Distribuição vertical nictemeral de Cladocera (Crustacea: Branchiopoda) no lago Tupé, Rio Negro, Amazonas, Brasil.

Abstract

Cladocerans are micro-crustaceans, with size range between 0,2 and 3,0 mm, living preferably in freshwater ecosystems. They are filtering herbivorous, with short life cycle and fast reproduction. Their strict relationship with the environment makes them sensible to alterations. Their distribution is heterogeneous and studies about their ecology shows an ability to perform vertical movements in the water column. These movements can be affected by food and oxygen distribution, and also by predator s presence. In neo-tropical ecosystems, especially in the Amazon Basin, the phenomena of vertical migration and its causes are still unknown. This study had the objective to understand the vertical distribution of Cladocera from the Tupé Lake, and its variations during a 24h cycle, during low and high water period and also detect the biomass for the most abundant species and its ecological implications. The Tupé Lake is a black water RIA lake, located close to Manaus, and a channel of connection with Negro river links the lake variations to the river s hydrological cycles. The study was performed during low-water period (November/2005) and high water period (june/2006), with sampling using a 55μm plankton net, every 4 hours, during the 24 hours cycle, every meter of dept of water column. The samples were fixed with formol 6%, and counted in Sedgewick-Rafter chamber, using optic microscope. Biomass were determined using dry weight. During the dry season, 17 species were registered, against 20 species during wet season. The most frequent species in dry season were Moina minuta (29,80%); Bosminopsis deitersi (29,21%) and Ceriodaphnia cornuta (18,34%). B. deitersi registered an heterogeneous distribution with higher densities in depths higher than 3m, an between 2-10pm. M. minuta e C. cornuta didn t show a clear pattern of vertical distribution, but an extreme decline in their populations between 2 and 6am were related to high Chaoborus sp. abundances, the last one being an important zooplankton predator. Holopedium amazonicum registered its higher densities at 3m depth, during most of the day. During high water period the species with higher occurrence were Bosminopsis deitersi (59,22%); Ceriodaphnia cornuta (12,42%) and Diaphanosoma polyspina (11,42%). The highest densities of B. deitersi during the high waters were registered at 6am, at 3m depth. D. polyspina registered its highest abundances in the same time period, at 5m depth. C. cornuta didn t show a clear distribution pattern. M.minuta and H. amazonicum only occurred in high densities during low water period. Between the environmental variables, only the light intensity related itself with the species distribution, due mainly to its relationship with predation. The biomasses of the more abundant species were close to others authors. C. cornuta and M. minuta were the species with higher biomass contribution to the total community during low waters, while during high waters; D. polyspina registered general biomass relatively close to B. deitersi, even though their densities were 6 times different. Differences in the size related distribution of the B. deitersi were registered.