Burkina Faso is a landlocked country located in West Africa with a marked dry season and very variable rainfall [1] . The country’s economy is mainly driven by agriculture, but unfortunately, crop production remains largely rainfed and limited to a growing season of 3 to 5 months from North to South. This reduced temporal window for agricultural activities, associated with population growth and the impacts of climate change, makes access to reliable water sources absolutely crucial for food security, rural livelihoods, and socio-economic development [2] . To face seasonal water deficits, many small reservoirs (over 970) have been constructed in Burkina Faso, with the majority built since the 1970s and serving for irrigation, livestock, fisheries, domestic needs, etc. [3] . These important ecosystems for local communities are becoming more vulnerable to challenges from growing agriculture, settlement, and changes in land use that come with them [4] . As a result, the hydrology changes, the habitat worsens, and the health of the overall ecosystem declines. Despite their significance, the ecological status and biodiversity of many small reservoirs in Burkina Faso remain poorly understood, particularly their benthic macroinvertebrate communities. These organisms are widely recognized as critical indicators of freshwater ecosystem integrity due to their ecological roles in nutrient cycling and organic matter breakdown, as well as their varied sensitivity to environmental stressors and anthropogenic disturbances [5] - [7] . Macroinvertebrate-based assessments offer significant advantages for water quality and ecosystem health monitoring because of their relative ubiquity, cost-effective sampling, and well-documented responses to pollution gradients, habitat alteration, and seasonal fluctuations [8] .

Recent research efforts across Burkina Faso have documented a high richness of benthic macroinvertebrates in rivers and reservoirs, with communities composed predominantly of insects, molluscs, and annelids [5] [9] .

They also demonstrated a strong relationship among environmental variables, seasonal hydrological cycles, land use types, and the composition and abundance of macroinvertebrate assemblages. Taxa known to be sensitive to pollution, such as Orders of Ephemeroptera, Plecoptera, and Trichoptera (EPT), are generally associated with clean and well-oxygenated waters. The tolerant taxa, like Chironomidae, Culicidae, and certain pulmonate snails, are more present in disturbed or eutrophic environments [10] .

Comparable techniques based on macroinvertebrates have been used in other tropical and subtropical regions, demonstrating the global applicability of this method. This is the case of [11] , who used macroinvertebrates to assess the ecological status of the Pinyinyi River in Tanzania during the dry and wet seasons. They showed that domestic water use, livestock grazing, and agricultural practices had a significant impact on macroinvertebrate assemblages and water chemistry. Furthermore, using a modified Biological Monitoring Working Party (BMWP) index, [12] also examined 36 watercourses in the Philippines to identify taxa that are sensitive and tolerant to important water quality variables such as faecal coliforms, BOD, and nitrates. Thus, the use of macroinvertebrates as bioindicators provides comparative frameworks for assessing ecosystem health and water quality in small freshwater systems, such as reservoirs in West Africa.

The reservoirs of Ladwenda and Bidiga, even modest in size, deliver essential ecosystem services including water provisioning, fisheries, and agricultural support for the local population around [2] .

However, activities conducted in reservoir catchments, such as intensive farming, livestock grazing, etc., threaten local biodiversity and alter water quality, resulting in the loss of sensitive taxa [3] . The diversity, assemblage structure, and response of macroinvertebrates to environmental change are not yet well known in Bidiga and Ladwenda reservoirs. To address this gap, the present study evaluates the dynamics of benthic macroinvertebrates in Bidiga and Ladwenda reservoirs. Specifically, it aims to: 1) determine the taxonomic composition of macroinvertebrates in these reservoirs; 2) analyze their richness and abundance dynamics; and 3) determine the influence of key environmental variables on the distribution of macroinvertebrates. By addressing these objectives, this study aspires to provide insights to support improved biomonitoring and inform the sustainable management of Burkina Faso’s increasingly vital small water bodies.

The physico-chemical conditions of the study sites and their variability are similar to those reported for small reservoirs across Burkina Faso and West Africa [3] [4] [22] [23] . The 30 taxa of macroinvertebrate families recorded and distributed across three phyla—Arthropoda, Annelida, and Mollusca—and five classes (Insecta, Malacostraca, Clitellata, Gastropoda, and Bivalvia) are indicative of a relatively diverse benthic community in the studied reservoirs. This significant richness is consistent with reports in other reservoirs in Burkina Faso. For example, in newly impounded and older small reservoirs, 25 to 34 macroinvertebrate taxa are observed, dominated by insects and molluscs [5] [6] .

