The data collection extended over a period of three consecutive years (from January 2020 to October 2022). Sampling was carried out four times per year, corresponding to one campaign per quarter, for a total of twelve sampling campaigns.This approach made it possible to take into account the two main climatic regimes present in the study area:

Sampling targeted the main course of the Tinguilinta River, from the offshore marine area near Kamsar to the vicinity of the Dapilon port. Following a reconnaissance phase, fifteen fixed sampling points were georeferenced using a GPS device (Garmin). These stations were selected to reflect the diversity of environments and the gradient of anthropogenic impact ( Table 1 ).

Each study site was sampled during each expedition, with operations conducted between 08:00 and 16:00.

The system under study is characterized by a semi-diurnal tidal regime, with two high and two low tides each day, separated by an interval of about six hours. To account for this temporal variability, plankton samples were collected at both high tide and low tide. The flow of the Tinguilinta River, measured at the Tanéné hydrometric station (located approximately 15 km upstream of Boké), averages 250 m³·s⁻¹, with substantial increases during the rainy season that can exceed 500 m³·s⁻¹.

A surface water sample was collected at each site during each mission, regardless of the tidal phase.

Temperature (˚C) and pH were measured in situ using a VWR pHenomenal pH1100 multiparameter probe. The other samples were preserved in polyethylene containers, transported in refrigerated devices, and then analyzed at CERE (Gamal Abdel Nasser University of Conakry). The quantified physicochemical parameters included: Temperature (˚C), pH, Nitrates ( ${\text{NO}}_3^{-}$), Nitrites ( ${\text{NO}}_2^{-}$), Phosphates ( ${\text{PO}}_4^{3-}$), Total Dissolved Solids (TDS), Suspended Particulate Matter (SPM), Turbidity (NTU), Dissolved Oxygen (mg∙L⁻¹), Salinity (g/L), and Conductivity (µS∙cm⁻¹).

Copepods were collected using a Hensen plankton net (mesh size: 55 µm; opening diameter: 70 cm) following a stratified procedure (vertical hauls covering 8 - 16 m depth per tow).

At each site:

Plankton samples were immediately fixed in a 4% formalin solution. Taxonomic identification was carried out in the laboratory using a stereomicroscope and a compound microscope, based on the taxonomic keys of [12] and [13] .

Statistical analyses included Principal Component Analysis (PCA) and Redundancy Analysis (RDA). PCA was applied to maximize the graphical representation of information contained in the dataset and to objectively discriminate between sampling stations or compare them pairwise [14] . The PCA matrix was used to highlight the main environmental factors by cross-referencing physicochemical parameters with copepod assemblages across the 15 stations.

RDA was employed to investigate the relationships between physicochemical variables and variations in the abundance of dominant copepod species.

Biodiversity was assessed using the Shannon-Wiener diversity index, calculated as:

$H^{\prime }=-{\displaystyle {\sum }_i^s\left(P_i\times \log 2P_i\right)}$; where P_i = Ni × N; where $H^{\prime }$ = representing the Shannon-Wiener diversity index; S = the total number of species in the sample; P_i = the relative abundance of species i; N_i = the number of individuals of species i in the sample; N = the total number of individuals in the sample.

The diversity index H′ ranges from 0 to 5. Values below 2 indicate an imbalanced or disturbed environment, whereas values above 2 suggest a relatively stable community structure.

Evenness (or equitability) E was calculated to compare the diversity of two or more communities comprising different numbers of species. It is determined using the following formula:

$\text{E}=H^{\prime }/{\text{og}}_2\text{S}\text{l}$

where H′ represents the Shannon-Wiener diversity index, log2 denotes the logarithm to base 2, and S corresponds to the total number of species present.

A survey conducted across fifteen sampling sites in the Tinguilinta Estuary revealed a total of 30 species of zooplanktonic copepods. The taxonomic composition of this assemblage was dominated by the order Calanoida (20 species), followed by Cyclopoida (6 species), Poecilostomatoida (3 species), and Harpacticoida (1 species). These 30 taxa were distributed among 16 families and 4 orders. The family exhibiting the highest species richness was Oithonidae (4 taxa), followed by Paracalanidae, Oncaeidae, and Candaciidae (3 taxa each), and then Acartiidae, Calanidae, Temoridae, Eucalanidae, and Corycaeidae (2 taxa each). The remaining families were represented by a single species (see Table 2 ).

