CAMEROON ESTUARINE SYSTEMS
C.E. Gabche and S.V. Smith
Budgets for the Cameroon estuary complex
Budgets for the Rio del Ray estuary complex
Cameroons coastal zone and estuarine systems
Cameroon (8-16°E; 2-13°N) is
situated on the extreme north-eastern end of the Gulf of Guinea with a surface area of
469,440 km². The main topographical regions
are: the low coastal plain covered by equatorial rain forests in the south, the mountain
forests peaking at the active Mount Cameroon (4,070 m) in the west, the transitional
plateau rising to the Adamaoua Mountains in the centre, and rolling savannah slopes
gradating down to the marshlands surrounding Lake Tchad to the north of the Adamaoua
Mountain range. Cameroon is drained by four
major drainage basins: Atlantic, Zaire/Congo, Niger and Tchad. A watershed exists along the southern Cameroon
plateau separating the coastal from the Congo system, with freshwater input into the
Atlantic drainage basin.
Cameroon (8-16°E; 2-13°N) is situated on the extreme north-eastern end of the Gulf of Guinea with a surface area of 469,440 km². The main topographical regions are: the low coastal plain covered by equatorial rain forests in the south, the mountain forests peaking at the active Mount Cameroon (4,070 m) in the west, the transitional plateau rising to the Adamaoua Mountains in the centre, and rolling savannah slopes gradating down to the marshlands surrounding Lake Tchad to the north of the Adamaoua Mountain range. Cameroon is drained by four major drainage basins: Atlantic, Zaire/Congo, Niger and Tchad. A watershed exists along the southern Cameroon plateau separating the coastal from the Congo system, with freshwater input into the Atlantic drainage basin.
Cameroons coastal zone (Figure 1), extends along 402 km (Sayer et al. 1992), from latitude 2.30°N at the Equatorial Guinea borders to 4.67°N at the Nigeria borders. The coastal zone area is estimated at 9,670 km² (Adam 1998) representing 22% of the Gulf of Guinea countries.
Figure 1. Cameroon and the Gulf of Guinea.
Cameroons coastal climate is of an equatorial type and is influenced by the meteorological equator, being the meeting point between the anticyclone of Azores (North Atlantic) and that of Saint Helen (South Atlantic). This climate results from the combined effect of convergence of the tropical oceanic low-pressure zone and the inter-tropical front within the continent. There are two distinct seasons: a long rainy season of more than 8 months (March-October) and a dry season of four months (November-February) exist. Air temperatures are high throughout the year. South-westerly monsoon winds predominate, modified by land sea breezes causing humidity values to almost saturation point. Wind speeds exceptionally reach values of 18 m sec-1 (April, 1993) with average values recorded over a period of 10 years (1983 1993) varying between 0.5-2.5 m sec-1. The rainy season is hot and dry with a north-easterly harmattan when the inter tropical convergence zone deviates from its normally southern position at 5-7°N.
Cameroons coastal tropical rainforest is interrupted at the active Cameroon Mountain and within the mangrove estuarine complexes. These complexes are characterized by very low altitudes (0-20 m), developed on low soils (generally less than 5 m high) with primary stages of mangroves developed at 0-5 m while mature ones reach 2 m. Mangrove estuarine complexes in Cameroon occupy approximately 30% (3,500 km2) of Cameroons coastal zone. There are about 38 species of mangrove, dominated by Rhizophora (R. racemosa and R. harrisanii) species (Gabche 1997). This is followed by the Atlantic forest dominated by families of Caesalpinacea and Guttiferae, Euphorbiaceae; swamp forest dominated by Rapphia spp., Matritia quadricorius, Clenolephon englerianus, and seasonally inundated forests of Guitbortia demeussei and Oxysttigma menil. Phytoplankton species (Folack 1991) are dominated by diatoms such as Chaetoceros testissimus, Nitzchia closterium, Diatoma vulgare, Trachyneis and Coscinodiscus.
Dense river networks flow into three estuarine systems along the coast. The West/Rio-del-Rey system has several rivers (Cross, Ndian and Meme) that discharge at the Rio-del-Rey Point (4.8°N; 8.3°E). The Cameroon estuary complex with several rivers (Mungo, Wouri, Dibamba etc) discharges at Douala Point (3.84.1°N and 9.2510°E). This extends towards the west at Bimbia and south to the Sanaga River estuary. The third estuary complex in the south is made up of several rivers (Nyong, Lokoundje, Kienke, Lobe and Ntem) which discharge independently into the Atlantic Ocean. Some physical characteristics of the Cameroon and Rio-del-Rey estuarine complexes are given in Table 1. The rivers of these estuaries have watersheds from high altitudes (2,0002,500 m) at the Adamawa plateau, Rumpi Hills and Manegumba Mountains. The mangroves of the Rio-del-Rey cover an area of about 1,500 km2 with 50 km of coastline and a landward extension of 30 km. The Cameroon estuary has a coastline of 60 km from the Sanaga to the Bimbia estuary and 30 km into the hinterlands giving area of 1,800 km2. The southern river systems at the Ntem also has estuarine mangrove swamps. The supplies from the dense river network, groundwater and rainfall are major sources of freshwater into the continental shelf (area = 15,400 km² ) (Gabche and Folack 1997). The gradual descent (10, 30, 50 and 100 m depth) of the continental shelf results in generally weak circulation with subsequent high sedimentation rates.
