1. Introduction
Human activities have greatly affected rivers globally, leading to disruptions in the movement of energy and matter, posing a threat to the survival of various organisms [
1,
2,
3,
4,
5,
6,
7,
8]. Multiple studies have indicated that human activities at the catchment level can be seen in the physical disruption of local habitats of river stations, leading to water pollution that negatively impacts aquatic communities [
9,
10,
11]. Due to their inherent functions, freshwater ecosystems not only rank among the most exploited systems globally but also directly bear the impacts of human activities within their catchments [
12]. Among the world’s freshwater ecosystems, Afrotropical freshwater ecosystems are among the most threatened ecosystems globally despite their unique biodiversity and high endemism [
13,
14]. Rural-to-urban migration across countries within sub-Saharan Africa is leading to a rapid increase in urban growth, resulting in intensified human activities in the catchment areas of forested riverine systems [
1,
15,
16,
17]. Concerted efforts have been made to use biological indicators to better understand and evaluate the health of freshwater ecosystems. Macrophytes, algae, protozoa, macroinvertebrates, and fish are examples of biological indicators that have been explored [
18]. This all-encompassing approach of using biological indicators together with physico-chemical variables enables a thorough assessment of the ecological status, providing a holistic perspective on the dynamics and overall health of freshwater ecosystems.
In Nigeria, basic assessments of stream biota are sometimes used to supplement physico-chemical analysis for the management and monitoring of water quality [
19]. Even while a physico-chemical analysis is widely used, it has limits, especially when it comes to providing detailed information about the state of degradation of water bodies. To track actual changes in freshwater environmental conditions, physico-chemical and biological parameters have been explored as early-warning indicators [
20]. Because of their responsiveness to both man-made and natural pressures, biological indicators, in particular, provide valuable insights. There are numerous biomonitoring techniques and instruments available for tracking how pollution affects riverine ecosystems. These strategies include multivariate approaches, biotic indices, functional feeding groups, and multimetric indices [
17,
21]. Multimetric indices (MMIs), in particular, have proven to be extremely effective, owing to their ability to incorporate data and information from both structural and functional assemblages of organisms and the entire ecosystem in which they inhabit [
21]. The MMIs have been developed and applied to a wide range of biological components, such as phytoplankton [
22], macroinvertebrates [
17,
23,
24,
25], and fish [
26]. There have been a few prior studies in the Afrotropical region, such as those conducted in South Africa [
27,
28], East Africa [
29,
30], and West Africa [
17,
24,
31]. While these studies set the pace for the development of MMIs in a continent like Africa that has been understudied, the emphasis on macroinvertebrate-based multimetric indices at the regional level is distinctive. The incorporation of regional factors ensures a nuanced understanding of the unique characteristics and challenges within Nigeria, contributing valuable insights to the broader context of biomonitoring using MMIs in sub-Saharan Africa.
Multimetric indices (MMIs) are a class of biotic indices that include measures of abundance, composition, richness, and variety, in addition to taxonomic, trait, and functional metrics. The resilience and effectiveness of this all-encompassing method have been observed to outperform that of conventional biomonitoring indicators [
32,
33]. In developing MMIs, two scoring methods have been widely explored, namely the continuous and discrete scoring system [
17,
34]. The continuous scoring system involves the scoring of metrics using numerical values (e.g., 0–1, 0–10, etc.), while the discrete scoring system involves the use of a range of numerical values that have been predetermined (e.g., 1,2,3,4,5, etc.; [
17,
34]). The statistical behaviour of continuous scores is less subjective as there are allowances for fractional values (e.g., 0.5, 1.5, etc.), unlike the discrete scoring system in which arbitrary ranges of scores are used with no allowance for fractional values. Further, for a continuous scoring system, some metric scores may increase with the level of disturbance in particular stations, and in such cases, the metric scores are rescaled by subtracting the said scores from a potential maximum score [
35]. This is not applicable to the discrete scoring system, as there is no allowance for rescaling scores of metrics should there be variation in their projected level of disturbance in stations before the scoring proper [
35]. Finally, due to the standardized nature of the continuous scoring system, biological condition classes (e.g., poor, fair, and good) are easily interpretable by river managers, unlike the discrete scoring system that may need an expert to interpret the biological condition classes if there is any variation in the level of disturbances from the initial projected disturbances [
36]. Several organisms have been used in the past to develop MMIs such as birds, fish, macroinvertebrates, plankton, and macrophytes employing either continuous or discrete scoring systems [
8,
17].
