ojms Open Journal of Marine Science 2161-7392 2161-7384 Scientific Research Publishing 10.4236/ojms.2026.164017 ojms-154631 Article Earth Environmental Sciences Study of the Spatio-Temporal Distribution of Zooplankton in the Estuary of the Fatala River, Boffa/Republic of Guinea Sylla Aboubacar 1 Diakite Sory 1 Guissé Ahmed 1 Sow Mamadou Djouldé 1 Keita Ibrahima 1 Konaté Lamine 1 Bérété Mamady 1 Keita Ansoumane 1 Diop Bouya 1 2 Plankton/Hydrobiology Laboratory, Marine and Coastal Research Center (CEREMAC, Formerly the Conakry Rogbanè Scientific Research Center (CERESCOR)), Conakry, Guinea Laboratory of Atmospheric and Ocean Sciences, Materials, Energy and Devices (LSAOMED), Training and Research Unit in Science and Technology (UFRSAT), Saint-Louis, Senegal

The authors declare no conflicts of interest regarding the publication of this paper.

 01 10 2026 10 2026 16 04 293 317 13 07 2026 17 10 2026 20 10 2026 © 2026 by the authors and Scientific Research Publishing Inc. 2026 This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( https://creativecommons.org/licenses/by/4.0/ ). https://doi.org/10.4236/ojms.2026.164017

The present study took place during the hydrological season 2022-2023 (dry season, intermediate season and rainy season) in the estuary of the Fatala River. It aims to evaluate the spatio-temporal distribution of zooplankton organisms in the estuary of the Fatala River. The samples were taken at eight (8) fixed stations distributed along the estuary. The samples were collected using a plankton net. The results reveal the presence of 57 species, grouped into 41 genera, 27 families, 17 orders, 11 groups and 68,419.72 individuals. This population is largely dominated by the order of Calanoida (41%) and slightly dominated by the orders of Decapods, Cyclopods, Ctenopods, Spumellaria, Leptothecates, and Halocyprids (1%). Copepods are the most abundant group (52%), followed by Malacostraca (13.71%) and Sagittoides (7.07%). Total zooplankton density varies seasonally: it reaches a maximum in the shoulder season (29,742.96 ind/m3), decreases in the rainy season (26,785.82 ind/m3) and remains minimum in the dry season (11,890.94 ind/m3). Depending on the stations, the highest density (19,233.26 ind/m3) was recorded at station 5, while the lowest density (2329.44 ind/m3) was observed at station 3 (upstream), an area characterized by strong water agitation due to the meeting of two arms of the sea.

 Estuary Zooplankton Spatio-Temporal Distribution Copepods Biomass Density 1. Introduction

Aquatic environments, whether marine (oceans, seas, and estuaries), brackish water, or freshwater (lakes, streams, rivers, and wetlands), play a fundamental role in the maintenance and development of biodiversity. They harbor a great diversity of species, both floral and faunal, and ensure ecological functions essential to the balance of ecosystems. The latter contribute to the nutritional, economic, social, and cultural progress, as well as the recreation, of human populations [1].

Estuaries are transition zones between continental waters and marine waters. They have always been favorable places for the development of economic activity. They contribute 33% of the economic value of marine systems [2]. They represent a growing scientific interest based on the variety of biotic and abiotic processes; they are also important for the richness of biological diversity on the one hand, and for the importance of socio-economic activities on the other.

The economic value of all these services has been estimated at approximately 30 trillion euros per year on average at the global scale [3].

Plankton is defined as the set of pelagic organisms, both plant and animal, that float between two waters and are carried by currents and water masses [4]-[12].

Photosynthesis thus makes it possible to recycle carbon into the biomass of primary producers, and then transfer it to consumers throughout the food chain [4][11][13]-[15]. Their movement capacities, linked to the presence of vibrating cilia, antennae, or swimming legs, do not allow them to resist currents (http://atelierpeche.free.fr/3-Habitants/zooplancton.pdf) [4][13][15]-[18].

Plankton has existed on Earth since the very beginnings of life, nearly 3.8 billion years ago, in the form of microscopic blue algae called cyanobacteria (http://educatif.eau-et-rivieres.asso.fr/pdf/plancton-generalites.pdf).

The importance of estuarine ecosystems for these marine species is also explained by the high abundance of zooplankton.

Zooplankton (Figure 1) comprises a group of heterogeneous organisms, including more than 7000 marine species belonging to about fifteen (15) phyla [19].

Figure 1. Classification of Zooplankton according to size classes. Source: Romagnan (2013).

Zooplankton occupies an important place in marine ecosystems [19]-[21]. A modification in zooplankton communities will therefore have direct and indirect repercussions on the entire trophic network that depends on them [20].

