1. Introduction

Waterfalls, most of which are formed from stream or river which cascade from a high elevation over a cliff or rock, had very little attention from researchers the world over. In Africa, the reason being that, waterfalls were named after deities and were used as places of traditional and ancestral worships. In other places, the intensity of waterfalls andpressure generated from it due to gravity has given the im- pression of a lifeless zone[1]. Knowledge of waterfalls systems in Africa are therefore limited to hydrology and geological features[2,3], as natural monuments for revenue generation, because of their ecotourism potentials[4] and as source for drinking, irrigation and other domestic purposes. Studies on the fisheries and aquaculture potentials, biodiversity conservation and aquatic ecology of waterfalls are scarce.

Knowledge of status of the fauna and flora of Agbokum waterfall is important in the development and management of conservation measures. It is also possible that the biodiversity of Cross river fishes is much higher than reported[5, 6] and could yield records of undiscovered and un-described species if the Agbokim Waterfalls and other water bodies such as Crater lakes, flood plains, lakes and mountain streams which link up with the Cross River, are investigated. The fundamental attributes of an aquatic ecosystem are the number of species present and their abundance[7]. Both attribute are dynamic and related[8]. In aquatic ecosystems interaction between the environment and population processes affects the distribution and abundance patterns of species[9,10]. The resources of the length of the waterfalls are yet to be tapped, more so due to the total neglect by stakeholders. This study presents an analysis of the occurrence, distribution and dynamics in the fauna and flora resources of Agbokim waterfalls to reduce our over- dependence on estuarine and open water body systems.

2. Methods

2.1. Study Area

The study area is Agbokim Waterfalls in Cross River State, Nigeria, latitude 5^°59’ North and longitude 8^°45’ East. It is bounded in the West by the Cross River and in the North by the Cameroon high forests. The climate is the tropical hinter-land type, with wet (May-November) and dry (December-April) seasons. Mean annual temperature ranged between 20℃ and 32℃ and annual total average rainfall, from 1450mm to 3015mm. The vegetation is the rainforest type with Soil consisting of deep laterite and dark fertile, clayey and loamy soils. The Agbokim waterfall as most others is a product of two rivers, River Ekim and River Bakue, which are tributaries of the Cross River system. River Ekim is divided into three streams, while River Bakue has four streams. These seven streams flow into a floodplain, from where they independently cascade over steep cliff which provides seven-faced falls into the casket or waterfalls. Of ecological importance are numerous small pools and swamps which are found along the length of the waterfalls. The high annual discharge and rainfall of the area provide excellent buffers against natural ecological stresses such as drought[9]. For the purpose of this study, the 6817.7m long waterfall is divided into three reaches; upstream, midstream (region of waterfalls) and downstream. Upstream is 2003.13 m long with substrate of gravel and rocks under fast water current and shoreline covered with high forest and cocoa farms, Midstream length of 807.42 m has substrates of sand and rocks under heavy water turbulence with shoreline sparsely shaded with vegetation while downstream length, 4007.15 m has fine sand and clay under slow water current with an extensive wide area.

2.2. Plankton Studies

Phytoplankton samples were collected monthly, from August 2005 – July 2007. These were collected in 250ml bottles[11]. The water samples, for plankton analysis was taken to the Fisheries Laboratory of Cross River University of Technology, Obubra Campus, Cross River State. Fresh sample was mixed gently and pipetted into 5ml plankton chamber containing 2 drops of Lugol’s solution. The Lugol’s solution served as the fixative while also enhancing sedimentation of organisms. The chamber was left overnight for complete sedimentation to take place. Plankton was analyzed using Zeiss inverted plankton microscope[12] using Plankton determination keys[13-15].

Zooplankton was sampled from a scoop plastic bucket of 10 liter capacity. Subsurface water (5m depth) was vertically collected with a bucket and filtered through plankton net of mesh size 100µ. Fifty scoop buckets of 10 liter capacity were filtered via the net. The net samples were washed into the 1 ml sample collecting bottles and fixed immediately in 2% formalin solution. After making up to 100 ml with distilled water, the samples were agitated and homogenized and 1 ml sub-samples placed in counting chamber for observation under 40-100 × magnification. Organisms were identified[16,17] and enumerated for subsequent statistical computations.

