ROTIFERS OF THE UPPER PARANÁ RIVER FLOODPLAIN : ADDITIONS TO THE CHECKLIST

Rotifers present a high diversity in freshwater ecosystems. This study registered 11 genera and 42 species, new records for the Upper Paraná River floodplain. These results showed an increase in rotifer diversity in this ecosystem from 184 to 230 species. Among them some were registered only in the rivers and others in the lagoons. Thirty-seven species occurred in the littoral zone and 34 species in the pelagic; 3 species were registered only in the former zone and 2 species only in the latter. The lagoons presented the greatest richness, probably because of the greater stability, low current velocity, and the extensive aquatic macrophyte banks in the littoral zone of these environments as compared to those of the rivers. The highest number of species in the littoral habitats occurred due to the greater influence of shoreline vegetation, which allows greater habitat diversification. This fact contributed to the occurrence of non-planktonic species in the zooplankton samples.


INTRODUCTION
Biodiversity is the variety of organisms considered at all levels and includes genetic and ecosystem variants, which comprise arrays of species, genera, and families, as well as communities of organisms within particular habitats and the physical conditions under which they live (Wilson, 1992).
Rotifers present a high diversity in freshwater ecosystems.According to Nogrady (1993), early rotifer distribution studies show that all rotifers are cosmopolitan, but recent research considers that endemism is significant in several genera, e.g., Notholca, and describes latitudinal zonation in certain planktonic rotifers and restricted distribution for some species (Green, 1972;Pjeler, 1977;Dumont, 1983).
The Paraná River segment discussed in this paper presents a wide braided channel with a low gradient (0.09 m/km) and a wide floodplain in Mato Grosso do Sul State.This segment extends 480 km and may reach up to 20 km in width.It is the last non-dammed section of the Paraná River in Brazil (Agostinho et al., 1995).
New records from lagoons and rivers of the Upper Paraná River floodplain are included in this study following research developed by Bonecker et al. (1994Bonecker et al. ( , 1998a)), and Lansac-Tôha et al. (1997).

Environments and habitats
Based on the horizontal distribution in each environment, some differences were verified.Thirtyseven species occurred in the littoral zone and 34 species in the pelagic.Among these, 3 (Trichocerca cf.tigris, Lecane unguitata, Mytilinia cf.unguipes) species were registered only in the former zone and 2 (Lecane elegans, Paradicranophorus cf.wockei) only in the latter (Table 1).
Comparing the environments, the lagoons presented the greatest richness, is probably owing to the greater stability and low current velocity of these environments compared to that of the rivers.
As for some limnological characteristics of the environments in the various floodplain environments, the lagoons are shallow (1.5-5.0 m) and present lowest mean values for temperature, conductivity, and dissolved oxygen, and the highest mean chlorophyll-a concentrations (Thomaz et al., 1992).They also have extensive banks of aquatic macrophytes in the littoral region, principally Eichhornia azurea (Schwartz) Kunth.These lowland lagoons occupy depressions formed by active or inactive channels, and have constant or intermittent communication with the Paraná River or secondary channels (Souza Filho & Stevaux, 1997).
The rivers also present aquatic macrophytes in the littoral area, chiefly Eichhornia azurea (Schwartz) Kunth.The Baía River is characterized by low current velocity (0.11-0.50 m/s) and high concentration of dissolved humic substances.The Ivinheima River has neutral to alkaline pH, high conductivity, complete alkalinity, dissolved oxygen, and low chlorophyll-a concentrations (Thomaz et al., 1991;Bonecker & Lansac-Tôha, 1996).
Although there is no great difference in rotifer richness between the habitats of these environments, the highest number of species in the littoral habitats occurred due to the greater influence of shoreline vegetation, which allows greater habitat diversification (Green, 1972).This fact contributed to non-planktonic species occurrence in the zooplankton samples (Bonecker et al., 1998a).The important role of emergent and submerged aquatic macrophytes in the community composition of rotifers was also verified in small lagoons and temporary ponds in the Pantanal Matogrossense (Bonecker et al., 1998b), in an artificial lake (Martínez et al., 2000) in the State of Mato Grosso do Sul, and in a New Zealand lake (Duggan et al., 1998).
The higher diversity observed in the lagoons was related to some characteristics of these environments, e.g., low velocity of current.According to Ward (1994), the reproducing populations of planktonic organisms are restricted to the slow-flowing lower reaches of these environments.
Study area and sampling stations.