This study was conducted in the Mouanko district located in the Department of Sanaga-Maritime, Littoral Region, Cameroon (Figure 1). The climate of this area is of the coastal tropical type with two seasons: the rainy season from March to October and the dry season from November to February [17] [18]. The soils are ferralitic and hydromorphic with vegetation dominated by mangroves of the genus Rhizophora [19]. Its hydrography is made up of numerous rivers including the Sanaga, Kwa-kwa and Wouri [20]. In total, 08 sampling stations (named ED1 to ED4 in freshwaters influenced zones and ES1 to ES4 in marinewater influenced zone) were selected. These stations were selected based on criteria such as wide area coverage, accessibility, and the presence of potential pollution sources.

Samples for physico-chemical and biological analysis were taken monthly in the 08 study stations for 12 months. Concerning physico-chemical analysis, the sampling were done using double-capped polyethylene bottles (250 and 1000 mL), without bubbles, and then transported to the laboratory in a refrigerated chamber. Concerning biological analysis, Rotifers and Cladocerans were collected at each station by filtering 100 L of water through a 64 µm mesh plankton filter. The filtrate obtained was collected in a 350 mL flask and then fixed and stored in 96˚ alcohol.

Measurements of physico-chemical variables were made both in the field and in the laboratory following the recommendations of [21]. In the field, temperature (˚C), salinity (mg/L), pH (UC) and electrical conductivity (µS/cm) were measured using a water test multiparameter; dissolved oxygen (%) was measured using a Hach HQ 30d oximeter. In the laboratory, suspended solids (mg/L), PO 4 3 − (mg/L), the forms of nitrogen ( NO 2 − , NO 3 − and NH 4 + ) were measured by colorimetry with the spectrophotometer of mark HACH DR/3900.

Regarding biological parameters, samples of Cladocerans and Rotifers previously fixed with 96˚ alcohol, were identified and counted using specific identification keys such as [7] [8] [22] - [29] under a stereoscopic microscope WILD M5 and an optical microscope of mark IVYMEN.

The Organic Pollution Index (OPI) was calculated from the quality classes obtained using the concentrations of three physicochemical parameters, namely, ammonia nitrogen ( NH 4 + ), nitrite ( NO 2 − ), and phosphate ( PO 4 3 − ) [30] to measure the level of pollution in the mangrove waters of Mouanko.

The frequency of occurrence (F) of a species, which represents the ratio of the number of samples where this species is noted to the total number of samples taken, was calculated according to the formula:

F = ( S i / S t ) × 100 (1)

where Si is the number of stations where the taxon i was sampled and St is the total number of stations sampled.

The Shannon-Weaver diversity index (H') was calculated according to the formula:

H ′ = − [ ( n i / N ) log 2 ( n i / N ) ] (2)

where H' represents the specific diversity, (in bits/individual), ni the number of species i, N the total number of individuals. It reflects the diversity of species that make up the community in the environment.

Pielou’s J-index was used to measure the equitability (or equi-partition) of species in the stand. It is obtained by the formula:

J = H ′ / log 2 S (3)

with H' the Shannon-Weaver index, log₂ the logarithm to base 2 and S the species richness. The index J varies from 0 (dominance of a single species) to 1 (even distribution of individuals in the community).

Canonical Correspondence Analysis (CCA) was used to highlight the relationships between physico-chemical variables and zooplankton taxa. These analyses were carried out with the software Past for Windows version 3.

The Kruskal Wallis test varried significantly (p < 0.05) between months for all parameters studied except in the case of nitrite in freshwater influenced zones, suspended solids in marinewater influenced zones and orthophosphate in both environments.

Temperature values in freshwater influenced zones ranged from 25.7˚C (ED1 in September) to 32˚C (ED4 in April) (Figure 2(a)). In marinewater influenced zones, data fluctuated from 25.7˚C (ES1 in July) to 31.3˚C (ES2 in May) (Figure 2(b)). Overall, Mouanko mangrove waters had a mean temperature value of

28.5˚C ± 1.5˚C. The highest values were observed between the months of November to May and the lowest values were recorded between the months of June to October in both sites. Suspended solids (SS) values ranged from 1 mg/L (ED2 in February) to 126 mg/L (ED1 in April) in freshwater influenced zones (Figure 2(c)); while they ranged from 1 mg/L (ES1 in November and February; ES2 in November and October; ES3 in February, April, May and September, ES4 in October) to 44 mg/L (ES1 in July). Overall, Mouanko mangrove waters had a mean suspended solids value of 14.42 ± 16.8 mg/L (Figure 2(d)).