The diversity of the ocean’s hydrobiological conditions has been at the origin of the remarkable marine biodiversity [22][23]. Mangrove forests [24] constitute nursery areas for numerous fish species, shrimps, and plankton, as well as breeding and feeding grounds for certain mammals (dolphins and manatees) and many migratory or resident birds [24]. Marine and coastal ecosystems are the reservoir of resources that support economic and social maritime activities [25] and constitute a valuable source of food [26][27].

The ocean harbors a community of plankton, composed of microscopic organisms that drift in the ocean according to the currents. It represents more than 95% of the marine biomass and includes a remarkable diversity: viruses, bacteria, microalgae, reproductive cells, fish larvae, and microcrustaceans [4][11][28].

Plankton plays a major role in aquatic environments [29].

Zooplankton also constitutes an important source of protein for fish fry [29]-[31].

However, global marine biodiversity is currently under increasing pressure from anthropogenic factors such as pollution, overexploitation of fishery resources, climate warming, and the expansion of economic activities [1][23][25][28].

In Africa, the continuous loss of biodiversity is driven by a set of anthropogenic factors [32][33]. According to [34] and [35], pollutants, which are contaminants, affect the most productive areas of the marine environment, including estuaries [36].

Microplastics also harm plankton, which is the essential component of the marine habitat [37].

Anthropogenic activities affecting the coastal and marine environment therefore have direct consequences on the living species inhabiting these environments. They modify their abundance, diversity, and distribution, as well as their feeding, development, and reproduction, in addition to the relationships among these species [28][38]. The climate changes observed at the global scale only exacerbate this problem [13][33].

The Guinean continental shelf, with an area of approximately 56,000 km2, together with the adjacent waters, forms a unit characterized by a great diversity of fauna and flora, among which fishery resources are among the most abundant on the West African coast [39][40].

However, this ecosystem is subjected to multiple severe natural and anthropogenic pressures [41]. The degradation of the coastal zone has intensified with the increase in the Guinean population, which rose from 4.507 million in 1980 to 12.609 million in 2015, and is estimated to reach 27.486 million by 2050 [41].

Several studies have already attempted to highlight the ecological status of Guinean aquatic ecosystems based on the zooplankton compartment: [40]-[44].

Our study has shown that anthropogenic activities have an impact on the spatio-temporal distribution of zooplankton in the Fatala River estuary. It indicates how the density and biomass of zooplankton vary in the Fatala River estuary over time and space in relation to environmental characteristics.

We assessed the abundance and distribution of zooplankton organisms in the Fatala River estuary over time and space in relation to some abiotic parameters. To achieve this, we:

Analyzed some physicochemical parameters (temperature, salinity, pH, dissolved oxygen, and turbidity) likely to have a major influence on the distribution of zooplankton in the Fatala River estuary. Determined the taxonomic composition and the spatio-temporal distribution of zooplankton. Evaluated the specific density of zooplankton.

Estuaries (Figure 2) have been the subject of several studies [45]. According to [46]-[48], a sedimentary perspective was added to this definition. The definition proposed by [49] takes biological aspects into account. [50], by modifying the definition of [47], provided a definition that is certainly the most satisfactory from a hydrodynamic point of view. The properties and specific characteristics of estuarine systems have been studied by [2][45][51]-[55].

Figure 2. Schematic Representation of an Estuary. Source: Deloffre et al. (2005).

Marine organisms are divided into two major groups shown in Figure 3: the benthos and the pelagos [4][9]. In Figure 3, we have the pelagos, the benthos, the nekton, and the seston.

Figure 3. Compartments of the different groups of living organisms in the water mass and the substrate. Source: Devreker et al. (2012).

The works of [15][56][57] have shown the external morphology and the appendages of an adult female copepod. Those of [15][58]-[61] on the study of the general architecture of a copepod appendage, the characteristics of the life cycle of a copepod, and the general diagram of an euphausiid show the main morphological characteristics that are defined according to the Source: http://www.obs-vlfr.fr/~gaspari/copepodes/cop_doc.htm.

1.1. Presentation of the Study Area

The Prefecture of Boffa

It is located in the west of the country on the Atlantic coast (Figure 4), in the natural region of Maritime Guinea. It is one of the ten prefectures that make up the administrative region of Boké. The Guinean coastal zone is characterized by the presence of an extensive continental shelf whose width is between 120 and 200 km [62]. The slope of this shelf is very low offshore from the mouth of the Fatala River [63][64].

Figure 4. Map of the Prefecture of Boffa. Source: Géomatic Laboratory, CERE-UGANC, 2026.

