Eutrophication has a greater influence on floodplain lake carbon cycling than dam installation across the middle Yangtze region

•C/N ratios and δ13C suggest regional similarities in floodplain lake carbon cycling one century ago.•Timings of shifts in sedimentary proxies indicate larger responses to eutrophication than dam installation in the 20th century.•Eutrophication influences production and burial of autochthonous carbon by stimulating aquatic plant or algal growth.