366
Kenneth A. Moore and Frederick T. Short
leaf width and length as well as increasing aboveground to belowground biomass ratios with increasing depth (Phillips and Backman, 1983; Short, 1983;
Bigley and Harrison, 1986; Conacher et al., 1994;
Curiel et al., 1996; Lee et al., 2000; Lerodiacounou
and Laurenson, 2002). Much of this morphological
change is related to differences in light availability
as the plants attempt to optimize photosynthesis. In
situ light manipulation experiments of many Zostera
species (Backman and Barilotti, 1976; Dennsion
and Alberte, 1982, 1986; Bulthuis, 1983; Philippart,
1995) and controlled system experiments (Harrison,
1982; Abal et al., 1994; Short et al., 1995; Moore
et al., 1997) have demonstrated higher densities
and productivity, but smaller shoots, under higher
light conditions. Much of the influence of depth on
Zostera plant size may also be related to different levels of sediment nitrogen and other sediment characteristics, which typically change with depth (Short,
1983; Dennison et al., 1987; Lee et al., 2000). Nutrient additions to sediment have long been observed to
increase individual Z. marina shoot size (Orth, 1977;
Dennison et al., 1987; Short et al., 1995), although
this was not observed in Z. tasmanica (Bulthuis and
Woelkerling, 1981). Physical factors such as current velocity, exposure to waves and relative water
depths can have significant effects on the structure
of Zostera meadows (Fonseca and Bell, 1998). For
example, Fonseca et al. (1983) found an inverse relationship between current velocity and aboveground
to belowground biomass allocation for Z. marina
beds. High current or wave regimes may also change
sediment conditions in seagrass beds by increasing
the diffusive loss of pore water nutrients into the
water column (Koch, 1999a; Nepf and Koch, 1999)
as well as loss of organic matter (Fonseca and Bell,
1998) – see also Koch et al., Chapter 8.
D. Physiology
Investigations of seagrass physiology, covered in
depth elsewhere (see Zimmerman, Chapter 13,
Larkum et al., Chapter 14) have been led by studies
on Zostera, including the early experiments on the influence of low salinity on seed germination (Arasaki,
1950) and Z. marina’s metabolic responses to light,
temperature and salinity (Biebl and McRoy, 1971).
Subsequent work has shown the response of Zostera
to temperature, with leaf respiration increasing more
rapidly than photosynthesis with rising temperature,
resulting in a steady decrease in the photosynthesisto-respiration ratio (Evans et al., 1986; Marsh et al.,
1986; Bulthuis, 1987) and the occurrence of a seasonal growth optimum. For species growing in locations with temperatures above the optimum for
growth, near the upper limit of thermal tolerance, an
increase in annual temperature decreases productivity and distribution. Z. marina at the southern end
of its distribution on the east coast of the US shows
summer suppression as in North Carolina (Thayer
et al., 1984) and Chesapeake Bay (Moore et al.,
1996) where Z. marina flourishes during the cooler
months but dies back during hot summer periods.
Temperature also affects flowering (De Cock, 1981)
and seed germination (Harrison, 1982; Phillips et al.,
1983, Hootsmans et al., 1987).
Much evidence for the physiology of light response in seagrasses is based on studies of Z. marina (Dennison and Alberte, 1986; Sand-Jensen,
1989; Short et al., 1995), Z. capricorni (Abal and
Dennison, 1996), and Z. tasmanica (formerly Heterozostera tasmanica, Bulthuis, 1983). Experimental studies have shown the effects of reduced light
on photosynthesis in single Zostera shoots and
parts of plants (Drew, 1979; Williams and McRoy,
1982; Wetzel and Penhale, 1983), documenting the
photosynthesis–irradiance (P–I) curve which describes a saturating relationship of the plants to increased light. The Diving-PAM (pulse amplitude
modulated fluorometer) has accelerated the investigation of in situ photosynthesis in seagrasses with
studies on Zostera (Beer et al., 1998; Ralph and
Short, 2002; Ralph et al., 2002; Campbell et al.,
2003), although most Diving-PAM work has been
done on other genera. For Z. marina, the effects
of decreased light are a reduction in not only shoot
morphology but shoot density, number of leaves per
shoot, and growth rate (Short et al., 1993a, 1995;
Moore and Wetzel, 2000). That light reduction ultimately reduces areal plant productivity has been
documented for Z. marina (Backman and Barilotti,
1976) and for Z. tasmanica (Bulthuis, 1983).