The predominance of Insecta is characteristic of West African reservoirs and rivers, and this aligns with findings elsewhere in the region [5] [24] . This group generally dominates macroinvertebrate assemblages in tropical standing waters because of their adaptive traits, life cycle strategies, and broad tolerance spectrum [5] [6] . Other key contributors to West African macroinvertebrate diversity are molluscs and annelids. The overall abundance and diversity of these organisms reflect the diverse microhabitats and productivity often observed in tropical ecosystems [4] [6] .

Oligochaetae were found only in Bidiga. They are known for their tolerance to organic enrichment and fine, soft sediments [5] [6] . Their absence in Ladwenda may indicate lower sediment organic matter, different substrate composition, or better oxygenation compared to Bidiga. The absence of Ecnomidae (Trichoptera), Gastropoda, and Bivalvia in Bidiga suggests that environmental conditions are not suitable for these groups. Gastropods and bivalves, for instance, require suitable substrates, stable water levels, and moderate levels of productivity [4] . Their absence might reflect lower calcium concentrations, more variable water levels, or greater disturbance—patterns echoed in studies across West African small water bodies, where mollusc abundance is closely tied to both water chemistry and habitat stability [4] [6] .

In studies of other Burkina Faso reservoirs, differences in macroinvertebrate community composition between sites have been attributed to spatial variability in habitat complexity, water quality (e.g., conductivity, pH, transparency), local catchment land use, and seasonal hydrology [4] - [6] . For example, [5] observed that certain mollusks and annelids were present in some semi-aquatic habitats but absent from others, correlating these patterns with resource availability, predation pressure, and disturbance gradients.

Reservoirs with higher macroinvertebrate family richness and a balance of sensitive and tolerant groups tend to support healthier ecosystems and greater water purification potential [5] [24] . The presence or absence of certain indicator families (e.g., Oligochaetae as pollution-tolerant, some caddisflies as sensitive) can be a useful tool for rapid ecological assessment and may reflect recent environmental changes or anthropogenic pressures [6] [24] . At the regional scale, such findings reinforce the importance of catchment management and the regular monitoring of physical habitats and water quality in sustaining reservoir biodiversity in the face of increasing human and climatic pressures across West Africa [3] [4] .

Seasonal and regional fluctuations in macroinvertebrate diversity are characteristic of small reservoirs and semi-arid water bodies in West Africa and around the world [4] - [6] [23] . In the Bidiga and Ladwenda reservoirs, species richness and the Shannon-Wiener index showed distinct seasonal maxima that coincided with or followed the rainy season. This is consistent with the traditional view that high water levels and post-rainfall intervals induce habitat heterogeneity, pollutant dilution, and an influx of organic matter—elements that facilitate the colonization, reproduction, and dispersal of more diverse macroinvertebrate assemblages [4] [5] [23] [25] . The J-equity index and Simpson’s diversity index showed similar seasonality, with both indices reaching maximum values during the rainy season. High values for the equity and Simpson indices indicate that the community is richer and more balanced, rather than being dominated by a few stress-tolerant species [6] [24] . Low values at the beginning of the dry season or at the onset of rains correspond to periods of ‘bottlenecks’ characterized by reduced water volume, increased disturbance, and the dominance of a few very robust species [4] [5] .

Macroinvertebrate abundance and composition in both reservoirs were highly dynamic. The dominance of Corixidae, followed by Notonectidae, Chironomidae, Baetidae, and Planorbidae, is indicative of sequential resource use and niche replacement, which are classic features of aquatic invertebrate succession in seasonally variable tropical waters [4] [23] [26] . The early and high Corixidae numbers likely reflect their opportunistic colonization in newly flooded, nutrient-enriched post-dry season waters—a pattern also seen in other West African and global reservoirs [5] [23] . The subsequent rise of predators such as Notonectidae and the later establishment of Planorbidae and further diversification as the season progressed align with successional models, where environmental stabilization (increasing water levels, declining disturbance) permits the establishment of less-tolerant taxa [4] [6] [23] . The persistent abundance of Chironomidae in both reservoirs—and especially during stress periods—confirms the genus’s reputation as a resilient, generalist group able to withstand wide changes in oxygen, temperature, and organic content [27] . Baetidae surges followed by abrupt declines hint at windows of optimal water quality or flow, paralleled in other global studies where mayflies are sensitive to even brief lapses in water quality [24] [23] . Bidiga’s bimodal Corixidae pattern, low Chironomidae, and the staggered resurgence of abundance toward the end of the season further demonstrate the interplay between hydrology, water quality, and biological adaptation [5] [25] .

