The effect of temperature on the response of Daphnia to toxic cyanobacteria

Abstract

1. In a set of life‐table experiments, cohorts of neonate Daphnia pulex were exposed to a toxic strain of Anabaena affinis and A. flos‐aquae, and to a pure cyanobacterial toxin (anatoxin‐a), at 12 or 14, 19, and 25 °C. The fecundity and survival of individual animals were assessed at 1‐, 2‐ or 3‐day intervals, depending on the temperature, through to the fifth brood of the control cohort. The sensitivity of D. pulex to the cyanobacteria and the toxin at each temperature was measured by determining its finite population growth rate (λ) in an experimental treatment as a fraction of that in a control treatment. Tests with three concentrations of cyanobacteria (1.0, 2.5 and 5.0 μg mL–1) and one concentration of anatoxin‐a (1 μg mL–1), and with two clones of D. pulex, showed a consistent and statistically significant pattern of increasing sensitivity at higher temperatures.2. Anabaena affinis affected both survivorship and fecundity, while A. flos‐aquae and its toxin, anatoxin‐a, primarily affected fecundity. Presence of cyanobacteria affected brood size, brood number, time to first reproduction and interclutch interval. Temperature affected time to first reproduction and interclutch interval at all concentrations of cyanobacteria. Brood number and brood size were little affected by temperature except at the highest concentrations of cyanobacteria. Increasing the concentration of A. flos‐aquae affected demographic parameters, especially 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 effect of toxic cyanobacteria on daphniid population growth rates.