Diversity of zooplankton in the urban lakes of Yamoussoukro (central Côte d'Ivoire)

 Corresponding author: Fréderic Gnanmien KOUAKOU Copyright © 2025 Author(s) retain the copyright of this article. This article is published under the terms of the Creative Commons Attribution Liscense 4.0. Diversity of zooplankton in the urban lakes of Yamoussoukro (central Côte d'Ivoire) Gnanmien Fréderic KOUAKOU *, Kouakou Roland DJENE, Bi Boly Valéry N’Do, Jean-Renaud ALLOUKO and Kotchi Yves BONY Department of Biodiversity and Sustainable Ecosystem Management, Faculty Environment, Jean Lorougnon Guede University of Daloa, Côte d'Ivoire. GSC Biological and Pharmaceutical Sciences, 2025, 32(03), 180-185 Publication history: Received on 05 August 2025; revised on 14 September 2025; accepted on 17 September 2025 Article DOI: https://doi.org/10.30574/gscbps.2025.32.3.0354 Abstract The urban lakes of Yamoussoukro, subject to strong anthropic pressure, present an ecological richness that is often overlooked, particularly in terms of zooplankton. This study aims to assess the zooplankton diversity of these aquatic environments in relation to their physico-chemical conditions and surrounding human activities. The six (6) sampling campaigns revealed the presence of four (4) taxonomic groups : Rotifera, Cladocera, Copepoda, and Ostracoda, each playing a specific ecological role in lake food webs. Among these groups, Rotifera are rich in taxa and the most abundant. The composition and abundance of taxa vary between lakes, influenced by factors such as temperature, pH, dissolved oxygen, transparency, and depth. Specific diversity, calculated using the Shannon index, shows low taxonomic richness in the urban lakes of Yamoussoukro, suggesting ecological imbalance. In conclusion, zooplankton appears to be a powerful tool for ecological diagnosis and sustainable management of urban aquatic environments. A better understanding of this biodiversity can contribute to the development of conservation strategies adapted to the local context of Yamoussoukro. Keywords: Diversity; Zooplankton; Artificial Lakes; Yamoussoukro 1. Introduction Urban aquatic ecosystems play a vital role in maintaining biodiversity, particularly in tropical regions where biological productivity is naturally high [1]. In Yamoussoukro, the political capital of Côte d'Ivoire, urban lakes are dynamic habitats subject to various anthropogenic pressures such as increasing urbanization, domestic waste disposal, and hydrological changes. Among aquatic communities, zooplankton occupies a strategic position in the trophic structure [2]. This important role is to transfer the energy fixed by producers to higher trophic levels [3]. Their short development cycle and small size make them easy to sample. Thus, studying zooplankton diversity in these lakes not only allows us to assess the ecological quality of the environments, but also to better understand the biological interactions that support their functioning. This research aims to characterize the specific diversity of zooplankton in the main urban lakes of Yamoussoukro, while exploring the environmental factors that influence their distribution and abundance. GSC Biological and Pharmaceutical Sciences, 2025, 32(03), 180-185 181 2. Materials and methods 2.1. Study site Yamoussoukro is located between 6°48' and 6°52' north latitude and between 5°12' and 5°18' west longitude [4]. It has a humid tropical climate characterized by two (2) main seasons. The dry season lasts from November to February, and the rainy season from March to October, with the highest rainfall in September [4], (Figure 1). The dense and ramified hydrographic network of the Yamoussoukro district consists mainly of the Bandama River and its tributaries, as well as the N'Zi and Kan rivers. The city of Yamoussoukro has ten (10) artificial lakes [5]. For this study, six (6) of these 10 lakes were selected for exploration (Figure 1). This choice was made based on their accessibility throughout the year, their geographical location, and the human activities carried out in their vicinity. A coding system was adopted to name these lakes. The lakes located in the city center have the codes L1, L2, and L3. Those located at the eastern, western, and southern ends have the codes L4, L5, and L6, respectively. Figure 1 Location of the six urban lakes sampled in the city of Yamoussoukro 2.2. Methods 2.2.1. Measurement of abiotic parameters Water temperature (°C), pH, conductivity (µS/Cm), and dissolved oxygen (mg/L) were measured using an HQ40d multiparameter meter. Water transparency (cm) was assessed using a 30 cm diameter Secchi disk attached to a long rope. Water depth (cm) was measured using an 8 m long board with regular graduations. The coverage of the lakes by macrophytes and the type of substrate were estimated visually and expressed as a percentage. 