Salinity-driven controls on carbon cycling along the Gambia River (West Africa)

The salinization of inland waters, driven by sea-level rise and anthropogenic activities, poses increasing threats to aquatic ecosystems and the human communities that depend on them. This process can alter the functioning of tidal rivers, particularly their biogeochemical cycles and their role as sources or sinks of carbon. The Gambia River, in West Africa, represents a key system for study: first, because the contribution of inland waters in this region—and in equatorial dry climates more broadly—to the global carbon cycle remains poorly quantified; second, due to its relatively intact hydrogeomorphology; and third, because its integrity is increasingly threatened by salinization linked to sea-level rise (~4 mm yr⁻¹ in the region), climate-driven changes in precipitation patterns, and upstream dam construction.

Within this context, we investigated how salinity influences key parameters of aquatic carbon cycling by combining seasonal and spatial sampling along the river–estuary continuum. We tested whether increasing salinity affects the concentrations, sources (δ¹³C isotopic signatures), and atmospheric emissions of dissolved greenhouse gases (CO₂ and CH₄), and how salinity gradients influence the availability and partitioning of carbon pools, including dissolved organic carbon (DOC), particulate organic carbon (POC), dissolved inorganic carbon (DIC), and alkalinity.

To measure these variables, along with ancillary parameters such as chlorophyll-a, total suspended solids, and nutrients, we conducted three sampling campaigns between 2024 and 2025 under dry, wet, and transitional seasonal conditions, covering 12 sites from freshwater reaches (~400 km inland) through the estuary to the coastal ocean. Each campaign also included intensive spatial sampling across salinity transition zones (67, 16, and 15 additional sites, respectively).

Preliminary analyses indicate that nutrients and ions exhibit relatively stable concentrations in freshwater reaches, increase at the onset of salinity intrusion, and stabilize again under fully saline conditions, with overall higher values in saline sections compared to freshwater. In contrast, CO₂ concentrations increase downstream in the river but decrease again across the salinity transition zone, remaining slightly higher in saline sections than in freshwater reaches, which can occasionally be undersaturated relative to the atmosphere, resulting in negative CO₂ fluxes. CO₂ patterns appear to be primarily associated with organic matter availability, closely following DOC distributions rather than salinity gradients, and showing an inverse relationship with chlorophyll-a, suggesting an important role of biological uptake in the upper river.

CH₄ dynamics, in contrast, show a stronger sensitivity to salinity, likely reflecting enhanced microbial competition with sulfate under saline conditions, which may reduce CH₄ production and emissions. Isotopic signatures indicate shifts in dominant methanogenic pathways, highlighting the role of organic matter composition and availability in controlling methane production pathways rather than absolute production rates.

Alkalinity was generally higher than DIC along the river–estuary continuum, and both variables deviated from conservative mixing at intermediate salinities (10–20), indicating the presence of in situ DIC production within the estuary. Together with the observed patterns in other carbon pools, these results demonstrate that salinity gradients exert differential controls on carbon species and associated biogeochemical processes along the Gambia River.