Assessing sedimentation rates and carbon fluxes in a West-African lagoon (Nokoué Lagoon, Benin)
Résumé
This study provides the first detailed evaluation of sedimentary processes and the dynamics of particulate matter deposition in the Nokoué Lagoon, the largest inland water body in Benin, which plays an important role in regional biodiversity and socio-economic activities. Sediment cores collected from four representative locations in the lagoon were dated using radionuclide tracers ( 137 Cs and excess 210 Pb) to assess sediment accumulation rates (SAR), mass accumulation rates (MAR), and carbon fluxes (including both organic (OC) and inorganic (IC) carbon). Our findings reveal significant spatial variability in SAR, ranging from 0.1 cm.y⁻¹ in the East to 1.5 cm.y⁻¹ in the West. The highest accumulation in the western lagoon is associated with weak water circulation, and is further enhanced by anthropogenic factors such as dense brush fish parks (acadjas), nearby lacustrine villages lacking wastewater treatment, and seasonal proliferation of water hyacinths, all of which enhance sediment trapping and deposition. In contrast, the lowest sedimentation rate observed in the eastern lagoon likely results from the strong hydrodynamic conditions in this area that limit sediment deposition, although this site is located near the mouth of the Ouémé River that transports a high load of suspended sediments. MAR, ranging from 0.1 to 2 g.cm⁻².y⁻¹, follow similar spatial trends, with up to a 13-fold increase from East to West, suggesting net westward sediment transport. Since the 1970s, MAR has increased across the lagoon. OC and IC fluxes in the lagoon are low, remaining below 0.1 g.cm⁻².y⁻¹. These low IC fluxe values indicate minimal carbonate deposition and point to a predominance of terrigenous sediment inputs. OC fluxe reaches its highest levels in the western area, likely due to organic detritus originating from acadjas, decomposing water hyacinths, and untreated domestic wastewater. 210 Pbₑₓ inventories reflect sediment texture, with higher values (7931 Bq.m⁻²) in fine-grained sediments and lower values (1547 Bq.m⁻²) in coarser zones. Sedimentary 210 Pbₑₓ fluxes (48.3 to 247.3 Bq.m⁻².y⁻¹) are generally lower than estimated atmospheric inputs (ca. 280 Bq.m⁻².y⁻¹) at most sites, suggesting active erosion and lateral redistribution processes within the lagoon. This study offers the first robust estimate of sedimentation rates in the Nokoué Lagoon based on an accurate geochronological method. The mean sedimentation rate (0.8 cm.y⁻¹) is substantially lower than the value of 2.3 cm.y⁻¹ previously estimated in earlier studies using bathymetric map comparisons. Our findings suggest the influence of water circulation patterns and human activities on the spatial distribution of SAR and OC inputs, providing an essential baseline for future environmental assessments and management strategies.