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Aim: The aim of this work is to diagnose the limnological conditions of the Castanhao Reservoir which is the largest and most important reservoir in the State of Ceara. This diagnosis is focused on physical-chemical variables, trophic state indices, and phytoplankton assemblages; Methods: Water samples were collected during six occasions from November/06 to July/07 from surface and bottom of the water column at three sites across the reservoir aiming to characterize physical-chemical (depth, transparency, suspended matter, temperature, dissolved oxygen, pH, redox potential, dissolved silicon, nitrogen and phosphorous) and biological (Chlorophyll-a, phytoplankton assemblages and total density) variables; Results: The trophic state index variables obtained for the reservoir showed Secchi disk depth ranging from 0.65 to 2.4 m, Chlorophyll-a from 0.71 to 15 µg.L -1 , total phosphorous from 11 to 369 µg.L -1 and soluble reactive phosphorous from 9.0 to 236 µg.L -1 . The results characterized a riverine-lacustrine transition area with low depth, transparency, dissolved oxygen with one anoxic event, and high values of suspended matter, total phosphorous and ammonium. The reduced light availability is probably the main factor inducing the low values for total phytoplankton density and Chlorophyll-a. The predominant class of phytoplankton was Bacillariophyceae with main species Aulacoseira distans var. distans as functional assemblage C. Typical lacustrine portion of the reservoir has higher depth and transparency than inlet reservoir what probably explains the higher total phytoplankton density. Cyanophyceae was the predominante phytoplankton class with main species Cylindrospermopsis raciborski as functional assemblage Sn. The phytoplankton assemblages in the Castanhao Reservoir were typical for mesotrophic/eutrophic aquatic environments confirming the trophic state of the reservoir; Conclusion: After five years completely flooded, the Castanhao Reservoir might be considered a mesotrophic, although depletion of dissolved oxygen and presence of a potentially toxic phytoplankton species (C. raciborskii) suggests the necessity of reservoir management focused on anthropogenic nutrient emission control and maintenance of natural ecological succession of the reservoir.

Trophic state, phytoplankton assemblages and limnological diagnosis of the Castanhão Reservoir, CE, Brazil