The analysis of the relative abundance of families revealed a marked predominance of Paracalanidae and Acartiidae, contributing respectively 35.76% and 13.51% of the total copepod abundance. Similar observations were made by [15] on pelagic ecosystems in relation to fluctuations in hydrological conditions along the southern Atlantic coast of Morocco.

The enumeration of 7 915 individuals revealed a predominance of Paracalanus scotti (1385 individuals), Acartia clausi (986 individuals), Oithona nana (918 individuals), Paracalanus parvus (733 individuals), Paracalanus aculeatus (712 individuals), Temora stylifera (638 individuals), Euterpina acutifrons (586 individuals), Centropages chierchiae (454 individuals), Eucalanus pileatus (369 individuals), and Calanus helgolandicus (311 individuals). This taxonomic profile closely resembles the community structure described by [16] in their assessment of species diversity and biomass of pelagic copepods in the Ras Kebdana region (Moroccan Mediterranean coast), as well as the findings reported by Kéita [17] on the zooplankton assemblages of the Guinean shelf and their principal ecological features (Bulletin du CERESCOR, No. 14).

Copepod abundance increased markedly during August (rainy season), reaching a first peak of 20265 ind/m² at Station S1, followed by a second peak of 20123 ind/m² at Station S2. Conversely, a pronounced decline was observed during the dry season in January, with abundance reaching a minimum of 1 120 ind/m² at Station S7 ( Table 3 ). These seasonal dynamics are consistent with the observations of [18] regarding zooplankton variability in Lake Geneva ( Figure 2 ).

The Shannon diversity index (H), which typically ranges from 0 to 5, is widely used as a metric to assess ecosystem complexity. Values below 2 generally indicate limited heterogeneity and may reflect ecological imbalance, whereas values above 2 are suggestive of a more balanced and presumably more stable biological community.

In the present study, the Shannon diversity index (H′) varied between 1.97 and 2.97 bits per individual (see Table 4 ). The maximum value (2.97) was recorded in January at station S1, while the minimum value (1.97) was observed in May at station S12. This range highlights a notable degree of diversification within the copepod assemblages under investigation. These findings are consistent with previous studies. For instance, [19] reported comparable values in Lake Kaby (Bongouanou, Côte d’Ivoire), with a maximum Shannon index of 2.72 bits per individual during the rainy season and a minimum of 1.94 bits per individual during the dry season. Similarly, [16] documented a maximum value of 3.49 in December in their study of species diversity and biomass of pelagic copepods in the Ras Kebdana region (Mediterranean coast of Morocco), with the lowest indices recorded during summer, reaching 1.69 in July ( Figure 3 ).

The Evenness Index further highlights the recurrent equilibrium of the ecosystem, with a minimum recorded value of 0.81. The extreme values ranged from 0.81, observed at stations S1, S2, and S3 during the months of August and October, to 0.97 at station S15 in January ( Table 5 ; Figure 4 ). These values indicate a well-balanced distribution of individuals among species, since they fall within the interval 0.80 < E < 0.97, which characterizes a community in equilibrium.

Comparable observations were reported by [20] in fishponds located in Bertoua (Eastern Cameroon), where Pielou’s Evenness Index revealed a relatively homogeneous distribution of individuals across the studied assemblages. The recorded values were broadly similar to those obtained in the present study, except for the Côte d’Azur pond (E = 0.65) and the Mopa pond (E = 0.70). Likewise, [21] in their study of the Kondi stream (Douala, Cameroon), found that the Evenness Index varied between 0.48 (station K3, April 2012) and 1.0 (station K1, October 2011 and February 2012). In that case, station K1, with a mean value of 0.91 ± 0.1, supported a community with the most evenly distributed individuals among species.