Analysis of temporal beta diversity (Bray-Curtis dissimilarity) quantifies the magnitude of community composition changes over time, revealing periods of disturbance and stabilization. The higher TBI values observed in the Ladwenda and Bidiga reservoirs during the rainy season transitions reflect substantial community turnover driven by new taxa entering as habitat diversity peaks and water quality improves [4] [6] [23] . These findings are mirrored in regional and international studies, which report that rainy season flooding in semi-arid zones causes sharp dissimilarity as new species colonize, migrants arrive, and habitat shifts disrupt resident assemblages [5] [6] [26] . Periods of low beta diversity (post-rainy season and lower dissimilarity in November-December) indicate community stabilization, mirroring the broader “flood pulse” ecological theory [4] [28] .

The clear relationships between environmental gradients and taxonomic composition are strongly supported by the regional and global literature. Multivariate analyses (PCA, CCA) in this study showed conductivity, TDS, and water level as primary structuring forces, with further influences from pH and transparency—a conclusion repeatedly confirmed for tropical and subtropical lentic systems. For instance, in Burkina Faso, [4] and [5] found that ionic load (conductivity/TDS) and water level predict not only the presence of particular families but also their abundance dynamics. Dytiscidae and Hydrophilidae preference for high-conductivity conditions is well documented [4] [29] , while Nepidae and Belostomatidae are known to partition sites along depth and water level axes, reflecting their different respiratory and microhabitat requirements [23] [30] .

The role of pH in segregating Corixidae and Chironomidae echoes the findings of [24] and Odume et al. [27] , who show that certain Hemiptera and Diptera have divergent tolerances to acidification and nutrient pulses. Strong species-environment correlations and permutation-test significance confirm that environmental heterogeneity is the main driver of temporal and spatial community structure in these reservoirs [4] [6] [23] .

The appearance and disappearance of sensitive (EPT) and tolerant (Oligochaetes, Chironomidae) groups at different times underline the power of functional group analysis in biomonitoring. Sensitive mayflies (Baetidae, Caenidae), caddisflies (Hydropsychidae, Ecnomidae), and burrowing mayflies (Polymitarcyidae) are internationally accepted as indicators of high-quality, stable aquatic habitats [23] [24] [31] . Their absence during the dry season or early rains, and increased appearance post-flood when physical and chemical stability improve, echoes studies from Burkina Faso, Ghana, Zimbabwe, and elsewhere [5] [6] [23] .

More consistent detection of sensitive taxa in Ladwenda, including Ecnomidae, suggests sustained or less disturbed ecological quality year-round compared to Bidiga—a pattern often attributed to smaller, less disturbed catchments, reduced pollutant input, or managed water abstraction [4] [6] . High abundances of tolerant groups coinciding with stressful, low-water, or early flood periods are a global feature of lentic macroinvertebrate succession in systems affected by hydrological variation and anthropogenic impacts [4] [27] .

This study was limited to two reservoirs and covered a single annual cycle. The interannual variability in the hydrology, water chemistry, and biological dynamics could not be well assessed. While these constraints reduce the extent to which the findings can be generalised, the results nonetheless provide valuable baseline information on the diversity and dynamics of macroinvertebrate communities in West African small reservoirs. They also highlight the importance of continued multi-year and multi-site monitoring to strengthen ecological assessments in the region.

This study shows that the diversity and dynamics of benthic macroinvertebrate assemblages in studied reservoirs are closely governed by seasonal environmental variation—especially hydrological regime, water chemistry, and physical habitat structure. High diversity, evenness, and the occurrence of sensitive taxa are linked to the rainy season and post-flood stabilization. Dry seasons and transitional periods favor more homogeneous and tolerant communities. Rapid ecological renewal, following environmental disturbances and recovery, is highlighted by substantial temporal beta diversity. Environmental gradients, particularly conductivity, TDS, water level, and pH, significantly shape community composition, as validated by robust multivariate analyses. The appearance and persistence of sensitive taxa can serve as effective indicators of ecosystem health and water quality.

This work was supported by the project titled Pathways out of Poverty for Reservoir-dependent Communities in Burkina Faso (2018-19), funded by ESRC-DFID’s Development Frontiers, and is a collaboration between Bioversity International, Université Joseph Ki-Zerbo, SNV World, and King’s College London.