Prymnesium parvum in inland waters : comparative toxicity, microcystin allelopathy, and toxin photodegradation.

The harmful algal species Prymnesium parvum, a toxin-producing mixotrophic haptophyte, has caused massive fish kills in slightly saline inland Texas water bodies. In the first study, standardized aquatic test models from multiple trophic levels were simultaneously employed to compare sensitivities to P. parvum toxins. Fish mortality was the most sensitive endpoint, though adverse reproduction effects in two invertebrate species were observed; a green algal species was not adversely affected. A second study employed a probabilistic risk assessment approach and examined potential allelopathy of the cyanotoxin microcystin-LR to P. parvum. A concentration of 4,392.8 μg l⁻¹ significantly inhibited P. parvum growth over a portion of the study, which corresponded to a 9% probability of detecting this concentration in the environment. Finally, the effect of sunlight on toxicity of P. parvum cell-free filtrate was assessed. Exposure to eight hours of full or ~50% sunlight eliminated toxicity to Pimephales promelas, suggesting toxin photodegradation.