Ecology of the freshwater phytoplankton assemblages from a tropical reservoir of Northeastern Brazil

The ecology of the freshwater phytoplankton assemblages was investigated in João Alves Reservoir of Parelhas, Rio Grande do Norte, from July, 2003 to June, 2004. The present study is important for the reservoir management and development of inland fisheries. This tropical shallow reservoir is located on the Serido River of Piranhas-Açu hydrographic basin and has a water holding capacity of 85.012.750 m³. Water and phytoplankton samples were collected on fortnightly basis from a fixed station where maximum depth was registered. Ecological indices, such as, Shannon-Weaver and Pielou were calculated for species diversity and evenness, besides Margalef's species richness index. The variables, such as, pH, temperature, electrical conductivity and dissolved oxygen concentrations were analyzed, besides nitrate-nitrogen, ammoniacal nitrogen, orthophosphate and chlorophyll content. Alkaline pH, reduced transparency, significant dissolved oxygen concentrations and moderate electrical conductivity were observed during an annual cycle. The trophic status of the reservoir shows a tendency of oligo to mesotrophic state, with low to moderate inorganic nutrients. The phytoplankton showed the dominance of diatom species, Aulacoseira granulata, A. distans and Cyclotella meneghiniana both during dry and wet period and correlated significantly with reactive silicate and nitrate-nitrogen. The Chlorophyceae species, particularly Oocystis borgei grew with the onset of rainy season and co-existed with the Cryptophyceae species. The proliferation of “r” strategists’ of unicellular and colonial forms of phytoplankton species contributed increased diversity and co-occurrence. The reduced cyanobacterial species was attributed to corrective management practice, which aimed to keep sustained development of cage culture of fishes under organic cultivation and limited nutrient levels.