The rate of dissolved oxygen of water varied between 32% (ED1 in November) and 97.6% (ED3 in September) with an average of 72.33% ± 14.7% in freshwater influenced zones (Figure 3(a)). In sites influenced by marinewater, they varied from 36% (ES3 in March) to 88% (ES2 in May) with an average of 66.73 ± 13.42% (Figure 3(b)). In general, the mangrove waters of Mouanko were moderately oxygenated (69.56% ± 14.29%). The pH ranged from 5.85 (ED2 in November) to 9.55 (ED1 in October) in freshwater influenced zones (Figure 3(c)) and from 5.92 (ES1 in November) to 9.37 (ES1 in October) in marinewater influenced sites (Figure 3(d)). The mangrove waters of Mouanko were thus characterized as slightly basic (7.64 ± 0.71 UC).

Water salinity ranged from 0.01‰ (ED1 in July) to 0.703‰ (ED2 in January)

with an average value of up to 0.15‰ in freshwaters influenced zones (Figure 4(a)). In marinewater influenced sites, it ranged from 23.3 mg/L (ES1 in October) to 13,220 mg/L (ES2 in April) with an average of up to 3.87‰ (Figure 4(b)). Spatially, the Mann-Whitney test showed a significant difference in the marinewater influenced sites between station ES1 with stations ES2 (p = 0.000), ES3 (p = 0.01) and ES4 (p = 0.04) while, station ES2 with station ES4 (p = 0.03). The variation curve of electrical conductivity showed a similar trend to that of salinity, varying from 21 µS/cm (ED1 in July) to 97.6 µS/cm (ED3 in September) with an average value of 142.58 ± 253.62 µS/cm in freshwater influenced sites (Figure 4(c)) and from 51 µS/cm (ES1 in October) to 23,300 µS/cm (ES2 in April) in marinewater influenced sites (Figure 4(d)). Spatially, the Mann-Whitney test showed a significant difference in the marinewater influenced sites between station ES1 with stations ES2 (p = 0.000), ES3 (p = 0.02) and ES4 (p = 0.04) and station ES2 with station ES4 (p = 0.052).

Recorded PO 4 3 − concentrations ranged from 0 mg/L (ED2 in February and ED3 in December) to 0.84 mg/L (ED4 in October) in freshwater influenced sites (Figure 5(a)). In contrast, they ranged from 0 mg/L (ES1 in March, July and August; ES2 in November and February; ES3 in November and September) to

0.91 mg/L (ES3 in October) in marinewater influenced zones (Figure 5(b)). Overall, the mangrove waters of Mouanko had on average PO 4 3 − values of 0.04 ± 0.12 mg/L.

For NH 4 + , freshwater concentrations ranged from 0 mg/L (ED2 in February) to 0.76 mg/L (ED2 in September) (Figure 6(a)). In marinewater influenced sites, they ranged from 0 mg/L (ES1 in February, ES3 in November and April) to 2.14 mg/L (ES2 in November) (Figure 6(b)). Globally, the mangrove waters of Mouanko had an average NH 4 + value of 0.39 ± 0.3 mg/L. Water NO 3 − levels ranged from 0.94 mg/L (ED1 in November) to 14.4 mg/L (ED4 in December) in freshwaters influenced sites (Figure 6(c)) and from 0.84 mg/L (ES1 in November) to 15.2 mg/L (ES4 in February) in marinewater influenced sites (Figure 6(d)). In general, the mangrove waters of Mouanko presented on average, nitrate values of 5.98 ± 3.14 mg/L. NO 2 − concentrations ranged from 0.01 mg/L (ED1 in September; ED2 in May and September; ED3 in April, May and September; ED4 in April and September) to 0.41 (ED2 in July) in freshwater influenced sites (Figure 6(e)). In marinewater influenced sites, they ranged from 0.01 mg/L (ES1 in September and October; ES2 in September; ES3 in August; ES4 in February and October) to 0.35 mg/L (ES3-May) (Figure 6(f)). In all, the mangrove waters of Mouanko presented average nitrate values of 0.06 ± 0.05 mg/L.