 1.2. Material and Methods

1.2.1. Material

ϖEquipment for measuring abiotic parameters

The equipment used for the measurement of physicochemical parameters consists of: a mercury thermometer graduated in ˚C, a 70+ WATERPROOF conductivity meter, a 70+ WATERPROOF oxygen meter, a 70+ WATERPROOF pH meter, a Lovibond Model TB210 IR turbidimeter, a Secchi disk, and a GPS of the Germain brand.

ϖEquipment for the collection and processing of zooplankton samples

For the collection and processing of zooplankton samples, a 15 HP motorized boat and a 55-micron Hensen plankton net were used. We also used 5% formalin, polyethylene bottles, a GPS, and a 40 m long plumb line. A binocular optical microscope, a counting chamber, and pipettes were also used.

1.2.2. Method

ϖField exploration

To obtain information about the study area and understand its specific characteristics, a reconnaissance mission of the study area was carried out. This visit allowed us to observe field realities, take photographs, and choose the sampling stations for the samples.

The choice of stations was made, on the one hand, by taking into account the distance between two stations and, on the other hand, by considering the movement of the water. Thus, the stations were placed in locations separated from each other by at least more than 2 km on the one hand, and in locations that were as little disturbed as possible on the other hand.

Sampling

For data collection, sampling of water samples and the analysis of samples were carried out. Sampling was conducted aboard a 15 HP motorized boat during three (3) missions at eight (8) fixed stations (Figure 5). Zooplankton samples were collected using a 55-micron Hensen plankton net with an opening diameter of 70 cm. To study the influence of tidal phenomena on the spatio-temporal distribution of zooplankton organisms, total sampling was carried out with a vertical haul from the bottom to the surface (bottom-surface) during the different tidal phases (high tide, intermediate phase or intermediate tide, and low tide).

Figure 5. Map of the Sampling Area. Source: Géomatic Laboratory, CERE-UGANC, 2026.

During each sampling campaign, one sample was collected at each station and during each tidal phase, giving 3 samples per station, therefore 24 samples per campaign. In total, for the three missions, 72 samples were collected, then concentrated and fixed with 5% formalin for laboratory analyses.

Alongside the water sample collections, the geographical coordinates of the stations (Table 1) were recorded using a Germain brand GPS, and depth measurements were carried out using a 40 m long plumb line.

The geographical coordinates and depths of the sampling stations are recorded in Table 1:

Table 1. Geographical coordinates and depths of the stations.

Stations Location Latitude Longitude Depth (m)
S1 Samansira 10.22195˚ −13.91125˚ 3.5
S2 Kamatambaya 10.20687˚ −13.95527˚ 8
S3 Baralandé 10.20795˚ −13.95519˚ 25
S4 Port de Boffa 10.17358˚ −14.02886˚ 7.5
S5 Bras_Motéba 10.13768˚ −14.08109˚ 4
S6 Dobiré 10.07194˚ −14.074118˚ 4.5
S7 Mouth 1 10.05605˚ −14.07255˚ 9
S8 Mouth 2 10.050065˚ −14.06911˚ 16

Sample Analysis

Biotic Parameter

The identification of zooplankton species, the calculation of density, and mass density were carried out in the laboratory.

Density Calculation

After homogenization of the sample by agitation, three sub-samples of 1 ml each were collected and observed separately in the counting chamber under an optical microscope. Each individual encountered was identified and counted. The planktonology manuals: [65]-[68] were used for species identification.

The study of certain zooplankton organisms requires a detailed examination of some body parts (cephalic region, post-abdominal region, thoracic appendages, etc.). In the laboratory, a careful dissection of some organisms was carried out, thus separating the parts to be observed in order to better see them under the microscope and/or binocular microscope.

The zooplankton density of each tide is calculated from the arithmetic mean of the number of individuals encountered in the three sub-samples: this mean is multiplied by 100 to give the number of individuals of the tide. The density is expressed as the number of individuals per unit volume of water. Its formula is as follows:

D = (ni × 100)/V

where:

V = H∙S withS= πR2

Therefore:

V = πH(d/2)2, knowing that R = d/2.

D is the zooplankton density;

ni is the number of individuals encountered;

V is the volume of water filtered;

H is the distance traveled by the net ;

S is the surface area of the net opening;

R is the radius of the net opening;

d is the diameter of the net opening.

Biomass Calculation

The tables of [69] were used for the determination of biomass according to the average individual weight of organisms. From these tables, the following formulas are derived:

For calanoids, the weight of the organism is:

W = 1 / 6 π L T 2 + 1 / 4 π l t 2

For Harpacticoids, it is calculated as follows:

W = 1 / 4 π L T + 1 / 12 π l ( T 2 + T t + t 2 )

where:

L andT represent respectively the length and width of the anterior body;

landt are the length and width of the posterior body.

The body length of zooplankton individuals was measured using a microscope equipped with a graduated eyepiece.

