Estudo comparativo da variação temporal da comunidade fitoplanctônica do açude Cruzeta inserido no bioma caatinga, Rio Grande do Norte

The Cruzeta impoundment, situated in the city of Cruzeta, hinterland of the state of the Rio Grande do Norte state has significant importance to the municipality as it represents the only source of supplying water to the region. It was hypothesized that the regional consequence of the global warming and the warming of surface water could substantially contribute the significant growth of the aquatic macrophytes in the years 2008-2009. The growth of these vegetation believed to be improved the degree of water transparency and as a consequence of the improved growth of phytoplankton species and chlorophyll biomass. At the same time the aquatic macrophyte could interact and compete potentially for the dissolved inorganic nutrients resources and the phytoplankton community. This study presents a comparison of years 2004-2005 when it did not have the expressive presence of the aquatic macrophyte community or restricted to the littoral region. In contrast, the years 2008-2009 showed a significant growth of the aquatic macrophyte in the Cruzeta/RN impoundments. The present study is an attempt to elucidate the significant presence of the aquatic macrophyte, Eichhornia crassipes, Ceratophyllum submersum, Nymphea sp and Pistia sp, and its interference on the ecology of phytoplankton. The samplings had been carried out from September of 2008 to April of 2009 and consistently between 10:00 h and 12:00 h with the aid of Van Dorn bottle and the plankton net of mesh size 20 Qm. The collections were made in three depths ie., surface, mid-column and bottom. The Physical-Chemical parameters such as pH, temperature, electrical conductivity and dissolved oxygen had been analyzed in situ. The samples for analysis of nutrients and chlorophyll were kept under refrigeration for posterior analysis in the laboratory. Phytoplankton samples were preserved in Lugol-iodine and kept for sedimentation for quali-quantitative analysis of phytoplankton. Enumeration of cells, colonies and filaments was done with the aid of Sedgwick-Rafter counting chamber and expressed as numbers/ml. Chlorophyll a was analyzed as a functional component of phytoplankton biomass and extracted with cold 90% acetone. The results indicate that the chlorophyll concentration varied between 5,65-8,08 Qg.L-1 for the dry period and 5,09-6,23 Qg.L-1 for the rainy period and showed considerable reduction when compared to the values to the 2004-2005 study period. The temperature was always presented higher in relation to the 2004-2005 study. Phytoplankton species showed a relative abundance of the Cyanophyceae for both the period of dry and rainy. The predominance species are filamentous Leptolymbya geophila Borzi (Planctolyngbya sp), Anabaena plankctônica Brunnthaler, Oscillatória limosa Ag. and Cylindrospermopsis raciborskii (Wolosz). The concentration of the nutrients such as nitrate and orto-phosfato had always presented higher values during the rainy period and the ammoniacal nitrogen retained moderate values in the dry period and a slight increase in rainy season. The main conclusions are the reduction of the concentration of chlorophyll, diversity of phytoplankton, and the increase in temperature and transparency of the water during the period of the study