Role of akinetes in the development of cyanobacterial populations in the lower Murray River, Australia

Planktonic populations and benthic resting stages (akinetes) of the common bloom-forming cyanobacteria Anabaena circinalis Rabenhorst and Anabaena flos-aquae f. flos-aquae (Lyngb.) Komarek were monitored in the Murray River near Nildottie and in adjacent floodplain wetlands (lagoons) from 1995 to 1997, to determine the extent of sporulation and the contribution of recruitment from the sediments to seasonal development of blooms. Physical and chemical characteristics of the water in the river and the lagoons were examined in relation to the succession of key life-cycle stages and growth. The warm, shallow lagoons supported considerably higher populations of Anabaena in the summer than did the river, with correspondingly higher incidence of sporulation. Viable akinetes were abundant in the sediments of both the river channel and the lagoons, providing a potentially significant inoculum for cyanobacterial growth. The apparent germination of akinetes early in summer and immediately following sporulation in mid summer indicated a strategy for both initiation and maintenance of populations. A. circinalis also persisted as a planktonic population throughout winter. Germination is considered more likely to occur in the shallow lagoons than in the main channel, principally because of frequent resuspension of sediments containing resting stages to the euphotic zone or because of direct penetration of light to the sediments.