Chapter 24 Human Impacts on Seagrasses
579
Studies in Asia and the Mediterranean need to be
conducted to gain a better understanding of global
trends.
4. Ecophysiology of Herbicides
Research into indirect impacts and level of contamination from agricultural herbicides over the past
30 years has covered a wide range of compounds and
seagrass species. Recent application of antifoulantherbicides has included the use of Irgarol 1051 and
Diuron (Thomas et al., 2000); however, limited research has been directed at understanding the impact of these compounds on non-target organisms,
such as seagrasses (Leadbitter et al., 1999; Scarlett
et al., 1999; Ralph, 2000). Herbicides move readily
into seagrass tissue, and uptake of atrazine has been
shown to occur through both the leaves and roots
(Schwarzschild et al., 1994). The effect on seagrass
is greatly reduced when the uptake pathway is via
the roots, similar to nitrogen toxicity as discussed
in the previous section. The impact sites of most
common herbicides are well documented for target
species, and are likely to be identical for seagrasses.
For example, diuron reversibly inhibits photosynthetic electron flow to plastoquinone in photosystem
II (PSII) by blocking the electron transport chain after the primary electron acceptor (Q A ) (Miles, 1991),
with similar effects shown for atrazine (Solomon
et al., 1996).
Despite the widely acknowledged risk of herbicide contamination, toxicity testing using estuarine
plants remains in its infancy (Lytle and Lytle, 2001).
Thus, there is only limited data available on the
sensitivities of different species to determine critical levels of toxicants for the conservation of these
ecosystems. For example, diuron at 10 µg L
−1 was
shown to reduce effective quantum yield of photosynthesis by approximately 50%, however these levels only occur during storm events, when the seagrass will be exposed to the additional impact of
freshwater stress and increased turbidity. These environmentally relevant interactions are presently unknown, and we may be underestimating the impact
of such pollution events (Haynes et al., 2000b).
Not all seagrasses respond to herbicides to the
same degree and a comparison of diuron effects
on three seagrass species by Haynes et al. (2000b)
demonstrated variation in effective quantum yield
responses. Halophila ovalis appeared more sensitive to lower concentrations of diuron than Zostera
capricorni and Cymodocea serrulata, which may
have implications for the long-term composition of a
mixed meadow. Competitive fitness of one seagrass
species may result in the change of the community
structure, as one species suffers a substantial decline, changing the trophic interactions based on that
meadow (El Jay et al., 1997). To further understand
the impact on community scale processes, we also
need to understand whether particular life-stages are
more sensitive than others, such as seedlings, meristematic tissue, and mature leaves.
5. Bioaccumulation of Herbicides
Seagrasses have been found to bioaccumulate herbicides, which creates problems throughout associated
food webs. The capacity of the seagrass to bioaccumulate a toxic compound will define its trophic
threat. For example, irgarol can be accumulated at
up to 25,000 times the background level (<0.003 µg
dm
−3 ) in Z. marina (Scarlett, 1999). Irgarol does not
appear to depurate in fresh seawater. This suggests
that the herbicide burden will accumulate during periods of elevated contaminant load, but will not diffuse back out of the seagrass when flushed with clean
water. Therefore, herbicides would be available for
consumption by grazers such as dugongs, which in
fact have been found to have elevated levels of herbicides (Haynes et al., 2000a). This is in contrast to the
rapid recovery of Z. capricorni after in situ exposure
to irgarol, diuron and atrazine on return to uncontaminated water (Macinnis-Ng and Ralph, 2003a), a
characteristic of PSII inhibitor herbicide impact on
aquatic macrophytes (Bowmer, 1986). Further investigations of detoxification processes may address
this anomaly between acclimation and physiological
responses.
Genotypic and phenotypic plasticity is a common
phenomenon within seagrass populations (Waycott, 1998; Waycott et al., Chapter 2). Site-specific
variation in herbicide tolerance means extrapolation of sensitivity/tolerance data should be treated
cautiously. For example, Macinnis-Ng and Ralph
(2003a) found that diuron toxicity to Z. capricorni
was more toxic both in situ and in the laboratory,
than Haynes et al. (2000b) found for the same
species occurring 1000 km north. A 5-day exposure period for the plants from the northern site
showed effective quantum yield (F/Fm) of around
0.2, whereas those from the southern population
showed values of 0.0 after only a few hours of
579
Studies in Asia and the Mediterranean need to be
conducted to gain a better understanding of global
trends.
