CYANOBACTERIA AND CYANOTOXINS IN RESERVOIRS OF RIO GRANDE DO NORT ESTATE AND THE POTENCIAL CONTROL OF BLOOM SBY NILE TILAPIA (Oreochromis niloticus).

Abstract

Cyanobacteria blooms in reservoirs result in loss of water quality and negative effects to human health. To reduce these impacts the monitoring of the cyanobacteria and cyanotoxin occurrence as well as the application of measures to counteract the cyanobacteria blooms are made necessary. The manipulation of filter-feeding omnivorous fish stock, as Nile tilapia, has been proposed as an strategy for cyanobacteria bloom control. The present work aimed to evaluate: I) the presence of cyanobacteria (composition, density and biovolume by sedimentation technique) and cianotoxins (mouse bioassay) in five reservoirs located at the semi-arid region of Rio Grande do Norte state, in five sampling campaigns between September 2002 and March 2004; II) the per capita consumption rates of filamentous cyanobacteria by Nile tilapia through laboratory experiments using a natural population of cyanobacteria (experiment I) and a culture of Cylindrospermopsis raciborskii (experiment II), after the juveniles tilapias being exposed to a gradient of cyanobacteria biomass. The phytoplankton of the investigated reservoirs were dominated by cyanobacteria, including various toxigenic species (C. raciborskii, Microcystis spp., Aphanizomenon e Anabaena circinalis). Cianotoxins were present in three out of five reservoirs. The consumption rate of cyanobacteria (?g chlorophyll-a.fish-1.day-1) by the tilapia was 0,29 in the experiment I and 0,5 in the experiment II. The human populations that use the studied reservoirs as drinking water supply are being potentially exposed to the negative effects of the cyanobacteria. The present research suggests that the stock of Nile tilapia to control cyanobacteria blooms is viable. Nevertheless, factors such as the ichthyo-eutrophication and the accumulation of cyanotoxins in the fish biomass should be taken into account before implementing a biomanipulation program.