide contamination in the UK and Queensland have demonstrated that herbicides accumulate around the sources and tend to be at much
lower concentrations in off-shore sediments (Haynes
et al., 2000a; Thomas et al., 2000). While sediment
and surface water concentrations give an indication
of amounts of herbicides in the environment, they
give no indication of the effects on biota. More
detailed studies are required in other parts of the
world to fully understand the extent of herbicide
contamination.
2. Processes of Herbicide Contamination
Herbicides are generally persistent compounds that,
due to their chemical structure, can be resistant
to microbial attack (Solomon et al., 1996). But
biodegradation and photodegradation in marine environments can considerably reduce the half-life of
these toxins. The most common herbicide, atrazine
has a half-life of 3 days in estuarine waters (Solomon
et al., 1996) and Irgarol 1051 has a half-life in seawater of 100 days (Scarlett et al., 1999). Yet degradation of a compound does not necessarily reduce
the risk, for example, the degradation product of
Irgarol 1051, M1, remains toxic to macrophytes and
algae (Okamura et al., 2000). Models of herbicide
fate predict that atrazine and Irgarol 1051 will predominantly remain in the water column, with very
little material adsorbing to the sediments or entering
biota (Rogers et al., 1996). This model is supported
by sediment concentration data for these and other
herbicides (Thomas et al., 2000). While atrazine remains in surface waters, its toxicity tends to be minimized, as it diffuses throughout the water column
(Mersie et al., 2000), effectively reducing the dose
in large water bodies. Nevertheless, despite the low
concentration of herbicides adsorbing to sediments,
concentrations of herbicides in sediments provide
the best history of previous exposure (Thomas et al.,
2001).
3. Status of Herbicide Contamination
Accumulation of herbicides in the marine environment has been noted in several areas around the
world. In the UK, antifouling biocides such as diuron
and Irgarol 1051 have been detected in sediments of
a number of estuarine and coastal areas (Thomas
et al., 2000; Thomas et al., 2001). Haynes et al.
(2000) surveyed the concentrations of a number of
herbicides in sediment and seagrass in Queensland.
Diuron was the substance of most concern, occurring at concentrations that may affect seagrasses.
However, the relationship between contamination
and resulting impacts needs to be more fully investigated. For example, concentrations of triazine
and polyurea herbicides in Wadden Sea sediments
showed a connection to seagrass condition (Bester,
2000), yet the concentrations of other contaminants
such as metals were not considered, so causality has
not been fully demonstrated. Investigation of herbicide contamination is limited to a few regions.
P. J. Ralph, D. Tomasko, K. Moore, S. Seddon, and C. M. O. Macinnis-Ng
6. Recovery
Very few studies have investigated recovery after
a sediment-induced seagrass decline, but as mentioned earlier, once sediments are resuspended they
can create a cycle of continued turbidity and sedimentation. In the case of Western Port Bay, the denuded tidal flats, where the abundance of fine silt
and a regime of strong tidal currents ensured that
fine sediments were continually resuspended, which
were not conditions conducive to seagrass recolonization, growth and survival. Of the 12,970 ha
of Heterozostera tasmanica, Zostera muelleri, and
Amphibolis antarctica that were lost between 1973
and 1984 (Bulthuis and Woelkerling, 1981; Bulthuis
et al., 1984a), a follow-up survey by Stephens (1995)
revealed that only 3,400 ha of the affected area had
recovered (less that 2% recovery per year). Most of
this was recovery by Z. muelleri colonizing intertidal mud banks that were previously dominated by
H. tasmanica, indicating that these beds are possibly too shallow now for H. tasmanica to survive increased periods of exposure to desiccation
(S. Seddon, personal observation). In Hervey Bay,
2 years after the cyclone and floods, limited recovery had occurred from surviving seed stock, but only
in deeper water (>10 m), whereas the lack of recovery in shallow water was attributed to the lack of
viable seed surviving after the cyclone (Preen et al.,
1995).
C. Herbicides
1. Sources of Herbicides
Herbicides are introduced to estuaries mainly as
runoff from agricultural and municipal applications, whilst several herbicides are directly introduced as biocides on ships hulls. Recent studies
of herbicide contamination in the UK and Queensland have demonstrated that herbicides accumulate around the sources and tend to be at much
lower concentrations in off-shore sediments (Haynes
et al., 2000a; Thomas et al., 2000). While sediment
and surface water concentrations give an indication
of amounts of herbicides in the environment, they
give no indication of the effects on biota. More
detailed studies are required in other parts of the
world to fully understand the extent of herbicide
contamination.
2. Processes of Herbicide Contamination
Herbicides are generally persistent compounds that,
due to their chemical structure, can be resistant
to microbial attack (Solomon et al., 1996). But
biodegradation and photodegradation in marine environments can considerably reduce the half-life of
these toxins. The most common herbicide, atrazine
has a half-life of 3 days in estuarine waters (Solomon
et al., 1996) and Irgarol 1051 has a half-life in seawater of 100 days (Scarlett et al., 1999). Yet degradation of a compound does not necessarily reduce
the risk, for example, the degradation product of
Irgarol 1051, M1, remains toxic to macrophytes and
algae (Okamura et al., 2000). Models of herbicide
fate predict that atrazine and Irgarol 1051 will predominantly remain in the water column, with very
little material adsorbing to the sediments or entering
biota (Rogers et al., 1996). This model is supported
by sediment concentration data for these and other
herbicides (Thomas et al., 2000). While atrazine remains in surface waters, its toxicity tends to be minimized, as it diffuses throughout the water column
(Mersie et al., 2000), effectively reducing the dose
in large water bodies. Nevertheless, despite the low
concentration of herbicides adsorbing to sediments,
concentrations of herbicides in sediments provide
the best history of previous exposure (Thomas et al.,
2001).
3. Status of Herbicide Contamination
Accumulation of herbicides in the marine environment has been noted in several areas around the
world. In the UK, antifouling biocides such as diuron
and Irgarol 1051 have been detected in sediments of
a number of estuarine and coastal areas (Thomas
et al., 2000; Thomas et al., 2001). Haynes et al.
(2000) surveyed the concentrations of a number of
herbicides in sediment and seagrass in Queensland.
Diuron was the substance of most concern, occurring at concentrations that may affect seagrasses.
However, the relationship between contamination
and resulting impacts needs to be more fully investigated. For example, concentrations of triazine
and polyurea herbicides in Wadden Sea sediments
showed a connection to seagrass condition (Bester,
2000), yet the concentrations of other contaminants
such as metals were not considered, so causality has
not been fully demonstrated. Investigation of herbicide contamination is limited to a few regions.