Mangroves

Introduction:

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 world’s coastline between 25°N and 25°S and are estimated to occupy between 0.17 and 0.20 10⁶km². 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.

The Chemical Oceanography Unit has carried out 10 field cruises in mangroves and tropical estuaries:

Nagada Creek, Papua New Guinea, 28/07-08/08/2000
46 measurements of salinity, water temperature, pH and Total Alkalinity
Norman’s Pond, Bahamas, 03/12-17/12/2000
Continuous measurements (acquisition every min) of pCO₂, water temperature and wind speed during two 24h cycles using the Floating Equilibrator System and 23 samples of Total Alkalinity. Press article on the Bahamas cruise published in "Le Quinzième jour" (in french)
Gautami-Godavari delta, India, 26/05-05/06/2001
70 measurements of salinity, water temperature, pH, Total Alkalinity, dissolved O₂, POC, DOC, chlorophyll-a, bacterial biomass and wind speed
Gazi Bay, Kenya, 17/07-01/08/2003
90 measurements of salinity, water temperature, pH, Total Alkalinity, dissolved O₂, inorganic nutrients (NO₃, Si, PO₄), bacterial biomass, chlorophyll-a and wind speed
Mekong delta, Vietnam, 29/11-05/12/2004
53 measurements of salinity, water temperature, pH, Total Alkalinity, dissolved O₂, inorganic nutrients (NO₃, Si, PO₄) , wind speed and ancillary data
Mekong delta, Vietnam, 02/04-07/04/2004
61 measurements of salinity, water temperature, pH, Total Alkalinity, dissolved O₂, inorganic nutrients (NO₃, Si, PO₄), wind speed and ancillary data
Tam Giang 3 & Kien Vang mangrove forests (Ngoc Hien & Nam Can districts, Ca Mau Province), Vietnam, 10/04-14/04/2004
36 measurements of salinity, water temperature, pH, Total Alkalinity, dissolved O₂, inorganic nutrients (NO₃, Si, PO₄), wind speed and ancillary data
Tana River, Kenya, 13/04-25/04/2004
in collaboration with the Mangrove Management Group (VUB)
67 measurements of salinity, water temperature, pH, Total Alkalinity, dissolved O₂, inorganic nutrients (NO₃, Si, PO₄) and ancillary data
Mekong delta, Vietnam, 14/10-19/10/2004
55 measurements of salinity, water temperature, pH, Total Alkalinity, dissolved O₂, inorganic nutrients (NO₃, Si, PO₄), wind speed and ancillary data
Tam Giang 3 & Kien Vang mangrove forests (Ngoc Hien & Nam Can districts, Ca Mau Province), Vietnam, 23/10-25/10/2004
28 measurements of salinity, water temperature, pH, Total Alkalinity, dissolved O₂, inorganic nutrients (NO₃, Si, PO₄), wind speed and ancillary data
Toumanguié and Bia Rivers, and Aby and Ebrié lagoons, Ivory Coast, 17/05-19/06/2005
82 measurements of salinity, water temperature, pH, Total Alkalinity, dissolved O₂, wind speed and ancillary data

Description of the studied sites:

