Fluctuación temporal del zooplancton en arroyos y esteros del Chaco Oriental (Argentina)

Resumen

The zooplankton of 20 streams and 4 tropical swamps located in an area locally called "Eastern Chaco" (comprised by the Pilcomayo river to 28°S and the Paraguay and Paraná rivers to 60°W) was studied from May to October, 1984, during high and low water periods. Three series of quantitative samples (53 in total; 100 liters each) were collected just below the surface at 24 stations placed along a transect paralell to the National Highway 11, with a 10 liters vessel and a 53 μm-mesh sieve.

Some of the streams which were subject to wide environmental fluctuations showed clear changes in the density and dominance conditions of the zooplankton in relation to the hydrologic regime and the conductivity of the water. During the high water period, characterized by conductivity values between 90 and 420 μS.cm^-1, collections from open water locales vielded low densities (10 to 100 individuals.liter^-1) and invariably contained littoral and periphytic taxa which were swept into the streams from other waterbodies (swamps, small pools). Maxima of both conductivity (11 OOOnS.cm -1) and zooplankton numbers (1 500 individuals.liter^-1) occurred during the dry season, with dominance of rotifers adapted to high salinity conditions (Brachionus plicatilis, B. angularis, Synchaeta sp., etc.). Other streams presented low values of conductivity (below 500μS.cm^-1) and a well developed aquatic vegetation during both high and low water periods; the zooplankton was here dominated by Bdelloidea, Polyarthra sp. and Filinia sp., the halophilous species being absent. The microcrustacean fauna of the streams was almost invariably characterized by large numbers of species and low densities, Ceriodaphnia sp. and Copepoda nauplii being the only organisms of some numerical importance.

In the swamps, both truly planktonic species (Polyarthra sp. (vulgaris-dolichoptera group), Keratella lenzi, K. serrulata, Filinia terminalis, F. longiseta, Diaphanosoma sp.) and a large number of adventitious (pseudo planktonic) forms were recorded; this assemblage and the relatively high density of organisms during the dry season (up to 300 individuals.liter^-1) contrast noticeably with the few available records from adjacent areas.