Chapter 18 The Biology of Thalassia
413
to the plants (Erftemeijer and Middelburg, 1995;
Stapel et al., 1996a; Koch et al., 2001). Jensen
et al. (1998), for example, reported that total sediment P in the upper 20 cm in subtropical carbonate sediment in Bermuda was 500-fold larger than
the pool of P dissolved in pore waters. Pore-water
NH
+
4 concentrations in meadows of both Thalassia
species usually vary between 2 and 200 µM (Erftemeijer, 1994; Lee and Dunton, 1999b; Table 1).
But exceptionally high figures (of up to 800 µM)
have been reported in T. hemprichii meadows from
the East African coast (Erftemeijer and Herman,
1994; Stapel et al., 1996a; Marb` a et. al., 2002), and
in T. testudinum beds in the Laguna madre (Lee
and Dunton, 2000). Pore-water PO
3−
4 concentrations
are generally higher in T. hemprichii beds (up to
35 µM) compared to T. testudinum meadows (<1.5
µM; Fourqurean et al., 1992a; McGlathery et al.,
2001). The grain size of the sediments, determining diffusion rates of nutrients, as well as the sediments’ origin (terrigenous or marine) and the so far
poorly studied biogeochemical interactions, may be
an important factor controlling nutrient availability
in pore water, as has been put forward by Erftemeijer et al. (1994), Erftemeijer and Middelburg
(1995) and Stapel et al. (1996a) for T. hemprichii.
The pore-water chemistry is also affected by current velocity of the overlaying water column. Under stagnant water conditions, sulfide levels may
increase in pore water, while high flow rates may lead
to reduced nutrient concentrations (Koch, 1999a).
Nutrient concentrations in Thalassia beds show annual fluctuations in sediment pore water, in the water
column, as well as in the plant material, which are
due to seasonal influences (Erftemeijer and Herman,
1994; Lee and Dunton, 1999a,b; Ziegler and Benner,
1999).
The oligotrophic waters in which Thalassia proliferates have very low concentrations of dissolved inorganic nutrients, but in these waters, total dissolved
nutrient pools (including dissolved organic nitrogen
and phosphorus) may be up to five times higher than
the total concentration of the dissolved inorganic nutrients (Hansell and Carlson, 2002). The availability
of the dissolved organic nutrients to Thalassia is not
well known and is likely to be variable because it
depends on the composition and molecular size of
the compounds. Little is known concerning the uptake of organic nutrients by Thalassia or any other
seagrass (see Romero et al., Chapter 9), but other
submerged primary producers such as marine algae
and bacteria and various macrophytes are known to
assimilate organic nutrients (Lipson and Nasholm,
2001).
B. Nutrient Limitation
Duarte (1990) suggested that seagrass plants with
nutrient levels in leaves below the median values for seagrasses of 1.8% N and 0.20% P (as %
DW) were nutrient-limited. Leaf N-concentrations
of T. testudinum and T. hemprichii ranged between 0.88–3.96% and 1.5–2.8% of DW, respectively (Fourqurean et al., 1992b; Stapel et al., 1997;
Jensen et al., 1998). For P, these values varied between 0.048 and 0.274% of DW for T. testudinum
and 0.11–0.23% of DW for T. hemprichii (Erftemeijer, 1994; Terrados et al., 1999a; Fourqurean and
Zieman, 2002). The lower values of tissue N- and
P-concentrations of both species are well below the
critical levels according to Duarte (1990), but these
levels alone cannot be used as indicators of N- or Plimitation in Thalassia (see Table 1). Experimental
additions of nutrients to the sediment have increased
seagrass size or growth, nutrient content, shoot density, and the above- to belowground biomass ratio in
the Philippines, occasionally in the Caribbean, but
not in Indonesia. Terrados et al. (1999a) found that
the N-content in leaves of T. hemprichii reflects the
availability of N in the sediment. Within the limits of
these the limits of these studies, this seems consistent with the values given in Table 1. The existence
of N limitation in T. testudinum beds probably depends on in situ sediment NH
+
4 concentration, and
hence this may also be used as an indicator of the N
status for seagrass growth (Lee and Dunton, 2000).
C. Nutrient Uptake by the Plants
Thalassia takes up nutrients from its environment
with its leaves and roots. Leaves and roots contribute
about equally to the plant’s total nutrient acquisition
(Erftemeijer and Middelburg, 1995; Lee and Dunton, 1999b). Significant differences in pore-water
NH
+
4 concentrations between two sites in the Gulf
of Mexico (averages respectively 87 and 26 µM) did
not affect the relative contribution of roots and leaves
to the total N requirements in T. testudinum (Lee and
Dunton, 1999b).