Of the varieties of organisms employed to generate MMIs, macroinvertebrates are particularly noteworthy due to their global focus. Their importance as primary consumers in the food chain and web of aquatic ecosystems, as well as their sampling simplicity and ubiquity, are the reasons for this attention [
31,
37,
38,
39]. In addition, aquatic ecosystems depend broadly on macroinvertebrates, which include a diversity of aquatic insects, mollusks, and crustaceans. As secondary producers, they have a significant impact on the flow of energy up and down the food chain, which affects the general well-being and dynamics of aquatic habitats [
31]. Aquatic macroinvertebrates such as snails, crustaceans, and insect larvae respond differentially to a variety of environmental conditions. They are sessile and sometimes not very mobile, and they live close to the bottom sediments and the water column, making them vulnerable to various stressors disrupting their natural habitat, such as agriculture, urbanization, and deforestation [
40]. Further, the outright removal of forests for agricultural and urban development purposes within the catchments of the freshwater ecosystem, most especially in the Nigerian’s Niger Delta region, calls for serious attention.
The relevance of the Niger Delta region for global biodiversity conservation efforts has been highlighted by its identification on a global scale as a hotspot for biodiversity [
41]. Before now, the bulk of riverine systems in Nigeria’s Niger Delta pass through wooded catchments that are home to mangrove swamps, with white and red mangroves predominating in particular [
24,
42,
43,
44,
45]. That has changed now due to urban and agricultural activities around some of the catchments of the rivers within the region. To this end, it is pertinent to assess the current health states of riverine systems in the region to ascertain the level of degradation the systems have undergone. The majority of the studies conducted to date on evaluating the health of rivers and streams in this region, despite its ecological significance, have concentrated on the composition, diversity, and abundance of the organisms [
19,
42,
43]. Furthermore, the application of multimetric indices is consistent with the current understanding of ecological health as a complex construct influenced by a range of interrelated factors. This is in contrast to the single biotic index methodology [
21,
44], which only considers data obtained from individual organisms. Hence, in this study, we developed a macroinvertebrate-based MMI for selected riverine ecosystems draining both urban and agricultural catchments in the Niger Delta. We also explored the effectiveness of using both continuous and discrete scoring systems in the development of MMI. Finally, we validated the developed MMI with separate datasets to test the applicability of the developed MMI.
In recent times, there have been serious debates on the kind of scoring system used in awarding ecological classes (e.g., fair, poor, moderate, and good) for MMI development. For instance, Ruaro et al., [
34] frowned at the use of a discrete scoring system, stressing that the discrete scoring system does not take into consideration the continuation of scores awarded to metrics during MMI development in a bid to award the appropriate ecological classes to metrics incorporated into MMIs. Also, Edegbene [
46] stressed the preference of continuous scoring over the discrete scoring systems in a study where MMI was developed for selected forested riverine ecosystems in Nigeria; hence, our exploration of the use of both continuous and discrete scoring systems to ascertain the one that is more effective for the development of MMIs. This novel contribution to the science of applied aquatic ecology is crucial due to the limited research on the development of biomonitoring tools using biotic indices in Africa, most especially in Nigeria where the current study was conducted. The study significantly enhances the scarce literature on the development of macroinvertebrate-based MMIs for monitoring riverine systems in Nigeria.
4. Discussion
The selected rivers stations in the Niger Delta region of Nigeria were classified into three impact classes, which include LIS, MIS, and HIS based on physico-chemical characteristics. This classification aimed to assess the diverse levels of impact on water quality related to predominant land-use types, including urban and agricultural land uses. Changes in water quality in the study area are mainly influenced by forestry, agriculture, and urbanization. Stations designated as MIS and HIS showed significantly higher nutrient contents in both urban and urban–agriculture catchments.
A thorough selection process resulted in the retention of only five measures for incorporation into the final MMI–urban–agriculture from a pool of 67 potential macroinvertebrate metrics tested. Following a thorough investigation, 18 of the 67 potential metrics demonstrated effective discriminatory abilities, clearly distinguishing between the LIS, the MIS, and the HIS, confirming their sensitivity. Notably, metrics such as Oligochaete, Chironomidae+Oligochaete, and Diptera/Chironomidae that have been widely reported to be pollution-tolerant were identified as critical contributors to the efficacy of abundance, composition, and richness measurements in determining impact levels [
17,
24,
33]. Chironomidae and Oligochaete composition and abundance have been consistently reported to dwell more in river stations subjected to increased urban and agricultural influences [
33]. These specific family/order of macroinvertebrates, recognized for their tolerance to a variety of contaminants, have increased in quantity or prevalence, acting as dependable indicators of environmental degradation in such environments. As revealed in previous studies e.g., [
19,
33,
65], the links between Oligochaete and Chironomidae and nutrient enrichment have been a focal point in freshwater biomonitoring. Chironomidae genera, such as
Chironomus, can trap oxygen from the air using blood tissues (haemoglobin) and, thus, resist biological enrichment and contamination in water [
65,
66]. Recent findings by Macedo et al. [
67] highlighted the distribution and diversity of Chironomidae in the headwaters of streams within the catchments of a Neotropical savanna affected by hydroelectric power plants, implying that catchment disturbances negatively impact stream functionality, resulting in the distribution and diversity of pollution-tolerant taxa such as Oligochaete and Chironomidae. These organisms thrive in nutrient-rich habitats influenced by anthropogenic activities such as agricultural runoff and urban pollution. The importance of researching the links between Chironomidae and Oligochaetes and nutrient levels stems from their potential use as bioindicators [
66].