2.3. Faunal Studies

Macro-invertebrate benthic fauna samples of the waterfalls were collected four times near the margin of the river at each reach by use of a corer 0.004m² already described[18] and modified[19]. Three replicate samples were taken from each reach and transferred to separately labeled polythene bags for laboratory analysis. Sieves with meshsizes 2mm and 1mm were used to screen the organisms that were later sorted and preserved in 5% buffered formalin. The individual organisms were identified using a stereoscopic microscope and identification guides[21-23]

The fish of the river was sampled in all the reaches using gill net (22-76mm stretched mesh size). Genus and species identifications was carried out for the Cyprinids[24], for the Bagrids[25]; for the Clariidae[26] and for the Clupeidae and Mugilidae[27]. Fish catch of each reach was presented as a numerical contribution by each species. This was determined by calculating the ratio each species, represented of the total catch for each reach based on the number of species and relative abundance.

2.4. Environmental Parameters

Water level was determined with a lead sinker attached to a calibrated rope. The sinker was lowered down into water until it reached the substrate. The depth was then read from the calibrated rope. The procedure was repeated in two other locations randomly selected from the edge and middle of water within the sampling site. River width was measured with a long calibrated rope in three places in each sampling sites and the average was taken. Water velocity (flow velocity) was determined to 0.03m/sec accuracy with Wagtech current flow meter, model WFM001 with 125mm diameter impella. Water discharge was determined[28] using the formula: WTD = CSA x WD x WV. Where WTD = Water discharge (m³s^-1), CSA = Cross Sectional Area (m); WD = Water dept (m) and WV = Water velocity ms^-1.Temperature values were recorded from a mercury-in glass thermometer graduated in units of ℃ (50℃) by immersing the thermometer slightly under the surface of water(2cm) for 5 minutes until mercury stood at one place). PyeUnicam Model 7065 electronic metre at 25℃ after standardization with buffer solution at pH 4, 7 and 9 was used for pH. The dissolved oxygen concentration of the water samples was determined with a Fischers digital oxygen analyzer.

2.5. Vegetation

Studies of the vegetation was carried out by selecting three populations from the three reaches to represent different degrees of disturbances. All populations were vigorous and healthy in all sampling sites, when the study began in 2007. In each reach, random stations, 20 × 20 in each were mapped out and labeled markers placed within each quadrat and mapped with the plants. At intervals of 4-6 months, all plants, within a quadrat, were mapped and the following figures recorded for each seedling or established shoot status number damaged, presence or absence of individuals and their zonation patterns were also noted.

2.6. Data Treatment and Analysis

The mean and standard deviation of each of the physico-chemical parameters were calculated. Analysis of variance (ANOVA) was used to test for statistical differences between the means of the physical and chemical parameters of the sampling sites. To calculate mean abundance, numbers in different samples were summed for each species and averaged across all sampling sites. Physico-chemical parameters were correlated with the abundances of fish species using Pearson product moment correlation coefficient analysis. Shannon-Wiener diversity function (H) was used to calculate heterogeneity for each site. Richness index was expressed using Margalef’s richness index.

E = d / S[31]

E = Equitability

d = Margalef’s richness index and H´ = Shannon-Wiener Diversity Function

S = total species number

p_i= proportion of each species in each sample

Relative abundance % = (n/N) × 100,

n refers to the number of individuals of the species in the samples and N to the total number of individuals of fish caught.

