Long-term observations on Anabaena circinalis Rabenhorst (Cyanophyta)

Field observations on Anabaena circinalis Rabenhorst over six summer seasonal appearances in two dams have shown that the frequency of occurence of heterocysts became fairly constant soon after the appearance of the species and decreased just before the end of the growing season. By contrast, akinete frequency reached a maximum, very early in the season, then decreased rapidly. Drought led to a decrease in occurrence of both Anabaena and of akinetes in several dams; this was possibly associated with an increasing concentration of NOx in the water. At Carcoar dam this akinete reduction was shown first in end-of-season figures. Drought also led to an end-of-season decrease there in the occurrence of heterocysts. Variations in morphology were noted. The coiling of the trichome, shape of akinete and relative position of heterocyst were all variable, although these characters are often assumed to be of taxonomic importance.