412
B. I. van Tussenbroek, J. A. Vonk, J. Stapel et al.
spend repeatedly extended times on site underwater.
On the other hand, new laboratory instrumentation
and techniques, such as stable isotope analysis and
molecular genetics, have opened up our view of community dynamics and evolution of the seagrasses.
In the present chapter, we will cover several fields
of seagrass research, where the genus Thalassia
has played a substantial role, and in which existing
paradigms have been challenged in the light of new
approaches and developments.
IV. Thalassia–Sediment Interactions
Thalassia, like many other seagrass species, has major effects on the detention and deposition of particles in the water column. The dissipation of wave
and current energy by the plant canopies results in
increased deposition rates of the particles (see Koch
et al., Chapter 8). The intensity of this canopy effect depends on a complex way on leaf morphology,
stiffness, and density. The rhizomes and roots, on the
other hand, stabilize the sediments and thus prevent
resuspension of the particles (see Koch et al., Chapter 8). The combined effect of reduced resuspension
and enhanced deposition results in enhanced deposition of particles in Thalassia beds (Almasi et al.,
1987; Koch, 1999b; Gacia et al., 2003). There is often organic matter associated with these particles,
and as a consequence non-seagrass derived organic
matter contributes significantly to sediment carbon
pools in seagrass beds. The total sediment organic
pool in seagrass meadows depends on seagrass litter production, non-seagrass local production (e.g.
benthic microalgae), and allochthonous organic matter, in combination with the relative utilization and
degradation of these carbon inputs (see Mateo et al.,
Chapter 7). The relative contribution of Thalassia
derived organic matter can easily be traced using
stable carbon isotope signatures and varies from a
small (<30%) contribution (Hemminga et al., 1994)
to predominant (Jones et al., 2003).
Thalassia also contributes to the sediment accumulation through calcium carbonate production
of the epiphyte community on the leaves (see also
Borowitzka et al., Chapter 19). Calcium carbonate
production of the epiphytes scales with seagrass production, and that of associated calcified algae such
as Halimeda spp. Gacia et al. (2003) reported calcium carbonate production rates associated with T.
testudinum leaves of 0.49–7.7 g CaCO 3 m
−2 day
−1 in
the tropics and 0.08–2.85 g CaCO 3 m
−2 day
−1 , in the
subtropics. For tropical and subtropical T. hemprichii
epiphyte calcium production rates were 0.92–6.52
and 0.38 g CaCO 3 m
−2 day
−1 , respectively. Calcium carbonate derived from Thalassia epiphytes
contributes variable to sediment deposition: from
negligible to significant (35%, Gacia et al., 2003).
However, dissolution of carbonates, related to carbon dioxide production during aerobic respiration
of organic material, has also been reported to occur
in seagrass beds. This aerobic respiration is maintained by root oxygen release. Burdige and Zimmerman (2002) reported significant carbonate dissolution in T. testudinum beds with rates (4–11 mmol
m
−2 day
−1 ) depending on seagrass bed density. Carbonate dissolution in T. testudinum beds is less than
carbonate production (5–77 mmol m
−2 day
−1 ; see
above) indicating that on the whole seagrass-derived
carbonate accumulates in the sediments.
V. Nutrient Dynamics
Nutrient research in Thalassia has mainly been focused on the dissolved inorganic nitrogen and phosphorus (Hemminga and Duarte, 2000a,b), although
micronutrients, such as iron may also play an important role, locally, in moderating the productivity of
Thalassia (Duarte et al., 1995). For a proper understanding of nutrient dynamics, the availability, net
uptake and re-use of nutrients by plants should be
taken into account, as well as processes of nutrient
losses (export of material, denitrification, and foraging fauna), gains in the community (sedimentation, N 2 -fixation and water enrichment; Hemminga
et al., 1991; Romero et al., Chapter 9) and the interaction with the abiotic compartment (Thomas and
Cornelisen, 2003).
A. Nutrient Availability
In the tropical and subtropical shallow coastal areas
where Thalassia occurs, water-column dissolved inorganic concentrations are generally low (PO
3−
4 <
1µM and NH
+
4 + NO
−
3 < 3µM; Tomasko and
Lapointe, 1991; Agawin et al., 1996; Ziegler and
Benner, 1999), with exception of areas with high
river inflow or which are frequently exposed to the
air during low tides (Erftemeijer and Herman, 1994;
Stapel et al., 1997). The majority of the N and P
pools in Thalassia meadows are present in the sediments, adsorbed to sediment particles or bound
in organic matter and therefore largely unavailable
B. I. van Tussenbroek, J. A. Vonk, J. Stapel et al.
spend repeatedly extended times on site underwater.
