Development of Anabaena Bory ex Bornet & Flahault (Cyanobacteria) blooms in a shallow, subtropical lake in southern 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 that states their relative importance in phytoplankton communities. Some species of cyanobacteria produce toxins and their blooms can cause the death of aquatic organisms and even humans. Species of cyanobacteria respond differently to environmental fluctuations in each habitat. The main goal of this paper was to record the occurrence of Anabaena circinalis and A. spiroides blooms in Itapeva Lake, as well as some factors involved in forming these blooms. Anabaena circinalis and Anabaena spiroides blooms were identified at Itapeva Lake during the period from December 1998 to August 1999, the former species blooming more intensely (duration and density) than the latter. Blooms of both species were found independent of the season of the year. Maximum density values recorded for both species occurred in autumn (May/99), a period in which blooms were recorded at three different regions of the lake. It should be stressed that the density used to consider the bloom was a minimum of 2,000 cel.mL^-1. The maximum bloom was preceded by an event of strong turbulence in the system, which made nutrients available and the cyanobacteria spores stored in the sediment were dispersed into the water column. The remaining time of the study period was characterized by a progressive reduction in wind velocity and longer stabilization period in the water column.

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