THE EFFECT OF TEMPERATURE ON THE RESPONSE OF DAPHNIA TO TOXIC CYANOBACTERIA

1. In a set of life-table experiments, cohorts of neonate Daphnia pule x were exposed to a toxic strain of Anabaena affinis and A. pos-aquae, and to a pure cyanobacterial toxin (anatoxin-a), at 12 or 14, 19, and 25 degrees C. The fecundity and survival of individual animals were a ssessed at 1-, 2-or 3-day intervals, depending on the temperature, thr ough to the fifth brood of the control cohort. The sensitivity of D, p ulex to the cyanobacteria and the toxin at each temperature was measur ed by determining its finite population growth rate (lambda) in an exp erimental treatment as a fraction of that in a control treatment. Test s with three concentrations of cyanobacteria (1.0, 2.5 and 5.0 mu m L- 1) and one concentration of anatoxin-a (1 mu g mL(-1)), and with two c lones of D. pulex, showed a consistent and statistically significant p attern of increasing sensitivity at higher temperatures. 2. Anabaena a ffinis affected both survivorship and fecundity, while A. flos-aquae a nd its toxin, anatoxin-a, primarily affected fecundity. Presence of cy anobacteria affected brood size, brood number, time to first reproduct ion and interclutch interval. Temperature affected. time to first repr oduction and interclutch interval at all concentrations of cyanobacter ia. Brood number and brood size were little affected by temperature ex cept at the highest concentrations of cyanobacteria. Increasing the co ncentration of A. flos-aquae affected demographic parameters, especial ly at the lower temperatures, while increasing the concentration of A. affinis had less effect. 3. The study suggests that increasing water temperatures in natural systems should exacerbate the inhibitory effec t of toxic cyanobacteria on daphniid population growth rates.