Key words: Rainforest, Cameroon, Biotope, Fish, Tropical Biodiversity
Introduction and Habitat Types
The focus area for this post was south of Douala in Cameroon. The hydrological setting was a catchment located between the Dibamba and Sanaga River systems. The study area consisted of tributaries of the Ndonga River watershed , an affluent of the Wouri/Cameroon Estuary system (Figure 3).
The landcover in the Ndonga River watershed consists of agricultural activities, minor villages and dense littoral tropical forest (Figure 2). This region is considered to be a coastal lowland plain, with low/gentle topography across the Ndonga River watershed. Elevation was 0 meters above mean sea level (mamsl) in the west and 139 mamsl in the east of the watershed (Figure 4). The low nature of the topography has allowed for tidal movements of water far inland with most of the lower Ndonga River consisting of mangrove habitats.
Habitat types in the Ndonga River watershed can be separated into 3 major classes. These include swamps (Figure 5), flowing streams (Figure 16) and mangroves (Figure 33), which will each be discussed below.
Swamps - Headwaters
Headwater swamps are the dominant freshwater habitat types located at the flat source zone of the Ndonga River system. Gradients (slope) within this habitat type allow for the dispersal of water across a large surface area, resulting in an increase of the wetted perimeter. This has caused the deposition of fine clay sediments and dense leafy substrates (Figure 6). The swamp conditions typically form a braided network of connected channels, submerged floodplain's and isolated pools. Typical plant species observed in these habitats included Lasimorpha senegalensis, Hallea, Anthocleista and Uapaca. Raphia palm are also abundant in this habitat type. The canopy cover of these habitat types is typically 85-100%.
Figure 7: Shallow swamps with Lasimorpha senegalensis in coastal Cameroon - January 2020
Swamps - Fish
A common fish species in the shallows of the swamps include Aphyosemion ahli (Figure 8), a colorful oviparous non-annual killifish. These species occur over dense layers of detritus and feed on insects which fall from the forest canopy. These species are sensitive to habitat modification based on their ecological preferences. Notopteridae were also found in the headwater swamp habitats, where they were observed to be using the leaf litter and undercut banks to hide during the day, but would be seen roaming the shallows in the evening. These species included both expected taxa Xenomystus nigri and Papyrocranus afer (Figure 9).
Swamps - Flowing Swamps and Swamp Forest
Where enough water collects but gradients remain gentle, flowing swamps occur, usually with wide floodplains and a deep meandering central channel. The flowing swamps move between large trees where buttress root systems form that are specifically adapted to hold trees steady in the swampy terrain (Figure 10). Multiple channels form in between the trees creating a braided network of highly variable habitats (Figure 11). Submerged logs, dense rooted river banks, low light adapted channel edge plants and pockets of dense leaf litter form the dominant cover in the habitat type. The canopy cover of these systems is typically 85-100%.
Flowing Swamps and Swamp Forest - Fish
The undercut banks and dense root wads are suitable habitat for a diverse array of catfish like Parauchenoglanis monkei, Malapterurus beninensis, Parachanna obscura and Schilbe brevianalis all of which were observed in this habitat (Figure 12 and Figure 13).
The detritus rich substrates of the swamps form important feeding grounds for the endangered Benitochromis nigrodorsalis (Figure 14), a mouthbrooding cichlid. The complex habitat types have allowed for the evolution of specialized fish species such as Pantodon buchholzi. It is expected that both these species would be recognized as sensitive species, susceptible to changes in forest canopy structure. The habitat types covered in the swamps are effectively illustrated in the video below.
Where steep gradients and enough water volume exist in the Ndonga River watershed, flowing streams with gravel and sand substrates form (Figure 16). The structure of these streams consist of primary and secondary channels which become isolated during the low flow periods. This structure and hydrological regime creates isolated pools adjacent the main river. Gallery riparian forest occur adjacent to the flowing streams. These large/tall trees provide woody debris and leafy material to the substrates of the rivers, providing the primary cover for aquatic biota. Forest cover in this habitat type varies from 70-100% cover. The open areas allow for the penetration of light and subsequent prefoliation of aquatic macrophytes.
Flowing Streams - Fish
The fish communities of the flowing streams consist of many rheophilic species such as the common Bryconalestes longipinnis (Figure 18) and the less common Phractura brevicauda (Figure 19) .
