Chapter 24 Human Impacts on Seagrasses
573
nutrient supply, therefore fertilizing N-limited seagrasses can lead to enhanced photosynthetic rates,
lower respiration rates and increased photosynthetic
efficiency.
The most common symptoms of eutrophicationrelated seagrass decline include high epiphytic loading (but see Mateo et al., Chapter 7, Borowitzka
et al., Chapter 19 and Valentine and Duffy, Chapter
20 for the offsetting effects of grazing) slower growth
rates, reduced biomass, and shoot thinning (Cambridge et al., 1986; Short et al., 1995); although some
of these variables do not always respond consistently to elevated nutrients. For example, in a study
of morphological and physiological characteristics
of Zostera capricorni in Moreton Bay (Australia),
there were no significant differences in leaf length,
shoot density, biomass, and growth rates relating
to increased nutrients, whereas leaf nutrient content
and amino acid concentration increased with greater
proximity to various nutrient sources, such as river
runoff, local septic systems, sewage, and prawn farm
effluent (Udy and Dennison, 1997a). Additionally,
research indicates that not all species are similarly
vulnerable to high levels of nutrients. Burkholder
et al. (1994) suggested that some species have developed mechanisms to regulate the metabolism of
nitrate. In this experiment, shoot production of Halodule wrightii and Ruppia maritima greatly increased
in response to nitrate enrichment, while Zostera marina responded more typically with reduced shoot
production, although at this stage the mechanisms
to enable nitrate regulation are not clear.
Factors such as light availability, temperature,
balance of C, N, P, and redox potential all affect the ability of seagrasses to deal with excessive nutrients. The impact of other factors such as
reduced salinity combined with elevated nutrients
is currently unknown. This is important because
many sources of nutrients are delivered in freshwater (e.g. river runoff and sewage). While many
species have proven to be fairly tolerant of a broad
range of salinities, negative symptoms such as loss
of biomass and lack of leaf production have occurred
in salinities lower than 10 ppt (Hillman et al., 1995;
Doering and Chamberlain, 1999) and it is not clear
whether these effects would be exacerbated in conditions of elevated nutrients. Interactive effects are
likely, as Tomasko and Hall (1999) found in a field
study that Thalassia testudinum productivity was depressed at salinities less than 20 ppt; however, in this
system, low salinity was associated with elevated
dissolved organic matter, rather than suspended
sediments.
5. Water Column Nutrients
In addition to the major effect of eutrophication
induced light attenuation outlined earlier, a direct
toxic effect of water-column nitrogen on the physiology of seagrasses was demonstrated by Burkholder
et al. (1992). They found signs of ‘crumbling’
shoot meristems preceding whole plant mortality for
Zostera marina in the medium and high enrichment
treatments exposed to daily doses of the nitrate (7 and
35 µM N-NO
−
3 respectively) over an 8 week period.
Burkholder et al. (1994) hypothesized that the lack
of an inhibition mechanism for nitrate uptake may
eventually result in internal imbalances of other essential nutrients (e.g. C and P) and that this could be
a result of the plants need to sequester carbon for the
synthesis of amino acids in order to prevent internal
accumulation of toxic compounds such as ammonia in leaf tissues. Leading on from this work, Brun
et al. (2002) also found evidence of nitrogen toxicity when they exposed Z. noltii to a varying number
of pulses of ammonium (from 1 to 8) delivering a
total dose of 200 µM ammonium, which resulted in
significant reductions of growth and primary production. When they validated this work in the field,
they obtained comparable inhibitory affects during
winter, but interestingly not in spring, which Brun
et al. (2002) suggest is due to higher sucrose levels
in the plants being able to meet the increased carbon
demand required for N assimilation.
Other studies (Short et al., 1995; Moore and
Wetzel, 2000) have not found similar responses for
Z. marina at more ecologically relevant levels of nutrient enrichment. Moore and Wetzel (2000) suggest that differences in methods of nutrient delivery
may have influenced these responses. Burkholder
et al. (1992) enriched once a day with a single pulse
of nitrate in a system with limited water turnover
(0.05–0.1 day
−1 ), maintaining ∼200–300 µM NO
−
3
for the highest treatment throughout the experiment,
whereas Moore and Wetzel (2000) provided a more
continuous application of nutrients and higher water exchange (2–16 day
−1 ). It should also be noted
that Z. marina, one of the species most frequently investigated, has the highest mean and widest range of
nitrogen and phosphorus content of the 27 species reviewed by Duarte (1990), and consequently its nutrient requirements and tolerances to eutrophication are
573
nutrient supply, therefore fertilizing N-limited seagrasses can lead to enhanced photosynthetic rates,
lower respiration rates and increased photosynthetic
efficiency.
