Graciela Ferrari* and Daniel Sienra**, * National Direction of Aquatic Resources, Constituyente 1497, CP 11200; ** Montevideo City Hall. Environmental Hygien Laboratory. Rambla Gandhi. Camino al Faro s/n. CP 11300. Montevideo, Uruguay; E-mail: gferrari@dinara.gub.uy
Abstract
The Uruguayan coast is exposed to frequent toxic blooms of dinoflagellates. The National Direction of Aquatic Resources has been carrying out the National Monitoring Program of harmful algae and mollusc toxicity since 1980. Harmful cyanobacteria blooms are recurrent phenomena that have been increased in the last years. The blooms are formed in fresh water and overtaking the whole Río de la Plata fluvio-marine system exposing the cells to a gradient of salinity until reaching the Atlantic Ocean. The Río de la Plata drains the second largest basin of South America. It flows into the Atlantic Ocean generating an estuarine system of about 35,000 km2, with only 5-15 m water depth, serving South American countries and supporting many activities including shipping, fishing, tourism and waste disposal. Microcystis aeruginosa commonly blooms in summer periods extending from the inner zone of the Río de la Plata to the oceanic area in Punta del Este, producing a large bright green discoloration like painting. Bloom extracts were analysed though mouse bioassays giving positive results. Toxicity levels were estimated between 100 y 1000 µg/g, resulting in a medium to high range of toxicity.
In summer 2001, a very extensive Microcystis aeruginosa bloom in different aquatic systems and in the coast of Río de la Plata was registered. This episode was accompanied by other cyanobacteria species: M. novacekii, M. flos-aquae, M. wessenbergii, Anabaena circinalis and A. cf. spiroides that have not been registered in the coast before. Microcystis aeruginosa was the dominant species during the bloom and the highest density was 2.5 x 107 cel·l-1, during calm weather, high temperature (27° C), 8 %o and soft wind condition. The Southern winds accumulated the scum on the beaches. When wind intensity increased the bloom was dispersed and dissolved. This phenomenon caused public impact, fish mortality and problems in drinkable water. Many people had skin problems, however neither of them were reported to the Public Health authorities or to the Toxicity centre. The authorities of the City Hall of Montevideo closed the beaches affected by the algae and warned about harmful effects. The National Direction of Aquatic Resources authorities recommended not to consume fish in affected areas.
The objective of the present study was to analyse the environmental conditions that favoured the development of harmful cyanobacteria blooms in the Uruguayan coast in January 2001 and to evaluate the general situation in order to apply preventive measures on these episodes.