Lability of riverine particulate organic carbon increased by water storage and extreme drought-heatwave events

Rivers are increasingly affected by dams and reservoirs, which impact the source and transport of riverine particulate organic carbon (POC), and by extreme events, which present an important climatic stressor for river ecosystems. How and to what extent water regulation and extreme events affect POC lability, however, remain poorly constrained. By studying the POC characteristics within the Yangtze River Basin in normal and extreme drought-heatwave years, we find that during a normal year water storage by the Three Gorges Reservoir increases the proportion of algae-derived POC (by 26 ± 4%) compared to water drainage, leading to a higher proportion of labile POC and younger POC age. During the extreme drought-heatwave year, the drought-heatwave event increases proportions of algae-derived POC and labile POC (by 19 ± 11% and 18 ± 8%, respectively) in the whole Yangtze River Basin, with a greater increase in the Three Gorges Reservoir (by 34 ± 7% and 29 ± 3%, respectively; compared with the non-Three Gorges Reservoir areas, P < 0.01), whilst resulting in a 29 ± 6% reduction in export POC flux to the East China Sea. We use our findings from the Yangtze River Basin to assess the global impact of dams and reservoirs, and estimate that the global riverine export POC flux has decreased from pre-dam 190 ± 5 Tg C/yr to post-dam 97 ± 48 Tg C/yr. We also posit that the additional increase in POC lability and decrease in export POC flux during an extreme drought-heatwave event in the Yangtze River Basin occurs in other major river systems as well. Combined we assert that more frequent extreme drought-heatwave events will compound the effects of damming, resulting in an increasingly labile and thus more readily remineralized riverine POC source and a decreasing export POC flux into the near-shore environment. Our results provide critical insights into assessing and mitigating the effects of combined damming and extreme drought-heatwave events on the organic carbon cycle in river systems.