Effects of a cyanobacterial bloom (Cylindrospermopsis raciborskii) on bacteria and zooplankton communities in Ingazeira reservoir (northeast Brazil)

Species composition and seasonal succession of some planktonic components w ere studied through monthly samplings during 2 yr (1997 and 1998) in Ingaze ira reservoir, northeast Brazil. Linked to the severe drought in this regio n (1997 El Nino event) was the dominance of the toxic filamentous cyanobact erium Cylindrospermopsis raciborskii in the phytoplankton in 1998 (96 to 10 0% of total phytoplankton biomass), with small proportions of heterocytes ( 12% of filaments). A great part of the variability of the particulate organ ic carbon (R-2 = 83.9%) was explained by changes in the C. raciborskii carb on biomass. A more significant change in bacterial communities was observed in the post-bloom phase when biomass increased due to the appearance of la rger size-classes of cell volume, This bacterial size structure may be the consequence of a strong pressure by bacterivores. Among the zooplanktonic g roups, rotifers were numerically more abundant throughout the survey, but m icrocrustaceans, especially the copepods, contributed the highest proportio n of the biomass. Despite the low edibility of C, raciborskii (large tricho mes; mean of 97 mum, n = 204), zooplankton diversity increased during and a fter the bloom (March to December 1998). Our data suggest that rotifers and copepods were able to cut up and shorten the filaments to edible size for other zooplankton species, especially the small-bodied herbivorous cladocer ans, Thus, in the studied ecosystem, the heterotrophic micro-organism commu nity appeared to be able to develop a strategy to cope with a dominant and relatively inedible algal food source.