This strategy of selecting final measurements based on their sensitivity to water-quality deterioration is consistent with comparable tactics used in studies such as the one conducted by Mereta et al. [
3]. By carefully evaluating the biological preferences and responses of various macroinvertebrate taxa, the chosen metrics become effective instruments for the assessment of the deteriorating impact of water quality on riverine ecosystems. This method improves the precision and reliability of multimetric indices, increasing their utility in biomonitoring and ecological assessments. Numerous research has looked into the ecological implications of these macroinvertebrate taxa in connection to nutrient levels [
33,
44]. The susceptibility of Chironomidae, a family of aquatic insects, and Oligochaete, an order of segmented worms, to nutrient-rich conditions is well known. Their abundance and distribution in aquatic habitats frequently reflect variations in nutrient concentrations, particularly when organic matter and fertilizer inputs are increased.
According to this study, agricultural activities and urbanization have a negative impact on rivers in the Niger Delta. Previous research has shown that industrial, municipal, and agricultural discharges can alter river and stream water quality [
68,
69]. This suggests that agricultural and urban pollution are influencing the composition and function of macroinvertebrates. This influence is visible in the current study, where taxa that are resistant to pollution, such as Chironomidae and Oligochaete, predominate.
4.1. Metric Validation: Applicability and Effectiveness of Continuous Versus Discrete Scoring Systems
The signal/noise test and subsequent removal of noisy metrics showed a methodical approach to improving the MMIs. Because of the noise in this setting, these measurements may exhibit considerable fluctuation or uncertainty, making them less credible indicators for ecological assessment. The validation process for the developed MMI–urban–agriculture, using different datasets, revealed excellent performance in the moderate-water-quality ecological class. The continuous scoring system achieved a 94.12% success rate, while the discrete scoring system achieved 75.47% for the moderate-water-quality ecological class. However, performance in the good- and poor-water-quality ecological classes was less impressive. This pattern highlights the prevalence of moderate pollution levels, indicating that the final multimetric index is particularly effective at assessing and categorizing river systems in urban–agricultural catchments with moderate pollution. The relatively high number of stations classified as having poor water quality raises concerns about the overall suitability of the discrete scoring system in defining the ecological status of river systems in urban–agricultural catchments in the current study area.
The findings support the urban river syndrome, which is characterized by increased nutrient levels, suspended particles, and changes in channel structure and stability in urban rivers [
70,
71,
72,
73]. Recognizing these effects highlights the impact of human activities on water quality in the rivers of the Niger Delta’s urban, urban–forestry, and urban–agriculture catchments. Hence, there is a need for the development of comprehensive tools in this region, like water-chemistry analysis, environmental integrity evaluation, and biological information [
74,
75,
76]. For instance, Stoddard et al. [
54] found that combining natural environmental variations with human-induced pressures introduced additional variability in multimetric indices and could significantly bias ecological assessments. Other research has demonstrated that natural landscape elements influence macroinvertebrate communities in Neotropical Savanna streams [
77], and that these natural influences can have a more pronounced effect on macroinvertebrate responses than human-induced pressures. Nonetheless, for the creation of an MMI, testing the stability of selected metrics with natural seasonal variations is essential [
57]. Some studies on seasonal variability supported the hypothesis that this stage is significant [
78].
