Physicochemical gradients shape zooplankton and bivalve communities along the Sô River–Lake Nokoué continuum in Benin: implications for environmental health monitoring

Tropical river–lagoon systems are dynamic environments in which physicochemical variability can influence aquatic community structure. This study characterized the composition, taxonomic richness, occurrence, and spatiotemporal variation of zooplankton and bivalve communities along the Sô River–Lake Nokoué continuum in southern Benin. Monthly biological sampling was conducted at 11 stations from August 2024 to March 2025, including four stations in the Sô River (S1–S4) and seven in Lake Nokoué (S5–S11). A total of 24 zooplankton families and nine bivalve families were recorded. Eurotatoria dominated the zooplankton community, accounting for 56% of total relative density and exhibiting the highest taxonomic richness. Bivalves showed lower family-level richness but were recorded across the study area. Principal component analysis revealed a marked physicochemical gradient associated primarily with salinity, electrical conductivity, and total dissolved solids, distinguishing riverine from lagoonal conditions. Redundancy analysis identified associations between zooplankton community composition and dissolved oxygen and salinity, whereas bivalve community composition was associated with pH and temperature. These results highlight the environmental heterogeneity of the Sô River–Lake Nokoué continuum and provide an ecological baseline for future aquatic monitoring. Because pathogens, human exposure, and disease outcomes were not assessed, the findings do not establish a direct relationship between the observed community patterns and waterborne disease risk.