Chapter 18 The Biology of Thalassia
415
column depends more on the uptake capacity of the
leaves (Stapel et al., 1996a). Stapel et al. (1996a)
determined the maximum uptake rate (V max ) and the
half-saturation constant (K m ; Michaelis-Menten) of
ammonium and phosphate uptake by the leaves of T.
hemprichii plants from three contrasting locations
in Indonesia, but did not find significant differences.
Lee and Dunton (1999a) studied the leaf-uptake rates
for NH
+
4 in T. testudinum in the USA and found much
lower values for the uptake characteristics. The V max
for NH
+
4 uptake by T. testudinum leaves varied between 7.8 and 17.5 µMol g
−1 DW h
−1 (Lee and
Dunton, 1999a), whereas that of T. hemprichii leaves
ranged between 25 and 48 µMol g
−1 DW h
−1 (Stapel
et al., 1996a). Additionally, K m for ammonium uptake by leaves of T. testudinum and T. hemprichii
ranged between 3.9 and 23.3 µM and 14–97 µM,
respectively (Stapel et al., 1996a; Lee and Dunton,
1999a). These figures indicate that T. testudinum
leaves have a lower capacity for NH
+
4 uptake, but a
higher uptake affinity than T. hemprichii, suggesting
that T. testudinum might be better adapted to growth
under low nitrogen conditions. V max for inorganic
phosphate by T. testudinum was 1.9–2.1 µMol g
−1
DW h
−1 for leaf uptake and 0.38–0.57 µMol g
−1
DW h
−1 for uptake by the roots (Gras et al., 2003).
Stapel et al. (1996a) reported somewhat higher rates
for phosphate uptake by T. hemprichii: ∼2.2–3.2 and
1 µMol g
−1 DW h
−1 for leaf and root uptake, respectively.
Abiotic parameters that influence the nutrient uptake by leaves are ambient irradiance and water
movement. Both current velocity and turbulent energy affect the nutrient uptake by the leaves of
Thalassia (Thomas et al., 2000; Thomas and Cornelisen, 2003; Koch et al., Chapter 8). An increase
in current velocity causes a decrease in the thickness
of the diffusion boundary layer and a gain in nutrient uptake by leaves (Koch, 1994). Recent work on
meadows of T. testudinum indicate that ammonium
uptake can even be predicted using empirically derived engineering equations that describe the transport of chemicals between a fluid and a rough surface
(Thomas et al., 2000). The influence of ambient irradiance on the uptake of inorganic N and P by T.
testudinum and epiphytes is less clear. Kraemer and
Hanisak (2000) found a significant positive correlation between irradiance and nutrient uptake, whereas
Lee and Dunton (1999b) found no difference in nutrient uptake in light or dark.
D. Nutrient Losses and Gains
in Thalassia Meadows
The loss of nutrients from a Thalassia meadow, like
that from other seagrass meadows, occurs primarily through denitrification (Shieh and Yang, 1997),
sediment resuspension (Koch, 1999b), the export
of Thalassia leaves (Zieman et al., 1979) and herbivory (Heck and Valentine, 1995; Valentine and
Duffy, Chapter 20). These processes are strongly related to leaf senescence (Hemminga et al., 1991),
water flow and turbulence (Koch and Gust, 1999)
and herbivore action (‘sloppy feeding’; Valentine
and Heck, 1999). Fauna can consume up to 25%
of the annual leaf production (Klumpp et al., 1993),
which is a substantially larger loss than the water
flow export of leaf material from Thalassia meadows, estimated to be maximally 10% (Greenway,
1976; Stapel et al., 1996b). Nutrients in Thalassia seagrass meadows are replenished by the accumulation of sestonic particles in the meadow
(Agawin and Duarte, 2002), sedimentation of
allochthonous material (Almasi et al., 1987) and
nutrient uptake by seagrasses and other primary producers (Wear et al., 1999), especially diatoms (Sterrenburg et al., 1995). These processes are all controlled by water movements (Koch, 1994; Thomas
et al., 2003). N 2 -fixation, which is positively related
to the belowground biomass (Capone and Taylor,
1980), may contribute up to 8% of the N requirement
of Thalassia (Hemminga et al., 1991; Moriarty and
O’Donohue, 1993). Patriquin (1972) argued that essentially all the fixed nitrogen requirements of T. testudinum meadow could be supplied by heterotrophic
nitrogen fixation within the rhizospheric sediments.
While processes of nutrient loss are relatively well
defined, those involved in nutrient gains and retention in Thalassia meadows are far less clear. The persistence of often highly productive Thalassia beds in
oligotrophic environments has therefore been considered as rather paradoxical. In these environments,
Thalassia should possess efficient nutrient conservation mechanisms, but Hemminga et al. (1999)
reported that T. hemprichii appeared to be rather
wasteful with nutrients, and the low efficiency of utilization of internal nutrient pools seemed to be more
characteristic for species of nutrient-rich areas rather
than of nutrient-poor habitats. The main strategies
for efficient internal nutrient conservation are considered to be a prolonged leaf lifespan (Hemminga
415
column depends more on the uptake capacity of the
leaves (Stapel et al., 1996a). Stapel et al. (1996a)
determined the maximum uptake rate (V max ) and the
half-saturation constant (K m ; Michaelis-Menten) of
ammonium and phosphate uptake by the leaves of T.
hemprichii plants from three contrasting locations
in Indonesia, but did not find significant differences.
