572
P. J. Ralph, D. Tomasko, K. Moore, S. Seddon, and C. M. O. Macinnis-Ng
activities (Lewis et al., 1991; Udy and Dennison,
1997a; Johansson and Greening, 2000). Although
studies by Borum (1985), Silberstein et al. (1986),
Neverauskas (1987) and Dixon (1999) demonstrate
the important role played by epiphytic algae in reducing light penetration to the leaves of seagrasses
in eutrophied waters, and overgrowth by macroalgae
has also been implicated as a mechanism of nutrientinduced seagrass declines (Lapointe et al., 1994; Valiela et al., 1997; Hauxwell et al., 2001; Deegan et al.,
2002). Regardless of the mechanism(s) of impact,
nutrient enrichment of nearshore marine and estuarine waters is thought to be the major reason for
seagrass loss worldwide (e.g. Larkum, 1976; Kemp
et al., 1983; Cambridge and McComb, 1984; Orth
and Moore, 1984; Bourcier, 1986; Silberstein et al.,
1986; Kenworthy et al., Chapter 25).
3. Status of Eutrophication
During the 1960s to 1980s, there were a number of significant eutrophication related declines of
sensitive species, including Posidonia spp. and
Amphibolis spp. in Australia (Higginson, 1970;
Cambridge and McComb, 1984; Bastyan, 1986;
Shepherd et al., 1989; Larkum and West, 1990;
Hillman et al., 1991) and Posidonia oceanica in
Europe (Peres and Picard, 1975; Pergent-Martini and
Pergent, 1996). This era represented a time where untreated sewage and industrial waste were discharged
directly into coastal waters and estuaries without understanding of the detrimental nature of nutrients on
seagrass meadows. In response to work investigating these declines, management of water resources
and wastewater has greatly improved in some areas,
although non-point source (i.e. stormwater runoff)
pollution loads remain much more difficult to control than point sources (e.g. Heyl, 1992). We are now
able to accurately predict that, for example, in Lemon
Bay a 45% increase in N loading by 2010 is likely
to increase both chlorophyll levels and light attenuation, resulting in an average reduction in the depth
limit of light-limited meadows of Thalassia testudinum by approximately 24%, from 1.2 m (MTL)
to 0.9 m (Tomasko et al., 2001). Unless future nutrient load reductions occur, seagrass meadows around
the world are likely to experience nutrient-induced
decreases in seagrass coverage (den Hartog and Polderman, 1975; Orth and Moore, 1983; Johansson
and Greening, 1999; Tomasko et al., 1996).
4. Ecophysiology of Nutrient Effects
on Seagrass
The detrimental effects of light attenuation on seagrass, whether due to direct shading from algal
epiphytes and macroalgal smothering or indirect effects of phytoplankton, was suggested by Shepherd
et al. (1989) as the key unifying hypothesis explaining the primary cause of seagrass declines worldwide. However, in the last decade the contribution
of direct nitrogen toxicity and sediment anoxia (sediments contaminated with organic matter waste) as
mechanisms of mortality have been receiving more
attention. Research into nitrogen as a toxicant per
se, has been a topic of inquiry since the study by
Burkholder et al. (1992) pointed out the potential
for a direct toxic effect of elevated levels of water
column nitrate to seagrass meadows in nearshore
waters of North Carolina (USA). While a small degree of nutrient enrichment can enhance seagrass
growth, further enrichment may lead to a situation
of nutrient saturation and even toxicity. Seagrasses
exposed to nutrient saturation have been shown to
exhibit high tissue nutrient content and amino acid
concentrations (Udy and Dennison, 1997a) and such
precursors can alert researchers and managers to the
possibility of broad-scale decline.
The paradox is that nutrients are essential for seagrass photosynthesis and growth. In fact, it has been
shown that seagrass meadows can be N, P, and/or Fe
limited, possibly to the detriment of the bed (Dennison et al., 1987; Udy and Dennison, 1997a). For
example, nitrogen limitation was a significant factor contributing to the sudden decline of Zostera
marina in Grevelingen, The Netherlands (van Lent
et al., 1995). Consequently, considerable effort has
been directed toward the determination of N and P
requirements and nutrient budgets in areas where
nutrient levels are sub-optimal and positive physiological and ecological effects have been reported in
response to nutrient enrichment experiments (e.g.
Udy and Dennison, 1997a). Typically, seagrasses
that are nutrient-limited, react positively to fertilization experiments showing responses including
increased growth rates, enhanced photosynthetic
performance, increased leaf length, higher shoot
densities, and increased above ground biomass (e.g.
Agawin et al., 1996; Udy and Dennison, 1997b).
