Mangrove forests are found from the highest level of spring tides down almost to mean sea level on sheltered sedimented shores throughout the tropics. They dominate approximately 75% of the worlds coastline between 25°N and 25°S and are estimated to occupy between 0.17 and 0.20 106 km2. They occur in fully saline waters but also penetrate considerable distances into estuaries.
In the tropics, many mangrove ecosystems are not only a source of edible fish, crustaceans and molluscs, but they also provide shelter, wood for fuel and the wood-chip industry, and a variety of natural products. Also, by trapping sediment they stabilize coastlines, affording protection from winds and storms. In the Indo-Pacifc region mangrove systems are being destroyed at an average rate of 1% per year and in some places the figure is much higher.
The net primary production of mangrove trees derived from indirect measurements is on average 58±7 mol C m-2 year-1 (Gattuso et al. 1997 Ann. Rev. Ecol. Syst. 29: 405-434). Most leaf production enters the detrital pathway as litter fall at an average rate of 32 mol C m-2 year-1. Although the overall mangrove ecosystem (submerged and aerial compartments) is net autotrophic, submerged primary production is often limited by high turbidity and changes in salinity. Thus, water column and sediment metabolism are largely heterotrophic. According to a recent review of literature (Jennerjahn and Ittekkot 2002 Naturwissenschaften 89: 23-30), the global estimation of leaf litter remineralized within mangroves is about 23 106 tC year-1, corresponding to 25% of the total litter fall.
Anaerobic processes are of major importance in mangrove sediments and sulfate reduction along with aerobic respiration account for almost all the diagenetic carbon degradation in mangroves. Generally, sulfate reduction is the major diagenetic pathway in mangroves (Alongi et al. 1998 J. Exp. Mar. Biol. Ecol. 225: 197-218; Alongi et al. 2000 Aq. Bot. 68: 97-122), but in some cases aerobic degradation predominates (Alongi et al. 2000 Aq. Bot. 68: 97-122; Alongi et al. 2001 Mar. Geol. 179: 85-103), and in one Thai mangrove, iron reduction was reported as the dominant process (Kristensen et al. 2000 Aquat. Microb. Ecol. 22: 199-213). Denitrification and methanogenesis are generally considered to have a negligible role in the mangrove diagenetic carbon degradation. For instance, in a Western Australia mangrove ecosystem, the major pathway of bacterial decomposition of organic carbon is sulfate reduction (74%), followed by aerobic respiration (22%), while the contribution from denitrification and methanogenesis is small (2% each) (Alongi 1998 Coastal Ecosystem Processes CRC).
The emission from mangrove sediments of the natural occurring greenhouse gases methane (CH4) and nitrous oxide (N2O) seems to be highly variable from one site to another. In Gazi Bay (Kenya), Middelburg et al. 1996 (Biogeochemistry 34: 133-155) observed very low or nil fluxes of these gases. However, in the mangrove wetlands in Queensland (Australia), Kreuzwieser et al. 2003 (Plant Biology 5: 423-431) report highly variable but significant fluxes of CH4 and N2O from the sediments.
Mangrove ecosystems can export organic matter to adjacent systems and/or accumulate organic carbon in the sediments. The average rate of carbon accumulation in the sediment is 23 mol C m-2 year-1. The quality and quantity of material exported from mangroves depend on forest type, productivity, physical constraints and biological forcings. Leaf litter export from mangroves ranges from 0.3 to 30% of litter fall (Gattuso et al. 1997 Ann. Rev. Ecol. Syst. 29:405-434).
Little dissolved inorganic carbon data are available and have been reported in two mangroves in the Bay of Bengal (Saptamukhi and Mooriganga) (Ghosh et al. 1987 Mahasagar 20:155-161), Septiba Bay (Brazil) (Ovalle et al. 1990 Estuar. coast. shelf sc. 31:639-650) and Florida Bay (USA) (Millero et al. 2001 Bull. Mar. Sc. 68:101-123). The range of reported partial pressure of CO2 (pCO2) values is huge, varying from 330 to 4000 ppm (present day atmospheric value is 370 ppm). However, in none of these publications the atmospheric fluxes of CO2 have been estimated or integrated.
A recent study on the CO2 dynamics during the pre-monsoon period, has shown that pCO2 values in the waters of the Coringa National Forest mangroves are much higher than in the adjacent systems (Godavari estuary and Kakinada Bay). Over-saturation of CO2 with respect to atmospheric equilibrium was on average 634% in the mangrove waters compared to an average over-saturation of 122% in the Godavari estuary and Kakinada Bay [Bouillon et al. 2003]. Another recent study that compiles pCO2 data in 7 mangrove systems worldwide, suggests that air-water efflux of CO2 converges to about 50 mmol m-2 day-1 [Borges et al. 2003]. The extrapolation of this conservative value to the surface area of worldwide mangrove ecosystems gives a global emission of CO2 to the atmosphere of about 50 106 tC year-1. On a regional scale, the subtropical and tropical open oceanic waters behave as a net source of CO2 of about 0.43 PgC year-1 [between 32°N and 32°S, based on Takahashi et al. 1997, Proc.Natl.Acad.Sci.USA 94:8292-8299]. Thus, mangrove surrounding waters would be an additional CO2 source of about 12% to the one of open oceanic waters, in tropical and subtropical latitudes, with a surface area about one thousand times smaller.
This research is funded by the Fonds National de la Recherche Scientifique (FNRS), Belgium (contract numbers 2.4521.96, 2.4594.01, 2.4596.01, 2.4545.02, 1.5.066.03).
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D. Bay, J. Bosire, J.-M. Bouquegneau, G. Castillo-Cabello, L. Chou, P. Cremer, Dala, V. Demoulin, I. De Mesel, Durussun, J.-P. Gattuso, C. Kalavati, J. Kairo, M. Kone, P. Leclerc, P. Le Hong, P. Luong Le, G. Lepoint, D. Nguyen Nguyen, H. Nguyen Van, I. Mallentjer, A.V. Raman, A.V.V.S. Rao, M. Rixen, B. Tran Quoc, M. Tsagaris, Simba, S. Satyanarayana, ...