The variability of plankton in reservoirs is still little known, mainly in those lake-like. Planktonic crustacean species structure was studied in the E.R. Mexia norpatagonic lake-like reservoir (39 degrees 30' South; 69 degrees West; A= 820 square kilometers; zmean= 25 meters; RT= 1 year). Temporal and macrospatial variations in number and biomass were also estimated and related with some environmental factors. The defined structural and phenologic model was contrasted with that of other lakes and reservoirs. Extensive samplings were carried out in the Limay River (to which the reservoir belongs) and the Neuquen River subbasins. Monthly vertical profiles in one sampling station (October 80 - March 83), and vertical hauls between bottom and surface were accomplished in six limnetic stations (November 83 - October 84) of the reservoir with an "ad hoc" developed sampler, with nets and flowmeter.

Changes in specific composition of the Limay River were evident from the upper end of the reservoir up to the rest of the river. The species registered in the reservoir were those with higher dispersion degree in the studied subbasins. The reservoir species number (8) was similar to those of temperate lakes. It corresponds to a ³copepod environment,²of BOECKELLA GRACILIS type. The reservoir taxocenosis showed high biocenotic resemblance with that of a glacial lake, similar in retention time and latitude. The mean annual biomass showed slight variations in three years and it agreed with the oligo-mesotrophic state established through classic parameters. Longitudinal gradients in density means (in area basis) of planktonic crustacean species and in turbidity (mainly inorganic) averages were observed. A gradient in primary productivity means (in area basis) was also inferred. Since predation doesn't seem to be responsible for these crustacean gradients, it is proposed that they would be linked to direct and indirect effects of the turbidity gradient. The temporal variation in density and in community structure exceeded the spatial one. The main change in community structure was displayed during summer due to an increased importance of cladocerans.