Nagada Creek (5°09'S 145°48'E) is a small tidal creek (3 km long, 400 m wide) in the northern Papua New Guinea coast, that opens to Nagada Lagoon. The latter is a wide lagoon (maximum width 5 km) containing numerous islands and submerged patch reefs separated from the Pacific Ocean by a continuously submerged barrier reef. Nagada Creek is fringed by well developed mangroves while seagrass beds and submerged reefs are mainly present at the mouth of the creek.
Gaderu Creek (16°50'N 82°19'E) is the main tidal creek connecting the Gautami Godavari estuary (Anhra Pradesh, east coast of India) to the semi-enclosed shallow Kakinada Bay, with a total length of 11 km (about 30 m wide and 4 m deep). The Gautami Godavari is the northern branch of the second largest Indian river, with an average annual discharge of 1.1 10 ¹¹ m³, of which 93-96 % occurs during the wet monsoon. North of the Gautami Godavari estuary, a network of tidal creeks forms a connection between the Bay of Bengal, Kakinada Bay, and the estuary proper. The area consists of intertidal mudflats, most of which are covered with mangroves, although, in some areas aquaculture has proliferated during the past decades. In contrast to Nagada Creek, no seagrass beds or coral reefs are found adjacent to the mangrove forest.
Norman’s Pond Cay (23°46'N 76°07'W) is one of the many Exuma Cays islands (Bahamas archipelago) which form the boundary between the shallow Great Bahama Bank and the deep Exuma Sound. The carbonate shallow shelf encompasses a variety of tropical environments: coral reefs, mud/sand flats with mangroves, seagrass beds and ooid shoals. The site of investigation, Norman’s Pond (about 500 m long and wide, 2 m deep), is a brackish to hypersaline pond where a dense mangrove is present. It is located within the southern part of the Norman’s Pond Cay and connected to the leeward shore of the island by a tidal channel which floods and drains the pond.
Gazi Bay (39°30'E 4.°22'S) is a shallow, tropical coastal water system with mean depth often less than 5.0 m. The Bay is in southern Kenya, approximately 47 km from Mombasa. The total area of the Bay, excluding the area covered by the mangroves, is 10 km². The mangrove forest, covering an area of about 5 km², is dominated by Rhizophora mucronata, Sonneratia alba, Ceriops tagal, Bruguieria gymnorrhiza and Xylocarpus granatum. The seagrass zone dominated by Thalassodendron ciliatum, is found in the central region of the Bay and covers an additional area of 7.0 km². The Bay is open to the Indian Ocean through a relatively wide (3500 m) and shallow entrance in the south. This entrance is rather shallow, with depths ranging between 3.0 and 8.0 m in the eastern and western regions, respectively. There are also a number of narrow and shallow cuts through the coral reef, which is submerged most of the time but partly emerges at spring low tide. There are two tidal creeks (Kidogoweni and Kinondo) draining the upper region of the Bay, which is dominated by mangrove vegetation. Kidogoweni Creek receives fresh water from Kidogoweni River. However, Kinondo Creek lacks direct surface freshwater input but there may be groundwater influx as water salinity fluctuates significantly in the wet season. Finally, there is Mkurumuji River which has higher flow rates and discharges into the south-western region of the Bay. The drainage basin of Mkurumuji and Kidogoweni Rivers extends into the coastal areas being 164 and 30 km² respectively. River discharge is important in wet seasons and occasionally reaches up to 5.0 and 17.à m³ s^-1 for the Kidogoweni and Mkurumuji Rivers, respectively. The climatic seasons are the south-east monson which is responsible for the “long rains” (March-May) and the north-east monsoon, which causes “short rains” (November-December). Rainy seasons are a result of the location of the intertropical convergence zone in the East Africa region. Total annual rainfall is between 1000 and 1500 mm. The rates of evaporation range between 1950 and 2200 mm per year. Wind blows into the area with an easterly component and is always onshore.
