Differential sensitivity of green algae to allelopathic substances from Chara

Three short-term laboratory experiments were conducted to investigate allelopathic effects of a mixture of Chara globularis var. globularis Thuillier and Chara contraria var. contraria A. Braun ex Kützing on three different green algae. Single phytoplankton species were exposed to filtered water originating from charophyte cultures. Phytoplankton growth was monitored by determination of chlorophyll concentrations in batch cultures. The change in chlorophyll concentration during the experiments was analysed with a logistic growth model, resulting in an estimate of the exponential growth rate and the duration of the lag phase of the single green algae. The results indicate allelopathic effects of Chara on the growth of the green algae Selenastrum capricornutum Printz and Chlorella minutissima Fott et Nováková, whereas Scenedesmus obliquus (Turpin) Kützing did not seem to be affected. The exponential growth rate of S. capricornutum decreased 7% in the presence of water from a charophyte culture, while the growth rate of C. minutissima decreased with 3%. The allelopathic effect of Chara did not increase when the green alga C. minutissima was P-limited. The effect of Chara was different when young sprouts were used. With young sprouts the duration of the lag phase of C. minutissima was extended (25%), whilst for old plants the growth rate of this green alga decreased. Although the inhibiting effect of charophytes on specific phytoplankton species is rather small, the differential sensitivity of the species to Chara might influence the composition and biomass of phytoplankton communities in the field.