There have been a series of debates that continuous scoring is more responsive, applicable, and less subjective than the discrete scoring system [
4,
34,
44,
79], but this study has shown that both scoring systems have almost similar levels of applicability and effectiveness in determining the ecological status of river-draining urban–agriculture catchments. For instance, Blocksom [
79] had opined that although discrete scoring system had an effect on the final multimetric index variability, it could alter the ecological categorisation in a fairly minor rate. This supports the fact that the issues of applicability and effectiveness of the developed multimetric indices are not with the metric scoring system adopted by researchers but with the criteria adopted for reference station selection, size of stations marked for a study, sample size, and metric selection criteria in building multimetric indices [
34,
74]. Conversely, Stoddard et al. [
54] stated that the continuous scoring system should be maintained in order to get metrics that are best performing to be integrated into multimetric indices. Also, consistency and reproducibility are required when deciding on the metric scoring approach to be adopted in developing multimetric indices [
52]. Overall, we suggest that the use of both continuous and discrete scoring systems in developing future multimetric indices to confirm their level of applicability and effectiveness in determining the ecological status of riverine systems draining urban–agriculture catchments and other land-use types around riverine-system catchments across the globe.
4.2. Correlating Physico-Chemical Variables with Integrated Metrics
The RDA findings provide a thorough examination of the correlation between physico-chemical parameters and macroinvertebrate metrics in urban–agricultural river systems. However, at the HIS on Axis 1, BOD, nitrate, and water temperature exhibited strong positive correlations with Oligochaete richness and Chironomidae/Diptera abundance, indicating that these metrics are influenced by pollution indicators associated with higher-impact categories when a station is influenced by a single dominating stressor classification based on abiotic factors, which is a reliable strategy. This was validated by the connection of %Chironomidae+Oligochaeta with water temperature, conductivity, BOD and nitrate, and in our study’s RDA. The inclusion of these measurements is consistent with earlier research that has included similar indications in multimetric indices [
3].
Because of their susceptibility to pollution, the metrics linked to abundance and composition are popular candidates for inclusion in multimetric indices. Several research in the field supports this recognition [
25,
33,
44]. Further, because of their sensitivity to disturbances, particularly pollution, abundance and composition metrics are significant indicators in biomonitoring studies. Findings of similar studies have emphasized the discriminating potential of several diversity indices [
23]. Diversity measures are important in biomonitoring studies because they provide information on the structure and variability of the communities of macroinvertebrates in response to environmental changes.
This study used different measures of diversity like Simpson diversity, Evenness, Shannon–Wiener, and Margalef’s indices, successfully distinguishing between various conditions. Specifically, the Evenness index showed particular sensitivity, indicating its usefulness in distinguishing between different types of impacts. This finding is congruent with previous studies e.g., [
17,
23,
24], which found that several diversity indices have significant discriminatory potential. Edegbene et al. [
17,
24] used the Shannon diversity and Margalef indices for the development of MMI for a Nigerian river, demonstrating the utility of diversity indices as biomonitoring tools for understanding river system ecological health. Furthermore, Aura et al. [
30] incorporated Shannon diversity and Margalef indices into multimetric indices, highlighting the versatility and application of diversity measures in evaluating water-quality status. The sensitivity of these indicators to various human-induced stressors is widely accepted [
80,
81]. They are frequently used in the assessment of wetlands [
82]. The inclusion of Chironomidae and Oligochaeta in the construction of the MMI emphasizes their complementary roles in increasing the index’s sensitivity. The MMI becomes more robust in its ability to reflect and respond to environmental stressors by integrating these two groups, boosting its effectiveness as a bioassessment tool. Numerous studies undertaken in various climatic zones have used these metrics as important components in the construction of MMI [
58,
83,
84]. While many studies have shown that the abundance of Chironomidae increases with habitat degradation [
85], several authors [
86] recommend identifying them at the genus or species level before using them as water-quality indicators. Notably, Chironomidae/Diptera and Oligochaeta have been found as bioindicators in freshwater systems that respond positively to rising anthropogenic activity. This was validated by the connection of %Chironomidae+Oligochaeta with water temperature, conductivity, BOD, and nutrient levels (nitrate) in our study’s Redundancy Analysis (RDA). The inclusion of these measurements is consistent with earlier research that has included similar indications in MMIs [
3,
17]. Diptera and Oligochaete presence have been noted to exhibit a negative correlation with a previously established MMI, as reported by Chowdhury et al. [
86].
Although some tropical studies have shown that metric measures related to functional assemblage (e.g., feeding types or habits) respond differently to increased anthropogenic influences [
87], many authors have recommended using trophic measures for biomonitoring, especially in multimetric systems [
5,
57,
88]. The impact of urbanization and agricultural activities on the quality of river water and biological communities have become major public concern [
10,
89]. This concern is warranted due to the documented negative effects on river biological balance in such areas. This is emphasized in this study, particularly with increasing rural–urban mobility and continuous agricultural activities in the Niger Delta catchments under investigation.