Wetlands are areas where water is the primary determining factor in the environment for plant and associated animal life ([25]). Wetlands are sometimes referred to as unique environments because they constitute entities distinct from terrestrial and aquatic ecosystems ([5]; [20]). They act like real sponges that release water just as they absorb it ([11]). Previous studies demonstrate that, in most regions of the world, wetlands are undergoing a continuous decline in their area and quality consequently, the ecosystem services they provide to populations are compromised ([16]). Population growth, the rise of industrialization and the transformation of human society related to the development of agriculture with the increasing use of phytosanitary products (chemical fertilizers and pesticides) are factors that have favored the pollution of aquatic ecosystems ([23]). In Cameroon, as in most developing countries, watercourses in the urban area are subject to untreated pollutant discharges such as domestic wastewater and industrial effluents. Soap effluents which degrade the quality of water while modifying the biodiversity of the ecosystems which constitute part of these pollutants ([4]; [24]). Human activities are therefore the major cause of ecosystem degradation ([16]; [14]). Watercourses are thus subject to physical and chemical disturbances which are at the origin of an increasing degradation of water quality ([4]). Soap factories generally produce small quantities of wastewater that cause local impacts in the surrounding waters. Indeed, soap factories in Cameroon are industrial installations which produce a wide range of hygiene products including soaps, detergents and cosmetics. These products require the use of chemical substances such as surfactants, bleaching agents and perfumes ([14]), which can affect aquatic environments and the health of populations living near the soap factories. The release of these chemicals into water has harmful consequences on flora and fauna, particularly aquatic macrophytes ([17]). This is the case of the soap factories in the localities of Bonaberi (Douala IV) and Bomono Ba Mbengue (in the district of Dibombari) located respectively near and at a considerable distance from a swamp (508 meters for Bomono marsh and Bonaberi marsh, which is only 113 meters). These swamps receive soap effluents which could affect the distribution and composition of the aquatic macrophyte communities found there and consequently, an intensification of eutrophication. Indeed, plants participate in the biotic and abiotic phenomena which take place in ecosystems and appear as visible markers of environmental disturbances ([22]; [20]). They can only partially escape an environment which would be unfavourable to them by “dispersing in space and time” according to [9]. The main role of macrophytes in wetlands is to provide oxygen and surfaces for the growth of biofilms, as well as to use nutrients from polluted water ([19]; [11]). Thus, plants are considered as one of the biological elements to be analyzed in order to assess the ecological state of waterways ([16]). Indeed, the city of Douala, the economic capital of Cameroon, is facing many environmental challenges related to rapid industrialization and increasing urbanization. Among these challenges, the pollution of surface water by industrial effluents, particularly those from soap factories, represents a major concern for the preservation of aquatic biodiversity. Aquatic macrophytes, biological indicators of the quality of aquatic ecosystems, are particularly sensitive to these environmental changes ([23]). The following hypothesis will be verified: swamps located near soap factories have high physico-chemical characteristics and a low distribution and abundance of macrophytes compared to those far from soap factories. The objective of this study is to evaluate the physico-chemical characteristics of the waters of two marshes receiving wastewater from soap factories as well as the distribution of their macrophytes in the Districts of Bonaberi and Dibombari.

