Development of Anabaena Bory (Cyanobacteria) blooms in a subtropical shallow lake, south Brazil

BECKER¹, V.; CARDOSO², L. DE S. & MOTTA MARQUES³, D. da

¹ Programa de Pós-Graduação em Ecologia

^2,3 Instituto de Pesquisas Hidráulicas – UFRGS.Av. Bento Gonçalves nº 9500, cep. 91540-000, Porto Alegre, RS, Brazil. e-mail: ¹vbecker@via-rs.net ; ²lu.scardoso@ig.com.br; ³dmm@iph.ufrgs.br

Abstract: Cyanobacteria have a number of special properties wich determine their relative importance in phytoplankton communities. Some species of cyanobacteria produce toxins and their blooms may cause the death of aquatic organisms or even of human beings. Species of cyanobacteria respond in different ways to environmental fluctuations in the habitat. The main purpose of this paper is to record the occurrence of blooms of Anabaena circinalis and A. spiroides in Itapeva Lake, as well as the factors involved in forming these blooms. Anabaena circinalis and Anabaena spiroides blooms were characterized for Itapeva Lake, during the period from December 1998 to August 1999, the first species bloom being more intense (duration and density) than the second. Blooms of both species were found independent of season of the year. The maximum values recorded for both species occurred in autumn (May/99), a period in which blooms were recorded at three different regions in the lake. It should be stressed that the density used to consider bloom was a minimum of 2,000 cel.mL^-1. This maximum bloom was preceded by an event with strong turbulence in the system, which made nutrients available and/or dispersed the spores of these cyanobacteria stored in the sediment into the water column. The remaining time of the studied period was characterized by an increasing reduction in wind velocity and longer stabilization period in the water column.

Key-Words: Cyanobacteria, Anabaena circinalis, Anabaena spiroides, bloom, shallow lake.