Figure 1. Map of Cross River State showing Agbokim Waterfalls

3. Results

3.1. Plankton

Table 1. Plankton spatial numerical composition and diversity of Agbokim Waterfalls in wet (W) and dry (D) season for the three sites, upstream, midstream and downstream for 2005 – 2007 Reaches Upstream Midstream Downstream Seasons D W D W D W Plankton taxa/species Phytoplankton Chlorophyceae Akinstrodesmus spiralis 46 - 57 4 32 16 Phacus quinquemarginatus 20 18 32 5 13 10 Trachelomonas volvocina 91 55 69 22 318 17 Trachelomonas volzii 163 123 91 432 23 Euglena acus 14 3 26 3 2 1 Astasia klebsii 8 2 13 3 3 4 Euglenopsis vorax 5 5 7 5 1 Spirogyra varians 9 7 3 11 Selenastrum gracile 25 5 13 88 13 Scenedesmus denticulatus 46 20 44 31 3 6 Zygenma insigne 5 7 4 55 Dictyosphaerium pulchellum 4 4 14 8 Tribonema bombycinum 2 87 13 2 22 Mougeotia transeaui 2 6 3 21 Volvox tertius 7 11 12 8 Ulothrix cylindricum 2 4 3 3 Eudorina elegans 1 2 5 7 Chlamydomonas ehrenbergii 2 8 4 2 1 22 Chlorella ellipsoidea 112 18 38 12 212 44 Cyanophyceae Anabaena bornetiana 11 15 19 27 Aphanizomeno holsaticum 22 15 11 32 Nostochopsis lobatus 12 3 19 2 5 Nostoc lobatus 49 10 43 8 Bacillariophyceae 13 Navicula radiosa 21 12 12 27 Nitzschia sigmoidea 34 12 12 21 88 19 Synedra ulna 54 16 12 4 76 10 Chaetoceros elmorei 4 4 37 8 Cryptophyceae 23 1 8 15 Cryptomonas erosa 9 11 14 3 Dinophyceae Gymnodinium palustre 13 4 10 5 Zooplankton Rotifera Filinia longiseta 4 2 12 16 Lecane bulla 12 3 2 6 27 Synchaeta oblonga 3 1 3 1 13 11 Bracchionus quadridentatus 4 6 4 2 19 94 Cladocera 138 Moina micrura 1 51 151 Bosmina longirostris 14 - 8 2 34 Daphnia magna 1 3 4 2 17 25 Diaphanosoma excisum 1 21 Copepoda Calanus finmarchicus 211 35 6 1 344 Pseudocalanus elongate 4 2 3 1 115 28 Acartia tonsa 22 1 1 4 7 91 Tisbe holothuriae 4 8 1 35 8 Schizopera elatensis 8 4 2 1 30 23

A total of 29 phytoplankton species sampled from five taxonomic groups; Chlorophyceae, Cyanophyceae, Bacillariophyceae, Cryptophyceae and Dinophyceae, represented phytoplankton community of the Agbokum waterfalls (Table 1). Chlorophyceae was the dominant group and Trachelomonas volzii most abundant species within the group. The dry season samples recorded higher values (718) of individual Trachelomonas volzii than the wet. Three major taxonomic forms of zooplankton; rotifer, cladocera and copepod, consisting of 13 species were recorded. Copepoda (51) was dominant zooplankton with Calanus finmarchicus as most abundant species in the dry season while dominant wet season zooplankton Moina micrura (256) represent 65% of total zooplankton. About 78.6% of total phytoplankton and zooplankton was contributed from downstream populations while only 10% from midstream.

3.2. Macro-faunal Composition and Distribution

Macro-invetebrates assemblages collected from the waterfalls consist of nine major taxa and 1878 individuals (Figure 2). Spatial distribution of the taxa and individuals recorded upstream, midstream and downstream reaches were 9 (413), 9 (113) and 9 (1346) respectively. Dipterans and nematodes were dominant groups representing 15.4 and 15.2 percent respectively; decapods and annelids represent 14.7 and 13.5 percent respectively. Coleoptera was the least group (6.5%). About 71.7 percent of the total macro- invertebrates were recorded from downstream reaches while only 6.3 percent were contributed by midstream. Seasonal variation of the invertebrates in the different reaches showed that the dry season samples were significantly higher (1416) (p < 0.05) than the wet (462) (Figure 3).

Figure 2. Spatial changes in the numerical composition of the dominant macro-fauna invertebrates

Figure 3. Seasonal changes in the numerical abundance of macro-fauna invetebrates

Figure 4. Seasonal changes in the percentage fish catch

Figure 5. Percentage cover of the most common vegetation types in the three reaches of Agbokum water falls

3.3. Fish Fauna

A total of 548 fish representing 13 species from 5 families were sampled during the dry and wet seasons, with only one family being restricted to the wet season, which was of very rare taxa (Figure 4). The number of taxa, and relative proportion of each species present, upstream, midstream and downstream reaches are shown in Table 2. Cichlidae, Clariidae and Cyprinidae were the most abundant families accounting for 56.7% of the total catch. Tilapia zillii, Clarias gariepinus and Labeo coubie dominated overall catch constituting 45.5%. Among the three dominant species, two benthic fishes (L. coubie and C. gariepinus) showed inverse distributional patterns. Clariid fish (C. gariepinus) was most abundant downstream and least represented, upstream. Cyprinid (L.coubie), on the other hand, dominated the upstream reaches and scarce downstream. Five species were site specific because of the association with the midstream portion of the river (L. senegalensis) and downstream reaches (A. occidentalis, H. longifilis, H. fasciatus and C. nigrodigitatus). Distribution of other species revealed no distinct trends.

