Land use, hydroclimate and damming influence organic carbon sedimentation in a flood pulse wetland, Malaysia

Water bodies located in floodplains and tropical forests are known to be important carbon stores, but many are subjected to intensive pressures from damming, land use and climate changes. Sedimentary records preserve long-term archives for understanding how such changes affect the quantity and quality of carbon stores. We analysed sediment cores from seven sites across a flood-pulse multi-basin wetland, Tasik Chini in Peninsular Malaysia (for percentage LOI550, sediment density and spheroidal carbonaceous particles), and conducted more analyses on three 210Pb-dated cores (X-ray fluorescence of elements, grain size analysis, carbon isotopes, C/N ratios, carotenoid pigments) to gain an understanding of the drivers of organic carbon accumulation rates (OCARs) since 1860 ce. The median OCAR of 85 g m-2 a-1 for the basin since 1945 ce was higher than in other floodplain and temperate lakes and in line with other tropical forest lakes. However, we found evidence for different mechanisms of OC deposition across the basin. In 'autochthonous mode', the site with minimal local land disturbance had lowest OCARs and OC was derived mainly from autochthonous production, which rose slightly around 1940 ce when regional land disturbance increased nutrient influx to the basin. The site with the most long-term and intensive land disturbance through forest removal (1940s) and then conversion to rubber and oil palm farming (1980s) functioned mainly in 'allochthonous mode'; that is, increases in OCARs after 1940 ce were driven by deposition of soil-derived OC. The highest OCARs were in the basin that was converted to oil palm after the 1980s and had increased iron mining activity in the 2000s; because this site was located distal from the flood pulse and became increasingly hydrologically disconnected after a low rainfall period in the 1970s, the lake responded strongly in 'autochthonous mode', through encroachment of fringing swamp, the spread of benthic algae and macrophytes, and efficient sediment retention. Weir installation in 1995 ce raised water levels and increased lentic conditions, promoting autochthonous OC production and sedimentation across all basins. The long-term fate of this more recently deposited OC remains uncertain because it is more labile. Overall Tasik Chini has responded strongly to land use changes since at least the 1940s, earlier than anticipated in this region of Southeast Asia, and the sedimentary proxies indicate large changes in the ecosystem function and capacity for C storage over the past ca. 80 years. Most of these shifts have increased OC accumulation by strengthening autochthonous production or allochthonous OC fluxes, but the implications for other aspects of the C cycle, including catchment soil C loss and greenhouse gas production, need to be accounted for when evaluating the overall impacts of land and hydrological disruption.