Chapter 6 Seagrasses and Biogeochemistry
143
Seagrass beds affect the metabolism of CaCO 3
by enhancing (i) carbonate dissolution in the sediments and (ii) carbonate precipitation by associated
epibionts. The effect of seagrass beds on deposition
of organic matter, and the release of O 2 by seagrass
roots and rhizomes into the sediments, enhances aerobic mineralization of organic matter, and, thus, generation of CO 2 available for carbonate dissolution in
the sediments colonized. Recent studies (Eldridge
and Morse, 2000; Burdige and Zimmerman, 2002)
demonstrate that the high levels of organic matter decomposition occurring in seagrass sediments
cannot be explained solely by the high concentration of DIC found in meadow pore waters. In addition, the excess of DIC in seagrass pore water observed cannot result from mineralization of organic
matter by sulfate reduction, since sulfate reduction
rates in seagrass carbonate sediments are low (e.g.
Eldridge and Morse, 2000; Burdige and Zimmerman, 2002; Holmer et al., 2003). Using a diagenic
model to explain the sediment profiles of concentrations of DOM, DIC, O 2 , nitrate, ammonium, and
sulfide in seagrass sediments from Laguna Madre,
and considering O 2 release by seagrass roots, Eldridge and Morse (2000) showed that carbonate dissolution should contribute more than 50% to the
pore water DIC concentration during early diagenesis in vegetated sediments. Burdige and Zimmerman (2002) obtained similar results by examining
vertical pore water profiles of alkalinity, total DIC,
Ca
2+ , sulfide and pH in seagrass and bare sediments
in Bahamas. They estimated that carbonate dissolution in shallow tropical sediments mostly colonized
by T. testudinum would occur at rates ranging from
4 to 11 mmol Ca 2 CO 3 m
−2 d
−1 , tending to increase
with increasing shoot density. In addition, the observed rates of carbonate dissolution in the Bahamas
seagrass sediments should consume about twice the
amount of CO 2 produced by aerobic respiration if
O 2 only was supplied by physical processes such
as diffusion, bioturbation, and pore water advection
(Burdige and Zimmerman, 2002). The rates of carbonate dissolution observed in seagrass sediments
could only be explained if part of sediment aerobic
respiration was driven by photosynthetically-derived
O 2 , released by seagrass roots (Burdige and Zimmerman, 2002). The effect of seagrass beds on sediment carbonate dissolution should, in turn, increase
phosphorus pore water concentration, since a significant fraction of the phosphorus in seagrass carbonate sediments is adsorbed to carbonate minerals
and can become available to the plants by acid dissolution (Burdige and Zimmerman, 2002). Therefore,
the enhancement of sediment carbonate dissolution
by seagrasses can increase phosphorus availability
for seagrass uptake, and, since this nutrient often
limits seagrass growth in carbonate sediments (see
Romero et al., Chapter 9), it may increase seagrass
production.
Most epibiota associated with the seagrass canopy
in carbonate environments comprise calcifying organisms (see Borowitzka et al., Chapter 19). In tropical seagrass beds, for instance, calcium carbonate accumulation on seagrass leaves ranged from
0.004 g CaCO 3 leaf
−1 (in Halodule uninervis) to
0.417 g CaCO 3 leaf
−1 (in E. acoroides, Gacia
et al., 2003). The rate of calcium carbonate production has been quantified for temperate and tropical seagrass communities, and it varies from 0.05 g
CaCO 3 m
−2 d
−1 to 7.67 g CaCO 3 m
−2 d
−1 (Table 1).
The large differences in calcium carbonate production among seagrass beds have been attributed to
the wide variability in seagrass production, but generally carbonate production increases with seagrass
productivity (Gacia et al., 2003). The amount of calcium carbonate produced by seagrass communities
may represent close to 35% of total calcium carbonate deposited in seagrass sediments in very productive meadows, such as those in the Philippines (Gacia
et al., 2003). Conversely, the contribution of seagrass
beds to deposition of calcium carbonate is negligible (<0.5%) in beds of low productivity, or in areas
with high calcium carbonate deposition from other
sources (Gacia et al., 2003). These estimates, however, do not account for potential losses of seagrass
biomass and associated minerals due to grazing or
leaf breaking, nor for the losses of sediment material from resuspension, which may vary significantly
among sites. Precipitation of CaCO 3 in seagrass beds
would increase CO 2 concentration in the water column, and, hence, it would stimulate seagrass photosynthesis. Carbonate precipitation, therefore, might
supply a significant fraction of inorganic carbon to
support high seagrass productivity in carbonate environments.