3.4. Vegetation

The dominant vegetation at the bank are of freshwater swamp type; Azolla africana, Nymphia lotus, Commelina sp, Bambusa vulgaris, Dryopteris sp. and Salvia nymphellula. The fringing trees are principally Raphia vinifera, Symphonia sp., Elaeis guinneesis, Havea brasiliensis, Bambusasp, Grewiasp and Cocos nucifera. The vegetation showed a distinct pattern in respect of percentage cover of species present. Total numbers of species varied significantly between reaches, but all reaches displayed similar change. Bambusasp, Symphonia and Elaeis guineesis displayed progressive increase from upstream reaching a maximum of 38.5 mean percent cover over the other species in the relatively undisturbed downstream. In contrast, the population in the highly disturbed midstream showed much smaller percentage cover values (Figure 5).

Table 2. Relative proportions of fish species for the three reaches (January 2006 to December, 2007) Reaches Up rive Mid river Down river Family/species pi pi pi Cichlidae Oreochromisniloticus 0.042 0.038 0.004 Tilapia zilli 0.1100.238 0.098 Tilapiazilli Hemichromisfasciatus 0.000 0.000 0.011 Pelmatochromis guntheri 0.010 0.006 0.024 Clariidae: Clariasanguil laris 0.031 0.005 0.057 Clariasgariepinus 0.023 0.014 0.315 Heterobranchus longifilis 0.000 0.000 0.111 Bagridae: Chrysichthys nigrodigitatus 0.000 0.000 0.013 Auchenogla nisoccidentalis 0.000 0.000 0.003 Mochokidae: Synodontis clarias 0.015 0.004 0.018 Cyprinidae: Labeo coubie 0.388 0.121 0.003 Labeo senegalensis 0.000 0.044 0.000 Barbus occidentalis 0.034 0.003 0.001

3.5. Richness and Diversity Indices

Changes in the plankton, flora and fauna community assemblages of the Agbokum waterfalls ecosystems were reflected in species richness and diversity. The Shannon-Weiner diversity index for the biodata investigated showed significant difference between reaches, with downstream reaches having highest values while midstream were least (Table 3). Seasonal differentiation in the diversity indices and species richness revealed higher values for the wet season samples of plankton, fish and vegetation than dry. On the other hand, dry season invertebrate samples showed higher richness index and diversity values. Throughout the year, samples from the waterfalls region of the stream (midstream) exhibited very lowest diversity and richness values.

3.6. Physico-chemical Characteristics

Water depth, water discharge, water velocity and dissolved oxygen vary significantly between reaches (p>0.05) (Table 4). All biological species studied correlated negatively with water discharge and water velocity except the Cyprinids (Table 5). On the other hand, all the species correlated positively with dissolved oxygen apart from annelids and the marginal vegetation.

Table 3. Diversity of macro and micro fauna and flora of Agbokum waterfalls during dry (D) and wet (W) seasons in the three sampling sites. r = richness index, d = Shannon weaner diversity index Reaches Seasons Items Diversity indices Upstream Midstream Downstream D W D W D W Phytoplankton r 52 12 12 9 153 17 d 0.55 0.28 0.03 0.03 0.69 0.28 Zooplankton r 29 16 7 12 79 49 d 0.12 0.28 0.16 0.05 0.63 0.16 Macro-invertebrates r 33 12 18 10 89 45 d 0.34 0.12 0.08 0.02 1.56 0.89 Fish species r 14 28 7 7 11 38 d 0.23 0.42 0.02 0.06 0.34 1.76 Vegetation r 26 49 8 12 45 78 d 0.12 0.42 0.02 0.04 0.54 1.55

