Print PageOver the last decade the southeastern Amazon has been one of the most active deforestation frontiers on the planet. Large-scale land cover and land use changes occurring in the region have the potential to degrade stream ecosystems through a variety of mechanisms, including fragmentation of the stream network by impoundments, changes in the amount of light and nutrient inputs from riparian areas, and alterations to both the quality and quantity of water flowing within streams. We evaluate some of these impacts in the headwaters of the Xingu Basin at two spatial scales. At the landscape scale, we use satellite time series data to understand the spatial and temporal dynamics of land use transitions and related changes to stream connectivity. Combining data from moderate and high resolution sensors, we characterize land use history, as well as the distribution of impoundments and riparian forests in the upper Xingu watershed. At the microwatershed scale, we use field data collected at 12 headwater streams in catchments dominated by pasture, soy croplands, and forest to evaluate the local impacts of these land uses on stream ecosystems.
Results/Conclusions
Landscape scale results reveal a stream network that is highly fragmented and increasingly dominated by agricultural land uses. Field measurements show that this fragmented landscape can have important consequences for headwater streams, including significant differences in stream temperatures across land uses. Streams draining pasture catchments were characterized by warmer (27.3°C) mean diel temperatures than those draining forest (24.5°C) and soy (26.3°C) catchments. Furthermore, the mean diel temperature range of pasture and soy streams was twice that of forest streams, indicating that agricultural catchments have greater variability and more extreme daily maximum temperatures than forested catchments. Temperature data collected upstream and downstream of impoundments indicate that these reservoirs have a pronounced warming effect on small streams, with temperature increases ranging from 2-4°C and persisting for at least 500 m downstream of a reservoir. Together with changes in connectivity at the landscape scale, such increases in temperature can exert a strong influence on the quality and distribution of stream habitat, thus negatively impacting stream biota and the overall integrity of aquatic ecosystems. Preliminary models suggest that these impacts could be substantially mitigated through improved land use practices, including the preservation and restoration of riparian areas and management of impoundments in the landscape.
