Zooplankton assemblages (Copepoda and Cladocera) in a cascade of reservoirs of a large tropical river (SE Brazil)

Abstract

Zooplankton assemblages (Copepoda and Cladocera) in a cascade of reservoirs of a large tropical river (SE Brazil) The construction of reservoirs for the production of electricity is one of the major human interferences on large tropical rivers. This study analyzes the structure of the microcrustacean assemblages (Copepoda and Cladocera) in a cascade of 8 reservoirs in the Paranapanema River. Samples were obtained during 8 campaigns through a vertical haul (from bottom to surface) with a net with 50 μm of mesh size in 19 stations distributed along the watershed (ca. 700 km). The 3 main tributaries were sampled downstream, just like the mouth zone ot the Paranapanema River into the Paran´ a River. Nineteen species of Copepoda and 56 of Cladocera were identified. This high richness confirms the importance of considering the entire watershed for the elaboration of regional inventories. The main species were Notodiaptomus henseni, N. iheringi, Thermocyclops minutus, T. decipiens, Mesocyclops longisetus, M. meridianus and M. ogunnus, among the copepods, and the cladocerans Daphnia gessneri, Ceriodaphnia cornuta rigaudi, C. cornuta cornuta, C. silvestrii, Diaphanosoma spinulosum, D. birgei, D. brevireme, D. fluviatile, Moina minuta, Bosmina hagmanni and Bosminopsis deitersi. The seasonal occurrence of the main species is discussed. A downstream increase in richness (River Continuum Concept) was not observed. The transition zones (riverine to lacustrine) and the zones connected to flooded areas and lagoons have a positive effect on the richness. Comparisons of the diaptomidae composition and dominance between decades indicate an important structural change, probably related to the increase in the trophic conditions. Two different conditions favor the increase in abundance: higher water retention time and also of the trophic conditions. Temporal and longitudinal (intra-reservoir) trends were observed but they could not be generalized. The variation of the relative abundance of the cladoceran families and Thermocyclops species, as well as of the Shannon diversity index, were related to the different trophic and hydrodynamic conditions in the cascade. The analysis of the microcrustacean composition and abundance has allowed the identification of important spatial patterns in the hydrographic watershed, including the effects of damming.