2.2.2. Sampling In the field, sampling of the zooplankton community in the urban lakes of Yamoussoukro was carried out between 6 a.m. and 9 a.m. for daytime sampling, and between 8 p.m. and 10 p.m. for nighttime sampling. Zooplankton collection consisted of filtering 30 L of water (3 buckets with a capacity of 10 L). This volume of water was then filtered using a plankton net with a mesh size of 20 μm [6]. The selected sample was collected and stored in 100 mL pill bottles labeled with the addition of 5 drops of 70% ethanol. In the laboratory, zooplankton organisms were observed by taking a 10 GSC Biological and Pharmaceutical Sciences, 2025, 32(03), 180-185 182 mL subsample using a micropipette. This subsample was mounted between a slide and cover slip and then observed under an Optika B-192 photonic microscope with a 40× objective lens. The zooplankton organisms were identified and counted. Identification was carried out using the keys provided by [7] for rotifers, [8] for copepods, and [9] for cladocerans. These species were counted using an Optika M226799 inverted microscope. 2.2.3. Data analysis Taxonomic richness, percentage of occurrence, Shannon and Equitability indices were used to determine the structure and dynamics of the zooplankton community. The percentage of occurrence (F) is obtained using the following formula : F = (Si / St) × 100, where Si is the number of samples in which taxon i was captured and St is the total number of samples. The classification of taxa based on their percentage of occurrence was carried out according to Dajoz (2000) : F ≥50 : constant taxon ; 25% ≤ F ˂ 50% accessory taxon and F ˂ 25 : accidental taxon. Seasonal variations in taxonomic richness and densities of zooplankton organisms were assessed using the Mann-Whitney U test with Statistica v 7.1 software. Zooplankton taxa and environmental variables were correlated with SOM. This is version 2 of the SOM Toolbox interface for Matlab®. 3. Results 3.1. Overall taxonomic composition of zooplankton The taxonomic composition and occurrences of zooplankton taxa in the urban lakes of Yamoussoukro are recorded in Table 1. A total of 31 taxa belonging to 17 families and 4 groups were collected from all lakes. The groups encountered are Rotifera, Copepoda, Cladocera, and Ostracoda. Among these groups, Rotifera has the largest number of taxa with 23 taxa. Next come Copepoda (4 taxa) and Cladocera (3 taxa). Finally, there are the ostracods (1 taxon). The rotifer group is dominated by the Brachionidae family. Within the rotifer group, taxa are most commonly found in the Brachionidae (seven taxa) and Trichocercidae (five taxa) families. The copepod group is dominated by the Cyclopidae family. The distribution of specific zooplankton richness by lake indicates that Lake 1 has more taxa (21 taxa). In contrast, Lake 4 has fewer taxa (10 taxa). Analysis of occurrences shows that three (3) taxa are common and constant in all lakes. These are Brachionus angularis angularis, Mesocyclops leuckarti leuckarti, and Tropodiaptomus sp. All other taxa are incidental or accidental. GSC Biological and Pharmaceutical Sciences, 2025, 32(03), 180-185 1 Table 1 Taxonomic compositions and occurrences of zooplankton species in urban lakes in Yamoussoukro (Ivory Coast) from July 2021 to May 2022 Groups Familles Taxa Acronyms Lake 1 Lake 2 Lake 3 Lake 4 Lake 5 Lake 6 Cladocera Bosminidae Bosmina longirostris Blo *** ** *** * - *** Chydoridae Leydigia acanthocercoides Lac - ** *** - - ** Daphniidae Simocephalus sp. Ssp *** - - - - * Copepoda Diaptomidae Tropodiaptomus sp. Tsp *** *** * ** ** * Pseudodiaptomidae Pseudodiaptomus gordioides Pgo - - - - - ** Cyclopidae Mesocyclops leuckarti Mle *** * *** ** ** *** Nauplii sp. Nsp *** ** ** *** ** Rotifera Conochilidae Conochilus hippocrepsis Chi - ** ** * ** *** Filiniidae Filinia opoliensis Fop - * * - - * Testudinellidae Testudinella patina Tpa * - * - * * Asplanchnidae Asplanchna sp. Asp *** ** - - ** - Brachionidae Anuraeopsis fissa Afi *** * * - - - Brachionus angularis Ban ** ** ** *** * * Brachionus caudatus Bca - * - - - - Brachionus forticula Brf - - - - ** - Brachionus plicatilis Bpl * - - ** ** - Brachionus