How a mega-dam in the Amazon’s largest tributary altered the drivers of planktonic communities

Community drivers shift over time in response to changes in landscape and environmental conditions. Although dams alter the landscape (e.g., by flooding areas) and environment (e.g., by increasing transparency), their impact on the influence of community drivers remains unclear. We investigated how the construction of a large Amazonian River dam altered the influence of environment and space on planktonic communities. From 2005 to 2015, we monitored environmental parameters and collected phytoplankton and zooplankton from 15 locations along the Madeira River and its tributaries. We conducted twenty-nine samplings upstream of the Jirau dam before, during filling, and after its construction, evaluating the influence of environment and space on communities and assessing any changes with damming. We documented the frequency of significant variables to determine if their importance (frequency) changed post-damming. Following damming, environmental heterogeneity decreased and communities comprised species adapted to lentic habitats, with the environment exerting the main influence. Damming did not significantly alter environmental influence but shifted specific drivers, with some factors decreasing (e.g., nutrients) and others increasing (e.g., oxygen) in importance, indicating changes in the environmental mechanisms underlying community assemblage. Monitoring programs in dam-affected regions should be regularly evaluated due to the temporal variability of aquatic community drivers.