Population dynamics of Aulascoseira granulata (EHR.) Simonson (Bacillariophyceae, Centrales), the dominant alga in the Murray River, Australia

Temporal and spatial changes in the population of Aulacoseira granulata, the dominant alga within the Murray River (south-eastern Australia), are described utilizing weekly data (1980-1992) from the algal, water quality and flow monitoring programs of the Murray-Darling Basin Commission. The population dynamics of A. granulata are related to flow conditions, turbidity and silica concentrations. Diatoms (A. granulata and other centric species) were the most abundant algae during the 12 years of phytoplankton record for the upper and central reaches of the Murray River, with A. granulata occurring at up to 77,700 cells/ml. In the slower flowing, lower reaches of the river, centric diatoms other than A. granulata occurred most frequently, followed by Scenedesmus spp., while A. granulata occurred intermittently at high densities. Turbidity values within the river generally ranged from 5-80 NTU (maximum value 470 NTU). Silica concentrations were generally 2-6 mg/l (maximum value 44.5 mg/l), but values less than 0.1 mg/l were not uncommon. The population dynamics of A. granulata were most strongly related to flow and silica concentration. In the upper half of the river, cell densities were reduced dramatically by floods or silica depletion but a substantial population was usually present under median discharge conditions, on some occasions when the silica concentration was less than 0.1 mg/l, A. granulata was less prevalent in the more slowly flowing lower parts of the river (the lower Murray is controlled by a lock and weir system), reaching high cell densities for short periods only when flow conditions provided sufficient turbulence, and never exhausting the supply of silica. The higher turbidities found in the lower parts of the river (median values around 60 NTU) appeared to impact significantly on cell densities only when above 100 NTU. The findings are discussed in relation to the known meroplanktonic lifestyle of A. granulata.