The Mekong River is one of the longest river system in the World with a length of about 4000 km, running through 6 countries : China, Myanmar, Laos, Thailand, Cambodia and Vietnam. The Mekong River runs through Vietnam for a distance of more than 200 km and basin area of 45000 km². The river divides into 2 branches : Tien and Hau (Bassac). The Hau runs into the ocean through three estuaries: DinhAn, TranhDe and BatXac. The Tien runs through six estuaries: Tieu, Dai, BaLai, HamLuong, CoChien and CungHau. The main water volume (over 79%) flows into the Tien River. The population density in the Mekong delta is relatively high with an average 400 inhabitants per km², which may reach as much as 1000 inhabitants per km² in many villages along the mai branches and canals of the delta. Most of sewage is discharged directly into the delta. Discharge from the Mekong River mainly depends on the rain regime on the basin, which follows two seasons: the wet season from May to November which provides about 75% of the total annual fresh water volume and the dry season from December to April.
Tam Giang 3 & Kien Vang mangrove forests are located Nam Can and Ngoc Hien Districts (Minh Hai Province, Vietnam). A large area of mangrove forest and associated intertidal mudflats around Cua Song Bay at the extreme southern tip of the Mekong Delta, bounded to the west by the Gulf of Thailand and to the southeast by the East Sea. The entire area is subject to tidal inundation twice daily, the tidal amplitude varying between 0.53m at neap tides and 1.09m at spring tides. The water in the channels is either brackish or saline, and there is no fresh water in the area. The average salinity at Ca Mau Cape is 24.5-26.0 p.p.t., decreasing to 18.0-20.0 p.p.t. after heavy rainfall. Accretion is occurring at a rapid rate, the mudflats extending westwards at about 60m per year. About 45,000 ha (40%) of the mangrove forest in Minh Hai Province were destroyed by toxic chemicals during the period 1968-1970, and since then many thousands of hectares have been cut down for timber and fuelwood or cleared for the construction of shrimp and fish ponds. Tropical monsoonal climate with a dry season from November to March and a rainy season during the southwest monsoon from April to October. The average annual rainfall is 2,250 mm (range 1,940 to 2,950 mm), the heaviest rainfall occurring in August and September. Mean monthly temperatures range from a minimum of 25.0C in January to a maximum of 27.8C in February; mean relative humidities range from 79% to 88%. Storms are frequent in April and May, with wind speeds sometimes exceeding 100 km/hr. Extensive mangrove forests dominated by species of Avicennia and Rhizophora. The following species have been recorded in the forests: Avicennia lanata, A. officinalis, A. alba, Rhizophora apiculata, R. mucronata, R. orientala, R. stylosa, Bruguiera parviflora, B. gymnorhiza, B. sexangula, Lumnitzera racemosa, Ceriops tagal, C. decandra, Xylocarpus granatum, Sonneratia ovata, Excoecaria agallocha, Phoenix paludosa, Nypa fruticans, Kandelia candel, Aegiceras corniculatum, Acanthus ilicifolius, A. ebracteatus, Pluchea pteropoda, Paspalum vaginatum, Acrostichum aureum, Derris trifolia and Sarcolobus globosus. The three main groups of algae are Baccilariophyta, Cyanophyta and Chlorophyta. Forty-four genera have been recorded, the commonest being Rhizozlenia and Cyclotella. The mangrove forests are very important for wildlife especially birds. At least two or three new breeding colonies of large waterbirds have been established in recent years. Resident waterfowl include Pelecanus philippensis, Phalacrocorax carbo, Anhinga melanogaster, Ixobrychus sinensis, I. cinnamomeus, I. flavicollis, Nycticorax nycticorax, Ardeola bacchus, A. speciosa, Bubulcus ibis, Butorides striatus, Egretta sacra, E. garzetta, E. intermedia, Ardea purpurea, A. cinerea, A. sumatrana, Anastomus oscitans, Ciconia episcopus, Threskiornis melanocephalus, Plegadis falcinellus, Dendrocygna javanica, Rallus striatus, Amaurornis phoenicurus, Gallicrex cinerea and Gallinula chloropus. The extensive intertidal mudflats are an important staging and wintering area for large numbers of migratory shorebirds such as Himantopus himantopus, Recurvirostra avosetta, Pluvialis dominica, P. squatarola, Charadrius dubius, C. mongolus, C. leschenaultii, Limosa limosa, Numenius phaeopus, N. arquata, Tringa erythropus, T. totanus, Xenus cinereus, Actitis hypoleucos, Gallinago stenura and G. gallinago. Mammals include Macaca mulatta, Lutra perspicillata, Viverra zibetha, Felis chaus, F. viverrina, Neofelis nebulosa, Sus scrofa and Cervus unicolor. Reptiles and amphibians include Python reticulatus, P. molurus, Enhydris spp, Varanus sp, Mabouya multifasciata, Bufo melanostictus and four species of Rana. Fifty-three species of fishes have been recorded, representing 13 orders and 29 families. Perciformes comprise 47% of the total (25 species).