Physicochemical characteristics of water

The analyses of the physico-chemical parameters highlighted significant differences between the two sites. Indeed, the pH, salinity and conductivity values obtained at Bonaberi are very high (respectively 11.62 ± 1.17, 591.67 ± 123.56 g/L and 1283.67 ± 214.43 μS/cm) compared to the standard which is respectively 6.5 - 8.5 ([26]; [7]; [24]), 200g/L and 1200 μS/cm ([13]). This shows that the waters of the Bonaberi marshes are unusable for agriculture because a high electrical conductivity makes water considered unusable in irrigated areas because it has an impact on the development of organisms and on chemical balances, depending on the affinity of each organism with the concentration of ions ([23]). Comparing the values of electrical conductivity and salinity in the analyzed wastewater with the standards for the quality of water intended for irrigation allows us to deduce that the waters of the Bomono marshes would be suitable for watering market garden crops which are most often grown along the watercourse. The pH value observed in Bomono would be linked to the strong presence of fixed macrophytes. Indeed, the humic acid which comes from the decomposition of dead macrophytes would contribute to lowering the pH ([2]). [10] reported that when the pH is high, ammoniacal nitrogen ( NH 4 + ) becomes toxic while for a pH below 8, ammoniacal nitrogen has little influence on the flora. There is no significant difference between the temperatures of the soap effluents of Bomono Ba Mbengue (25.53˚C ± 0.57˚C) and those of Bonaberi (24.3 ± 3.39). These values are very close to the ambient temperature, this could be explained by the close connection between the temperature of the water and that of the ambient environment as well as possible discharges of waste water as is the case with waste water from soap factories. These results are similar to those of [13] who found temperatures between 29˚C and 32˚C in surface water and groundwater at the level of two continental wetlands in Mauritania. The ANOVA test performed shows that there is a significant difference between the values of NO 3 − (3543.60 ± 2834.85 mg/L), PO 4 3 − (11.85 ± 11.92 mg/L), BOD5 (45.00 mg/L ± 22.91) and Fe²⁺ (24.07 ± 15.72 mg/L) of the Bonaberi marshes and those obtained at Bomono Ba Mbengue which are respectively 8.53 ± 0.70 mg/L; 1.26 ± 1.13 mg/L; 16.67 ± 2.87 mg/L; 1.11 ± 0.21 mg/L. The values of nitrates, phosphorus and BOD5 obtained at Bonaberi are higher than those found at Bomono and also higher than the threshold set by [26] which are respectively: nitrate < 1 mg/l, 0.03 mg/l of phosphate and <30 mg/l of O₂. This result indicates that the waters studied are subject to a risk of pollution by nitrates, and this is much more accentuated at Bonaberi. The dissolved phosphate level in both sites is above the threshold of 0.03 mg/l, which would indicate eutrophication of these environments but much more accelerated in the Bonaberi marshes which directly receives wastewater from soap factories as well as that of nearby households. The practice of agriculture with the use of chemical fertilizers (market gardening and food crops) in the surrounding area should also constitute a significant source of phosphate transported into these various marshes by runoff water ([24]). This also shows that most of the PO 4 2 − measured in these marshes would have an exogenous origin. The high values of the BOD5 content in Bonaberi compared to Bomono constitute a direct indicator of the presence of a significant quantity of biodegradable organic matter in the water leading to excessive consumption of oxygen by microorganisms for its decomposition. Organic pollution therefore leads to eutrophication ([25]). The high iron values in Bonaberi compared to Bomono also indicate pollution due to soap effluents which are directly drained into these marshes compared to those of Bomono, it can also be said that the Bonaberi marsh is subject to pollution by nitrates and phosphates. Analyses of the physico-chemical parameters obtained in the two marshes show that the waters of the Bomono marshes are less polluted compared to those of Bonaberi. This notable difference can be attributed to the distance of the Bomono Ba mbengue marshes, located at a significant distance from the soap factory (508 meters), unlike the Bonaberi marsh, which is only 113 meters from the latter including the households located nearby.