urceolaris Bur - - - * * - Platyias quadridentatus Pqu ** - - - - - Lecanidae Lecane bulla Lbu *** - * ** *** * Lecane furcata Lfu ** - - - - - Lecane lunaris Llu * - ** - - * Lepadellidae Colurella uncinata Cun - - * - - - Groups Familles Taxa Acronyms Lake 1 Lake 2 Lake 3 Lake 4 Lake 5 Lake 6 Rotifera Notommatidae Cephalodella gibba Cgi - * - - - - GSC Biological and Pharmaceutical Sciences, 2025, 32(03), 180-185 2 Philodinidae Philodina vorax Pvo * - * - * - Rotaria neptunia Rne *** * * - - * Trichocercidae Trichocerca capucina Tca * - * - - * Trichocerca longiseta Tlo * - - - - - Trichocerca pusila Tpu *** ** - - - - Trichocerca similis Tsi ** *** - - *** Trichocerca weberi Twe ** * - * - - Ostracoda Cyprididae Hemicypris anomala Han * - - - * - Total = 4 17 31 21 16 17 10 12 16 GSC Biological and Pharmaceutical Sciences, 2025, 32(03), 180-185 180 3.1.1. Seasonal variation in zooplankton populations Seasonal variation in taxonomic richness does not vary significantly from one season to another (Mann-Whitney U test, p > 0.05). The median value ranges from 13 taxa recorded in Lake 1 during the rainy season to 3 taxa obtained in Lakes 2 and 4 during the dry season (Table 2). Table 2 Seasonal variation in the taxonomic richness of zooplankton in urban lakes in Yamoussoukro (Ivory Coast) from July 2021 to May 2022 Urban lakes of Yamoussoukro Seasons Lake 1 Lake 2 Lake 3 Lake 4 Lake 5 Lake 6 Dry season Min 3 1 3 2 1 2 Med 9 3 5 3 4 4 Max 13 11 6 7 13 16 Rainy season Min 12 4 3 4 6 5 Med 13 8 4 5 7 6 Max 14 9 5 6 8 7 Extreme median values are in bold 3.2. Overall analysis of zooplankton populations The proportions of zooplankton groups collected in urban lakes in Yamoussoukro based on their abundance are shown in Figure 2. The Rotifera group (57%) is the most abundant. The least abundant group is Ostracoda (1%). Figure 2 Quantitative contributions of zooplankton groups collected in the lakes of Yamoussoukro (Ivory Coast) from July 2021 to May 2022 3.3. Zooplankton abundance The seasonal variation in zooplankton density does not vary significantly from one season to another (Mann-Whitney test, p > 0.05). The median values for density range from 600 ind./L obtained at Lake 4 during the rainy season to 7,000 ind./L recorded at Lake 1 during the dry season (Table 3). GSC Biological and Pharmaceutical Sciences, 2025, 32(03), 180-185 181 Table 3 Seasonal variation in zooplankton density in urban lakes in Yamoussoukro (Ivory Coast) from July 2021 to May 2022 Urban lakes of Yamoussoukro Seasons Lake 1 Lake 2 Lake 3 Lake 4 Lake 5 Lake 6 Dry season Min 2000 100 1000 500 100 500 Med 7000 1050 1150 800 3000 2000 Max 9000 9000 2500 4000 8000 7000 Rainy season Min 4000 1000 1000 500 2000 1800 Med 5000 1000 1100 600 1000 1800 Max 8000 2500 1200 1000 3500 4000 Extreme median values are in bold 3.4. Seasonal variations in Shannon diversity and Piélou evenness indices Seasonal variations in Shannon diversity and Piélou evenness indices do not differ significantly from one season to another (Mann-Whitney U test, p > 0.05). Table 4 shows that the median values of the Shannon diversity index range from 1 bit/ind. (lakes 2 and 3) to 2 bit/ind. (lake 1). Table 5 shows that the zooplankton community is evenly distributed across all lakes. The median values of the Piélou equitability index range from 0.8 (lakes 1, 2, and 5) to 0.95 (lake 3). Table 4 Seasonal variation in the zooplankton diversity index of urban lakes in Yamoussoukro (Ivory Coast) from July 2021 to May 2022 Urban lake of Yamoussoukro Seasons Lake 1 Lake 2 Lake 3 Lake 4 Lake 5 Lake 6 Dry season Min 1,9 1,4 1,1 0,7 0,8 0,7 Med 2 1,8 1,3 1,2 1,6 1,7 Max 2,2 2,1 1,4 1,5 2,4 2,5 Rainy season Min 1 1 0,8 1,1 1,3 1,2 Med 1,9 1 1 1,1 1,5 1,4 Max 2 1,9 1,3 1,6 1,9 1,8 Extreme median values are in bold Table 5 Seasonal variation in the zooplankton equity index for urban lakes in Yamoussoukro (Ivory Coast) from July 2021 to May 2022 Urban lakes of Yamoussoukro Seasons Lake 1 Lake 2 Lake 3 Lake 4 Lake 5 Lake 6 Dry season Min 0,82 0,9 0,9 0,8 0,85 0,9 Med 0,85 0,93 0,95 0,9 0,9 0,92 Max 0,87 0,97 0,98 0,98 0,95 0,95 Rainy season Min 0,78 0,4 0,84 0,75 0,3 0,88 Med 0,8 0,8 0,87 0,9 0,8 0,92 Max 0,85 0,9 0,98 0,98 0,9 0,95 Extreme median values are in bold GSC Biological and Pharmaceutical Sciences, 2025, 32(03), 180-185 182 3.5. Zooplankton distribution profile The SOM (Self-Organizing Map) analysis enabled the 36 samples (6 lakes x 6 campaigns) to be classified based on the presence/absence matrix of zooplankton taxa in the urban