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 flo s-aquae f. flos-aquae (Lyngb.) Komarek were monitored in the Murray River n ear Nildottie and in adjacent floodplain wetlands (lagoons) from 1995 to 19 97, to determine the extent of sporulation and the contribution of recruitm ent from the sediments to seasonal development of blooms. Physical and chem ical characteristics of the water in the river and the lagoons were examine d in relation to the succession of key life-cycle stages and growth. The wa rm, shallow lagoons supported considerably higher populations of Anabaena i n the summer than did the river, with correspondingly higher incidence of s porulation. Viable akinetes were abundant in the sediments of both the rive r channel and the lagoons, providing a potentially significant inoculum for cyanobacterial growth. The apparent germination of akinetes early in summe r and immediately following sporulation in mid summer indicated a strategy for both initiation and maintenance of populations. A. circinalis also pers isted as a planktonic population throughout winter. Germination is consider ed more likely to occur in the shallow lagoons than in the main channel, pr incipally because of frequent resuspension of sediments containing resting stages to the euphotic zone or because of direct penetration of light to th e sediments.