Chapter 24 Human Impacts on Seagrasses
581
limited bio-availability. Consequently, much of the
contaminant load is not available to the seagrass.
Simply measuring concentrations of metals in sediment or water will not accurately indicate the impact,
because the physicochemical conditions in a particular habitat will determine the bio-available fraction.
There may be high concentrations of metal in the sediment, but if the material is bound to the sediment,
it may not have a toxic impact on biota. Variation in
sediment conditions outlined above can change how
metals are bound and thus, a site with low concentrations of metals in sediment may indicate low binding ability, as much as low contamination. Relating
environmental concentrations to concentrations in
tissues (and therefore impact levels) is problematic
due to the variability in accumulation phenomena,
as outlined below.
Pergent-Martini and Pergent (2000) have pursued
the question of metal accumulation in seagrasses
for many years, providing a detailed understanding
of the distribution and some mechanisms of metal
uptake. They found accumulation varies according
to the specific heavy metal, the species of seagrass
being assessed, the tissue-type (including compartment, and age), the sampling period and the level of
contamination. Thus, assessing the abundance and
distribution of metals in seagrass meadows is relatively straightforward, but determining the source,
uptake and impact of these metals requires more research. Heavy metal uptake is widely believed to
be controlled by exposure concentration, which is a
passive, surface area-dependent process (Ward,
1989; Malea et al., 1995). Heavy metal uptake
into a seagrass can follow two pathways: from surrounding water to leaves then rhizomes or acropetal
translocation from interstitial water (in the sediment) into roots to rhizomes and leaves. Very
little is known about the cellular uptake mechanisms, toxicology, and heavy metal acclimation in
seagrasses.
Uptake of heavy metal contaminants also appears
to be influenced by season. This has been previously
attributed to variations in growth rates that dilute the
accumulated metals (Lyngby and Brix, 1982); however, new evidence suggests that climatic patterns
show an equal or greater influence on the seasonality of metal accumulation. Prange and Dennison
(2000) found the seasonal changes in concentration
were associated with seasonal rainfall events, leading to changes in metal load in the water/sediment.
Schlacher-Hoenlinger and Schlacher (1998) suggested that storm frequency and seasonal rainfall
influence heavy metal accumulation, in conjunction
with the annual growth cycle. Seasonal weather patterns are also linked to sediment re-suspension and
microbial degradation of heavy metals.
As noted earlier, toxicity testing using estuarine
plants is a relatively new area of research (Lytle and
Lytle, 2001). Thus, there is only limited data on the
sensitivities of different species and there is sparse
data available for determining critical levels of toxic
metals for the conservation of these ecosystems. For
example, two species of Halophila were found to be
impacted by iron and copper exposure, while Zostera
capricorni was only affected by copper and Halodule
uninervis and Cymodocea serrulata remained unaffected (Prange and Dennison, 2000). More studies
that compare a number of species are required to
fully understand metal effects on different taxa, as
opposed to measuring metal accumulation.
3. Status of Heavy Metal Contamination
Heavy metal contamination in estuarine areas has
been identified across the world. Mediterranean
studies were reviewed by Pergent-Martini and Pergent (2000) and this seems to be the area with the
most information available in the literature. There
are some Australian studies such as Fabris et al.
(1982), Batley (1987), Ward (1987), and Prange and
Dennison (2000).
4. Ecophysiology of Heavy Metals Impact
on Seagrass
Most of the heavy metal research has focused on
the accumulation of metals into seagrasses (Sanchiz
et al., 2001). Seagrasses are good as biomonitors, as
their tissue metal content often reflects bio-available
water/sediment content (Sanchiz et al., 2001), but
this is not true for all species for all metals. Few
studies have considered the physiological impact
of the metals on the seagrass (Ralph and Burchett,
1998a; Prange and Dennison, 2000; Macinnis-Ng
and Ralph, 2002). Understanding the ecophysiology
will allow us to understand the role of contaminant
metals in the marine environment, not simply identify the presence and location of the contaminants.
Since ample evidence has been collected over the
past years to show that seagrasses accumulate heavy
metals, we now need to understand whether metals
can kill, permanently damage or merely cause stress
581
limited bio-availability. Consequently, much of the
contaminant load is not available to the seagrass.
