Freshwater environments represent complex and dynamic ecosystems. They play essential roles in biodiversity conservation, organismal functioning, and the cycling of organic matter and nutrients, as well as in energy transfer [1]. However, they are exposed to anthropogenic pressures related to the exploitation of mineral, agricultural, and other resources, resulting in significant changes to the structure of the plant and animal communities that inhabit them, including benthic macroinvertebrates. Threats to the extinction of species and ecosystems have been observed, more alarming than ever in recent decades [2], to the point where intensive human exploitation of natural resources has led to the depletion, or even near-disappearance, of some of them. Nutrient accumulation in waterways is one of the most widespread ecological problems, responsible for freshwater pollution worldwide [3].

At the same time, the silting up of waterways by natural phenomena and/or human activity has serious consequences for their proper functioning, namely the decline of aquatic species, the considerable reduction of spawning grounds, and the significant deterioration of the biological functioning of aquatic invertebrates [4]. It alters the physical habitat of waterways. Formerly biogenic, habitats experience the accumulation of a layer of sand, degrading their potential. Siltation also leads to a decrease in the river’s capacity to support aquatic fauna, and the services it provides are also impaired. Similar to the decrease in the river’s self-purification capacity, which is directly linked to the quality of water for human consumption [4]. Moreover, excessive siltation or over-siltation, due to urbanization, runoff, agriculture, livestock farming, and reduced morphogenic flows [5][6], is often considered an indicator of poor biological quality of the substrate [7] and can affect all aquatic communities, cause the clogging of interstices between coarse sediments, leading to a decrease in oxygenation of the hyporheic zone and threatening benthic macroinvertebrates [8]. Consequently, these profound changes deteriorate the integrity of these ecosystems, which are relatively homogeneous functional units in which a collection of living organisms interact with each other and their environment. Ecological integrity refers to the state of an ecosystem whose structure and functions are not altered by stresses attributable to human activity, and which retains its capacity to adapt. The closer an ecosystem is to this condition, the more it is considered to be intact.

Thus, ecological studies are of paramount importance in the management of natural systems on the one hand, and in the assessment of the ecological health of aquatic systems on the other hand [1]. This explains the valued use of aquatic macroinvertebrates in assessing the health of aquatic ecosystems, as they offer numerous advantages due to their great diversity and variable tolerance to pollution and habitat degradation, as well as their sedentary nature [9]. They are excellent bioindicators and reflect the ecological status of surface waters, reacting very quickly to changes in their environment [10], through the accumulation of pollutants and environmental disturbances over relatively long periods, from a few months to several years [11].

Consequently, several indices have been established [12], including indices of benthic macroinvertebrate tolerance to pollution, among which are the normalized global biological index (IBGN) and the Hilsenhofff family biotic index [13]-[15].

In Congo, as elsewhere, certain aquatic ecosystems are experiencing intense anthropogenic pressures, and the Mfilou River is no exception. However, the assessment of the level of degradation of these ecosystems, specifically the assessment of pollution, has so far been carried out at the national level only through a physicochemical approach. Therefore, we undertook a preliminary study focusing on the biological assessment of water pollution in the Mfilou River, using benthic macroinvertebrates. Previous studies [16] have shown that the waters of the Mfilou River are devoid of pollution-sensitive taxa (Mayflies, Stoneflies, and Trichoptera). The aim of this study is to highlight the different levels of pollution in the Mfilou River using a biological approach, specifically through benthic macroinvertebrates. It should be noted that the type and severity of pollution were determined from biological indices, without concomitant physico-chemical validation.

Taxonomic diversity

The benthic macroinvertebrates of Mfilou comprise 129 taxa belonging to 62 families, 16 orders, 6 classes, and 4 phyla. They are unevenly distributed across the different stations, with the Diata station being the richest in taxa. Despite the observed anthropogenic pressure (market gardening, household and industrial wastewater discharges, siltation, and even over-siltation, etc.), this taxonomic richness remains high. This situation is similar to the observations made by [36], who showed that taxonomic richness is high in the most heterogeneous areas of the foothills and lower mountains (200 to 500 m altitude). Conversely, our results contrast with those obtained by [37], who identified 22 taxa belonging to 18 families, 8 orders, 3 classes, and 2 phyla among 365 specimens collected in a reserve where increased human pressure was observed. However, the high taxonomic richness of benthic macroinvertebrates observed in the Mfilou River is not synonymous with good hydrological quality of its waters. Indeed, the benthic macroinvertebrates community of the Mfilou River is composed of only 57.21% insects. The absence of Trichoptera and Plecoptera, and the near-total absence of Ephemeroptera (only 2 taxa, representing 3.23% of the taxonomic richness, were identified), clearly demonstrate the severe degradation of this river [16]. In fact, the individuals in this group are pollution-sensitive. It should be noted that insects are an indicator group of the ecological integrity of any ecosystem, due to their plasticity, and that they represent the most important animal group in terms of taxonomic richness, far exceeding that of all other groups present, even combined, in the same ecosystem. because they represent nearly 95% of the organisms present in the environment [38][39]. Among the 129 taxa listed, the Diptera Chironomus sp. (Chironomidae), the Neotaenioglossa Melonia tuberculata (Thiaridae), and the Haplotaxida Naidiidae are the most numerous groups. Some authors, including [40], attest that these taxa, which colonize all environments, are ubiquitous and are often present in very large numbers.