Floristic diversity of marshes

The results of the floristic richness observed within the analyzed marshes show a total of 65 identified species. The Acanthaceae and Poaceae families are particularly distinguished by their high representation, testifying to a specific diversity that is both rich and varied. The accumulation curves by the rarefaction method of the marshes of Bomono Ba Mbengue and Bonaberi tend towards practically a plateau. This suggests that the specific richness seems to be total in these two types of marshes, also showing that the sampling is approaching a state of satisfaction. The accumulation curves of the two marshes indicate that these two sites are very diverse because the more we sample, the more new species we collect. The Shannon, Pielou and Simpson diversity indices reveal a particularly high floristic richness in the Bomono Ba Mbengue swamps compared to that of Bonaberi, thus testifying to a site that is not only distinguished by its number of species, but also by a harmonious balance in the relative abundance of the different species present. These results reinforce the results of [16] as well as [9], who had observed a similar gradient in the distribution of macrophytes along the Kambo and Kondi rivers. On the other hand, the Sorensen index, which displays a value of 27%, indicates a low similarity between the two sites, suggesting that the effluents from the activities of the Bonaberi soap factory could have a significant impact on the biodiversity of the marsh. This very low value (<50%), underlines that the floristic funds are distinct at these different water points ([16]). This diversity would be largely attributed to their capacity for adaptation and propagation, influenced by various climatic factors and by the dispersion caused by both water currents and human activity ([5]). In addition, previous research has highlighted that Poaceae are among the plants most tolerant to pollution, particularly that caused by heavy metals ([24]). In particular, the Bomono Ba Mbengue swamp stands out with 40 species, surpassing Bonaberi, which has only 33 species. The work of [16] indicates that the most polluted sites generally harbor the fewest macrophytes. The results of the floristic inventories show that certain species are sensitive to water pollution and stand out for their frequency of appearance and their abundance-dominance in the different swamps studied. These species can be described as indicator species of pollution of the swamps in the study area. Thus, the most abundant species in Bomono Ba Mbengue are: Costus aferand Cyrtosperma senegalensisand a predominance of species such as, Costus afer(100%), Asystasia gangetica(100%), Cyrtosperma senegalensis(100%), Commelina benghalensisand Nymphaea alba (87%), Nephrolepis bisserataand Oplismenus hirtellus(73%) which undoubtedly dominate the plant landscape, while in Bonaberi, species such as Dianthera secundaand Onoclonea struthiopterisalso show a strong presence, although their recovery values are lower. There is also a predominance of species such as Panicum virgatum,Ipomoea aquaticaand Vachellia farnesiana(87%), Commelina benghalensis,Commelina diffusa(80%) and Alternanthera sessilis(73%). The Bomono Ba Mbengue marsh is distinguished from that of Bonaberi by its floristic diversity. Many studies have shown that the Poaceae family is among the families most tolerant to pollution, particularly by heavy metals ([1]). These results corroborate those of [24] and those of [18] who also noted that the dominant species of the floristic inventory belong mainly to the Commelinaceae family and constitute an indicator of pollution of aquatic environments. Similarly, [14] by characterizing the impact of agri-food industries on receiving environments find that Poaceae and Commelinaceae appear as two families most represented in the fermenteries of Cameroon located in the city of Douala.

Principal component analysis of the different physicochemical variables in the Bomono Ba Mbengue and Bonaberi marshes reveals that the parameters are highly correlated. The strong correlation may be due to common water pollution from various sources but also to interdependent physicochemical interactions between the parameters themselves. Indeed, organic pollution that affects BOD5 and nutrient inputs such as nitrates and phosphates most often comes from identical human activities ([13]). Thus, temperature and high pH directly influence the biodegradation (BOD5) of organic matter present in the water, while conductivity reflects the presence of dissolved ions from practically the same pollution sources ([25]) such as wastewater from soap factories, surrounding households and agricultural runoff. These wastewaters can also jointly introduce salts and heavy metals such as iron into the water and the higher the salinity in the water the more conductive it is ([13]). All these physicochemical parameters are strongly correlated with the Bonaberi marsh showing that this site is highly polluted compared to that of Bomono Mba Mbengue. Furthermore, the specific richness, as well as the diversity indices (Shannon, Pielou, Simpson) are strongly correlated with the Bomono Mba Mbengue site showing that it is not only rich in species but also very diverse in terms of distribution of individuals and therefore less polluted. These results show that the physicochemical parameters the distribution of macrophytes that the absence or presence of a species in a swamp is inherent to the composition of the environment in nutrients ([23]). This study was conducted only during the rainy season, which differs from the work of [24] and [25] who worked in both the rainy and dry seasons.

The analyses of this study show that soap effluents, wastewater from surrounding households and agricultural runoff contain large quantities of nitrates, phosphorus, salts and heavy metals which are found in the different swamps, but much higher in the Bonaberi marsh compared to that of Bomono Ba Mbengue. High temperatures and pH directly influence the biodegradation (BOD5) of organic matter present in the water. This suggests that these effluents should not be discharged into aquatic environments without treatment. Similarly, the distance between swamps and sources of pollution influences the degree of nutrients introduced into the marsh. Furthermore, diversity indices are strongly correlated with the Bomono Mba Mbengue site showing that it is not only rich in species but also very diverse in terms of individual distribution and therefore less polluted. Therefore, these wetlands remain highly vulnerable to pollution and degradation, requiring urgent and clear-sighted integrated management, protection and conservation measures.

We sincerely thank the Wastewater Analysis Laboratories of the University of Yaounde I, the Plant biology and Physiology laboratory of the University of Douala for the various analyses carried out as part of this work. We also thank the chiefs of the localities of Bomono Ba Mbengue and Bonaberi for who facilitated our fieldwork by providing vital information about the Marshes.