Connected reservoirs: modelling aquatic ecosystems along a cascade system in Brazil

Connected reservoirs along cascade systems have been constructed along large rivers worldwide, establishing a network of aquatic environments. To date, modeling studies largely ignore the ecological feedback on water quality in connected reservoirs, thus limiting the ecosystem representation and missing the mechanisms acting between them. Here, we applied a novel modelling framework that fully links reservoir processes along the cascade system, considering the input from each water body to the next in the system. The one-dimensional GLM-AED model was applied to simulate hydrodynamics and biogeochemical processes in six reservoirs along the Tietê River (Brazil) situated in the most populous Brazilian state (24 million inhabitants), playing a relevant role for the energy generation and water supply in the region. The model was run for the 2008-2016 period, calibrated and validated against measured field data. Eighteen scenarios of reducing nutrient loads were simulated to assess how restoration strategies modify the ecological state downstream. All six reservoirs were sensitive to nutrient load reductions, which changed the nutrient retention capacity, and triggered a domino effect along the cascade system, improving the ecological conditions further downstream. It reveals that local restoration strategies devoted to the uppermost reservoir in a cascade system are propagated and amplified along the system. This finding is of primary interest to water managers since the improvements from local strategies into the uppermost reservoir, rather than site-specific, act at catchment scale.