The uptake rate of nutrients from the pore
water by Thalassia roots is largely controlled by
sediment parameters and is generally diffusion limited, whereas the uptake of nutrients from the water
413
to the plants (Erftemeijer and Middelburg, 1995;
Stapel et al., 1996a; Koch et al., 2001). Jensen
et al. (1998), for example, reported that total sediment P in the upper 20 cm in subtropical carbonate sediment in Bermuda was 500-fold larger than
the pool of P dissolved in pore waters. Pore-water
NH
+
4 concentrations in meadows of both Thalassia
species usually vary between 2 and 200 µM (Erftemeijer, 1994; Lee and Dunton, 1999b; Table 1).
But exceptionally high figures (of up to 800 µM)
have been reported in T. hemprichii meadows from
the East African coast (Erftemeijer and Herman,
1994; Stapel et al., 1996a; Marb` a et. al., 2002), and
in T. testudinum beds in the Laguna madre (Lee
and Dunton, 2000). Pore-water PO
3−
4 concentrations
are generally higher in T. hemprichii beds (up to
35 µM) compared to T. testudinum meadows (<1.5
µM; Fourqurean et al., 1992a; McGlathery et al.,
2001). The grain size of the sediments, determining diffusion rates of nutrients, as well as the sediments’ origin (terrigenous or marine) and the so far
poorly studied biogeochemical interactions, may be
an important factor controlling nutrient availability
in pore water, as has been put forward by Erftemeijer et al. (1994), Erftemeijer and Middelburg
(1995) and Stapel et al. (1996a) for T. hemprichii.
The pore-water chemistry is also affected by current velocity of the overlaying water column. Under stagnant water conditions, sulfide levels may
increase in pore water, while high flow rates may lead
to reduced nutrient concentrations (Koch, 1999a).
Nutrient concentrations in Thalassia beds show annual fluctuations in sediment pore water, in the water
column, as well as in the plant material, which are
due to seasonal influences (Erftemeijer and Herman,
1994; Lee and Dunton, 1999a,b; Ziegler and Benner,
1999).
The oligotrophic waters in which Thalassia proliferates have very low concentrations of dissolved inorganic nutrients, but in these waters, total dissolved
nutrient pools (including dissolved organic nitrogen
and phosphorus) may be up to five times higher than
the total concentration of the dissolved inorganic nutrients (Hansell and Carlson, 2002). The availability
of the dissolved organic nutrients to Thalassia is not
well known and is likely to be variable because it
depends on the composition and molecular size of
the compounds. Little is known concerning the uptake of organic nutrients by Thalassia or any other
seagrass (see Romero et al., Chapter 9), but other
submerged primary producers such as marine algae
and bacteria and various macrophytes are known to
assimilate organic nutrients (Lipson and Nasholm,
2001).
B. Nutrient Limitation
Duarte (1990) suggested that seagrass plants with
nutrient levels in leaves below the median values for seagrasses of 1.8% N and 0.20% P (as %
DW) were nutrient-limited. Leaf N-concentrations
of T. testudinum and T. hemprichii ranged between 0.88–3.96% and 1.5–2.8% of DW, respectively (Fourqurean et al., 1992b; Stapel et al., 1997;
Jensen et al., 1998). For P, these values varied between 0.048 and 0.274% of DW for T. testudinum
and 0.11–0.23% of DW for T. hemprichii (Erftemeijer, 1994; Terrados et al., 1999a; Fourqurean and
Zieman, 2002). The lower values of tissue N- and
P-concentrations of both species are well below the
critical levels according to Duarte (1990), but these
levels alone cannot be used as indicators of N- or Plimitation in Thalassia (see Table 1). Experimental
additions of nutrients to the sediment have increased
seagrass size or growth, nutrient content, shoot density, and the above- to belowground biomass ratio in
the Philippines, occasionally in the Caribbean, but
not in Indonesia. Terrados et al. (1999a) found that
the N-content in leaves of T. hemprichii reflects the
availability of N in the sediment. Within the limits of
these the limits of these studies, this seems consistent with the values given in Table 1. The existence
of N limitation in T. testudinum beds probably depends on in situ sediment NH
+
4 concentration, and
hence this may also be used as an indicator of the N
status for seagrass growth (Lee and Dunton, 2000).
C. Nutrient Uptake by the Plants
Thalassia takes up nutrients from its environment
with its leaves and roots. Leaves and roots contribute
about equally to the plant’s total nutrient acquisition
(Erftemeijer and Middelburg, 1995; Lee and Dunton, 1999b). Significant differences in pore-water
NH
+
4 concentrations between two sites in the Gulf
of Mexico (averages respectively 87 and 26 µM) did
not affect the relative contribution of roots and leaves
to the total N requirements in T. testudinum (Lee and
Dunton, 1999b).
The uptake rate of nutrients from the pore
water by Thalassia roots is largely controlled by
sediment parameters and is generally diffusion limited, whereas the uptake of nutrients from the water