Available estimates of carbonate dissolution
(Burdige and Zimmerman, 2002) and production
rates (Table 1) indicate that carbonate metabolism,
indeed, may be an important source of inorganic carbon to support seagrass productivity. However, the
lack of published studies comparing net primary production and net carbonate metabolism in seagrass
143
Seagrass beds affect the metabolism of CaCO 3
by enhancing (i) carbonate dissolution in the sediments and (ii) carbonate precipitation by associated
epibionts. The effect of seagrass beds on deposition
of organic matter, and the release of O 2 by seagrass
roots and rhizomes into the sediments, enhances aerobic mineralization of organic matter, and, thus, generation of CO 2 available for carbonate dissolution in
the sediments colonized. Recent studies (Eldridge
and Morse, 2000; Burdige and Zimmerman, 2002)
demonstrate that the high levels of organic matter decomposition occurring in seagrass sediments
cannot be explained solely by the high concentration of DIC found in meadow pore waters. In addition, the excess of DIC in seagrass pore water observed cannot result from mineralization of organic
matter by sulfate reduction, since sulfate reduction
rates in seagrass carbonate sediments are low (e.g.
Eldridge and Morse, 2000; Burdige and Zimmerman, 2002; Holmer et al., 2003). Using a diagenic
model to explain the sediment profiles of concentrations of DOM, DIC, O 2 , nitrate, ammonium, and
sulfide in seagrass sediments from Laguna Madre,
and considering O 2 release by seagrass roots, Eldridge and Morse (2000) showed that carbonate dissolution should contribute more than 50% to the
pore water DIC concentration during early diagenesis in vegetated sediments. Burdige and Zimmerman (2002) obtained similar results by examining
vertical pore water profiles of alkalinity, total DIC,
Ca
2+ , sulfide and pH in seagrass and bare sediments
in Bahamas. They estimated that carbonate dissolution in shallow tropical sediments mostly colonized
by T. testudinum would occur at rates ranging from
4 to 11 mmol Ca 2 CO 3 m
−2 d
−1 , tending to increase
with increasing shoot density. In addition, the observed rates of carbonate dissolution in the Bahamas
seagrass sediments should consume about twice the
amount of CO 2 produced by aerobic respiration if
O 2 only was supplied by physical processes such
as diffusion, bioturbation, and pore water advection
(Burdige and Zimmerman, 2002). The rates of carbonate dissolution observed in seagrass sediments
could only be explained if part of sediment aerobic
respiration was driven by photosynthetically-derived
O 2 , released by seagrass roots (Burdige and Zimmerman, 2002). The effect of seagrass beds on sediment carbonate dissolution should, in turn, increase
phosphorus pore water concentration, since a significant fraction of the phosphorus in seagrass carbonate sediments is adsorbed to carbonate minerals
and can become available to the plants by acid dissolution (Burdige and Zimmerman, 2002). Therefore,
the enhancement of sediment carbonate dissolution
by seagrasses can increase phosphorus availability
for seagrass uptake, and, since this nutrient often
limits seagrass growth in carbonate sediments (see
Romero et al., Chapter 9), it may increase seagrass
production.
Most epibiota associated with the seagrass canopy
in carbonate environments comprise calcifying organisms (see Borowitzka et al., Chapter 19). In tropical seagrass beds, for instance, calcium carbonate accumulation on seagrass leaves ranged from
0.004 g CaCO 3 leaf
−1 (in Halodule uninervis) to
0.417 g CaCO 3 leaf
−1 (in E. acoroides, Gacia
et al., 2003). The rate of calcium carbonate production has been quantified for temperate and tropical seagrass communities, and it varies from 0.05 g
CaCO 3 m
−2 d
−1 to 7.67 g CaCO 3 m
−2 d
−1 (Table 1).
The large differences in calcium carbonate production among seagrass beds have been attributed to
the wide variability in seagrass production, but generally carbonate production increases with seagrass
productivity (Gacia et al., 2003). The amount of calcium carbonate produced by seagrass communities
may represent close to 35% of total calcium carbonate deposited in seagrass sediments in very productive meadows, such as those in the Philippines (Gacia
et al., 2003). Conversely, the contribution of seagrass
beds to deposition of calcium carbonate is negligible (<0.5%) in beds of low productivity, or in areas
with high calcium carbonate deposition from other
sources (Gacia et al., 2003). These estimates, however, do not account for potential losses of seagrass
biomass and associated minerals due to grazing or
leaf breaking, nor for the losses of sediment material from resuspension, which may vary significantly
among sites. Precipitation of CaCO 3 in seagrass beds
would increase CO 2 concentration in the water column, and, hence, it would stimulate seagrass photosynthesis. Carbonate precipitation, therefore, might
supply a significant fraction of inorganic carbon to
support high seagrass productivity in carbonate environments.
Available estimates of carbonate dissolution
(Burdige and Zimmerman, 2002) and production
rates (Table 1) indicate that carbonate metabolism,
indeed, may be an important source of inorganic carbon to support seagrass productivity. However, the
lack of published studies comparing net primary production and net carbonate metabolism in seagrass