Phytoplankton community and physical-chemical characteristics of water in the public reservoir of Cruzeta, RN, Brazil.

The Phytoplankton community and the abiotic factors of the Cruzeta reservoir were studied at three depths, surface, middle (2 m) and bottom, from September, 2004 to June, 2005, in order to characterize the environment and assess the possible factors that influence the compositional change of phytoplankton. Ninety species belonging to 6 classes (Chlorophyceae, Bacillariophyceae, Cyanophyceae, Dinophyceae, Chrysophyceae and Euglenophyceae) were identified with 66 and 80 taxonomic units in the dry and rainy season. The most representative class in terms of species richness was Chlorophyceae and dominated by Scendesmus quadricauda, Oocystis sp. and Chlorella sp. The group cyanobacteria were represented by 18 species of diverse morphological characteristics and the dominance of Cylindrospermopsis raciborskii in September 2004. The other major group, Bacillariophyceae is represented by 21 species with the predominance of Aulacoseira granulata in mid-column and bottom waters. The other dominant species was Phacus acuminatus of Euglenophyceae. The species diversity and evenness indices were high, moderate and low in relation to the three hydroperiod registered during the 2004-2005 annual cycle. The reservoir exhibits high electrical conductivity (290-550 microS x cm-1), alkaline pH (7.3-9.4), mean temperature of 28 degrees C, varying concentrations of dissolved oxygen (3.29-7.6 mg x L-l) and the greatest concentration of nutrients at the bottom (orthophosphate, 0.22-0.62 mg x L-1) with the general tendency of oligo-mesotrophic status during sampling periods. The chlorophyll a fluctuated to a minimum of 1.34 microg x L-l at the bottom in April, 2005 and a maximum of 14.3 microg x L-l in mid-column water in September, 2004. The reservoir is characteristically an oligo-mesotrophic environment.