On the other hand, new laboratory instrumentation
and techniques, such as stable isotope analysis and
molecular genetics, have opened up our view of community dynamics and evolution of the seagrasses.
In the present chapter, we will cover several fields
of seagrass research, where the genus Thalassia
has played a substantial role, and in which existing
paradigms have been challenged in the light of new
approaches and developments.
IV. Thalassia–Sediment Interactions
Thalassia, like many other seagrass species, has major effects on the detention and deposition of particles in the water column. The dissipation of wave
and current energy by the plant canopies results in
increased deposition rates of the particles (see Koch
et al., Chapter 8). The intensity of this canopy effect depends on a complex way on leaf morphology,
stiffness, and density. The rhizomes and roots, on the
other hand, stabilize the sediments and thus prevent
resuspension of the particles (see Koch et al., Chapter 8). The combined effect of reduced resuspension
and enhanced deposition results in enhanced deposition of particles in Thalassia beds (Almasi et al.,
1987; Koch, 1999b; Gacia et al., 2003). There is often organic matter associated with these particles,
and as a consequence non-seagrass derived organic
matter contributes significantly to sediment carbon
pools in seagrass beds. The total sediment organic
pool in seagrass meadows depends on seagrass litter production, non-seagrass local production (e.g.
benthic microalgae), and allochthonous organic matter, in combination with the relative utilization and
degradation of these carbon inputs (see Mateo et al.,
Chapter 7). The relative contribution of Thalassia
derived organic matter can easily be traced using
stable carbon isotope signatures and varies from a
small (<30%) contribution (Hemminga et al., 1994)
to predominant (Jones et al., 2003).
Thalassia also contributes to the sediment accumulation through calcium carbonate production
of the epiphyte community on the leaves (see also
Borowitzka et al., Chapter 19). Calcium carbonate
production of the epiphytes scales with seagrass production, and that of associated calcified algae such
as Halimeda spp. Gacia et al. (2003) reported calcium carbonate production rates associated with T.
testudinum leaves of 0.49–7.7 g CaCO 3 m
−2 day
−1 in
the tropics and 0.08–2.85 g CaCO 3 m
−2 day
−1 , in the
subtropics. For tropical and subtropical T. hemprichii
epiphyte calcium production rates were 0.92–6.52
and 0.38 g CaCO 3 m
−2 day
−1 , respectively. Calcium carbonate derived from Thalassia epiphytes
contributes variable to sediment deposition: from
negligible to significant (35%, Gacia et al., 2003).
However, dissolution of carbonates, related to carbon dioxide production during aerobic respiration
of organic material, has also been reported to occur
in seagrass beds. This aerobic respiration is maintained by root oxygen release. Burdige and Zimmerman (2002) reported significant carbonate dissolution in T. testudinum beds with rates (4–11 mmol
m
−2 day
−1 ) depending on seagrass bed density. Carbonate dissolution in T. testudinum beds is less than
carbonate production (5–77 mmol m
−2 day
−1 ; see
above) indicating that on the whole seagrass-derived
carbonate accumulates in the sediments.
V. Nutrient Dynamics
Nutrient research in Thalassia has mainly been focused on the dissolved inorganic nitrogen and phosphorus (Hemminga and Duarte, 2000a,b), although
micronutrients, such as iron may also play an important role, locally, in moderating the productivity of
Thalassia (Duarte et al., 1995). For a proper understanding of nutrient dynamics, the availability, net
uptake and re-use of nutrients by plants should be
taken into account, as well as processes of nutrient
losses (export of material, denitrification, and foraging fauna), gains in the community (sedimentation, N 2 -fixation and water enrichment; Hemminga
et al., 1991; Romero et al., Chapter 9) and the interaction with the abiotic compartment (Thomas and
Cornelisen, 2003).
A. Nutrient Availability
In the tropical and subtropical shallow coastal areas
where Thalassia occurs, water-column dissolved inorganic concentrations are generally low (PO
3−
4 <
1µM and NH
+
4 + NO
−
3 < 3µM; Tomasko and
Lapointe, 1991; Agawin et al., 1996; Ziegler and
Benner, 1999), with exception of areas with high
river inflow or which are frequently exposed to the
air during low tides (Erftemeijer and Herman, 1994;
Stapel et al., 1997). The majority of the N and P
pools in Thalassia meadows are present in the sediments, adsorbed to sediment particles or bound
in organic matter and therefore largely unavailable