Collapsing Aphanizomenon flos-aquae blooms: possible contributions of photo-oxidation, O2 toxicity, and cyanophages

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Abstract
Triggering mechanisms for collapse of Aphanizomenon flos-aquae (L.) Ralfs blooms in three shallow eutrophic pothole lakes (L 885, L 958, and L 522), located within an aquaculture project study area in southwestern Manitoba, Canada, were examined. Three of the collapses observed (L 885, mid-July 1979; L 958, mid-August 1979; and L 522, mid-July 1979) were initiated during periods of lake thermal stability when conditions conducive to photo-oxidation and (or) death due to O2 toxicity were operable. A fourth collapse (L 958, mid-August 1973) was initiated during a period of lake thermal instability when photo-oxidation and O2 toxicity could be dismissed as triggering mechanisms. The possibility of cyanophage-induced algal lysis causing bloom collapse was considered and morphological evidence for the occurrence of viruslike particles (vlps) within Aphanizomenon cells from L 885 (1979) and L 958 (1978, 1979) are presented. Since transmission and isolation of the vlps has not been substantiated, the verification of a virus infection of the Aphanizomenon populations studied is not yet possible. It is demonstrated that no single triggering mechanism can account for all of the algal collapses described.

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