Hydrochemistry and trophic state change in a large reservoir in the Brazilian northeast region under intense drought conditions

Abstract

Semiarid region following decreasing reservoir volume due to rainfall shortage during four consecutive years. The monitoring pe-riod, between November 2011 and May 2014, assessed approximately 50% water volume reduction and 10 meters’ decrease ofreservoir water level that degraded water quality. Decrease in reservoir volume, strong evaporation and the permanent influenceof anthropogenic activities, favored the concentration of salts and accumulation of nutrients and of increasing pH. Thermal strat-ification of the water column occurred when volume was maximum and lead to a significant reduction in dissolved oxygen in thehypolimnion (0.07 to 2.62 mg L–1). Diminishing volume resulted in mixing of the hypolimnion nutrient-rich and oxygen-poor watersin the entre water column and changed the initial oligotrophic condition to eutrophic. However, the temporal scale of the responseof the reservoir’s trophic state differs in the different areas of the reservoir. Whereas deeper areas accumulating nutrients fromaquaculture and agriculture progressively became mesotrophic and eventually eutrophic; shallower regions far from direct an-thropogenic influences, changed their trophic sate much later, but rapidly turned into super-eutrophic conditions, probably due tomore intense sediment resuspension and water mixing. Trophic State Index followed nutrient increase during most of the period.However, it also responded to an increase in chlorophyll a concentrations when the reservoir achieved its minimum volume, inparticular in the shallower areas. The results suggest that this type of reservoir systems are vulnerable to eutrophication duringextended drought periods and that a better assessment of the maximum support capacity for reservoir activities, particularly aqua-culture, must be re-assessed taking into consideration worst case scenarios forecasted by global climate change.Key words: