Résumé
The relationship between the trophic state of 39 reservoirs located in the semi-arid region of north-east Brazil and their planktonic structure was examined by a sampling survey during the 1998 drought (September to November). A total of 30 environmental and biological parameters, with special attention 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 eutrophication revealed a classification in which 90 % of the sites were eutrophic-hypertrophic. Among the 202 taxa identified in the phytoplankton assemblages,the cyanobacterium Cylindrospermopsis (Wolsz.) Seenayya et Subba Raju represented more than 80 % of the total phytoplankton density in 10 reservoirs and more than 50 % in 17 reservoirs. The rarity of other phytoplanktonic groups in the case of the Cylindrospermopsis dominance is the most significant biological feature of these reservoirs. Although rotifers were more closely linked to hyper-eutrophic conditions than copepods or cladocerans, this dominant group did not seem to play an efficient role in cyanobacterial control. A statistical approach based on the analysis of cross tables (co-inertia analysis) allows us to describe species-environment relationships with a special focus on the factors governing the presence of Cylindrospermopsis. As determined by hierarchical analysis, a major group of studied reservoirs (70 %) corresponded to deeper, warmer and hypertrophic reservoirs characterized by the combination of (1) low ammonia concentration, high temperature and pH values, and (2) high percentages of Cylindrospermopsis abundances. Lack of water renewal in 1998 linked to the 1997 EI Nino consequences seems to be a major determinant for both trophic conditions and Cylindrospermopsis dominance in the reservoirs studied.