Chironomidae Index

Often considered indicators of organic and/or metallic pollution, in this study, Chironomidae were also used to investigate other types of pollution, such as sediment-related pollution. The approach involved calculating the Chironomidae index, the values of which allow for the identification of the degree of sediment pollution at different sampling stations along a watercourse [26]. Thus, the Chironomidae index values obtained at the five study stations show that the waters present a dual diagnosis: clean water at the Egout Laiterie, Diata, Mâ Campagne, and Confluence stations, and moderate pollution at the Zone Maraîchère station. This station is characterized by a strong dominance of sediments, specifically sand-silt, which represented 87% of the substrate at this station during the study period. This could explain the overwhelming dominance of Chironomus larvae (Chironomidae, Chironominae, Chironomini) at this site, where they represent nearly 98.00% of the sample size and 22.15% of the specimens collected in the Mfilou River. Indeed, it is likely that they prefer soft, homogeneous sediments for building their tubes. [41] support this hypothesis, as the sedentarization of the larvae ensures good representation in their environment, since they are in contact with the sediments throughout their larval (2 to 4) and pupal stages. This preference would explain their proliferation at the Zone maraîchère statiton [42].

Normalized Global Biological Index (IBGN)

The values of the Normalized Global Biological Index allow the waters of the Mfilou River to be grouped into two quality classes. Class 2, characteristic of waters of average hydrological quality, reflects significant organic pollution at the Zone maraîchère, Diata, Mâ Campagne, and Confluence stations, where IBGN scores range from 9/20 to 11/20. The other class is designated as Class 3, characteristic of waters of poor hydrological quality, indicating high organic pollution at the Egout Laiterie station, where the IBGN score is 7/20. These results show a low number of taxa at the Egout Laiterie station due to more challenging environmental conditions, including high levels of sand-silt and suspended solids (SS). The indicator faunal groups are Chironomus sp. and Melanoidestuberculata. This high taxonomic richness, present across all stations and ranging from 17 to 34 taxa, is strongly dominated by Melanoidestubercluta (67.73%) and Chironomus sp. (22.15%).

HilsenhoffFamily Biotic Index (FBI)

The Hilsenhoff Family Biotic Index (FBI) used in this study incorporates all taxa present in the benthic macroinvertebrates community to demonstrate the level of degradation experienced by the ecosystem under study [13]. Thus, the FBI values obtained for the waters of the Mfilou River during this study highlighted the existence of three distinct hydrological qualities, including (i) very poor water quality, indicating severe organic pollution in the Egout laiterie and Zone Maraîchère stations; (ii) The water quality at the Diata station is rather poor, indicating substantial organic pollution, and (iii) the water quality at the Ma Campagne and Confluence stations is poor, illustrating very substantial organic pollution. The waters of the Mfilou River are therefore polluted from upstream to downstream, albeit to varying degrees.

The level of pollution in the Zone maraîchère station clearly illustrates the pressure exerted by human activities and the silting or over-silting due to runoff water on the Mfilou River, which would explain the proliferation of Chironomidae in this station. This situation reveals the nature of the pollution present: sediment-related pollution and organic pollution. The latter was evidenced by the presence of taxa that are highly tolerant to pollution, such as Diptera, Annelida, and Gastropod Molluscs. Thus, [43] state that the biodiversity of aquatic insect communities in a given ecosystem often reflects its ecological state; sensitive species colonizing these habitats due to unfavorable environmental conditions are gradually eliminated, while tolerant species establish colonies and thrive. Our results are consistent with observations made by [32] in the Oued El Abiod (Eastern Algeria) and [44] in the Kahuwa River (in Democratique Republic of Congo), which are of poor quality, evidence of severe pollution in downstream stations where human activity is significant. The same is true for the studies by [45], who observed significant pollution at the station crossed by the pipelines, and for those by [37], who found substantial, very substantial, and severe organic pollution at water stations in the Upper Bandama Fauna and Flora Reserve (RFFHB), located in the central part of northern Côte d’Ivoire, where hunting, non-timber forest product harvesting, fishing, mining, livestock farming, and agriculture were recorded.

The biological assessment of water pollution in the Mfilou River using the IBGN and FBI revealed varied trends. The observed discrepancies could be explained by the difference in quality class levels and categorization systems: 5 levels for the IBGN classes versus 7 for the FBI classes [32]. However, both indices highlight a worrying situation for the waters of the Mfilou River and establish that they are unfit for human consumption.

This publication aims to highlight the different levels of alteration in the integrity of the Mfilou River using a biological approach, specifically benthic macroinvertebrates.

Despite threats of all kinds (siltation of the main channel, market gardening, wastewater discharge, etc.), the waters of the Mfilou River maintain a high level of benthic macroinvertebrate diversity: 129 taxa were identified, unevenly distributed among 62 families belonging to 16 orders, 6 classes, and 4 phyla, out of a total of 17,325 collected individuals. Melanoidestuberculaand Chironomus sp. were the two dominant taxa in the studied benthic communities.

The biological assessment of pollution in the Mfilou River, based on the collected benthic macroinvertebrates, revealed pollution related to sediments (sand-silt) at the Zone maraîchère station and organic pollution from upstream to downstream of the studied river. The scores obtained from the Normalized Global Biological Index (IBGN) highlighted two levels of organic pollution: i) significant organic pollution at the Zone maraîchère, Diata, Mâ Campagne, and Confluence stations, and ii) severe organic pollution at the Egout laiterie station. Molluscs and Baetidae (Mayflies, Insects) are the indicator faunal groups for this river. The Hilsenhoff index (Biological Family Index or FBI) revealed three degrees of organic pollution: i) severe organic pollution at the Egout laiterie and Zone maraîchère stations; ii) substantial organic pollution at the Diata station; and (iii) very substantial organic pollution at the Ma Campagne and Confluence stations. The waters of the Mfilou River are all polluted and therefore unfit for direct or indirect human consumption.