The most common symptoms of eutrophicationrelated seagrass decline include high epiphytic loading (but see Mateo et al., Chapter 7, Borowitzka
et al., Chapter 19 and Valentine and Duffy, Chapter
20 for the offsetting effects of grazing) slower growth
rates, reduced biomass, and shoot thinning (Cambridge et al., 1986; Short et al., 1995); although some
of these variables do not always respond consistently to elevated nutrients. For example, in a study
of morphological and physiological characteristics
of Zostera capricorni in Moreton Bay (Australia),
there were no significant differences in leaf length,
shoot density, biomass, and growth rates relating
to increased nutrients, whereas leaf nutrient content
and amino acid concentration increased with greater
proximity to various nutrient sources, such as river
runoff, local septic systems, sewage, and prawn farm
effluent (Udy and Dennison, 1997a). Additionally,
research indicates that not all species are similarly
vulnerable to high levels of nutrients. Burkholder
et al. (1994) suggested that some species have developed mechanisms to regulate the metabolism of
nitrate. In this experiment, shoot production of Halodule wrightii and Ruppia maritima greatly increased
in response to nitrate enrichment, while Zostera marina responded more typically with reduced shoot
production, although at this stage the mechanisms
to enable nitrate regulation are not clear.
Factors such as light availability, temperature,
balance of C, N, P, and redox potential all affect the ability of seagrasses to deal with excessive nutrients. The impact of other factors such as
reduced salinity combined with elevated nutrients
is currently unknown. This is important because
many sources of nutrients are delivered in freshwater (e.g. river runoff and sewage). While many
species have proven to be fairly tolerant of a broad
range of salinities, negative symptoms such as loss
of biomass and lack of leaf production have occurred
in salinities lower than 10 ppt (Hillman et al., 1995;
Doering and Chamberlain, 1999) and it is not clear
whether these effects would be exacerbated in conditions of elevated nutrients. Interactive effects are
likely, as Tomasko and Hall (1999) found in a field
study that Thalassia testudinum productivity was depressed at salinities less than 20 ppt; however, in this
system, low salinity was associated with elevated
dissolved organic matter, rather than suspended
sediments.
5. Water Column Nutrients
In addition to the major effect of eutrophication
induced light attenuation outlined earlier, a direct
toxic effect of water-column nitrogen on the physiology of seagrasses was demonstrated by Burkholder
et al. (1992). They found signs of ‘crumbling’
shoot meristems preceding whole plant mortality for
Zostera marina in the medium and high enrichment
treatments exposed to daily doses of the nitrate (7 and
35 µM N-NO
−
3 respectively) over an 8 week period.
Burkholder et al. (1994) hypothesized that the lack
of an inhibition mechanism for nitrate uptake may
eventually result in internal imbalances of other essential nutrients (e.g. C and P) and that this could be
a result of the plants need to sequester carbon for the
synthesis of amino acids in order to prevent internal
accumulation of toxic compounds such as ammonia in leaf tissues. Leading on from this work, Brun
et al. (2002) also found evidence of nitrogen toxicity when they exposed Z. noltii to a varying number
of pulses of ammonium (from 1 to 8) delivering a
total dose of 200 µM ammonium, which resulted in
significant reductions of growth and primary production. When they validated this work in the field,
they obtained comparable inhibitory affects during
winter, but interestingly not in spring, which Brun
et al. (2002) suggest is due to higher sucrose levels
in the plants being able to meet the increased carbon
demand required for N assimilation.
Other studies (Short et al., 1995; Moore and
Wetzel, 2000) have not found similar responses for
Z. marina at more ecologically relevant levels of nutrient enrichment. Moore and Wetzel (2000) suggest that differences in methods of nutrient delivery
may have influenced these responses. Burkholder
et al. (1992) enriched once a day with a single pulse
of nitrate in a system with limited water turnover
(0.05–0.1 day
−1 ), maintaining ∼200–300 µM NO
−
3
for the highest treatment throughout the experiment,
whereas Moore and Wetzel (2000) provided a more
continuous application of nutrients and higher water exchange (2–16 day
−1 ). It should also be noted
that Z. marina, one of the species most frequently investigated, has the highest mean and widest range of
nitrogen and phosphorus content of the 27 species reviewed by Duarte (1990), and consequently its nutrient requirements and tolerances to eutrophication are