Carbon Sources, Transport and Sequestration in Tropical River Floodplains of Sabaki and Tana, Kenya

Rivers form a crucial component of the global carbon (C) cycle. They not only link terrestrial and oceanic pools, but their floodplains and channels also act as C sources and sinks, and areas of C biogeochemical processing. Quantification of C fluxes is often challenging because estimates can be biased if measurements do not adequately capture the high spatial (upstream vs downstream, channel vs floodplain) and temporal (day vs night, dry vs wet seasons, year to year) variability. This study focuses on closing existing knowledge gaps on the influence of river geomorphology on biogeochemical C processes and lateral C exchanges across river reaches and seasons in tropical river systems by quantifying C processes, sources and storage in two tropical river floodplain systems. Rivers Sabaki and Tana both originate from Kenya’s central highlands and drain into the Indian Ocean, but they differ strongly in their geomorphology and the degree of impact by agriculture, reservoirs, industries and nutrient inputs. We characterized C pools and sources (using C and N stable isotope ratios as proxies) in river water and floodplain sediments during different field campaigns in 2024 and 2025 during the dry season (September - October), as well as regular sampling of river biogeochemistry throughout the year. Additionally, we measured in situ benthic and pelagic respiration rates and concentrations of dissolved greenhouse gases (GHG: CO₂, N₂O, CH₄). Sediment organic carbon (OC) appeared to be mainly derived from riverine suspended matter, with localized contributions of floodplain vegetation in particular along the Tana River floodplains and in overbank floodplains of the Sabaki River. In the case of Sabaki, the sources of OC transported shows extreme contrasts between wet and dry periods, which are dominated by terrestrial runoff (mix of C4 and C3-derived C) and autochthonous production, respectively. The average sediment OC content showed a clear decline with depth (0.492% at <10 cm, 0.495% at 10-50 cm, 0.362% at 50-100 cm, 0.193% at 100-320 cm). Lower OC levels and preaged OC deposits within the top layer also supports the hypothesis that the floodplain OC is largely deposition from riverine particulate organic carbon (POC) during wet season. A strong correlation was observed between OC and clay content (r = 0.60, p < 0.001), and between OC and distance from the channel (r = 0.669 , p < 0.001).  Clay provides reactive surface area for OC sorption, and lower flow energy and fine sediments settle furthest. During the dry season Sabaki system is strongly autotrophic, characterized by strong CO₂ undersaturation and suspended matter dominated by photosynthetic biomass with a high OC content (on average 16.5%). Overall, our findings demonstrate that tropical river systems are highly dynamic component of the C cycle, in which geomorphology, seasonality, and land use strongly regulate C sources, storage and processing. Low land floodplains are primarily depositional sinks for in situ plant derived OC and allochthonous POC, with spatial patterns controlled by hydrodynamics and sediment texture, while temporal variability reflects shifts between terrestrial inputs during wet seasons and autochthonous production during dry periods.