 2. Statistical Processing Method 2.1. Principal Component Analysis (PCA)

The R software allowed us to perform Principal Component Analysis (PCA).

 2.2. Species Richness

Species richness is the number of species counted in a sampling station. It measures the biodiversity of an entire ecosystem by indicating the number of species present in the environment. This richness can be expressed as total richness, that is to say, the total number of species present in a given station, or as average richness, that is to say, the average number of species present in the samples of a studied community [41].

 2.3. Species Diversity (Shannon Index)

The Excel software was used to calculate the Shannon index, which is the most commonly used diversity index. It is also called the Shannon-Wiener index. It is defined by the following formula:

H ′ = − ∑ i = 1 S P i ⋅ log 2 ( P i )

where:

H' corresponds to the Shannon index;

S is the number of species;

Pi is the relative frequency of the ith species: Pi = ni/N, where ni = the number of individuals counted for a given species present;

N = the total number of individuals counted, all species combined.

The Shannon index makes it possible to express the species diversity of a studied community. Species diversity characterizes the more or less large number of species present in a community. If it is homogeneous (composed of a single and same species), then the index H' = 0. The more we have different species, the more its value increases logarithmically.

 3. Results, Interpretation and Discussion 3.1. Biotic Parameters

3.1.1. Structure of the Zooplankton Community

The study of the zooplankton community in the Fatala River estuary carried out during this study made it possible to determine 57 species (Table 2) distributed into 41 genera, 27 families, 17 orders, and 11 groups.

This community is largely dominated by the Copepod groups represented by 21 species, 13 genera, 7 families, and 2 orders, which are the most numerous. They constitute 52% of the total number of identified species (Table 2). Among the 57 species counted, 52 species were identified and 5 species could not be identified because of their state of decomposition: Paracalanus sp., Penilia sp., Sagitta sp., Tomopteris sp., Phyllorhiza sp.

The Copepod group represents 52%, followed by the Malacostraca (23%) and the Sagittoida (8%). Next come the Thaliacea (3%), followed by the Polychaeta (3%), the Scyphozoa (3%), and the Branchiopoda (3%). There are the Polycystina (1%), the Hydrozoa (1%), the Appendicularia (1%), and the Ostracoda (1%). These percentages are presented in Figure 6.

3.1.2. Taxonomic Composition

The main zooplankton species encountered in the Fatala River estuary are found in Table 2.

Figure 6. Percentage of the different zooplankton groups.

Table 2. List of the main zooplankton species encountered in the Fatala.