4. Ecophysiology of Herbicides
Research into indirect impacts and level of contamination from agricultural herbicides over the past
30 years has covered a wide range of compounds and
seagrass species. Recent application of antifoulantherbicides has included the use of Irgarol 1051 and
Diuron (Thomas et al., 2000); however, limited research has been directed at understanding the impact of these compounds on non-target organisms,
such as seagrasses (Leadbitter et al., 1999; Scarlett
et al., 1999; Ralph, 2000). Herbicides move readily
into seagrass tissue, and uptake of atrazine has been
shown to occur through both the leaves and roots
(Schwarzschild et al., 1994). The effect on seagrass
is greatly reduced when the uptake pathway is via
the roots, similar to nitrogen toxicity as discussed
in the previous section. The impact sites of most
common herbicides are well documented for target
species, and are likely to be identical for seagrasses.
For example, diuron reversibly inhibits photosynthetic electron flow to plastoquinone in photosystem
II (PSII) by blocking the electron transport chain after the primary electron acceptor (Q A ) (Miles, 1991),
with similar effects shown for atrazine (Solomon
et al., 1996).
Despite the widely acknowledged risk of herbicide contamination, toxicity testing using estuarine
plants remains in its infancy (Lytle and Lytle, 2001).
Thus, there is only limited data available on the
sensitivities of different species to determine critical levels of toxicants for the conservation of these
ecosystems. For example, diuron at 10 µg L
−1 was
shown to reduce effective quantum yield of photosynthesis by approximately 50%, however these levels only occur during storm events, when the seagrass will be exposed to the additional impact of
freshwater stress and increased turbidity. These environmentally relevant interactions are presently unknown, and we may be underestimating the impact
of such pollution events (Haynes et al., 2000b).
Not all seagrasses respond to herbicides to the
same degree and a comparison of diuron effects
on three seagrass species by Haynes et al. (2000b)
demonstrated variation in effective quantum yield
responses. Halophila ovalis appeared more sensitive to lower concentrations of diuron than Zostera
capricorni and Cymodocea serrulata, which may
have implications for the long-term composition of a
mixed meadow. Competitive fitness of one seagrass
species may result in the change of the community
structure, as one species suffers a substantial decline, changing the trophic interactions based on that
meadow (El Jay et al., 1997). To further understand
the impact on community scale processes, we also
need to understand whether particular life-stages are
more sensitive than others, such as seedlings, meristematic tissue, and mature leaves.
5. Bioaccumulation of Herbicides
Seagrasses have been found to bioaccumulate herbicides, which creates problems throughout associated
food webs. The capacity of the seagrass to bioaccumulate a toxic compound will define its trophic
threat. For example, irgarol can be accumulated at
up to 25,000 times the background level (<0.003 µg
dm
−3 ) in Z. marina (Scarlett, 1999). Irgarol does not
appear to depurate in fresh seawater. This suggests
that the herbicide burden will accumulate during periods of elevated contaminant load, but will not diffuse back out of the seagrass when flushed with clean
water. Therefore, herbicides would be available for
consumption by grazers such as dugongs, which in
fact have been found to have elevated levels of herbicides (Haynes et al., 2000a). This is in contrast to the
rapid recovery of Z. capricorni after in situ exposure
to irgarol, diuron and atrazine on return to uncontaminated water (Macinnis-Ng and Ralph, 2003a), a
characteristic of PSII inhibitor herbicide impact on
aquatic macrophytes (Bowmer, 1986). Further investigations of detoxification processes may address
this anomaly between acclimation and physiological
responses.
Genotypic and phenotypic plasticity is a common
phenomenon within seagrass populations (Waycott, 1998; Waycott et al., Chapter 2). Site-specific
variation in herbicide tolerance means extrapolation of sensitivity/tolerance data should be treated
cautiously. For example, Macinnis-Ng and Ralph
(2003a) found that diuron toxicity to Z. capricorni
was more toxic both in situ and in the laboratory,
than Haynes et al. (2000b) found for the same
species occurring 1000 km north. A 5-day exposure period for the plants from the northern site
showed effective quantum yield (F/Fm) of around
0.2, whereas those from the southern population
showed values of 0.0 after only a few hours of