Chapter 24 Human Impacts on Seagrasses
583
E. Petrochemicals
1. Sources of Petrochemicals
Petrochemicals are introduced into the marine environment as a result of hazardous materials disposal,
leakage from oil storage facilities, activities in refineries, ports and harbors, and in stormwater runoff
(Thorhaug, 1992). Since seagrasses are commonly
found along the coastal fringe, they are always under threat from oil spills. Many large oil spills have
been carefully monitored for long-term impacts on
seagrasses, such as the Exxon Valdez and the ‘first’
Gulf War (Burns et al., 1993; Dean et al., 1998) and
have shown few lasting impacts. Dispersants used
to clean up spills can also pose a threat to seagrass
both alone and in combination with the oil. Dispersants encourage the oil to spread and increase the
bioavailable fraction of oil by increasing the concentration of petroleum-derived hydrocarbons in the
water column and altering the interaction of these
compounds with biological membranes (Wolfe et al.,
1998).
2. Process of Petrochemicals Contamination
Generally, when seagrasses are exposed to petrochemicals, sub-lethal quantities are incorporated
into the tissue, causing a reduction in tolerance to
other stress factors (Zieman et al., 1984). Smothering of seagrass occurs when oil is stranded on areas of intertidal seagrass, leading to reduced growth
rates, blackened leaves and mortality (Howard et al.,
1989). This is only a problem for intertidal seagrasses. Oil appears to mainly affect flowering, and
if plants are able to spread through apical meristem growth, flowering impacts are not a significant
problem over the medium term for a well-established
meadow (Dean et al., 1998). The Exxon Valdez oil
spill in 1989 caused seagrass blades below oiled
beaches to be bleached white and ultimately killed
(Juday and Foster, 1990). Another study of this spill
noted a decrease in the density of leaves and flowering shoots of Z. marina after the event (Dean et al.,
1998). Dispersants consist of a surfactant in a carrier or solvent (Hatcher and Larkum, 1982). Dispersants can be toxic in their own right, but the solvent
can also encourage the breakdown of the waxy cuticle, allowing greater penetration of oils into seagrass
leaves and increasing phytotoxicity.
3. Status of Petrochemicals Contamination
Investigations of recent spills (Exxon Valdez; Dean
et al., 1998) have found no overall impact on seagrass biomass, density, flowering, or seed production
1 year after the spill. No difference was found between oiled and reference sites. Similarly, as a result
of the Gulf War (1991) vast areas of the coastline of
Saudi Arabia were heavily oiled; however, one year
after the initial event, the productivity of oiled and
un-oiled seagrass meadows were the same (Burns
et al., 1993). These are typical results for oil spills
and reflect the general lack of field data to support
any substantial long-term impact of petrochemical
contamination.
4. Ecophysiology of Petrochemicals Impact
on Seagrass
The primary phytotoxic effect of oil is induced by
the absorption of the water-soluble fraction (WSF).
The toxic components of petrochemicals are thought
to be the poly-aromatic hydrocarbons (PAH), which
are lipophilic, so they are able to pass through lipid
membranes and tend to accumulate in the thylakoid
membranes of the chloroplasts (Ren et al., 1994).
Thorhaug et al. (1986) found that dispersed oil had
a greater effect on the growth of Halodule wrightii,
Syringodium filiforme, and Thalassia testudium than
oil alone. Similarly, Hatcher and Larkum (1982)
found that stress levels in Posidonia australis were
more severe when the oil was dispersed, in comparison to oil alone. Ralph and Burchett (1998b) found
that oil + dispersant had an initial impact on effective quantum yield (F/Fm) in Halophila ovalis,
but some recovery occurred after 96 hours of constant exposure to the mixture. There are also examples of an oil + dispersant mixture being less toxic
than oil or dispersant alone (Macinnis-Ng and Ralph,
2003b).
It is difficult to compare results from studies
testing different oils and dispersants on different
species (Hatcher and Larkum, 1982; Thorhaug et al.,
1986; Thorhaug, 1988; Thorhaug, 1992; Ralph and
Burchett, 1998b). Any one of these variables could
be solely responsible for the variations in the results,
or a complex interaction of the three factors could be
involved. Further controlled experiments are needed
to understand this complex interaction.