Organic Pollution Index (OPI) values ranged from 3 (ED2 in August and October, ED4 in October) to 4.33 (ED1 in March and July; ED2 in January, February, May and September; ED3 in December, September and October) in freshwater influenced sites (Figure 7(a)) whereas, in sites under marinewater influence, they ranged from 2.33 (ED3 in October) to 4.66 (ED3 in May) (Figure 7(b)). Overall, the mangrove waters of Mouanko presented an average OPI value of 3.78 ± 0.4. This index showed no spatially significant difference (p > 0.05).

The mean water temperatures at the two study sites fluctuated around an average of 28.5˚C ± 1.5˚C. These results are in line with those of [31] (28.66˚C ± 0.15˚C and 29.49˚C ± 0.16˚C) in the mangrove waters of Mbiako and Tiko in Cameroon respectively; [32] (29.67˚C ± 0.76˚C) in the mangrove waters of Mouanko and Manoka in Cameroon. These values do not exceed 30˚C, and are therefore in conformity with Cameroonian standards for surface water [33]. The mean suspended solids value (14.42 ± 16.8 mg/L) obtained in these waters are mainly due to mangrove foliage falling, decomposing and leaving debris in the water. This result is comparable to that of [34] who showed that the vegetation in the mangrove contributes to increase the rate of suspended matter in the water. Nevertheless, the very high content observed at station ED1 in April (126 mg/L) could be justified by exogenous inputs from rainwater.

In freshwater influences sites, the dissolved oxygen saturation rate hovered around an average of 72.33% ± 14.7%. This result is close to those generally obtained in forest and peri-urban streams (above 73%) [35] [36]. In contrast, in marinewater influenced sites, this mean value was 66.73% ± 13.42%, close to that obtained by [32] (64.80% ± 7.94%) in the mangrove waters of Mouanko and Manoka in Cameroon. This low oxygenation in marinewater influenced sites of mangroves would be due to the fact that they are constantly subjected to tidal phenomena. In 2004, Villanueva revealed that the arrival of water poor in oxygen in the mangroves by the tide is at the origin of the low oxygenation of the waters of this ecosystem [37]. The average pH value (7.64 ± 0.71 UC) obtained in the mangrove waters during this study is slightly basic. This basicity could be due to inputs of alkaline ions from the sea water to the mangrove water via the tides. Indeed, according to [38], the pH of the marine environment is generally basic (varies between 7.5 and 8.4 UC). However, the high values of this parameter recorded in the month of October (around 9 CU) could be due to the abundance of oyster shells in the waters linked to the intensive fishing of oysters by local residents. According to [39], the high amount of shell material influences the pH through the release of carbonate and the neutralization of H⁺ ions that make the waters basic. The salinity values of the water at the two sites remain largely lower than those recognized for sea water in Cameroon [33] (23‰ and 24‰) throughout the study period. This may be as a result of the estuarine position of the two sites which benefit from high rainfall and freshwater inflows from rivers such as the Sanaga, Kwa-Kwa and Wouri which dilute the salinity of seawater. The low average values of the forms of nitrogen in the water could be linked to the purification capacity of the mangrove areas because they are subject to domestic and agricultural pollution brought by the tides, rivers and run-off water. According to [40], this can be explained by the high bioremediation capacity of these environments. Nonetheless, the nitrite peak observed in June at the ED2 station (0.41 mg/L) could be due to bacterial action, by transforming ammonia from organic waste (leaves and mangrove tree trunks, excreta and animal corpses) observed at this station, produces nitrite. [41] points out that nitrosomonas use ammoniacal elements ( NH 4 + and NH₃) and transform them into nitrite. The orthophosphate values recorded in the mangrove waters of Mouanko are very low (0.04 ± 0.12 mg/L) compared to those obtained in the Kondi stream located in the urban area (1.5 ± 1.43 mg/L) [13]. This low value could be due to the low anthropization of the area as well as the low practice of agricultural activities and uses of phosphorus materials. According to [40], orthophosphates can be contributed by pesticides used in agricultural activities and by domestic detergents.

The moderate level of organic pollution revealed by the IPO in the mangrove waters of Mouanko (3.78 ± 0.4) could be due to the bio-purifying nature of the mangrove. Indeed, mangroves play a role similar to that of filtering plants with regard to forms of nitrogen, orthophosphates, etc. [40].

The low levels of organic pollution indicator parameters (Nitrates, Nitrites, Ammonium and Phosphate), the high average values of dissolved oxygen and the low temperatures of the waters of the Mouanko mangroves show the low anthropic impact of this ecosystem [36] [41] [42] and confer on the waters of these mangroves an ecological quality that is moderately good, with characteristics that are favourable to the development of aquatic communities [43].

