Résumé
The species composition and the seasonal succession of phytoplankton were analyzed in a eutrophic drinking water reservoir located in the semi-arid region of northeast Brazil. Investigations were based on bimonthly or monthly sampling over 2 yr (1997 to 1998) conducted at 1 representative station with 2 sampled depths (0.5 and 5 m near the bottom). Limnological parameters (temperature, pH, dissolved oxygen, conductivity, light, dissolved inorganic nutrients) were simultaneously measured to determine the possible factors affecting the phytoplankton composition. We determined 30 taxa during the survey which were numerically dominated by the class Chlorophyceae. However, both in terms of abundance and biomass, Cyanobacteria dominated the phytoplankton community with Cylindrospermopsis raciborskii (Wolsz.) Seenayya et Subba Raju. This species can represent biomass close to 96-100% of total phytoplankton biomass, with values reaching 70 mg l(-1) (fresh weight) between April and November 1998. Over the survey, the filaments of C. raciborskii were coiled (average of 97%) with a mean proportion of 12.3% of terminal heterocytes. The species toxicity was determined from bioassay analysis and the presence of neurotoxins was revealed during the bloom. By March 1998, chlorophyll concentration reached 135 mu g l(-1) at the surface level, inducing a sharp decrease of the euphotic zone depth. Favorable environmental conditions were observed for the bloom with high temperatures, high pH, low N/P ratio, and absence of efficient predators. The nutrient context seemed to play a role in the cyanobacterial bloom despite the absence of external nutrient supply. However, annual rain deficit and lack of water renewal in 1998 linked to the 1997 EI Nino consequences seem to be the major factors responsible for both hypereutrophic conditions and cyanobacterial blooms in the reservoir studied. Thus, global climate change can influence phytoplankton population dynamics in continental waters, as demonstrated frequently in oceanic ecosystems.