Table 4. Mean variation and F-values of the analysis of variance (ANOVA) of physico-chemical parameters of water measured at three sampling sites. I: Upriver, II: Mid-river, III: Downriver Properties Upriver Midriver Downriver Properties F-value ANOVA Physical Conductivity (uScm-1) 36.6±11.0 38.1±11.5 25.3-58.2 0.77 p>0.05 Water temperature (℃) 28.0±1.1 26.6±1.2 27.2±0.8 3.36 P>0.05 Water depth (m) 1.4±0.7 2.2±0.5 4.8±0.6 4.32 P<0.05 Water discharge(m3s-1) 156.71±12 1496.46±82 189.8±13 5.8 p<0.05 Water velocity (m/s) 1.94±0.25 1.01±0.3 0.5±0.14 4.32 p<0.05 Transparency (cm) 26.6±13.9 29.5±14 28.8±13.4 1.56 p>0.05 Physical Conductivity (uScm-1) 36.6±11.0 38.1±11.5 25.3-58.2 0.77 p>0.05 Water temperature (℃) 28.0±1.1 26.6±1.2 27.2±0.8 3.36 P>0.05 Chemical Dissolved oxygen (mg/l) 6.6±0.3 9.61±0.2 4.34±0.5 3.98 P<0.05 pH 7.0±0.2 7.0±0.2 7.1±0.2 1.43 p>0.05

Table 5. Results of Pearson correlation analysis using physico-chemcal and biological parameters from Agbokum waterfalls Item Water depth Water discharge Water velocity Dissolved O2 Chlorophytes -0.675 -0.564 -0.654 0.654 Cyanophyceae -0.766 -0.843 -0.321 0.435 Rotifers -0.492 -0.477 -0.765 0.755 Cladocerans -0.566 -0.777 -0.465 0.665 Cichlids 0.586 -0.345 -0.234 0.788 Cichlids 0.897 -0.657 -0.654 0.234 Cyprinids 0.675 0.768 0.876 0.888 Decapods 0.456 -0.543 -0.287 0.654 Annelids 0.654 -0.254 -0.432 -0.765 Azolla -0.564 -0.671 -0.654 -0.234 Nympha -0.444 -0.876 -0.324 0.564 Commelina 0.398 -0.432 -0.487 -0.675

4. Discussion

4.1. Phytoplankton

The phytoplankton composition in this study agreed with results of other workers[32,33] that blue-green algae and green algae dominate most tropical African waters. The dominance of Chlorophyceae in respect of species number and population density in this study had also been observed elsewhere[34-37] and the overwhelming presence in the dry season, attributed to the presence of bright sunshine and extensive catchment area, draining calcium rich agriculture land[38-41]. Physiological and behavioural flexibility of Chlorophyceae can accommodate environmental stresses better than most fast growing species[42]. Second in prominence was Cyanophyceae, which had also been found to be prominent in Bulgaria[43], Hungary[44] and Sanabria Lake (Spain)[45]. Cyanophyta dominance, and sometimes bloom are amongst the most visible symptoms of pollution[46]. The observation in the waterfalls is similar to findings[47] that the abundance of phytoplankton increases with increase in transparency, which normally associated with black flood (dry season), while the high turbidity associated with the white flood (wet season) results in a decrease in its abundance. It was also reported, during monsoon, mostly in ponds and reservoirs in Asia, where phytoplankton minimum can be observed during the wet months[48]. According to reports[49], most of the reservoirs in India have three plankton pulses coinciding with the post-monsoon (October to November), winter (December to February) and summer (March to May) seasons, all within the dry season. During the wet months (June-August) flushing disturbs the standing crop of plankton. However, when the destabilising effects wean away, the nutrient input favours an accelerated plankton growth in November. The annual black and white flood pattern of water bodies is the most important factor regulating phytoplankton production[50,51]. Phytoplankton abundance, vary with flood condition. The high temperature, bright sunlight and rapid tropholytic activities by the decrease in water level and the movement of the deep, nutrient-rich areas into the fold of tropholytic zone, increase plankton biomass during dry month of November and April. The waterfalls region of the study area recorded least density of phytoplankton due to high level of disturbance caused by the impact of high water discharge on the environment. Re-suspension of particles often observed in this region leads to high water turbidity and therefore low light penetration, low degree of algal sedimentation, and therefore reduced phytoplankton production. Benthic algal production is also hampered by the turbulence effect of water in this region constantly bringing to the top all materials at the bottom[52]. Planktons were moved swiftly from upstream with high flow velocity and water turbulence to downstream with stable ecology, resulting in higher plankton diversity and abundance downstream.

