The aim of this work is to diagnose the limnological conditions of the Castanhão Reservoir which is the largest and most important reservoir in the State of Ceará. 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 Castanhão Reservoir were typical for mesotrophic/eutrophic aquatic environments confirming the trophic state of the reservoir; Conclusion: After five years completely flooded, the Castanhão 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.
This study evaluated changes in phytoplankton and nutrients after an increase in river discharge in four estuaries of northeastern Brazil. Samples were taken during a markedly freshwater-deprived period (Nov/2010, dry season), and during a rainy season (May/2011). While two estuaries are located in urban areas (Ceará -CE and Cocó -CO), the others are markedly freshwater-deprived (Pacoti -PAC and Pirangi -PIR). Our findings indicate that increased freshwater had opposing effects on estuaries impacted by anthropogenic discharges (CO, CE and PIR) compared with the other, less impacted (PAC). In CO and CE, the freshwater flow during the rainy period had a nutrient diluting effect and controlled phytoplankton blooms. In PIR, only phosphorus and inorganic nitrogen decreased in the rainy season. In contrast, an increase of all nutrients was observed in PAC, and of TN in PIR, followed by an increase in phytoplankton biomass. In the rainy season, eutrophic freshwater species dominated in all the estuaries. In the dry season, blooms of Cryptomonas/Rhodomonas and Synechocystis aquatilis were, respectively, observed in CE and CO. Also in the dry season, PAC and PIR had a predominance of marine species under a condition of hypersalinity. Canonical correspondence analysis suggested the variation observed for salinity, silica, and TN are the main driver of phytoplankton structure.
AbstrAct
Environmental variability and hazards on the coastal and continental shelf regions of South America and the Caribbean
Special Article CollectionNutrients and chlorophyll across a coral reef
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