Influence of physicochemical parameters on the spatial distribution of pelagic copepods in the Fatala River estuary (Boffa, Republic of Guinea)

This study, carried out between July and October 2020 in the Fatala River estuary (Boffa prefecture, Republic of Guinea), aimed to assess the specific diversity of pelagic copepods and to analyze their spatial distribution in relation to the physicochemical parameters of the environment. This estuary, subject to significant anthropogenic pressures linked to mining, agricultural, fishing and domestic activities, presents a high vulnerability to water quality degradation. Copepod samples were collected during four field campaigns using a zooplankton net and then identified in the laboratory. Physicochemical parameters (temperature, salinity, conductivity, dissolved oxygen, pH) were measured in situ, while suspended solids, turbidity, nitrates and phosphates were analyzed in the laboratory. The taxonomic analysis identified 59 species, distributed in two orders: Calanoida (89.83%) and Cyclopoida (10.17%). These species belong to 17 families and 27 genera. The community is largely dominated by Paracalanus aculeatus (25.14%), Aetideus armatus (18.93%), Paracalanus scotti (15.07%) and Paracalanus parvus (9.83%), together representing nearly 69% of the total abundance. The results show a clear spatial heterogeneity. In the upstream stations (S1 to S5), copepod densities vary between 0.00 and 499.34 individuals/m³, with an average of 258.352 individuals/m3, a reduced specific diversity (Shannon index H' ≤ 0.19) and the total absence of copepods at some stations. These low densities are explained by unfavorable environmental conditions, characterized by an acidic pH (6.3–6.7), low salinity, reduced mineralization, as well as very high phosphate concentrations (up to 28.6 mg/L). Conversely, the downstream stations (S6 to S14) present much higher densities, between 1733.51 and 4565.77 individuals/m³, with an average of 3248.89 individuals/m3, a higher specific richness and more favorable physicochemical conditions: alkaline pH, increased salinity and conductivity, moderate turbidity. The dominance of a limited number of euryecious species reflects an ecological imbalance typical of estuarine environments subject to multiple anthropogenic pressures.