SUMMARY 1. From measurements at several weir pool sites along the turbid and freshwater Barwon‐Darling River, Australia, the development of persistent stratification (for periods of >5 days) was related to river discharge. For the sites examined, the required discharge to allow the development of persistent stratification was between 100 and 450 ML day−1 during the hotter months. High discharge during the hotter months did not allow the formation of persistent stratification, although diel stratification did occur. Low discharge through the cooler months resulted in diel stratification, although persistent stratification lasting for a few days could occur at times. 2. The growth and dominance of Anabaena circinalis at these sites was closely related to the establishment and maintenance of persistent and strong thermal stratification. Growth only occurred during extended periods (>5 days) of persistent stratification. These conditions not only restrict the displacement of A. circinalis downstream, they also allowed the alga to accumulate in surface waters. 3. The discharge levels required to suppress the formation of persistent stratification at the study sites were variable because of large differences in channel cross‐sectional area. To compensate for this variation, the discharges were converted to flow velocities. A critical velocity of 0.05 ms−1 was sufficient for the suppression of persistent thermal stratification and concurrent A. circinalis growth for all sites. The turbulent velocity (u*) under weak wind mixing at the study locations varied between 2.66 × 10−3 and 2.91 × 10−3 ms−1 at the critical flow velocities. These values may have potential to be applied to other rivers in similar climatic zones to suppress nuisance cyanobacterial growth.
A turbid lowland river in Australia was studied to describe factors influencing the light conditions for phytoplankton growth. Vertical attenuation coefficients correlated with nepholometric turbidity enabling estimation of euphotic depths (z eu ) from long term turbidity monitoring. Light conditions were assessed from the ratio of z eu to the maximum water depth (z m ). Predominantly z eu /z m ratios were below 0.2, a value indicating the minimum light conditions required to support phytoplankton growth. A transitional state with z eu /z m between 0.2 and 0.35 occurred 15% of the time, while light sufficiency occurred for 30% of the time. Peaks in eukaryotic phytoplankton biomass developed when z eu /z m was at or above transitional values. Large increases in cyanobacterial numbers (Anabaena sp.) only occurred when z eu /z m exceeded 0.35. Turbidity increased quickly with elevated flows but did not decline substantially as flows reduced and light limiting conditions extended into low flow periods otherwise conducive to phytoplankton growth. However, during extended periods of reduced flows conductivity increased causing a substantial reduction in turbidity with concomitant improvements in light penetration. A turbidity of ca. 100 NTU marked the transition to light sufficiency at the study site and occurred at a conductivity of ca. 300 mS cm À1 demonstrating that small changes in salinity can have major effects on light penetration. These results show that flow, salinity and turbidity all play a part in determining the growth conditions for phytoplankton in turbid rivers.
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