The effects of the cyanobacterium Microcystis aeruginosa, the cyanobacterial hepatotoxin microcystin–LR, and ammonia on growth rate and ionic regulation of brown trout

Brown trout were exposed for 63 days to five treatments: a control; the purified cyanobacterial hepatotoxin microcystin—LR (MC—LR) (41—57 μg MC—LR 1^?1); lysed toxic Microcystis aeruginosa cells (41–68 μg MC—LR 1^?1 and 288 μg chlorophyll a 1^?1); lysed non—toxic M. aeruginosa cells (non—MC—LR containing and 288 μg chlorophyll a 1^?1); ammonia (65–325 μg NH₃ 1^?1). All treatments produced significantly reduced growth compared to controls (P<0·05, Fisher test). Exposure to ammonia resulted weight loss over the first 7 days followed by weight increase, though at a significantly lower level than in the other treatments. First exposed to lysed toxic M. aeruginosa cells grew less than those exposed to lysed non—toxic cyanobacteria or purified MC—LR. Sodium influx rates after 63 days exposure to purified MC—LR, lysed toxic M. aeruginosa cells, or ammonia showed a significant increase compared to control fish or those exposed to lysed non—toxic M. aeruginosa cells. There were no significant differences in Na⁺ efflux or net Na⁺ uptake rates between treatments. Significant increases in body Na⁺ and Cl^— were seen in fish exposed to lysed toxic M. aeruginosa cells or ammonia. Only fish exposed to ammonia showed a significant increase in body ammonia. Short—term exposure, over 4 h, to lysed toxic cells, non—toxic cells or purified MC—LR resulted in insignificant changes in Na⁺ flux rates compared to controls although there was a significant net Na⁺ loss in fish exposed to ammonia. Chronic exposure of fish to toxic cyanobacterial blooms may result in ionic imbalance and reduced growth.