lakes of Yamoussoukro. This classification was based on the distribution and probability of occurrence of the taxa. This allowed us to select the 30-cell map (5 rows x 6 columns) with the lowest topography and quantification errors. Four groups (I to IV) were determined using the cells of the self-organizing map following a detailed examination of the Ascending Hierarchical Classification (AHC) using Ward's method and Euclidean distance. Different figures on the Kohonen map were used to illustrate the groups (Figures 3 and 4). Thus, the samples collected from lakes 3 and 4 constitute group I. It represents 9.67% of all samples (G test : p < 0.05). Group II is characterized by samples from lake 1. It represents 38.7% of the samples (G test : p < 0.05). Group III includes samples from lakes 5 and 6. It represents 19.35% of all samples. Group IV includes samples from lake 2. This group represents 32.25% of the samples (G test : p < 0.05). Figure 3 Ascending hierarchical classification of cells based on zooplankton species in urban lakes in Yamoussoukro (Ivory Coast) from July 2021 to May 2022 : A = Ascending hierarchical classification of cells in the Kohonen map using the Ward method and Euclidean distance as the assembly distance (the numbers [1 to 30] correspond to the cell numbers in the Kohonen map; the Roman numerals [I to V] represent the groups selected). B = Kohonen map with cells numbered from 1 to 30 Figure 4 Distribution of samples according to the presence/absence data of zooplankton taxa collected in urban lakes in Yamoussoukro (Ivory Coast) from July 2021 to May 2022 : Roman numerals (I to IV) represent the defined groups ; Arabic numerals (1 to 9) designate the lakes, no = November, de = December, ja = January, ma = May, ju = July, se = September. Group I, the least diverse, comprises three taxa (Brachionus plicatilis, Brachionus forticula, and Brachionus urceolaris). Group II, the most diverse, contains 12 taxa (Anuraeopsis fissa, Asplanchna sp., Lecane bulla, Lecane furcata, GSC Biological and Pharmaceutical Sciences, 2025, 32(03), 180-185 183 Mesocyclops leuckarti, Platyias quadridentatus, Philodina vorax, Rotaria neptunia, Simocephalus sp., Trichocerca longiseta, Trichocerca pusila, and Trichocerca weberi. Group III consists of six taxa (Bosmina longirostris, Conochilus hippocrepsis, Colurella uncinata, Leydigia acanthocercoides, Nauplii sp., and Trichocerca similis). Group IV has 10 taxa (Brachionus angularis, Brachionus caudatus, Cephalodella gibba, Filinia opoliensis, Hemicypris anomala, Lecane lunaris, Pseudodiaptomus gordioides, Trichocerca capucina, Testudinella patina, Tropodiaptomus sp.). The number of taxa recorded in the different groups differs significantly from one group to another (Mann-Whitney test, p < 0.05). 3.6. Abiotic parameters determining zooplankton distribution Variations in pH, conductivity, temperature, dissolved oxygen, transparency, and depth for groups I to IV defined by Kohonen's self-organizing map are illustrated in Figure 5. The distribution of taxa in group I is determined by temperature. The distribution of taxa in group II is characterized by dissolved oxygen, transparency, and depth. As for taxa in group III, their distribution is influenced by pH. The distribution of taxa in group IV is determined by conductivity. Figure 5 Kohonen self-organizing map (SOM) showing variations in abiotic parameters in groups I to IV: dark color = sites with very high abiotic parameter gradients ; d = scale 4. Discussion Zooplankton sampling in the urban lakes of Yamoussoukro yielded 31 taxa. This taxonomic richness is identical to that observed by [10] in Lake Kaby in eastern Côte d'Ivoire (31 taxa) and by [11] in Lake Nokoué in southern Benin (31 taxa). This could be explained by the fact that these lakes, located in the middle of urban areas and subject to domestic waste that can pollute the environment, may have the same ecological characteristics. However, this diversity is lower than that reported by [12], who identified 49 species of zooplankton in Lake Léré in Chad. The distribution of taxonomic richness among groups shows that rotifers (57% of the taxa inventoried) largely dominate the zooplankton population. According to [6], this zooplankton group is the most abundant in freshwater. They can consume fine particles, such as bacteria and organic detritus, which are abundant in eutrophic aquatic ecosystems. He concludes that the predominance of rotifers in freshwater environments is a sign of eutrophication. This