Groupe Ordre Famille Genres Species AR AS
Copépode Calanoida Paracalanidés Paracalanus Paracalanus scotti (Fruchtl, 1923) A SS; SP
Paracalanus denudatus (Sewell, 1929) P SS; SP
Paracalanus nanus (Sars G.O., 1925) A SS; SP
Paracalanus aculéatus (Sars G.O., 1925) A SS; SP
Paracalanus parvus (Sars G.O., 1925) P SS; SP
Paracalanus sp. A SS; SP
Paracalanus crassirostris (Dahl F., 1894) P SS; SP
Calocalanus Calocalanus pavo (Dana, 1852)
Pseudocalanus Pseudocalanus elongatus (Brady, 1865) A SS; SP
Acrocalanus Acrocalanus andersoni (Bowman, 1958) A SS; SP
Acrocalanus longicornis (Giesbrecht, 1888) P SP
Clausocalanidés Clausocalanus Clausocalanus furcatus (Brady, 1883) R SP
Calanidés Nannocalanus Nannocalanus minor (Claus, 1863) R SS; SP
Calanus Calanus carinatus (Kroyer,1849) R SS; SP
Calanoides Calanoides carinatus (Kroyer, 1849) R SS; SP
Undinula Undinula vulgaris (Dana, 1849) A SP
Spinocalanidés Spinocalanus Spinocalanus abyssalis (Giesbrecht, 1888) R SP
Scolecitrichidés Scottocalanus Scottocalanus helenae (Lubbock, 1856) A SS; SP
Aetideidés Aetideus Aetideus armatus (Boeck, 1872) A SS; SP
cyclopoida Oncaeidés Oncaea Oncaea media (Giesbrecht, 1891) R SP
Onceaea venusta (Philippi, 1843) P SP
Malacostraca Mysida Mysidés Mysis Mysis relicta (Lovèn, 1862) R SP
Boreomysis Boreomysis microps (O. Sars, 1883) P SS; SP
Siriella Siriella thrompsonii (H.Milne Edwards, 1837) R SP
Mysidella Mysidella typhlops (GO Sars, 1872) R SP
Praunus Praunus inermis (Rathke, 1843) R SP
Hemimysis Hemimysis lamornae (Couch, 1856) R SP
Euphausiacés Euphausiidés Thysanoessa Thysanoessa gregaria (GO Sars, 1883) R SS; SP
Euphausia Euphausia krohnii (Brandt, 1851) P SP
Euphausia brevis (Hansen, 1905) P SS; SP
Amphipodes Lestrigonidés Hyperioides Hyperioides longipes (Chevreux, 1900) R SP
Lycaeidés Lycaea Lycaea pulex (Marion, 1874) A SS; SP
Oxycephalidés Rhabdosoma Rhabdosoma brevicandatum (Stebbing, 1888) P SS; SP
Phronimidés Phronima Phronima sedentaria (Forskål, 1775) R SP
Hyperiidés Hyperoche Hyperoche medusarum (Kröyer, 1838) R SP
Décapodes Luciferidés Lucifer Lucifer faxoni (Borradaile, 1915) A SS; SP
Branchiopodes CTénopodes Sididés Penilia Penilia sp. A SS; SP
sagittoides Phragmophora Eukrohniidés Eukrohnia Eukrohnia hamata (Möbius, 1875) P SS; SP
Eukrohnia Eukrohnia fowleri (Rittet-Zahony, 1909) A SS; SP
Aphragmophora Sagittidés Sagitta Sagitta minima (Grassi, 1881) P SP
Sagitta enflata (Grassi, 1881) P SP
Sagitta serratodentata (Krohn, 1853) P SS; SP
Sagitta friderici (Ritter-Záhony, 1911) P SS; SP
Sagitta hexaptera (Orbigny, 1836) P SS; SP
Sagitta planctonis (Steinhaus, 1896) R SP
Sagitta macrocephala (Fowler, 1904) P SP
Sagitta sp. P SP
Appendiculaires Copelata Oikopleuridés Oikopleura Oikopleura dioica (Fol, 1872) A SP
Thaliacés Salpides Salpidés Thalia Thalia democratica (Forskal, 1775) P SP
Salpa Salpa democratica (Forskal,1775) P SP
Salpa zonaria (Pallas, 1774) R SS; SP
Doliolides Doliolidés Doliolum Doliolum denticulatum (Gaimard, 1834) R SP
Polychètes Phyllodocida Syllidés Myrianida Myrianida fasciata (Milne Edwards, 1845) P SP
Tomopteridés Tomopteris Tomopteris sp. P SP
Hydrozoaires Leptothecata Phialellidés Phialella Phialella quadrata (Forbes, 1848) R SP
Scyphozoaires Rhizostomeae Mastigiidés Phyllorhiza Phyllorhiza sp. P SP
Polycystines Spumellaria Actinommidés Acanthosphaera Acanthosphaera actinota (Haeckel, 1860) A SP
Ostracodes Halocyprida Halocyprididés Conchoecia Conchoecia elegans (G.O. Sars, 1866) P SS; SP

Legend: A = Abundant; P = Present; R = Rare; DS = Dry season; RS = Rainy season.

3.1.3. Variation of Zooplankton Community Density

Density is expressed as the number of individuals per volume of filtered water (ind/m3). The study showed that, in the Fatala River estuary, the density of the zooplankton community varies according to the months, the seasons, and the tides (Table 3). The total density is 68,419.67 ind/m3.

Table 3. Relative frequency of the different zooplankton groups according to the seasons.

Zooplan kton Groups of zooplankton Dry Season Intermediate Season Rainy Season Total (%)
Copépodes 6214.37 16501.24 18347.66 41063.26 52%
Malacostraca 2747.73 5645.78 3626.94 12020.45 23%
Sagittoides 942.14 4394.43 1971.44 7308.01 8%
Thaliacés 320.29 803.23 655.33 1778.85 3%
Polychètes 394.58 456.61 709.49 1560.68 3%
Scyphozoa 403.94 404.61 593.95 1402.50 3%
Branchiopodes 362.12 389.04 131.79 882.95 3%
Polycistina 97.40 85.78 420.64 603.82 1%
Hydrozoa 64.94 479.99 180.54 725.46 1%
Appendicularia 278.46 482.63 115.54 876.63 2%
Ostracodes 64.94 99.64 32.50 197.07 1%
Total 11890 . 90 29742 . 96 26785 . 82 68419 . 67 100%

The variations in zooplankton density according to the seasons are recorded in Tables 4-6 and Figure 7.

Table 4. Variation in zooplankton density during the dry season.

