First assessment of cyanobacterial blooms and microcystin-LR in the Canadian portion of Lake of the Woods

Abstract

Cyanobacterial blooms containing the hepatotoxin microcystin-LR (MC-LR) occurred at least once at each of 5 sites sampled in June, July and August 2004 in the Boreal Shield (Ont., Canada) portion of Lake of the Woods. In June, cyanobacteria constituted 3.5–49% (median 25%) of total phytoplankton biomass and consisted largely of Aphanothece spp. (median 98% of total cyanobacterial biomass). In July and August, cyanobacteria comprised 54–98% (median 77%) of total phytoplankton biomass in surface water samples and consisted largely of Aphanizomenon flos-aquae (median 82% of total cyanobacterial biomass). Three species of Anabaena (A. flos-aquae, A. lemmermannii, and A. mendotae), as well as Homeothrix janthina, Pseudanabaena spp., Aphanocapsa spp., and Woronichinia spp. were also present during the study period. Among study sites, total phosphorus concentrations in surface grab samples ranged from 11 to 31 μg/L and were positively associated with total cyanobacterial biomass (r = 0.64, P = 0.01). MC-LR concentrations (μg/g dry weight) in bloom material collected with a 64-μm tow net and analyzed by High Performance Liquid Chromatography were positively related to ammonium concentrations in surface grab samples (r = 0.94, P = 0.001), but not to the total biomass of cyanobacteria or any cyanobacterial taxon. In the isolated Boreal Shield basins of this lake, cyanobacterial blooms and cyanotoxins like MC-LR may have ecological and human health consequences and may be sensitive indicators of human disturbance in the drainage basin.