Abstract
Natural lake systems represent important reservoirs for residential water supply, fish production, recreational activities and enjoyment of their natural beauty. Nevertheless, human impacts may affect their health status resulting in degradation and loss of biodiversity. The aim of the present study was to obtain data on the health status of a natural lake located in an indigenous reservation in the Brazilian Amazon, using the phytoplankton community changes along the rainy (June) and dry (November) seasons of 2006. We collected water (temperature, pH, Secchi depth and conductivity) and phytoplankton samples from the subsurface, middle of the water column, and approximately 30 cm above the bottom, over 24-hour sampling periods, from a central station in the lake. Samples taken from biotic and abiotic variables were correlated using canonical correspondence analysis (CCA). Results showed that the lake exhibited high temperatures in both seasons, and showed thermal stratification only during the rainy season. Dissolved oxygen exhibited a clinograde pattern in the rainy season and high oxygen in the hypolimnion in the dry season. In the rainy season, the water near the bottom was acidic, turbid and had a greater concentration of phosphorus. Dissolved oxygen, conductivity, pH, nitrite, total phosphorus and total dissolved phosphorus exhibited diel variations in the rainy season, whereas water temperature, dissolved oxygen, total nitrogen and total dissolved phosphorus exhibited significant differences between hours of the day in the dry season. The phytoplankton was represented by 39 taxa, and Chlorophyta showed the greatest species richness, totaling 25 taxa. Among Chlorophyta, desmids were the most diverse, accounting 52%. Bacillariophyta (nine species) was the second most diverse group. Cyanophyta was represented by three species, including Merismopedia tenuissima, the most abundant taxon. Despite the occurrence of taxa that indicate organic pollution, their biomass and frequency indicate that the system is not currently threatened. Lake Caracaranã is an oligotrophic system, with low algal density and isolated blooming episodes due to its shallow depth.
Highlights
The state of Roraima is located in the Brazilian Amazon and has a hydrographic basin with a large variety of aquatic systems, which are distinguished by their combination of physical, chemical and biological characteristics
The species best adapted to the new ecological situation become more abundant and there is a reorganization of the structure and dynamics of the community
Mean monthly rainfall during the study period ranged from 7mm (February) to 643.7mm (June) and temperature ranged from 26.4oC (July) to 29.4oC (November) (Brazilian National Meteorology Institute, 2006)
Summary
The state of Roraima is located in the Brazilian Amazon and has a hydrographic basin with a large variety of aquatic systems, which are distinguished by their combination of physical, chemical and biological characteristics. A number of studies have been carried out on the floodplains of hydrographic basins in Argentina, Mexico, the United States, Spain, Japan and Colombia, addressing the structure and dynamics of the phytoplankton community and the main effects of variations in water level on the composition and functioning of these communities (Izaguirre et al 2001, Takano et al 2001; Maberly et al 2002; Komárková & Tavera 2003; Rodrigo et al 2003; Domitrovic 2003; Schagerl & Oduor 2003; Kangurt et al 2003; Roozen et al 2003; Schemel et al 2004; Walks & Cyr 2004; Pinilla 2006) The results of these studies demonstrate that cyanobacteria and Chlorophyta are important groups and often dominant groups in such environments
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