Occurrence of Cylindrospermopsis (Cyanobacteria) in 39 Brazilian tropical reservoirs during the 1998 drought

The relationship between the trophic state of 39 reservoirs located in the semi-arid region of north-east Brazil and their planktonic structure was ex amined by a sampling survey during the 1998 drought (September to November) . A total of 30 environmental and biological parameters, with special atten tion paid to phytoplankton, were studied once at each reservoir to obtain a synoptic view at a state scale (Pernambuco State). The application of the chlorophyll a-total phosphorus standard relationship for grades of eutrophi cation revealed a classification in which 90 % of the sites were eutrophic- hypertrophic. Among the 202 taxa identified in the phytoplankton assemblage s,the cyanobacterium Cylindrospermopsis (Wolsz.) Seenayya et Subba Raju rep resented more than 80 % of the total phytoplankton density in 10 reservoirs and more than 50 % in 17 reservoirs. The rarity of other phytoplanktonic g roups in the case of the Cylindrospermopsis dominance is the most significa nt biological feature of these reservoirs. Although rotifers were more clos ely linked to hyper-eutrophic conditions than copepods or cladocerans, this dominant group did not seem to play an efficient role in cyanobacterial co ntrol. A statistical approach based on the analysis of cross tables (co-ine rtia analysis) allows us to describe species-environment relationships with a special focus on the factors governing the presence of Cylindrospermopsi s. As determined by hierarchical analysis, a major group of studied reservo irs (70 %) corresponded to deeper, warmer and hypertrophic reservoirs chara cterized by the combination of (1) low ammonia concentration, high temperat ure and pH values, and (2) high percentages of Cylindrospermopsis abundance s. Lack of water renewal in 1998 linked to the 1997 EI Nino consequences se ems to be a major determinant for both trophic conditions and Cylindrosperm opsis dominance in the reservoirs studied.