First report of a microcystin-containing bloom of the cyanobacterium microcystis aeruginosa in the La Plata River, South America.

This communication is the first registration of a toxincontaining bloom of the cyanobacterium Microcystis aeruginosa Kutzing in the La Plata River, Uruguay, ̈ South America. The occurrence of the species M. aeruginosa in the La Plata River was first registered in Ž . 1981 by CARP 1989 but without any toxicological evaluation. During the summer of 1999, short-term M. aeruginosa blooms were observed at Colonia, in the Ž . country’s southwest 34 27 S, 057 20 W , in two locations on the Uruguayan side of the La Plata River. Microcystis blooms coincided with an important tourist influx to the area and a peak of water use for recreational and domestic purposes. Two sites were chosen in the area for sampling, a protected area within a Ž . harbor, Puerto Juan Lacaze JL , and an open area Ž . within a beach resort, Playa Fomento see Fig. 1 . Samples were taken on 5, 8, 10, 22, and 24 February 1999 at both sites. The water during the bloom had a Ž . Ž high pH average 8.3 , low transparency 0.15 0.3 m . Ž . Secchi , variable surface temperatures 21 29 C , and a N:P ratio also favorable for the event, 7:1 to 19:1. No Ž 1 . rainfalls and very slow winds 0 34 km h were ideal conditions for M. aeruginosa bloom development and accumulation. During the 5 days of monitoring, phytoplankton density was within the range of 600 6000