Toxin production by cyanobacteria in water supply reservoirs can have important human, livestock and ecological health consequences. This research aimed to identify effects of potential climate change on growth and toxin production by the potentially toxic cyanobacteria, Cylindros permo psis raciborskii and Microcystis aeruginosa. CSIRO’s Limited Area Climate Models for Queensland were used as a basis to identify predicted climate changes resulting from global warming. Those predicted to effect phytoplankton were: increases in temperature, with minimum temperature increasing more rapidly than maximum temperature and changes to rainfall and the EI-Nifio Southern Oscillation (ENSO), which are predicted to decrease mean rainfall and frequency of rainfall events and increase the intensity of rainfall events. Higher intensity rainfall events are likely to result in greater erosion and increased nutrients as well as decreased light available for phytoplankton growth. A combination of culture experiments, field based mesocosm studies and analysis of historical data was undertaken.
Batch culture experiments with C. raciborskii and M. aeruginosa, examined the effects of varying light intensity on growth and toxin production. The effects of varying night time temperatures at constant diurnal temperatures were also examined at two light intensities. Mesocosm studies were undertaken at two locations, to investigate effects on cyanobacterial populations, of shading, dosing with nutrients, and a combination, to simulate an intense rainfall event.
C. raciborskii demonstrated wide tolerance levels with respect to light with growth rate and toxin production not being limited by light down to 18 µmol photons m-2s-1, M aeruginosa had narrower tolerance limits with respect to light intensity, indicating that C. raciborskii is a better competitor at a range of light intensities. Cylindrospermopsin (CYN) production rate was independent of light intensity over the range 18-100 µmol photons m-2s-1. The CYN quota per cell was significantly higher towards the end of the exponential phase for the lowest light treatment than for the highest light treatment. Increasing the night time temperatures from 15 to 25 °C increased the growth rate for both species studied. The CYN production rate was positively correlated with nighttime temperature although the CYN quota per cell was higher at 15 °C than at 25 °C night time temperature. The microcystin (MCYST) quota was more variable and showed no statistical difference between the various treatments; the MCYST production rate was greatest at 20 °C nocturnal temperature.
Mesocosm experiments showed that C. raciborskii growth or toxin production was not limited by shading. Nitrogen and phosphours dosing at the ambient ratio, resulted in increased C. raciborskii cell numbers, however this was subsequent to an initial decrease. Shading at both 50% and 70% did not reduce C. raciborskii below the level of the control, but prevented the development of the biomass peak observed in the dosed treatment. Nutrient dosing at lowered N:P ratios (~10:1) also resulted in an increase of C. raciborskii after an initial decrease. At the lower N:P ratio however, the non-heterocytic cyanobacteria increased to a greater extent than C. raciborskii and total cyanobacterial biomass did not increase significantly compared to control. Results indicate that even though elevated nutrients result in increased C. raciborskii biomass. C. raciborskii appears to be a better competitor when nutrients begin to be limiting, rather than when nutrients are abundant. There was no significant difference for the CYN quota between mesocosm treatments but it was significantly higher in the tropical reservoir than the sub-tropical reservoir. Analysis of historical data showed some relationships between the SOI and M. aeroginosa cell numbers. Also C. raciborskii peak biomass was found to occur earlier in the growing season over time, although this may be influenced by artificial destratification.
Increasing night time temperature has the potential to increase the speed at which blooms of these two species can develop. High nutrient input following high intensity rainfall events, is predicted to result in an increase of C. raciborskii populations and CYN, The strains present at the tropical study site exhibited a higher CYN per cell quota than the sub-tropical study site. If it is temperature that controls the distribution of these strains of C. raciborskii, an expansion southwards of their distribution is possible under enhanced greenhouse conditions.