The impact of salinity has been shown in the
laboratory, demonstrating that low salinities stimulate germination of Z. japonica (formerly Zostera
nana), Z. marina, Z. noltii, and Z. capricorni seeds
(Arasaki, 1950; Phillips et al., 1983; Churchill et al.,
1985; Hootsmans et al., 1987; Loques et al., 1990;
Conacher et al., 1994), although in a field study of
Z. marina, low sediment oxygen and decrease in
temperature were found to be more important than
Kenneth A. Moore and Frederick T. Short
leaf width and length as well as increasing aboveground to belowground biomass ratios with increasing depth (Phillips and Backman, 1983; Short, 1983;
Bigley and Harrison, 1986; Conacher et al., 1994;
Curiel et al., 1996; Lee et al., 2000; Lerodiacounou
and Laurenson, 2002). Much of this morphological
change is related to differences in light availability
as the plants attempt to optimize photosynthesis. In
situ light manipulation experiments of many Zostera
species (Backman and Barilotti, 1976; Dennsion
and Alberte, 1982, 1986; Bulthuis, 1983; Philippart,
1995) and controlled system experiments (Harrison,
1982; Abal et al., 1994; Short et al., 1995; Moore
et al., 1997) have demonstrated higher densities
and productivity, but smaller shoots, under higher
light conditions. Much of the influence of depth on
Zostera plant size may also be related to different levels of sediment nitrogen and other sediment characteristics, which typically change with depth (Short,
1983; Dennison et al., 1987; Lee et al., 2000). Nutrient additions to sediment have long been observed to
increase individual Z. marina shoot size (Orth, 1977;
Dennison et al., 1987; Short et al., 1995), although
this was not observed in Z. tasmanica (Bulthuis and
Woelkerling, 1981). Physical factors such as current velocity, exposure to waves and relative water
depths can have significant effects on the structure
of Zostera meadows (Fonseca and Bell, 1998). For
example, Fonseca et al. (1983) found an inverse relationship between current velocity and aboveground
to belowground biomass allocation for Z. marina
beds. High current or wave regimes may also change
sediment conditions in seagrass beds by increasing
the diffusive loss of pore water nutrients into the
water column (Koch, 1999a; Nepf and Koch, 1999)
as well as loss of organic matter (Fonseca and Bell,
1998) – see also Koch et al., Chapter 8.
D. Physiology
Investigations of seagrass physiology, covered in
depth elsewhere (see Zimmerman, Chapter 13,
Larkum et al., Chapter 14) have been led by studies
on Zostera, including the early experiments on the influence of low salinity on seed germination (Arasaki,
1950) and Z. marina’s metabolic responses to light,
temperature and salinity (Biebl and McRoy, 1971).
Subsequent work has shown the response of Zostera
to temperature, with leaf respiration increasing more
rapidly than photosynthesis with rising temperature,
resulting in a steady decrease in the photosynthesisto-respiration ratio (Evans et al., 1986; Marsh et al.,
1986; Bulthuis, 1987) and the occurrence of a seasonal growth optimum. For species growing in locations with temperatures above the optimum for
growth, near the upper limit of thermal tolerance, an
increase in annual temperature decreases productivity and distribution. Z. marina at the southern end
of its distribution on the east coast of the US shows
summer suppression as in North Carolina (Thayer
et al., 1984) and Chesapeake Bay (Moore et al.,
1996) where Z. marina flourishes during the cooler
months but dies back during hot summer periods.
Temperature also affects flowering (De Cock, 1981)
and seed germination (Harrison, 1982; Phillips et al.,
1983, Hootsmans et al., 1987).
Much evidence for the physiology of light response in seagrasses is based on studies of Z. marina (Dennison and Alberte, 1986; Sand-Jensen,
1989; Short et al., 1995), Z. capricorni (Abal and
Dennison, 1996), and Z. tasmanica (formerly Heterozostera tasmanica, Bulthuis, 1983). Experimental studies have shown the effects of reduced light
on photosynthesis in single Zostera shoots and
parts of plants (Drew, 1979; Williams and McRoy,
1982; Wetzel and Penhale, 1983), documenting the
photosynthesis–irradiance (P–I) curve which describes a saturating relationship of the plants to increased light. The Diving-PAM (pulse amplitude
modulated fluorometer) has accelerated the investigation of in situ photosynthesis in seagrasses with
studies on Zostera (Beer et al., 1998; Ralph and
Short, 2002; Ralph et al., 2002; Campbell et al.,
2003), although most Diving-PAM work has been
done on other genera. For Z. marina, the effects
of decreased light are a reduction in not only shoot
morphology but shoot density, number of leaves per
shoot, and growth rate (Short et al., 1993a, 1995;
Moore and Wetzel, 2000). That light reduction ultimately reduces areal plant productivity has been
documented for Z. marina (Backman and Barilotti,
1976) and for Z. tasmanica (Bulthuis, 1983).
The impact of salinity has been shown in the
laboratory, demonstrating that low salinities stimulate germination of Z. japonica (formerly Zostera
nana), Z. marina, Z. noltii, and Z. capricorni seeds
(Arasaki, 1950; Phillips et al., 1983; Churchill et al.,
1985; Hootsmans et al., 1987; Loques et al., 1990;
Conacher et al., 1994), although in a field study of
Z. marina, low sediment oxygen and decrease in
temperature were found to be more important than