This thesis examines the growth and dominance of two species of cyanobacteria (Anabaena circinalis and Microcystis aeruginosa) and other phytoplankton in two rivers in New South Wales, Australia. Investigations described within this thesis determined persistent thermal stratification (over a period of weeks) to be an important factor in, and prerequisite, to the development of A. circinalis growth in the Darling River at Bourke. After the establishment of persistent thermal stratification in the river, A. circinalis growth was maintained so long as the persistent stratification was maintained. Occasional overnight mixing of the water column did not disrupt growth. No other phytoplankton were found to co-dominate under these conditions. Persistent thermal stratification only occurred within the river during the hotter months (October to April) provided flows were below 450 ML/day. Flows above this resulted in diel mixing of the water column. Diel mixing during the hotter months and periods of low flow and increased euphotic depth, resulted in dominance by non-cyanobacterial phytoplankton.

The quantification of the advantage to A. circinalis growth through buoyancy regulation, relative to an evenly distributed hypothetical A. circinalis population was determined for the Darling River at Trevallyn. The buoyant population was found to increase in growth by five fold in comparison to the evenly distributed population. The vertical migration of A. circinalis using buoyancy during periods of persistent thermal stratification appears to be a strong positive selective force. The higher growth potential (through increased light quanta attainment) compared to neutrally or negatively buoyant phytoplankton, allows Anabaena growth to exceed that predicted by the ratio of euphotic depth to mixing depth.

The growth of M. aeruginosa in the Hawkesbury River at Sackville was found to have a significant (P<0.05) negative relationship with nitrate concentration. As nitrate levels decreased M. aeruginosa gained dominance over the Chlorophyceae. Nutrient addition experiments revealed M. aeruginosa and some other cyanobacteria to be the only phytoplankton able to grow during periods of M. aeruginosa dominance without the addition of nutrients (with the exception of Scenedesmus on one occasion). M. aeruginosa growth was also positively correlated to water temperature (P<0.05). Nitrate and water temperature did not have an interactive effect on M. aeruginosa growth. It is suggested that the reasons dominance of M. aeruginosa occurs under low NOx concentrations is a greater competitiveness for nitrogen than other phytoplankton.

No significant effect was recorded for phytoplankton exposed to variable light fields when compared to static tests. Some differences were found between the responses of different phytoplankton groups. The results from these tests suggest no disadvantage to cyanobacteria in a fluctuating light field environment. In fact, the Chlorophytes, if any group, appear to be disadvantaged. This is consistent with the continued growth of M. aeruginosa at Sackville under the mixed water column, fluctuating light environment.