DYNAMICS OF CYANOBACTERIA IN EUTROPHIC WATER RESERVOIRS OF A SEMI-ARID REGION IN BRAZIL.

Abstract

revista vol 13 nº 2.indd

The present paper reports on the temporal dynamics of phytoplankton in six eutrophic water reservoirs in Rio Grande do Norte State, Brazil, during the dry and rainy periods between September/2002 and March/2004. The eutrophication state of the reservoirs oscillated between mesotrophic and hipertrophic, thus reflecting significant variation in the nutrient composition and local biomass of algae over time. Mean depth of the reservoirs, water specific conductance, and TN/TP ratio were determinant causes for the variation of phytoplankton species. The phytoplankton community was dominated by cyanobacteria, reflecting in poor species richness and elevated biomass. Most cyanobacteria were potentially toxin-producing species, and hepatotoxin-producing blooms were recurrent. The phytoplankton was basically composed by M-group species (SR- strategists -Microcystis spp.) during the rainy period, and of an association of S1 and Sn species (R-strategists - Planktothrix and Cylindrospermopsis) during the dry period. Alternation between dominant species were also observed, which took place with associated species H, H1 S1, (R-philamentous heterocystic algae - Anabaena spp. and Aphanizomenon), K species and Lo species (C-strategists) represented by small colonies of Aphanocapsa and Snowella, however with smaller relative biomass. Elevated cyanobacterial populations and hepatotoxin-producing blooms in these water reservoirs represent a threat to human health, and controlling these blooms is strongly recommended.