Algal Blooms

The massive bloom of toxic blue green algae along the length of the Darling River in 1992 focused attention on the threat these microscopic plants pose to water quality in Australia's rivers, lakes and reservoirs. The bloom in the Darling was just the latest example of what has become a perennial problem for water managers. Much public concern has centred around the role of excess nutrients in the river systems from agricultural run-off and town sewage disposal, but nutrients are only one of several factors that can influence the growth of an algal population. 

While nutrients such as phosphorus are necessary for algae to flourish, water flow, turbulence, turbidity (muddiness), and temperature are all important. Work in CSIRO Land and Water is concentrating on the role played by these physical factors. By world standards, Australian water bodies tend to be highly turbid, our river flows are very intermittent and water temperatures high. As a result, a good deal of overseas experience cannot be applied easily to the Australian situation, and home-grown solutions must be found.

The work has followed parallel paths in the laboratory and in the field. A large shallow water tank has been installed in a boundary-layer wind tunnel. There, a powerful computer- controlled laser system is used to track the velocity and position of thousands of individual algal cells, so that their movement under the combined influence of wind-driven waves and turbulence and their own buoyancy can be studied. 

Measurements are revealing a complicated sequence that begins with surface wave build-up, and leads to the triggering of energetic, deeply penetrating turbulent eddies. These eddies appear to carry energy away from the surface, and also to carry the buoyant algal particles into the return flow at the bottom of the tank where they are carried back upwind. The cycle repeats itself in the time taken for four or five waves to pass by. 

A crucial finding is that there is a critical wind speed of about ten kph below which the process is not energetic enough to move buoyant algal cells off the surface and into the return flow. Instead they accumulate on the surface to form a scum, and are blown downwind. An unexpected result was the gradual appearance of a surface film under these conditions which acted like oil on water, damping down the surface waves and suppressing the mixing process. This effect has also been observed in natural conditions and may be important in the formation of algal mats.

Other laboratory tests which simulated the generation of turbulence by river flow suggest that, unlike wind-driven turbulence, this bottom-generated turbulence is ineffective in mixing buoyant cells off the surface.

The field study site of Rushy Billabong, located alongside the River Murray, is a turbid shallow lake subject to a high daily input of solar energy, making it typical of many inland Australian water bodies. There, the mixing effects of wind-driven turbulence are limited by thermal stratification in the water. Since stratification is determined by inputs and outputs of solar energy, the study of mixing in the billabong requires measurements of solar radiation, evaporation, radiative cooling of the water surface, and heat transfer between the atmosphere and the water, as well as wind speed, to be recorded on a continuous basis.

To do this, scientists from CSIRO Land and Water and their colleagues from the Murray-Darling Freshwater Research Centre have set up an extensive array of sensors in and above the billabong where all of these quantities are continuously logged. These physical measurements have been combined with intensive sampling of the concentrations and type of algae in the water to provide a view of the evolving spatial patterns of the algal populations.

These turn out to be surprisingly complicated, and over a 24 hour period, closely linked to changes in physical conditions in the billabong. For example, on one occasion measurements near midday showed the algae to have concentrated near the thermocline, the sharp boundary between hot surface and cold deeper waters that develops during the day. Twelve hours later the algae were much more evenly distributed. Like all plants algae need light to grow, and this changing distribution in the turbid water has a large effect on how much light the algae see and consequently on how fast they multiply.

Understanding the links between algal distribution and physical factors like wind and stratification is equally important when it comes to estimating algal concentrations from a few routine measurements.

Measurements of algal concentration through the growing season have revealed another interesting feature: large variations in algal biomass through the period, with different types of algae dominating the population at different times. Further studies are trying to link this succession to environmental conditions of nutrient availability, turbidity and mixing.

The long-term goal of this work is to devise strategies for minimising the development and impact of algal blooms on Australia's rivers and inland water bodies. The understanding gained in the laboratory and billabong studies is being encapsulated in predictive computer models which will be tested first in the readily controlled conditions of the wind-water tunnel and then in the field. 

One set of theoretical predictions, which has already been confirmed in the tunnel and is now being applied to the billabong data, is that the difference in algal concentration between the upwind and downwind end of a lake should decrease as wind speed increases, that it should decrease in deeper or less sharply stratified lakes, and increase as the flotation rate of the algae increases. These predictive physical models can form the basis of more complete algal growth models and also suggest sensible strategies for algal sampling.

CSIRO Land and Water's algal bloom work entered a new phase with a series of studies in a Murrumbidgee weir pool, where the knowledge gained will be applied to understand the effects of river flow rates and different water withdrawal strategies on algal populations and to using this knowledge for better river management.

Dr Ian Webster

Collaborators

Australian National University
CSIRO Land and Water
Murray-Darling Freshwater Research Centre

Funding

Australian Water Research Advisory Council
Murray-Darling Freshwater Research Centre