Planktonic rotifers from a tropical estuary under high marine influence (Passos River, PE, Brazil).

Passos River estuary, which is very influenced by marine waters, were carried out monthly from August/2009 to September/2010. A 64µm mesh size plankton net was hauled during low tide at two fixed stations in spring tides. Physical parameters as temperature, salinity, transparency and light extinction coefficient were obtained simultaneously. Statistical tests (Man-Whitney) were performed to evaluate possible seasonal and spatial variations. The rotifers were represented by only four species. Although there was no significant seasonal variation, the highest abundances were registered in the rainy season, with an expressive predominance of Brachionus plicatilis Muller, 1786. However, a population pick was registered in the dry season (September/2009; 13182,03 org.m-3 ) and the lowest value was registered in the rainy season (2010 May; 7,56 org.m-3 ). Brachionus plicatilis Muller, 1786 was highlighted as salinity indicator as well as its extremely opportunistic condition in eutrophic ecossistems, while Rotaria rotatoria (Pallas, 1766) and Rotaria sp. indicated organic pollution. Due to the constant high salinity regime the estuary showed few species richness. Keywords: Indicator, Rotifers, Salinity, Northeastern Brazil.