Simply measuring concentrations of metals in sediment or water will not accurately indicate the impact,
because the physicochemical conditions in a particular habitat will determine the bio-available fraction.
There may be high concentrations of metal in the sediment, but if the material is bound to the sediment,
it may not have a toxic impact on biota. Variation in
sediment conditions outlined above can change how
metals are bound and thus, a site with low concentrations of metals in sediment may indicate low binding ability, as much as low contamination. Relating
environmental concentrations to concentrations in
tissues (and therefore impact levels) is problematic
due to the variability in accumulation phenomena,
as outlined below.
Pergent-Martini and Pergent (2000) have pursued
the question of metal accumulation in seagrasses
for many years, providing a detailed understanding
of the distribution and some mechanisms of metal
uptake. They found accumulation varies according
to the specific heavy metal, the species of seagrass
being assessed, the tissue-type (including compartment, and age), the sampling period and the level of
contamination. Thus, assessing the abundance and
distribution of metals in seagrass meadows is relatively straightforward, but determining the source,
uptake and impact of these metals requires more research. Heavy metal uptake is widely believed to
be controlled by exposure concentration, which is a
passive, surface area-dependent process (Ward,
1989; Malea et al., 1995). Heavy metal uptake
into a seagrass can follow two pathways: from surrounding water to leaves then rhizomes or acropetal
translocation from interstitial water (in the sediment) into roots to rhizomes and leaves. Very
little is known about the cellular uptake mechanisms, toxicology, and heavy metal acclimation in
seagrasses.
Uptake of heavy metal contaminants also appears
to be influenced by season. This has been previously
attributed to variations in growth rates that dilute the
accumulated metals (Lyngby and Brix, 1982); however, new evidence suggests that climatic patterns
show an equal or greater influence on the seasonality of metal accumulation. Prange and Dennison
(2000) found the seasonal changes in concentration
were associated with seasonal rainfall events, leading to changes in metal load in the water/sediment.
Schlacher-Hoenlinger and Schlacher (1998) suggested that storm frequency and seasonal rainfall
influence heavy metal accumulation, in conjunction
with the annual growth cycle. Seasonal weather patterns are also linked to sediment re-suspension and
microbial degradation of heavy metals.
As noted earlier, toxicity testing using estuarine
plants is a relatively new area of research (Lytle and
Lytle, 2001). Thus, there is only limited data on the
sensitivities of different species and there is sparse
data available for determining critical levels of toxic
metals for the conservation of these ecosystems. For
example, two species of Halophila were found to be
impacted by iron and copper exposure, while Zostera
capricorni was only affected by copper and Halodule
uninervis and Cymodocea serrulata remained unaffected (Prange and Dennison, 2000). More studies
that compare a number of species are required to
fully understand metal effects on different taxa, as
opposed to measuring metal accumulation.
3. Status of Heavy Metal Contamination
Heavy metal contamination in estuarine areas has
been identified across the world. Mediterranean
studies were reviewed by Pergent-Martini and Pergent (2000) and this seems to be the area with the
most information available in the literature. There
are some Australian studies such as Fabris et al.
(1982), Batley (1987), Ward (1987), and Prange and
Dennison (2000).
4. Ecophysiology of Heavy Metals Impact
on Seagrass
Most of the heavy metal research has focused on
the accumulation of metals into seagrasses (Sanchiz
et al., 2001). Seagrasses are good as biomonitors, as
their tissue metal content often reflects bio-available
water/sediment content (Sanchiz et al., 2001), but
this is not true for all species for all metals. Few
studies have considered the physiological impact
of the metals on the seagrass (Ralph and Burchett,
1998a; Prange and Dennison, 2000; Macinnis-Ng
and Ralph, 2002). Understanding the ecophysiology
will allow us to understand the role of contaminant
metals in the marine environment, not simply identify the presence and location of the contaminants.
Since ample evidence has been collected over the
past years to show that seagrasses accumulate heavy
metals, we now need to understand whether metals
can kill, permanently damage or merely cause stress