4.2. Zooplankton

The zooplankton population dominated by copepods and cladocerans had also been documented[53]. There may be alternation in abundance between crustaceans and rotifers as reflected in the distribution and abundance of zooplankton in the sampled parts of the river and seasons. These alternation in the abundance of species in water body was regarded as a booster of all year round food for fish in the lake[54].The zooplankton forms respond similarly to disturbance in water environment as phytoplankton. Low densities of all zooplankton species observed in the middle reaches (waterfalls) could be due to high water discharge. Discharge rate is important both directly or indirectly, as it influence the environment by creating turbulence and high turbidity in this region[55]. Suspended particles have been found to hinder metabolism in copepods[56] and turbulence prevent larval forms from settling and developing to adult stages and hence die shortly before metamorphosis[57].

This study showed that the seasonal variation in zooplankton concentration could largely be due to the rotifers which normally constitute major diet items of larger zooplankton during the dry season. Species of rotifers and crustaceans considered good indicators of the trophic state of the water bodies were identified in the zooplankton community. Dominant rotifers species of Filinia longiseta, Bracchionus quadridentatus and Synchaeta oblonga and crustacean zooplankton community populated mainly by copepods and cladocerans, as a result of increase in cyclopoid copepodids and mesocyclops, are indicative of good water quality[58]. Temperature and the availability of food are about the most important factors controlling the abundance of zooplankton in water[59]. In this study, with higher temperature regimes during the dry season coupled with high level of food in the water as a result of high primary productivity (phytoplankton), can be responsible for the high populations of zooplankton. In the dry season the zooplankton population appeared to have great stability and in the rainy season the population is suspected to lack stability. This may depend on the residence time of water and on the abrupt water change which occur frequently during the rainy season[60]. The increased turbidity of the flood (wet season) destroys the periphytic algae and causes a decline in the amount of phytoplankton[61,62] which in turn reduces the standing crop of the herbivorous zooplankton fish food[63]. Zooplankton is restricted to aerate upper water layers and littoral regions of the water during dry season[64, 65].

4.3. Macro-Invertebrates

The gradual decrease of current velocity was evident from upstream to the middle reaches, then decrease abruptly to the downstream reaches where the velocity was least during the period of study. According to findings[66], flow velocity influences the type of river bed, amount of silt deposition, which in turn affects macro-invertebrate abundance. The significant differences in population densities of macro- faunal species between reaches of the waterfalls may therefore be due, in part, to differences in composition of the substratum which was muddy and silt downstream with sandy and rocky upstream and midstream reaches. The larval forms of most deposit-feeding macro-benthic fauna require fine sediments in which to burrow. The greater quantity of organic matter present in downstream reaches[67], is probably instrumental to decreasing the compartment of the sediments, thereby facilitating the burrowing of larval forms of decapods and polychaetes which formed dominant macro-fauna downstream. Another reason for higher density of macro-invertebrates downstream could be due to large surface area of the reaches which expose the site to air and sunlight as well as abundance of organic debris which favor the photosynthetic activity of aquatic plants[68]. The macro-invertebrate assemblages downstream, is striking in the fact that the decapods crustaceans dominated and were present in high numbers. Research findings had earlier implicated the presence of decapods to relatively unpolluted segment of the river since they are not tolerant of polluted water[69]. On the other hand upstream reaches where the velocity was rather high, the macro-invertebrate community was dominated by Odonata and Ephemeroptera (Baetissp) which according to findings[70,62], may be high flow adopted invertebrates. Midstream reaches had consistently lower density of invertebrates than other reaches due to higher water discharge rate which permanently suspended particles in water and in turn reduced to minimum invertebrate density in the region. The higher density of macro- invertebrates during the dry season could be as a result of the unstable nature of the substrate during the wet season arising from inputs of storm water[63-65].

4.4. Fish Species

Seasonal differentiation evident in higher number of species and individuals caught during wet months of the study period, agree with other results which described larger ichthyofaunal densities in water bodies in Grahamstown in the rainy season[66-68]. Reasons for the variation were ascribed to the connection of the water bodies to the sea which allows free movement of species across the two habitats during flood and these species being able to recruit during flood condition. Agbokim Waterfalls being drained by two small rivers, Ekue and Bakue, which are tributaries of the Cross River system with linkage to Cross River estuary from the lower reaches, could have exhibited the same variation. Also because of the considerable seasonal differences in dissolved oxygen concentration in the system, both at low water and during the floods, this factor appears to have played an essential role in determining the distribution of fish within the system. In general the more active the species the more it tends to avoid de-oxygenated areas[69]. Most species encountered downstream during dry season have adaptation for survival in low dissolved oxygen conditions like presence of external gills (Clarias, Heterobranchus). These species form a group that is well adapted to swamps life and tend to concentrate in the more de-oxygenated small pools and swamps of the floodplain during low water, when other more active species like Tilapia and carp are to be found in the mid- stream (water fall) and upstream respectively.

