574
P. J. Ralph, D. Tomasko, K. Moore, S. Seddon, and C. M. O. Macinnis-Ng
unlikely to be indicative of the majority of species,
particularly the more nutrient sensitive species found
in the Mediterranean and Australia. A greater range
of species must be examined for evidence of nitrogen toxicity before it will be possible to draw any
solid conclusions about the generality of this process. In the oligotrophic open waters of south and
south-western Australia, there is evidence from seagrass losses in the past indicating that some species,
such as Amphibolis antarctica, are highly sensitive
to elevated nutrients (Shepherd et al., 1989). Walker
and McComb (1988) demonstrated that A. antarctica
meadows growing in Shark Bay (Western Australia)
could sustain relatively high rates of production in
an extremely low nutrient environment. In fact, it has
been shown that A. antarctica leaves can take up ammonium at rates 5–38 times higher than rhizomes, in
addition to having the ability for surge-uptake during
pulses of very high ammonium concentrations over
short (2–4 h) periods (Pedersen et al., 1997). In the
absence of mechanisms to exclude or dump excess
N, it is interesting to speculate that eutrophication
may cause species like A. antarctica to quickly reach
tissue N saturation and possibly toxicity, although
this is yet to be investigated.
6. Sediment-Bound Nutrients
There are different tolerances and abilities for seagrasses to deal with nutrient enrichment, depending
on whether the source of nutrients is delivered in
the sediments or water-column and whether the sediments are predominantly carbonate or terrigenous.
For instance, Zostera marina appears to be more susceptible to water column ammonium toxicity when
grown on sand compared with mud, possibly because
carbon supply is more limited on sand (van Katwijk
et al., 1997). In south Sulawesi (Indonesia) mixed
seagrass meadows growing on N and P rich terrigenous sediments from coastal areas (including Thalassia hemprichii, Enhalus acoroides, and Halodule
uninervis) had higher algal biomass (phytoplankton, epiphytic algae and macroalgae), lower belowground biomass relative to above-ground biomass,
and higher tissue N and P content compared with
similar seagrasses growing on carbonate sediments
in reef flat environments (Erftemeijer, 1994). This
work also supported the conclusions of Short (1987)
that seagrasses growing on terrigenous sediments
are typically N-limited, while those growing on carbonate sediments in tropical environments are generally P-limited, due to the adsorption of phosphate
onto carbonate mineral surfaces. Although Udy et al.
(1999) found evidence that N rather than P was the
main limiting nutrient for Halodule uninervis and
Syringodium isoetifolium at Green Island (Great Barrier Reef), this was possibly related to sewage from
the outfall on the island which has a low N:P ratio.
Seagrasses also seem to be more tolerant of higher
levels of nutrients in the sediments than the water
column. Peralta et al. (2003) demonstrated a high
tolerance of Z. marina to nutrient enrichment of pore
water up to 30 mM ammonium, compared with only
25 µM ammonium (van Katwijk et al., 1997) or 200–
300 µM nitrate (Burkholder et al., 1992) required
in the water column to produce symptoms of nitrogen toxicity. While Van Lent et al. (1995) enriched
sediments up to ∼60 µM ammonium and still reported positive effects for a perennial population of
Z. marina growing in mesotrophic lagoon waters in
Grevelingen (The Netherlands) and Dennison et al.
(1987) reported ammonium saturation levels for
Z. marina growing in Great Harbor, Woods Hole,
as high as 100 µM, although they did obtain a clear
(but statistically non-significant) trend of decreasing root production rate with increasing sediment
ammonium concentration for plants growing in shallow water and mentioned the possibility of toxic effects at high ammonium levels. Several factors have
been proposed to explain why ammonium is less
toxic to roots than leaves including: a lower ammonium uptake rate in the roots, lower pH of sediments
compared with the water column and the release of
oxygen by the roots which reduce nitrification in the
surrounding rhizosphere (van Katwijk et al., 1997).
However, it is important to consider that Australian
and Mediterranean seagrasses have evolved in relatively nutrient poor environments, with little terrestrial runoff and these levels of tolerance to pore water
enrichment may not apply in these situations.