Hydrodynamic processes within the estuarine complexes indicate that semi-diurnal tidal wave action can be felt a long distance from the sea in the rivers (40 km in the Wouri; 35 km up the Dibamba), with wave height recordings ranging from 1.54.5 m. There is an enormous propagation of waves and ebb-tides through the estuarine complexes (Olivry 1986; Morin et al. 1989). Tidal currents are strong: 1-1.5 m s-1 for flood and up to 2.6 m s-1 for ebb. Chaubert et al. (1977) noted that sea swells in vicinity of the Rio-del-Rey are from south to south-west and distant in origin. This peculiarity results from the double obstacle created by Bioko Island and the wide continental shelf at the Rio-del-Rey (80 km as compared to 40 km at the Kribi coast). Swells of greater magnitude (226 m long) are common between June and September with lesser ones between November and April.
Salinity distribution within Cameroons estuarine complexes is determined by huge inputs of freshwater from rivers, rainfall and groundwater. Salinity is generally low with values at the Douala Port of 9-12 psu. Lafond (1967) showed maximum values of 20 psu at 15 km from the port offshore during the dry season and less than 12 psu in the rainy season. These values decrease towards the port to average values of 0 psu for every 100 m (2.6 psu for each km) near Japoma on the Dibamba River, and maximum values as low as 6.5 psu at low tide. Values of between 12.0-17.5 psu have been recorded within the Mungo River, with increased values due to seawater intrusion during the dry season and mixture with freshwater. Salinity distributions are in line with regional surface values which show significant fresh water in the Gulf of Guinea and in particular, in the Bight of Biafra, with values lower than 29 psu (ICITA 1973; GATE 1980).
|Table 1. Physical characteristics of some Cameroons coastal zone estuarine systems.|
Mean Depth (m)
|Rio del Ray||8.28||4.83||Cross||800||14|
The high nutrient loads derived from land support high productivity and relatively large fish catches (more than 60,000 tons per year) as compared to other countries of the Gulf of Guinea (Schneider 1992). These are comparable to those in the countries where upwelling occurs. In recent years (1980 to present) there has been a trend of decreasing marine fish catch in Cameroon. This is partially due to reduced fishing effort. The decline may also be partly due to pollution by agricultural and industrial waste and municipal discharge into the ocean. A detailed elaboration using methods in WHO (1989) on this is given in UNEP (1984) and Angwe and Gabche (1997).
Leaf litter from mangroves and estuaries forms an important nutrient base for food webs leading to commercially important food fishes and invertebrates (Snedaker and Snedaker 1984). The mangrove leaves become nutritious through microbial enrichment processes. Higher rates of leaf fall occur in the dry season than in the rainy season. However, because studies have not been carried out on mangrove litter decomposition and nutrient enrichment, in this study they are assumed to be zero.
Faunal species within the mangroves and estuarine complexes are dominated by the forest elephant (Eoxondonta africana), the giant forest hog (Hylochohrus meinertz hageni), the endangered drill Mandrillus leucophaeus), the highly vulnerable black colobus (Colobus satanus) and Upper Guinea primates (Cercopithecus mictitans martini, C. erythrotis camerunensis and C. pogomius pogonius). There is a significant population of the highly vulnerable African manatee (Trichechus senegalesis) within the Sanaga estuary.
Cameroons estuarine complexes and mangroves serve as habitats for meiofauno taxa such as nematodes, copepods, amphipods and protozoans which assist in the conversion of mangrove primary production to detritus. The benthic fauna is made up of polychaetes (Amphiura sp., Nephthys, etc), bivalves (Arca nuculana, Aloidis, Nsa sp., oysters (Crassostrea gasar) etc) and sponges (e.g. Holothurids). They also serve as breeding grounds and nurseries for crustaceans (crabs e.g. Grapsidae, Ocypodidae and Portunidae; shrimps e.g. Peneidae and Palaenonidae) and fin-fish species including mud skippers Periopthalnus sp., Cichlidae, Scianidae, Polynemidae, Clupeidae and Drepanidae.
The physical characteristics of Cameroons estuarine systems were determined from the scientific literature (Gabche and Folack 1997; Angwe and Gabche 1997; UNEP 1984; ICITA 1973; GATE 1980; Van den Bosche and Bernacsell 1990; Sayer et al. 1992; Mahé 1987; Folack 1988, 1989; Gabche and Hockey 1995; Folack et al. 1999). Hydrological data such as river discharge, rainfall and evaporation came from Cameroons annual hydrological handbook (1997) and the meteorological services in Douala with some modifications. Data on nutrient levels came from monitoring by government services such as the Ministry of Environment and Forestry in Douala and the Research Station for Fisheries and Oceanography Limbe, Cameroon.
Budgetary estimations for the Cameroon and Rio-del-Rey estuary systems were separated into four months (120 days: November February) of dry season and eight months (245 days: March October) of rainy season.
Figure 2. The Cameroon and Rio-del-Rey estuarine systems.
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Last Updated 21 May 2006 by DPS