Although on a community basis the three reaches did not separate out, some species when analysed individually revealed specific range preferences. Two heterologous species between two genera, C. gariepinus and L. coubie, the two most common and economically viable benthic fishes found in this study, demonstrated opposite habitat preference, with C. gariepinus dominating the lower reach and L. coubie more abundant in the upper reach. This opposing habitat preference and the attendant ecological and trophic heterogeneity reduce competition, and may be responsible for the overwhelming success of the two species in the Agbokum waterfalls. The relatively higher number of C. gariepinus in the lower reach during this study may be due to the fact that being mud-dwelling species, most individuals found their natural habitat downstream. The common carp, L. coubie adapted to live in shallow rocky bottom, assemble in the rocky upstream. Another dominant freshwater species (T. zilli) demonstrated great preference to the turbulent well aerated midstream reaches. This distributional trend, in a similar study[70] was attributed to several factors including fast water current, suitable breeding area, marginal vegetation and the absence of competitors and piscivorous predators.

4.5. Vegetation

Quantitative difference in numerical abundance of vegetation types between reaches, in this study, implies variation in control on a very local scale[71]. In the present study, experimental field study had been used to assess the impact of waterfall disturbance on marginal vegetation population in the different reaches of the waterfalls. The assessment of the plant size was difficult because individuals vary greatly in the proportion of their stem. Although there was general deterioration in the numerical abundance in the middle reaches of the waterfalls, Azolla africana, and probably Nymphia lotus and Commelina sp were endemic forming permanent marginal vegetation along all stretches of the midstream. The distribution pattern and composition of the vegetation change in a non-random way during the study period with Raphia vinifera, Havea brasiliensis, Grewia sp and Cocos nucifera shrinking in size from 34% to 8% at the midstream stretches of the river while the nymphia- rich vegetation becomes more frequent than expected. Nymphia has become a biological invader and has exhibited a full range of impact and abundance in this disturbed midstream reaches.

4.6. Diversity

Shannon Weaner diversity function for samples of phytoplankton, zooplankton and macro-invertebrates studied, reveal that dry season samples of these species were more diversified and stable. On the other hand fish and vegetation appear to be more stable during the wet season considering the higher richness and diversity during this period. However, communities in the waterfall region (midstream) of the river may be unstable throughout the year as reflected in their low values of richness and diversity during both seasons. If the integrity of such waterfalls in the tropics are protected, they will support growth and survival of organisms, throughout the year.

4.7. Physico-chemical Properties

The distribution and abundance of macro-invertebrates, as the results show, were probably affected by the physico-chemistry of the waterfalls, such as dissolved oxygen, water velocity, water discharge and water depth. Pearson’s correlation analysis showed that some of these parameters correlated significantly with some species of Chlorophytes, Cyanophyceae, rotifers, cladocerans, cichlids, clariids, cyprinids, decapods, annelids, azolla, Nympha and Commelina. The high correlation that occurred between these groups of organisms and physico-chemical properties of the water suggest that the physic-chemical parameters influenced the distribution and abundance of organisms as already reported[71,72].

5. Conclusions

Waterfalls can also be important for fisheries and biodiversity conservation, as rich biome for indigenous and exotic species which are swept upstream and over the waterfalls, settle downstream in a more stable environment. The waterfalls reaches are not completely lifeless as assumed by many researchers in Africa, but harbor some turbulent tolerant macro-biota. Among the plankton community, waterfalls contributed significant percentage of Trachelomonas sp., Scenedesmus denticulatus, Chlorella ellipsoidea and Nostoc lobatus. Labeo coubie and Tilapia zilli were the fish species that were most abundant in the waterfall region. Nymphia vegetation has become a biological invader and has exhibited a full range of impact and abundance in this disturbed midstream reaches. The disturbance regimes of the midstream reaches of Agbokum waterfalls combined with its very low faunal and floral diversity has made the environment unstable therefore susceptible to the invasion of disturbance tolerant biota.

ACKNOWLEDGEMENTS

The authors wish to thank the Management of Cross River University of Technology for the financial support and the use of the laboratory facilities. We also thank the Vice Chancellor Prof. Enebong E. Enebong and management of the University for the encouragement.