In the slower reaches where the forest canopy opens, aquatic macrophytes can be found and included Nymphae lilies instream, and Lasimorpha senegalensis on the channel edge habitat. This was suitable habitat for Pelvicachromis pulcher (Figure 21) which was also recorded on underwater footage. Sampling of the dense vegetation in the instream macrophytes would yield Polycentropsis abbreviata which was also common in the area (Figure 22).
Figure 20: Flowing streams with Nymphea lillies and Lasimorpha senegalensis, Cameroon
The secondary channels, which at the time (December 2018) had formed isolated pools (Figure 23), contained specialist fish species with highly specific habitat requirements such as Fundulopanchax kribianus/fallax (Figure 24). This species is listed as Endangered by the IUCN (IUCN, 2021) and was only found in one location, out of the +- 25 sample locations visited. The species is considered susceptible to modification given its very specific habitat specifications. Changes in land-cover of the catchment areas of these isolated pools may alter flow patterns resulting in modification to inundation state frequency and duration in the secondary channels. Changes to riparian forest would have a similar significant impact to these species. The maintenance of landcover in the upper catchment and implementation of buffers for riparian protection is considered of utmost importance in areas these fish are found. Neolebias ansorgii were also found in this habitat type.
The larger river systems within the Ndonga catchment consisted of the Mbongo, Mbanda and Mbimbe Rivers, as well as several unnamed watercourses. These larger river systems were typically lined with gallery riparian forest but had varied open and closed canopies. The open nature of the watercourses, and clear water conditions allowed for the proliferation of dense stands of Nymphaea lilies and emergent Commelina. Woody material and root wads from the adjacent forest was often observed in these larger river systems.
Figure 26: Larger rivers - the Mbimbe and Mbanda Rivers, draining into the Ndonga River system, Cameroon.
Larger Rivers - FISH
In Africa fish species richness has been associated with Arrgenius's Law, whereby the species richness increases in proportion to the function of surface area. In the case of freshwater ecology, species richness has been associated with catchment area, drainage length and most importantly overall discharge volume (Hugueny, 1989). In addition to the existing physical catchment characteristics, historical geological factors can have an effect on the diversity of aquatic fauna (Skelton, 2019). On a local scale, previous studies have shown fish community diversity is richer in wider, deeper rivers with open canopies (Angermeier and Karr 1983). The same observations were made during surveys completed in the Ndonga River system, whereby wider, higher Strahler Order watercourses tended to have higher diversities of fish species. A total of 138 species representing 26 families are expected to occur in the study area.
The shallow flowing waters of the lower Mbimbe and Mbongo River's contained dense schools Poropanchax stigmatopygus which were usually found over the sandy substrates. Numerous Barboides gracilis were found against the river bank as is seen in the underwater footage at the sites. The sampling of the concentrated stands of submerged vegetation typically resulted in the observations of Chromidotilapia guntheri, a common Cichlid across Lower Guinea.
The woody debris created from falling allochthonous materials is a difficult habitat type to sample effectively, with fish often hiding between the material. In this habitat type, electrofishing or overnight baited traps usually work best. Mastacembelus eels, and diverse species of Mormyridae are found associated with the fallen debris. Predatory species such as Hydrocynus forskahlii and Hepsetus odoe, which are likely feeding on the numerous smaller taxa, are also common in the the larger river systems, usually in the deeper pools of the larger rivers. in the deeper habitat types. In overnight traps set in the larger rivers the omnivorous Chrysichthys nigrodigitatus can also be found. The larger rivers systems are not without benthopelagic and top water feeding species which were commonly found in abundance, these included species such as Epiplatys infrafasciatus and Enteromius callipterus. The larger river systems are not without the estuarine and migratory species such as Pelmatolapia mariae which are often observed with fry in the freshwaters.
Extensive mangrove habitat can be found where the Ndonga River meets the Atlantic Ocean in the Doula-Eda Wildlife Reserve. Over the period of the study, I was fortunate enough to encounter a low tide where freshwater from a small swamp was flowing into the mangrove. This allowed a short time frame to conduct easy sampling where common species were found as presented below.
As more data is produced and analysed in this catchment, this page will be updated.