Abstract
This work reviews the limnology of the largest multi-use reservoir in NE Brazil, the Castanhão Reservoir in Ceará State, during 5 years of an extended drought when the reservoir's volume decreased from 88% to about 30%. Major physical and chemical parameters of the water column, phytoplankton community, trophic state and sediment geochemistry were monitored, as well as the impact from extensive aquaculture. Water quality of the full reservoir was maintained due to hydrodynamics, which transport nutrients to the hypolimnion of a stratified water column, rendering an oligotrophic state to the reservoir, notwithstanding the large nutrient inputs from aquaculture and irrigated agriculture. However, with the extension of the drought period, the reservoir volume reduced, decreasing water depth leading to breaking of the thermocline due to wind forcing, and mixing the entire water column. This increased turbidity, nutrient availability and primary productivity, also changed phytoplankton functional groups. As a result, at the end of the monitoring period, when the reservoir attained its lowest volume, its trophic state became eutrophic. Under a scenario of climate change, where annual precipitation is decreasing, human uses of reservoirs in the semiarid should be very restricted to maintain water quality proper for human use.
Highlights
The growing scarcity of water in the 21st century is considered a most serious global environmental problem
The observed deterioration of the trophic state index (TSI) suggests that the first process to occur is the concentration and accumulation of nutrients from anthropogenic activities, following the decrease in the reservoir volume, breaking thermal stratification, mixing of the water column, and, at least in the shallowest stations, sediment resuspension
When integrating the observed distribution of nutrients in water and sediments and the reservoir hydrodynamics, a model of the evolution of eutrophication and its triggering processes can be detailed integrating the role of aquaculture and the changing volume of the reservoir (Figure 12)
Summary
The growing scarcity of water in the 21st century is considered a most serious global environmental problem. During the past 90 years extended drought occurred in three periods
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