Agawin et al. (1996) pointed out a link between
C incorporation and nutrient deficiency, proposing
that since chlorophyll synthesis is dependent upon
P. J. Ralph, D. Tomasko, K. Moore, S. Seddon, and C. M. O. Macinnis-Ng
activities (Lewis et al., 1991; Udy and Dennison,
1997a; Johansson and Greening, 2000). Although
studies by Borum (1985), Silberstein et al. (1986),
Neverauskas (1987) and Dixon (1999) demonstrate
the important role played by epiphytic algae in reducing light penetration to the leaves of seagrasses
in eutrophied waters, and overgrowth by macroalgae
has also been implicated as a mechanism of nutrientinduced seagrass declines (Lapointe et al., 1994; Valiela et al., 1997; Hauxwell et al., 2001; Deegan et al.,
2002). Regardless of the mechanism(s) of impact,
nutrient enrichment of nearshore marine and estuarine waters is thought to be the major reason for
seagrass loss worldwide (e.g. Larkum, 1976; Kemp
et al., 1983; Cambridge and McComb, 1984; Orth
and Moore, 1984; Bourcier, 1986; Silberstein et al.,
1986; Kenworthy et al., Chapter 25).
3. Status of Eutrophication
During the 1960s to 1980s, there were a number of significant eutrophication related declines of
sensitive species, including Posidonia spp. and
Amphibolis spp. in Australia (Higginson, 1970;
Cambridge and McComb, 1984; Bastyan, 1986;
Shepherd et al., 1989; Larkum and West, 1990;
Hillman et al., 1991) and Posidonia oceanica in
Europe (Peres and Picard, 1975; Pergent-Martini and
Pergent, 1996). This era represented a time where untreated sewage and industrial waste were discharged
directly into coastal waters and estuaries without understanding of the detrimental nature of nutrients on
seagrass meadows. In response to work investigating these declines, management of water resources
and wastewater has greatly improved in some areas,
although non-point source (i.e. stormwater runoff)
pollution loads remain much more difficult to control than point sources (e.g. Heyl, 1992). We are now
able to accurately predict that, for example, in Lemon
Bay a 45% increase in N loading by 2010 is likely
to increase both chlorophyll levels and light attenuation, resulting in an average reduction in the depth
limit of light-limited meadows of Thalassia testudinum by approximately 24%, from 1.2 m (MTL)
to 0.9 m (Tomasko et al., 2001). Unless future nutrient load reductions occur, seagrass meadows around
the world are likely to experience nutrient-induced
decreases in seagrass coverage (den Hartog and Polderman, 1975; Orth and Moore, 1983; Johansson
and Greening, 1999; Tomasko et al., 1996).
4. Ecophysiology of Nutrient Effects
on Seagrass
The detrimental effects of light attenuation on seagrass, whether due to direct shading from algal
epiphytes and macroalgal smothering or indirect effects of phytoplankton, was suggested by Shepherd
et al. (1989) as the key unifying hypothesis explaining the primary cause of seagrass declines worldwide. However, in the last decade the contribution
of direct nitrogen toxicity and sediment anoxia (sediments contaminated with organic matter waste) as
mechanisms of mortality have been receiving more
attention. Research into nitrogen as a toxicant per
se, has been a topic of inquiry since the study by
Burkholder et al. (1992) pointed out the potential
for a direct toxic effect of elevated levels of water
column nitrate to seagrass meadows in nearshore
waters of North Carolina (USA). While a small degree of nutrient enrichment can enhance seagrass
growth, further enrichment may lead to a situation
of nutrient saturation and even toxicity. Seagrasses
exposed to nutrient saturation have been shown to
exhibit high tissue nutrient content and amino acid
concentrations (Udy and Dennison, 1997a) and such
precursors can alert researchers and managers to the
possibility of broad-scale decline.
The paradox is that nutrients are essential for seagrass photosynthesis and growth. In fact, it has been
shown that seagrass meadows can be N, P, and/or Fe
limited, possibly to the detriment of the bed (Dennison et al., 1987; Udy and Dennison, 1997a). For
example, nitrogen limitation was a significant factor contributing to the sudden decline of Zostera
marina in Grevelingen, The Netherlands (van Lent
et al., 1995). Consequently, considerable effort has
been directed toward the determination of N and P
requirements and nutrient budgets in areas where
nutrient levels are sub-optimal and positive physiological and ecological effects have been reported in
response to nutrient enrichment experiments (e.g.
Udy and Dennison, 1997a). Typically, seagrasses
that are nutrient-limited, react positively to fertilization experiments showing responses including
increased growth rates, enhanced photosynthetic
performance, increased leaf length, higher shoot
densities, and increased above ground biomass (e.g.
Agawin et al., 1996; Udy and Dennison, 1997b).
Agawin et al. (1996) pointed out a link between
C incorporation and nutrient deficiency, proposing
that since chlorophyll synthesis is dependent upon