Lee and Dunton (1999a) studied the leaf-uptake rates
for NH
+
4 in T. testudinum in the USA and found much
lower values for the uptake characteristics. The V max
for NH
+
4 uptake by T. testudinum leaves varied between 7.8 and 17.5 µMol g
−1 DW h
−1 (Lee and
Dunton, 1999a), whereas that of T. hemprichii leaves
ranged between 25 and 48 µMol g
−1 DW h
−1 (Stapel
et al., 1996a). Additionally, K m for ammonium uptake by leaves of T. testudinum and T. hemprichii
ranged between 3.9 and 23.3 µM and 14–97 µM,
respectively (Stapel et al., 1996a; Lee and Dunton,
1999a). These figures indicate that T. testudinum
leaves have a lower capacity for NH
+
4 uptake, but a
higher uptake affinity than T. hemprichii, suggesting
that T. testudinum might be better adapted to growth
under low nitrogen conditions. V max for inorganic
phosphate by T. testudinum was 1.9–2.1 µMol g
−1
DW h
−1 for leaf uptake and 0.38–0.57 µMol g
−1
DW h
−1 for uptake by the roots (Gras et al., 2003).
Stapel et al. (1996a) reported somewhat higher rates
for phosphate uptake by T. hemprichii: ∼2.2–3.2 and
1 µMol g
−1 DW h
−1 for leaf and root uptake, respectively.
Abiotic parameters that influence the nutrient uptake by leaves are ambient irradiance and water
movement. Both current velocity and turbulent energy affect the nutrient uptake by the leaves of
Thalassia (Thomas et al., 2000; Thomas and Cornelisen, 2003; Koch et al., Chapter 8). An increase
in current velocity causes a decrease in the thickness
of the diffusion boundary layer and a gain in nutrient uptake by leaves (Koch, 1994). Recent work on
meadows of T. testudinum indicate that ammonium
uptake can even be predicted using empirically derived engineering equations that describe the transport of chemicals between a fluid and a rough surface
(Thomas et al., 2000). The influence of ambient irradiance on the uptake of inorganic N and P by T.
testudinum and epiphytes is less clear. Kraemer and
Hanisak (2000) found a significant positive correlation between irradiance and nutrient uptake, whereas
Lee and Dunton (1999b) found no difference in nutrient uptake in light or dark.
D. Nutrient Losses and Gains
in Thalassia Meadows
The loss of nutrients from a Thalassia meadow, like
that from other seagrass meadows, occurs primarily through denitrification (Shieh and Yang, 1997),
sediment resuspension (Koch, 1999b), the export
of Thalassia leaves (Zieman et al., 1979) and herbivory (Heck and Valentine, 1995; Valentine and
Duffy, Chapter 20). These processes are strongly related to leaf senescence (Hemminga et al., 1991),
water flow and turbulence (Koch and Gust, 1999)
and herbivore action (‘sloppy feeding’; Valentine
and Heck, 1999). Fauna can consume up to 25%
of the annual leaf production (Klumpp et al., 1993),
which is a substantially larger loss than the water
flow export of leaf material from Thalassia meadows, estimated to be maximally 10% (Greenway,
1976; Stapel et al., 1996b). Nutrients in Thalassia seagrass meadows are replenished by the accumulation of sestonic particles in the meadow
(Agawin and Duarte, 2002), sedimentation of
allochthonous material (Almasi et al., 1987) and
nutrient uptake by seagrasses and other primary producers (Wear et al., 1999), especially diatoms (Sterrenburg et al., 1995). These processes are all controlled by water movements (Koch, 1994; Thomas
et al., 2003). N 2 -fixation, which is positively related
to the belowground biomass (Capone and Taylor,
1980), may contribute up to 8% of the N requirement
of Thalassia (Hemminga et al., 1991; Moriarty and
O’Donohue, 1993). Patriquin (1972) argued that essentially all the fixed nitrogen requirements of T. testudinum meadow could be supplied by heterotrophic
nitrogen fixation within the rhizospheric sediments.
While processes of nutrient loss are relatively well
defined, those involved in nutrient gains and retention in Thalassia meadows are far less clear. The persistence of often highly productive Thalassia beds in
oligotrophic environments has therefore been considered as rather paradoxical. In these environments,
Thalassia should possess efficient nutrient conservation mechanisms, but Hemminga et al. (1999)
reported that T. hemprichii appeared to be rather
wasteful with nutrients, and the low efficiency of utilization of internal nutrient pools seemed to be more
characteristic for species of nutrient-rich areas rather
than of nutrient-poor habitats. The main strategies
for efficient internal nutrient conservation are considered to be a prolonged leaf lifespan (Hemminga