During This study, 45 taxa of Cladocerans and Rotifers were collected. This taxonomic richness is largely suprior to that obtained by [44] (0 taxon) in the Sundarbans estuarine system in India. However, it is still lower than the 61 species obtained by [11] in the Upper Bandama Basin in northern Ivory Coast. These differences in taxonomic richness between these different environments could be related to the difference in mesh size of the net used by [44] (100 µm) and [11] (30 µm). They may also be due to the types of environment, physicochemical properties of the water, regional climate to and hydrology of the water bodies [45]. This specific richness was lower marinewater influenced sites (17 taxa) than in fresh-waters influenced sites (40 taxa). This would be due to the high levels of electrical conductivity and salinity recorded in marine environment. According to [46], when the electrical conductivity and salinity of the water increases, the high osmotic pressure created can induce migration or mortality of organisms.

Of the two zooplankton groups studied, Rotifers constitute 66.5% of the total diversity against 33.5% for Cladocera. This high diversity of Rotifers could be explained by their opportunistic trait that aids their resistance to environmental variations. According to [47], Rotifers form a highly diverse and cosmopolitan group in the animal kingdom. They are more adaptable to environmental changes and their short reproductive cycle largely compensates for their low relative fecundity [48]. In addition, they better escape fry grazing due to their small size [47].

Rotifers (67%) were the most abundant in our freshwater influenced stations. This high abundance of rotifers in this environment may well be due the fact that they represent the most important zooplanktonic group in freshwater [49] [50]. Temporally, the high abundance of the population in the months of April and July is justified by the fact that during this period, rainwater carries new nutrients (bacteria and organic detritus) into the mangroves and dilutes the salt water brought by tidal movements, thus making the environment favorable to the development of zooplankton [50]. On the other hand, during the months of November - February (dry season) the rise of the sea water thanks to the movements of the tides, floods the mangroves thus rendering the environment unsuitable for the development of a great number of individuals. Moreover, the low abundance of cladocerans (33%) could be as a result of the strong predation pressure exerted by fry and planktivorous fish. [51], emphasizes in this opinion that planktivorous fish preferentially consume cladocerans, because they are large prey, with reduced mobility, rich in energy and can be found without difficulty. The decrease in abundance of organisms in March is thought to be due to a transition between the dry and wet seasons. During this transition, the change in water quality would lead to a decrease in populations and species richness, but a little less for Rotifers which are more tolerant to environmental variations. This seems to confirm the hypothesis of a seasonal succession, under the influence of predation by fish fry, degrading water quality and competition between species [52]. Among the taxa collected, Peniliaavirostris present only in marinewater influenced areas showed the highest abundance. This marine species would have migrated from the ocean to the mangroves thanks to tidal phenomena. According to [53], it is the only cosmopolitan marine cladocera frequently found in warm coastal waters.

The CCA results show that the variables strongly associated with the distribution of taxa are salinity, electrical conductivity, suspended solids and dissolved oxygen. The first group characterized by high salinity and electrical conductivity presented only 3 taxa. This would mean that the high values of salinity and electrical conductivity would disadvantage the development of continental Cladocera and Rotifers. On the other hand, the 13 taxa obtained in the second group would be due to low values of salinity and conductivity. Similar results have already been observed by [54] in the Ebrié lagoons in Ivory Coast. According to [55], salinity plays a crucial role in the estuarine environment by controlling the composition and diversity of certain organisms.

The strong spatio-temporal fluctuations of the Shannon and Weaver index (H) and of the Pielou equitability coefficient reveal a great instability in the structuring of the communities studied, thus showing the variability of the environmental conditions that prevailed in this ecosystem throughout the study. In addition, the low values of the equitability index recorded at station ES4 might be due to the high relative abundance of the species Peniliaavirostris, which accounts for nearly 80% of the individuals at this station. Indeed, according to [56] [57] the Piélou index is low when one or a few taxa have a very high relative abundance. Similarly, the low diversity obtained in March, May and June is due to the instability of the environment linked to tidal movements that impact the stations by increasing the salinity and electrical conductivity of the water and causing the leaching of zooplanktonic organisms. [58] emphasizes in this regard that floods create conditions of instability that cause the drift of zooplankton populations. In general, salinity and electrical conductivity of the water seem to be the most important ecological parameters in determining the structure of the zooplanktonic community, as has been demonstrated in different estuarine or lagoon environments [59] [60].