583
E. Petrochemicals
1. Sources of Petrochemicals
Petrochemicals are introduced into the marine environment as a result of hazardous materials disposal,
leakage from oil storage facilities, activities in refineries, ports and harbors, and in stormwater runoff
(Thorhaug, 1992). Since seagrasses are commonly
found along the coastal fringe, they are always under threat from oil spills. Many large oil spills have
been carefully monitored for long-term impacts on
seagrasses, such as the Exxon Valdez and the ‘first’
Gulf War (Burns et al., 1993; Dean et al., 1998) and
have shown few lasting impacts. Dispersants used
to clean up spills can also pose a threat to seagrass
both alone and in combination with the oil. Dispersants encourage the oil to spread and increase the
bioavailable fraction of oil by increasing the concentration of petroleum-derived hydrocarbons in the
water column and altering the interaction of these
compounds with biological membranes (Wolfe et al.,
1998).
2. Process of Petrochemicals Contamination
Generally, when seagrasses are exposed to petrochemicals, sub-lethal quantities are incorporated
into the tissue, causing a reduction in tolerance to
other stress factors (Zieman et al., 1984). Smothering of seagrass occurs when oil is stranded on areas of intertidal seagrass, leading to reduced growth
rates, blackened leaves and mortality (Howard et al.,
1989). This is only a problem for intertidal seagrasses. Oil appears to mainly affect flowering, and
if plants are able to spread through apical meristem growth, flowering impacts are not a significant
problem over the medium term for a well-established
meadow (Dean et al., 1998). The Exxon Valdez oil
spill in 1989 caused seagrass blades below oiled
beaches to be bleached white and ultimately killed
(Juday and Foster, 1990). Another study of this spill
noted a decrease in the density of leaves and flowering shoots of Z. marina after the event (Dean et al.,
1998). Dispersants consist of a surfactant in a carrier or solvent (Hatcher and Larkum, 1982). Dispersants can be toxic in their own right, but the solvent
can also encourage the breakdown of the waxy cuticle, allowing greater penetration of oils into seagrass
leaves and increasing phytotoxicity.
3. Status of Petrochemicals Contamination
Investigations of recent spills (Exxon Valdez; Dean
et al., 1998) have found no overall impact on seagrass biomass, density, flowering, or seed production
1 year after the spill. No difference was found between oiled and reference sites. Similarly, as a result
of the Gulf War (1991) vast areas of the coastline of
Saudi Arabia were heavily oiled; however, one year
after the initial event, the productivity of oiled and
un-oiled seagrass meadows were the same (Burns
et al., 1993). These are typical results for oil spills
and reflect the general lack of field data to support
any substantial long-term impact of petrochemical
contamination.
4. Ecophysiology of Petrochemicals Impact
on Seagrass
The primary phytotoxic effect of oil is induced by
the absorption of the water-soluble fraction (WSF).
The toxic components of petrochemicals are thought
to be the poly-aromatic hydrocarbons (PAH), which
are lipophilic, so they are able to pass through lipid
membranes and tend to accumulate in the thylakoid
membranes of the chloroplasts (Ren et al., 1994).
Thorhaug et al. (1986) found that dispersed oil had
a greater effect on the growth of Halodule wrightii,
Syringodium filiforme, and Thalassia testudium than
oil alone. Similarly, Hatcher and Larkum (1982)
found that stress levels in Posidonia australis were
more severe when the oil was dispersed, in comparison to oil alone. Ralph and Burchett (1998b) found
that oil + dispersant had an initial impact on effective quantum yield (F/Fm) in Halophila ovalis,
but some recovery occurred after 96 hours of constant exposure to the mixture. There are also examples of an oil + dispersant mixture being less toxic
than oil or dispersant alone (Macinnis-Ng and Ralph,
2003b).
It is difficult to compare results from studies
testing different oils and dispersants on different
species (Hatcher and Larkum, 1982; Thorhaug et al.,
1986; Thorhaug, 1988; Thorhaug, 1992; Ralph and
Burchett, 1998b). Any one of these variables could
be solely responsible for the variations in the results,
or a complex interaction of the three factors could be
involved. Further controlled experiments are needed
to understand this complex interaction.