How artificial drought generated by the Balbina hydropower dam has transformed the floristic structure of downstream floodplain forests

The central Amazon basin has been affected by the two lowest water levels on instrumental records in 2023/2024, inducing severe hydrological droughts affecting human populations and floodplain ecosystems. However, the impacts of severe droughts on Amazonian floodplains are poorly studied. Here we investigate the extent of an artificial drought in the blackwater floodplains (igapó) downstream of the Balbina dam, caused by the damming of the Uatumã River to fill the reservoir in the period 1987–1989. Our hypothesis is that the severe prolonged hydrological drought conditions killed the igapó forests downstream the dam. Therefore, we established at regular fluvial distance of 10 ± 1 km a plot along the main course of the Uatumã River to analyze changes in floristic composition, diversity, structure and biomass of the igapó forests downstream of the Balbina dam. In 15 plots (total area 3,75 ha) a total of 2559 trees with diameter at breast height (DBH) ≥ 10 cm were inventoried, identified, and allometric equations were applied to estimate aboveground biomass. Based on floristic dissimilarity, a non-metric multidimensional scaling suggests two plot clusters, separated by the fluvial distance of about 120 km downstream of the dam. ... mehrWhile the plot cluster close to the dam is characterized by a high abundance of resprouting trees and biomass of disturbance-indicator species, plot cluster 2 presents a higher species richness and basal area and is dominated by late-successional species. We discuss these findings in the background of increased frequency and magnitude of severe droughts in recent years and the planned hydropower plants which are threats for this ecosystems harboring an unique biodiversity.