7. Sediment Anoxia
Point sources of eutrophication such as finfish cage
culture, not only add nutrients to the water column, promoting phytoplankton growth and associated light attenuation, but also contribute organic
matter. When organic matter accumulates in the underlying substrate, it causes a reduction in the redox potential of the sediments leading to conditions of anoxia. It is suggested that this in turn
places a greater oxygen demand on the roots and can
P. J. Ralph, D. Tomasko, K. Moore, S. Seddon, and C. M. O. Macinnis-Ng
unlikely to be indicative of the majority of species,
particularly the more nutrient sensitive species found
in the Mediterranean and Australia. A greater range
of species must be examined for evidence of nitrogen toxicity before it will be possible to draw any
solid conclusions about the generality of this process. In the oligotrophic open waters of south and
south-western Australia, there is evidence from seagrass losses in the past indicating that some species,
such as Amphibolis antarctica, are highly sensitive
to elevated nutrients (Shepherd et al., 1989). Walker
and McComb (1988) demonstrated that A. antarctica
meadows growing in Shark Bay (Western Australia)
could sustain relatively high rates of production in
an extremely low nutrient environment. In fact, it has
been shown that A. antarctica leaves can take up ammonium at rates 5–38 times higher than rhizomes, in
addition to having the ability for surge-uptake during
pulses of very high ammonium concentrations over
short (2–4 h) periods (Pedersen et al., 1997). In the
absence of mechanisms to exclude or dump excess
N, it is interesting to speculate that eutrophication
may cause species like A. antarctica to quickly reach
tissue N saturation and possibly toxicity, although
this is yet to be investigated.
6. Sediment-Bound Nutrients
There are different tolerances and abilities for seagrasses to deal with nutrient enrichment, depending
on whether the source of nutrients is delivered in
the sediments or water-column and whether the sediments are predominantly carbonate or terrigenous.
For instance, Zostera marina appears to be more susceptible to water column ammonium toxicity when
grown on sand compared with mud, possibly because
carbon supply is more limited on sand (van Katwijk
et al., 1997). In south Sulawesi (Indonesia) mixed
seagrass meadows growing on N and P rich terrigenous sediments from coastal areas (including Thalassia hemprichii, Enhalus acoroides, and Halodule
uninervis) had higher algal biomass (phytoplankton, epiphytic algae and macroalgae), lower belowground biomass relative to above-ground biomass,
and higher tissue N and P content compared with
similar seagrasses growing on carbonate sediments
in reef flat environments (Erftemeijer, 1994). This
work also supported the conclusions of Short (1987)
that seagrasses growing on terrigenous sediments
are typically N-limited, while those growing on carbonate sediments in tropical environments are generally P-limited, due to the adsorption of phosphate
onto carbonate mineral surfaces. Although Udy et al.
(1999) found evidence that N rather than P was the
main limiting nutrient for Halodule uninervis and
Syringodium isoetifolium at Green Island (Great Barrier Reef), this was possibly related to sewage from
the outfall on the island which has a low N:P ratio.
Seagrasses also seem to be more tolerant of higher
levels of nutrients in the sediments than the water
column. Peralta et al. (2003) demonstrated a high
tolerance of Z. marina to nutrient enrichment of pore
water up to 30 mM ammonium, compared with only
25 µM ammonium (van Katwijk et al., 1997) or 200–
300 µM nitrate (Burkholder et al., 1992) required
in the water column to produce symptoms of nitrogen toxicity. While Van Lent et al. (1995) enriched
sediments up to ∼60 µM ammonium and still reported positive effects for a perennial population of
Z. marina growing in mesotrophic lagoon waters in
Grevelingen (The Netherlands) and Dennison et al.
(1987) reported ammonium saturation levels for
Z. marina growing in Great Harbor, Woods Hole,
as high as 100 µM, although they did obtain a clear
(but statistically non-significant) trend of decreasing root production rate with increasing sediment
ammonium concentration for plants growing in shallow water and mentioned the possibility of toxic effects at high ammonium levels. Several factors have
been proposed to explain why ammonium is less
toxic to roots than leaves including: a lower ammonium uptake rate in the roots, lower pH of sediments
compared with the water column and the release of
oxygen by the roots which reduce nitrification in the
surrounding rhizosphere (van Katwijk et al., 1997).
However, it is important to consider that Australian
and Mediterranean seagrasses have evolved in relatively nutrient poor environments, with little terrestrial runoff and these levels of tolerance to pore water
enrichment may not apply in these situations.
7. Sediment Anoxia
Point sources of eutrophication such as finfish cage
culture, not only add nutrients to the water column, promoting phytoplankton growth and associated light attenuation, but also contribute organic
matter. When organic matter accumulates in the underlying substrate, it causes a reduction in the redox potential of the sediments leading to conditions of anoxia. It is suggested that this in turn
places a greater oxygen demand on the roots and can