Abstract
Species composition, density, and temporal dynamics of zooplankton were studied in six reservoirs in a semi-arid region in tropical northeastern Brazil (Rio Grande do Norte state). All the reservoirs are highly eutrophic, with high contents of total nitrogen (minimum of 1200 µg.L(-1)) and total phosphorus (minimum of 10 µg.L(-1)), and extremely high algal biomass was registered (surpassing 20 µg Chl a.L(-1)). All the reservoirs showed an enduring condition of high turbidity and phytoplankton assemblages dominated by cyanobacteria. Zooplankton also showed quantitative patterns suggestive of eutrophic conditions, expressed by high densities, mainly in Passagem das Traíras and Sabugi reservoirs. A spatial differentiation in the composition of the zooplankton community was registered. Rotifers (especially Keratella tropica, Brachionus havanensis, and Keratella americana) were the dominant forms in the zooplankton community of Itans, Passagem das Traíras, and Sabugi reservoirs, while calanoid copepods (mainly Notodiaptomus cearensis) dominated in the Armando Ribeiro, Gargalheiras, and Parelhas systems. The existence of novel relationships in zooplankton community composition in eutrophic reservoirs in this tropical semi-arid region must be considered in designating zooplankton indicators of eutrophic conditions.
Highlights
Species composition, density, and temporal dynamics of zooplankton were studied in six reservoirs in a semi-arid region in tropical northeastern Brazil (Rio Grande do Norte state)
The depth of the euphotic zone was smaller in Parelhas and Armando Ribeiro Gonçalves reservoirs
Sabugi and Gargalheiras reservoirs were less affected by temporal fluctuations in transparency (1.0-1.4 m)
Summary
Density, and temporal dynamics of zooplankton were studied in six reservoirs in a semi-arid region in tropical northeastern Brazil (Rio Grande do Norte state). The cyanobacteria can form large colonies or aggregates, which are generally unpalatable to the majority of planktonic herbivores because of the direct physical interference with their filtration apparatus (Webster & Peters, 1978; De Bernardi and Giussani, 1990; De Mott et al, 2001) Because of their chemical composition, the cyanobacteria are considered to be nutritionally deficient as a food for zooplankton (Brett and Müller-Navarra, 1997). As a function of their short life cycles, changes in the phytoplankton community are rapidly reflected by the zooplankton, which comes to indicate the intensity and conditions established during and after the consolidation of these disturbances All these characteristics make the zooplankton communities key elements for the understanding of the changes occurring in aquatic ecosystems due to eutrophication, in understanding the potential for propagation of these disturbances along the food chains
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