Principal Component Analysis (PCA) of the physicochemical parameters of estuarine waters during the dry season revealed that the first two axes, F1 (41.45%) and F2 (19.21%), together account for more than half of the total variance (60.66%) associated with the environmental variables under investigation. Examination of the correlation circle ( Figure 5 ) indicates a strong negative correlation between axis F1 and eight physicochemical parameters: Total Dissolved Solids (TDS, r = −0.94), Salinity (r = −0.90), Electrical Conductivity (EC, r = −0.89), pH (r = −0.66), Temperature (r = −0.66), Dissolved Oxygen (DO, r = −0.66), Nitrate ( ${\text{NO}}_3^{-}$, r = −0.57), and Turbidity (Tu, r = −0.32). In contrast, only Nitrite concentration ( ${\text{NO}}_2^{-}$, r ≈ 0.32) exhibited a positive correlation with axis F1. Axis F2 showed a positive correlation with Phosphate ( ${\text{PO}}_4^{3-}$, r = 0.76) and a negative correlation with Suspended Solids (SS, r = −0.82).

Principal Component Analysis (PCA) on axes F1 and F2 reveals a contrast between two groups of stations. On the one hand, stations S12, S11, and S15, characterized by high concentrations of dissolved substances and reduced turbidity, suggest mineralization of natural or anthropogenic origin. On the other hand, stations S1, S5, S6, S7, S8, and S9, showing low overall mineralization but an enrichment in nitrites, indicate recent pollution or incomplete degradation of nitrogen compounds. Stations S2, S13, and S14, with high phosphate concentrations, highlight pollution of residential or agricultural origin. Finally, stations S3, S4, and S10, dominated by Suspended Matter (SM), reflect significant organic pollution. ( Figure 6 ).

These findings are consistent with those reported by [22] for surface waters of the Moulouya River (Eastern Morocco), where high ${\text{PO}}_4^{3-}$ and suspended solid concentrations were similarly identified as indicators of organic pollution of domestic origin.

Principal Component Analysis (PCA) revealed that the first two principal components, F1 (explaining 62.60% of the variance) and F2 (explaining 11.23% of the variance), together accounted for 73.83% of the total variability observed in the environmental parameters. The variable correlation circle ( Figure 7 ) highlights a strong inverse association between F1 and five physicochemical parameters: turbidity (r ≈ −0.92), nitrites ( ${\text{NO}}_2^{-}$) (r ≈ −0.92), nitrates ( ${\text{NO}}_3^{-}$) (r ≈ −0.63), suspended particulate matter (SPM) (r ≈ −0.62), and phosphates ( ${\text{PO}}_4^{3-}$) (r ≈ −0.57). Conversely, F1 exhibited strong positive correlations with five other variables, namely electrical conductivity (EC) (r = 0.97), dissolved oxygen (DO) (r = 0.97), salinity (r = 0.95), pH (r = 0.81), and total dissolved solids (TDS) (r = 0.63). With respect to the second component (F2), only temperature (T, ˚C) showed a negative correlation (r ≈ −0.67).

Axis F1 reveals a contrast between sites most affected by human activities (S12, S13, S14, S15: characterized by high concentrations of nitrites, phosphates, suspended matter, and turbidity) and sites with lower anthropogenic influence, associated with better oxygenation (S1, S2, S3: correlated with higher salinity, electrical conductivity, and total dissolved solids). Stations S4 and S11 occupy an intermediate position (low concentrations of suspended matter), while stations S5 to S10 are distinguished along Axis F2 due to their geographical location in deeper or thermally cooler areas ( Figure 8 ).

Comparable findings were reported by [23] in their assessment of the physicochemical quality of the Merzeg River (peri-urban area of Casablanca, Morocco). Their results similarly showed that elevated concentrations of dissolved oxygen, pH, and electrical conductivity reflect mineralized and well-oxygenated waters, generally associated with reduced anthropogenic influence.

Influence of physicochemical parameters on the distribution of aquatic copepod communities in the Tinguilinta River estuary (Rio Nunez)

The RDA analysis reveals a clear distinction between different types of stations ( Figure 9 ):

Overall, the RDA results demonstrate that conductivity, temperature, salinity, pH, and dissolved oxygen are the key variables influencing copepod distribution. Similar patterns have been reported by [24] in four coastal rivers of southeastern Côte d’Ivoire and by [25] in the waters of the Upper Bandama River (Côte d’Ivoire).