Zooplankton Fauna Dry Season Total Average Density (ind/m 3 ) %
March April
Copépodes 3714.36 2500.01 6214.37 3107.19 51.43
Malacostraca 1708.77 1038.97 2747.74 1373.87 23.23
Sagittoida 520.06 422.08 942.14 471.07 7.75
Thaliacea 222.88 97.41 320.29 160.15 2.53
Polychaeta 297.18 97.41 394.59 197.29 3.07
Scyphozoa 371.47 32.47 403.94 201.97 3.31
Branchiopoda 297.18 64.94 362.12 181.06 2.52
Polycystina 0 97.41 97.41 48.705 2.03
Hydrozoa 0 64.94 64.94 32.47 1.27
Appendicularia 148.59 129.87 278.46 139.23 1.51
Ostracode 0 64.94 64.94 32.47 1.25
Total 7280 . 49 4610 . 45 11890 . 94 5945 . 475 100 . 00

Table 4 shows that, during the dry season, the highest value is observed in March (7280.49 ind/m3) and the lowest (4610.45 ind/m3) in April. [41], showed that the highest value of zooplankton density is observed in April in Tabounsou Bay. Copepods (51.43%), still predominant, are followed by Malacostraca, Sagittoidea, Scyphozoa, Polychaetes, Thaliaceans, Branchiopods, Polycystina, Appendicularians, Ostracods, and Hydrozoans during the dry season.

During the intermediate season, the highest value (19,233.26 ind/m3) is observed in November and the lowest (10,509.69 ind/m3) in October (Table 5). The group with the highest value is the Copepods group, which has a mean density of 8250.62 ind/m3 and represents 54.51% of the zooplankton organisms recorded during this season. They are followed successively by Malacostraca, Sagittoidea and Thaliaceans, Appendicularia, Hydrozoa, and Polychaetes.

Table 5. Zooplankton density during the intermediate season.

Zooplankton Fauna Intermediate Season Total Average Density (ind/m 3 ) %
October November
Copépodes 5909.94717 10591.29305 16501.2402 8250.6201 54.51
Malacostraca 2266.96119 3378.817414 5645.7786 2822.8893 18.53
Branchiopodes 194.11 194.93 389.04 194.52 1.07
Sagittoida 1015.61306 3378.817414 4394.43047 2197.2152 15.03
Appendicularia 287.69636 194.93 482.62636 241.3132 2.13
Thaliacea 478.340547 324.8862898 803.226837 401.6134 2.69
Polychaeta 131.720097 324.8862898 456.606387 228.3032 1.83
Hydrozoa 90.122759 389.8635478 479.986307 239.9932 2.05
Scyphozoa 79.72342 324.8862898 404.609714 202.3049 1.25
Polycystina 20.7986688 64.97725796 85.7759268 42.8879 0.42
Ostracode 34.66204506 64.97725796 99.639303 49.8197 0.49
Total 10509.6953 19233.2648 29742.9601 14871.4801 100.00

Table 6. Zooplankton density during the rainy season.

Zooplankton Fauna Rainy Season Total Average Density (ind/m 3 ) %
July August
Copépodes 10167.53322 8180.12623 18347.6594 9173.8297 67.56
Malacostraca 1617.562103 2009.37351 3626.93561 1813.4678 13.71
Branchiopoda 57.77 74.02 131.79 65.895 0.43
Sagittoida 924.3212016 1047.11781 1971.43902 985.7195 7.07
Appendicularia 57.77 57.77 115.54 57.77 0.25
Thaliacea 404.3905257 250.944044 655.33457 327.6673 1.66
Polychaeta 404.3905257 305099969 709.490495 354.7453 3.09
Hydrozoa 0 180.538525 180.538525 90.2693 2.07
Scyphozoa 346.6204506 247.329894 593.950345 296.9752 1.97
Polycystina 288.8503755 131.789744 420.640119 210.3201 1.72
Ostracode 0 32.49918752 32.4991875 16.2496 0.47
Total 14269 . 2084 12516 . 6089 26785 . 8173 13392 . 9088 100 . 00

The analysis of Tables 4-6 shows that copepods are characterized by their high numbers, with very unequal seasonal mean densities: 3107.19 ind/m3, i.e., 51.43% during the dry season, 8250.62 ind/m3, i.e., 54.51% during the intermediate season, and 9173.83 ind/m3, i.e., 67.56% during the rainy season. These results confirm those of [41][43][70]. After the copepods come successively the Malacostraca (13.71%), the Sagittoidea (7.07%), and the Polychaetes (3.09%). Hydrozoa and Ostracods were recorded only in August. They represent 2.07% and 0.47% of the total density, respectively. Appendicularia, with a mean density of 57.77 ind/m3, represent 0.25%. They represent the lowest mean density during this season. During the rainy season, the month of July recorded the maximum value (14,269.21 ind/m3), and the minimum value (12,516.61 ind/m3) was observed in August. In the Mellakoré estuary, along the Conakry-Benty axis, [43] found, in the area of highest concentration, the maximum density (140,000 ind/m3) in July.

