Sunlight amelioration of Prymnesium parvum acute toxicity to fish

Although a number of physical and chemical factors can influence ambient toxicity associated with Prymnesium parvum blooms in aquatic systems, the role of natural sunlight on the magnitude and duration of aquatic toxicity is less understood, particularly in inland waters. We performed two experiments exposing cell-free filtrates from P. parvum cultures to different magnitudes and durations of natural sunlight. During one study, fish acute toxicity from cell-free filtrates exposed to full and partial (50%) natural sunlight for 8 h was completely removed. However, acute toxicity of a dark treatment level in the field (26°C) did not differ from toxicity of a lab dark control (4°C), indicating that temperature did not influence the stability of acutely toxic P. parvum toxins in this study. In a second experiment, cell-free filtrates were exposed to full sunlight for different periods of time, resulting in a monotonic decrease in the magnitude of acute toxicity with an increased duration of sunlight exposure. Acute toxicity to fish was completely ameliorated after just 2 h of exposure to full sunlight, highlighting the potential importance of site-specific conditions on the duration of P. parvum impacts to aquatic life. Further studies are warranted to understand site-specific influences and direct and indirect photolysis mechanisms on the stability of P. parvum toxins in aquatic systems.