Print PagePrymnesium parvum is a harmful algal species that causes widespread fish kills. It is usually motile, but non-motile stages have been observed that may represent an encysted stage. We studied the relationship of these life history changes to the abundance and acute toxicity to fish of P. parvum. To test a hypothesis that seasonal variations trigger formation of non-motile cells, nutrient-limited (40 uM N, 1.5 uM P) laboratory cultures contrasted summer and winter conditions typical of Texas inland waters (summer -- 300 C, salinity 4 psu, 14:10 h photoperiod; winter -- 140 C, salinity 2, 10:14 h photoperiod). Another experiment addressed variations of salinity and temperature in a full factorial design with two other factors that might affect life history and acute toxicity to fish: silica availability (cell walls of cysts might be silicified), and mixing by aeration. Another experiment compared nutrient-sufficient and nutrient-limited cultures. Nitrogen and phosphorous were supplied at f/2 levels (880 um N, 36 uM P) for the nutrient-sufficient cultures and at 40 uM N and 1.5 uM P for the nutrient-limited cultures. An additional treatment added glucose as a carbon source for nutrient-limited cultures, to encourage mixotrophy which is potentially related to toxic activity.
Results/Conclusions
Acute toxicity to fish in simulated summer and winter cultures continuously increased for 20 days and then remained very high for another 22 days. This toxic activity did not differ significantly between summer and winter conditions until days 35 and 42, when winter cultures were more toxic. Non-motile cells remained sparse in both treatments, while motile cells dominated. In the four-factor experiment, acute toxicity to fish was significantly related to several interactions: (1) temperature and salinity; (2) temperature and aeration; (3) salinity and silica additions; and (4) temperature and salinity and aeration. In the remaining experiment, nutrient-sufficient cultures with highest abundance produced the greatest proportion of non-motile cells. In these cultures, acute toxicity to fish varied over time in relation to the numbers of motile cells and dropped when non-motile cells became dominant. In contrast, nutrient-limited cultures maintained lower total abundance, dominance by motile cells, and higher toxicity to fish. These observations suggest that high population density induces the non-motile phase, rather than stressful conditions of nutrient limitation or seasonal variation, which induce higher toxic activity.