Figure 7 shows that the total density of zooplankton organisms is higher during the intermediate season than during the rainy season and the dry season, with 29,742.96, 26,785.82, and 11,890.94 ind/m3, respectively.

Figure 7. Total zooplankton density according to the seasons.

The increase in zooplankton density during the intermediate season (October-November) in an estuary can be explained by the improvement of ecological conditions after the rainy season. Indeed, heavy rainfall brings many nutrients into the estuary through runoff, but high turbidity and floods can disrupt the development of zooplankton. When rainfall decreases, these nutrients remain available in the water and promote the proliferation of phytoplankton, the main food source of zooplankton. Furthermore, the progressive stabilization of physicochemical parameters (salinity, temperature, turbidity) and the balanced mixing of freshwater and marine waters create favorable conditions for the reproduction and development of zooplankton organisms. These results differ from those of [44], who observed a maximum zooplankton density during the rainy season, followed by the intermediate season and finally the dry season.

The density of zooplankton in the Fatala River estuary by station is presented in Figure 8.

Figure 8. Zooplankton density by station.

Figure 8 shows that zooplankton density differs from one station to another and from upstream to downstream. It is lower upstream (at stations 1, 2, and 3) than downstream.

This situation could be related to the excessive cutting of mangrove wood by woodcutters, as well as agricultural and mining activities that contribute to the degradation of this ecosystem. Mangrove forests, which have a dense and intertwined root system serving as ecological niches for various species, are increasingly degraded upstream. The excessive destruction of mangroves dangerously compromises their role in protecting the coasts against waves and wind-induced erosion. Their role is to preserve biological diversity, provide spawning grounds, and supply nutrients for many species of fish and crustaceans [71].

The lowest zooplankton density (2329.44 ind/m3) was obtained at station 3 (Figure 8). This seems to be related to the fact that the pH of the water at this station is pH = 6.3. This slight acidity of the water could be attributed to pollution caused by agricultural activities (which are more intense at station 3), mining activities, and port activities. The highest zooplankton density (19,233.26 ind/m3) was recorded at station 5, located downstream. This high abundance could be explained by the low anthropogenic activity, particularly mining, agricultural, and port activities, in this part of the estuary. Similar results were observed by [71] and [44]. However, the lowest zooplankton density downstream was observed at station 8. This situation could be related to the high water turbidity recorded at this station (31.7 NTU). Indeed, high turbidity reduces light penetration into the water column, which limits the photosynthetic activity of phytoplankton. However, phytoplankton constitutes the main food source for zooplankton; therefore, a decrease in its production can lead to a reduction in zooplankton density.

3.1.4. Biomass of the Zooplankton Community

The mass density, the total mass of zooplankton organisms living in a given place, is expressed in mg/m3. During this study, the biomass of zooplankton organisms varied according to the seasons.

Figure 9 indicates the variation of zooplankton biomass in the Fatala River estuary.

Figure 9. Zooplankton mass density according to the seasons.

Figure 9 shows that the zooplankton biomass in the Fatala River estuary varies according to the seasons. The minimum value (50.129 mg/m3) was recorded during the dry season and the maximum value (114.894 mg/m3) was observed during the intermediate season. This is related to several ecological and physicochemical factors favorable to their development during this period. During the intermediate season:

Temperatures are moderate, which promotes biological growth (photosynthesis, reproduction);Water masses may present moderate stratification; they allow good circulation of nutrients while maintaining favorable areas for zooplankton development.

This intermediate season benefits from sufficient sunshine without being extreme (https://fr.weatherspark.com). During the rainy season, heavy rainfall dilutes nutrients and disrupts aquatic communities. During the dry season, evaporation increases salinity, temperature rises, and nutrients are often limited, which can slow down primary production. The intermediate season therefore seems to represent a balance between these two extremes. Many zooplankton species have reproduction cycles synchronized with the intermediate seasons, where conditions appear optimal for the survival of juveniles.

3.1.5. Distribution of Some Main Zooplankton Species

ϖPseudocalanus elongatus

Pseudocalanus elongatus (Brady, 1865), a copepod of the order Calanoida, was recorded in all stations during all sampling campaigns. Its constant presence seems to indicate its good adaptation to local environmental conditions. Stations 1 and 2 recorded the highest densities, 148.59 ind/m3 and 129.87 ind/m3, respectively.

ϖAcrocalanus andersoni

Acrocalanus andersoni (Browman, 1958) is a copepod species of the order Calanoida, family Paracalanidae. This species is abundant at the mouth of the estuary during all seasons. It presents a high density of 3292.89 ind/m3 at station 6.

ϖAetideus armatus

Aetideus armatus (Boeck, 1872) is a copepod species of the family Aetideidae. This species is abundant at the mouth of the estuary during all seasons and absent upstream at stations 1, 2, and 3. It presents a high density of 584.79 ind/m3 at station 5.

