Respostas biogeoquímicas de um grande rio amazônico a mudanças ambientais

Abstract

Climate change and river regulation for hydropower generation are two of the main stressors pressuring the Amazon biome. Understanding the responses of Amazonian ecosystems to these stressors is key to support future strategies of socioeconomic development, and control and mitigation of associated socioenvironmental impacts. This thesis investigates the hydrobiogeochemical effects of environmental changes on the Madeira River, the largest tributary to the Amazon in terms of water and sediment discharge. The first chapter investigates the relationship between extreme floods and carbon dioxide (CO2) outgassing from the Madeira River. We show data from 9 field campaigns performed between 2009 and 2011, complemented with data from one additional campaign in April 2014 that is representative of the largest flood on record. CO2 outgassing fit an exponential relationship with water level. CO2 outgassing per unit area in 2014 was 50% higher than the other highest rate in our dataset. Reconstruction of CO2 fluxes since 1968 indicates that extreme-flood years outgas 20% more CO2 per unit area than years without reported occurrence of extreme floods. The second and third chapters examine the upstream and downstream hydrobiogeochemical effects of the Santo Antônio dam, a run-of-river hydropower dam newly built on the Madeira River. Whereas storage dams typically retain most sediments, the extent to which modern run-of-river dams do so is uncertain. Contrary to expectations, the Madeira dams have not detectably affected downstream transport of total suspended sediments, total organic carbon, and associated nutrients (phosphorus, potassium, calcium, and magnesium) critical for downstream river and floodplain productivity. This is likely because the reservoir’s short residence time and rapid water velocity prevent substantial sedimentation of the predominantly fine-grained sediments. Temperature depth profiles demonstrate that unlike the mainstem, some backflooded tributaries periodically developed thermal stratification. The water chemistry in backflooded tributaries became more similar to that of the mainstem after damming. In contrast, the overall water chemistry in the mainstem did not significantly change. While the effects of this run-of-river dam design on the main river are minimal, lateral bays created by backflooded tributaries are more susceptible to changes in water chemistry. The results of the chapters of this thesis suggest that alterations in the water level and flow – caused either by natural phenomena or anthropogenic regulation – affect ecosystem processes through changes in the connectivity of aquatic systems with adjoining terrestrial and aquatic-terrestrial transition zone habitats. The findings of this thesis have implications for decision making and management, especially considering the large number of hydropower dams planned in the Amazon basin.