This research is carried out in collaboration with the following institutions:

Laboratoire d’Océanologie, Université de Liège, Belgium
Laboratoire de Biodiversité Végétale, Université de Liège, Belgium
GeoHydrodynamics and Environment Research, Université de Liège, Belgium
Department of Analytical and Environmental Chemistry, Vrije Universiteit Brussel , Belgium
Laboratoire d'Océanographie Chimique, Université Libre de Bruxelles, Belgium
Institut für Medizinische Biologie, Arbeitsgruppe Allgemeine Mikrobiologie, Universität Wien, Austria
Département de Géologie et Océanographie, Université de Bordeaux 1, France
Centre for Estuarine and Coastal Ecology, The Netherlands
Caribbean Marine Research Centre, Bahamas
Kenya Marine and Fisheries Research Institute, Kenya
Research Institute for Aquaculture n°2, Vietnam
Ben Tre Fishery Department, Vietnam
Andhra University, India

This research is so far the subject of 5 peer reviewed publications:

This research is so far the subject of 10 presentations at international meetings/workshops:

Borges A.V., B. Delille & M. Frankignoulle,CO₂ emission from mangroves’ surrounding waters, Gas Exchange: on Land, at Sea and in the Air, Symposium in the context of SOLAS of IGBP/SCOR, Koninklijke Nederlandse Akademie van Wetenschappen, Amsterdam, The Netherlands, 13 March 2003, poster
Borges A.V., B. Delille & M. Frankignoulle, CO₂ emission from mangroves’ surrounding waters, European Geophysical Society / American Geophysical Union / European Geosciences Union, Nice, France, 6-11 April 2003, poster in pdf
Bouillon S., N. Koedam, A.V. Borges, M. Frankignoulle & F. Dehairs, Carbon dynamics in mangrove ecosystems: interactions between intertidal and adjacent aquatic habitats , European Geophysical Society / American Geophysical Union / European Geosciences Union, Nice, France, 6-11 April 2003, oral presentation
Borges A.V. A carbon budget for Gazy bay ecosystem, Gazi-2: Mini-workshop meeting on large-scale processes in mangrove research in Gazi, KMFRI, Mombasa, Kenya, 30 July 2003, oral presentation
Borges A.V., M. Frankignoulle, B. Delille and S. Bouillon, DIC dynamics in a tropical estuary (Kidogoweni, Kenya), European Geosciences Union, 1st General Assembly, Nice, France, 25 - 30 April 2004, poster in pdf
Bouillon S., F. Dehairs, G. Abril & A.V. Borges, Distribution and sources of organic carbon in a mangrove-seagrass ecosystem (Gazi Bay, Kenya), European Geosciences Union General Assembly, 24-29 April 2005, Vienna, Austria, poster
Bouillon S., F. Dehairs & A.V. Borges, Biogeochemstry in the Tana estuary and delta (Northern Kenya), European Geosciences Union General Assembly, 24-29 April 2005, Vienna, Austria, poster
Borges A.V., Y.M. Kone, L.-S. Schiettecatte, B. Delille, M. Frankignoulle & S. Bouillon, Preliminary results on the biogeochemistry in the Mekong estuary and delta (Vietnam), European Geosciences Union General Assembly, 24-29 April 2005, Vienna, Austria, poster
Borges A.V., Diversity of Ecosystems and Coastal Ocean CO2 Fluxes, XXXVII International Li�ge Colloquium on Ocean Dynamics, Gas Transfer At Water Surfaces, 2-6 May 2005, Li�ge, Belgium, oral presentation
Borges A.V., Do we have enough pieces of the jigsaw to integrate CO2 fluxes in the Coastal Ocean ?, ASLO 2005 Summer Meeting, 14-24 June 2005, Santiago de Compostela, Spain

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).

This research greatly benefited in cruise preparation, field sampling, laboratory analysis, manuscript elaboration and html tips, from the help of the following persons:

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, ...

Links to related web pages:

Carbon cycle in mangroves and tropical estuaries