The Mosaic Nature of Toxic Blooms of Cyanobacteria

Abstract

The incidence of toxic populations of planktonic Cyanobacteria in hypertrophic Hastings Lake, Alberta (located 40 km east of Edmonton) was monitored during July and August of 1975, 1976 and 1977. Samples were collected at intervals from different stations, concentrated from 1 to 20 (exceptionally to 200 or 400) times with a plankton net, as required, and tested for toxicity by 1.0 ml intraperitoneal injection into 20 g mice. Species composition was determined microscopically and biomass densities were determined from dry weights. Strains of two of the three predominant species, Microcystis aeruginosa Kütz. emend. Elenkin and Anabaena flos-aquae (Lyngb.) de Bréb. were isolated, cultured and tested for toxicity. Based on signs of poisoning produced in mice and chicks by populations and isolates, Microcystis aeruginosa type-c toxin was found to be primarily responsible for the toxicity observed at different stations during all three seasons. Only one collection (June 27, 1976, concentrated 200 times) indicated the presence of anatoxin-b. On a single day, as well as on different days, population density, species composition and toxicity varied greatly from station to station. On some days, big differences occurred between stations that were only 10 m apart. In July, 1975, localized pulses of type-c toxin were detected for a few days which ranged in titer from 0.4 to 1.4 times the lethal oral dose of anatoxin-a for a 60 kg calf (estimated as 16,000 mouse units (MU) per 10 liters, equivalent to 0.5 ml (i. p.) per 20 g mouse of the most dense population composed entirely of type-a). In 1976, one, and in 1977, two localized pulses of type-c toxin of much lower titer were detected. The use of toxicity as a marker has revealed that blooms of Cyanobacteria tend to have a dynamic and mosaic structure. Remarkably localized differences in strain as well as species composition can occur in response to morphometry and to prevailing nutritional and other environmental factors that affect growth, buoyancy, accumulation and dispersal. The dynamic and mosaic nature of blooms contributes to the unpredictable occurrence of lethal doses of cyanotoxins that may be ingested by livestock and wildlife. Other contributing factors are differences in type, productions, accumulation, release and inactivation of toxins and in animal susceptibility.