 3.2. Statistical Study

3.2.1. Principal Component Analysis (PCA)

The Principal Component Analysis carried out on the zooplankton data (Figure 10) highlights the distribution of species according to the sampling stations.

The F1 axis seems to oppose the stations located on the right side of the factorial plane (stations 1, 2, 4, and 6) to those located on the left side (stations 3, 5, 7, and 8). The stations located on the right are associated with several species such as Paracalanus parvus, Acrocalanus andersoni, Oikopleura dioica, and Oncaea media. This distribution suggests that these stations could present environmental conditions favorable to the development of these species, particularly in terms of salinity, food availability, or hydrological stability.

Figure 10. PCA of the variables.

Conversely, the stations located on the left side of the F1 axis (stations 3, 5, 7, and 8) are associated with species such as Sagitta planktonis, Sagitta macrocephala, Euphausia brevis, or Paracalanus aculeatus. This distribution could reflect different environmental conditions, possibly related to higher turbidity, continental inputs, or the influence of anthropogenic activities.

The F2 axis, meanwhile, makes it possible to distinguish the stations located in the upper part of the graph (stations 3 and 4) from those located in the lower part (stations 6, 7, and 8). The upper stations are associated with species such as Paracalanus nanus or Boreomysis microps, whereas those in the lower part are linked to species such as Paracalanus aculeatus, Tomopteris sp., or Salpa zonaria. This organization could reflect an ecological gradient related to certain environmental parameters, such as turbidity, salinity, or trophic availability.

3.2.2. Species Richness

Species richness is the total number of species present in the community. The species richness (S) of zooplankton is represented in Figure 11. This figure shows that station 5 is the richest in zooplankton species (S = 28). It is followed by station 6 with a species richness (S = 21), then by station 7 (S = 13). Station 3 (S = 3) has the lowest species richness.

Figure 11. Zooplankton species richness by station.

3.2.3. Shannon Index

The Shannon index of the zooplankton groups is represented in Figure 12. The analysis of this figure shows that in the Fatala River estuary, the Copepods (H' = 0.6) are the most diversified. They are followed by Malacostraca (H' = 0.18), then Sagittoidea (H' = 0.11). The Branchiopods, Appendicularians, Hydrozoa, Polycystina, and Ostracods (H' = 0.01) are the least diversified.

Figure 12. Shannon index of zooplankton groups.

Figure 13 shows the Shannon Index of the main zooplankton species in the study area.

Figure 13 shows that in the Fatala River estuary, the species Acrocalanus andersoni with the Shannon index (H' = 0.29) is the most diversified in the copepod zooplankton group. It is followed by Paracalanus nanus (H' = 0.23) and Lycaea pulex (H' = 0.22). Several species encountered have low specific diversity with H’ = 0.01. These include, among others: Doliolum denticulatum, Salpa zonaria, and Hyperioides longipes.

Figure 13. Shannon index of the main zooplankton species in the study area.

 4. Conclusions and Perspectives

The results of this study reveal that the pH of 6.3 slightly exceeds the quality standards established by the WHO in certain stations.

The study shows that zooplankton density increases progressively from upstream to downstream and reaches its maximum during the intermediate season, especially in October. This seasonal variation is closely related to environmental and ecological factors (water quality, resource availability, and the stability of physical conditions).

Within the holoplankton group, copepods appear as the most dominant group, representing 52% of the total density, followed by Malacostraca (23%) and Sagittoidea (8%). These three groups also present the greatest diversity. The most constant species observed are Acrocalanus andersoni, Paracalanus nanus, and Lycaea pulex.

Zooplankton mass density also varies according to the seasons, with a minimum value of 50.129 mg/m3 recorded during the dry season and a maximum value of 114.894 mg/m3 observed during the intermediate season.

In perspective, we will focus on the other links of the aquatic food chain.

 Author Contributions

Sylla and Diakité: Conducting sampling and collecting samples. Sylla: Data analysis and processing, interpretation and discussion of the results. Guissé, Sow, Konaté and Diakité: Supervision of laboratory analysis. Sylla, Diakité and Bérété: Contributed to the writing of the scientific article. Keita and Bouya: Overall supervision of the research work.

 Websites

http://www.obs-vlfr.fr/~gaspari/copepodes/cop_doc.htm. Accessed on 11/03/2024.

http://www.plancton-du-monde.org/module-formation/phyto_02.html. Accessed on 07/04/2024.

http://educatif.eau-et-rivieres.asso.fr/pdf/plancton-generalites.pdf. Accessed on 05/07/2024.

http://atelierpeche.free.fr/3-Habitants/zooplancton.pdf. Accessed on 09/03/25.

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