Myriam Bormans

Onset and persistence of cyanobacterial blooms in a large impounded tropical river, Australia

The dynamic interplay between physical, chemical and biological factors in the development and persistence of cyanobacterial blooms in impounded rivers is an important topic. Over a 3-year study period, variable climatic conditions were recorded in the Fitzroy River, Queensland, Australia, which is a typical, impounded lowland tropical river. Post-flood turbidity reduced the available light in the well-mixed water column to levels insufficient for cyanobacterial growth. Only when the water column stratified and the slowly sinking particles dropped from the surface layer did the ratio of surface mixed layer depth to euphotic depth approach 1, allowing cyanobacterial growth. By the time the light climate became favorable, most of the dissolved nutrients had been scavenged from the water column by settling particles or sequestered by fringing macrophytes and other biogeochemical processes. Cyanobacterial blooms dominated by Cylindrospermopsis raciborskii persisted for several months until the next flood flushed the system. The cyanobacterial species dominating that environment were very small and had high specific phosphorus uptake rates. Their nutrient requirement was met by transfer across the oxycline driven by regular high wind mixing events, entraining nutrient-rich bottom waters. Nutrient fluxes from the sediments into the anoxic bottom layer were sufficient to replace the bottom nutrients lost to the surface layer.

Modelling the distribution of Anabaena and Melosira in a stratified river weir pool

We have developed a model to predict the stratification dynamics and its influence on the distribution of algal species in a river with strong diurnal changes in temperaturestratification. While the phytoplankton concentrations vary inresponse to buoyancy and light limited growth, the populationsare also redistributed vertically due to diurnal mixing causedby surface evaporation and night time cooling. Theparameterization of the vertical eddy diffusivity coefficientwithin the model takes into account the generation of mixing byheat losses at the surface and the suppression of mixing bythermal stratification.The model is successfully tested against extensive temperatureand algal concentration data in Maude weir pool on theMurrumbidgee River, Australia. Given literature values offloating/sinking rates and maximum growth rates for thecyanobacterium Anabaena and the diatom Melosira, wewere able to reproduce extremely well the time evolution ofconcentrations of each species over long periods of low flow.

Predicting eutrophication effects in the Burrinjuck Reservoir (Australia) by means of the deterministic model SALMO and the recurrent neural network model ANNA

Abstract Two modelling paradigms were applied to the prediction of phytoplankton abundance in the Burrinjuck Reservoir: the deductive model SALMO and the inductive model ANNA. While SALMO is driven by process-based differential equations, the model ANNA is designed as recurrent feedforward neural network trained by time series data. Predictions of chlorophyll-a for the years 1979–1982 by both models were validated by means of measured data. Results showed that SALMO is able to predict annual average trends not only of chlorophyll-a but other chemical and biological state variables as well. It supports decision making by evaluating alternative scenarios for strategic eutrophication control. The model ANNA achieved reasonable accuracy in predicting timing and magnitudes of algal biomass up to 7 days ahead. The recurrent feedforward architecture of ANNA proved to be most efficient in order to model and predict seasonal dynamics of chlorophyll-a and its forecasting results can be utilized for early warning and tactical control of algal blooms in freshwater lakes. A sensitivity analysis conducted by ANNA revealed that algal abundance in Burrinjuck Reservoir is not only driven by physical and chemical characteristics of the water body but to a large extend by hydrological characteristics such as water depth as well.

Management strategies for cyanobacterial blooms in an impounded lowland river.

The incidence of blooms of the toxic cyanobacterium Anabaena circinalis in the major rivers of the Murray–Darling Basin has been strongly related to river discharge, because they are more prevalent during times of low flows. This study into the causes of cyanobacterial blooms in Maude Weir pool on the Murrumbidgee River demonstrates that for Anabaena blooms to form, the water column needs to be persistently stratified, a condition that occurs only during times of low discharge. Based on the relationship between discharge, stratification, and bloom formation, four strategies that might be implemented to minimize the occurrence or impacts of cyanobacterial blooms in weir pools are suggested. These strategies include setting a minimum discharge, pulsing the discharge, changing the discharge height, and altering the depth of water withdrawal. These strategies are evaluated in turn using results from the field study and from the predictions of a numerical model of the weir pool system. Copyright

Quantifying ecosystem metabolism in the middle reaches of Murrumbidgee River during irrigation flow releases

The relative importance of floodplain carbon inputs and in-stream metabolic processes have not been well quantified in major Australian rivers. We quantified seasonal phytoplankton primary production and net ecosystem production during irrigation flow regimes at four sites each located ∼100 km apart in the middle Murrumbidgee River. During flow periods dominated by storage release, ecosystem gross primary productivity, system respiration and phytoplankton chlorophyll concentrations all increased downstream so that overall net ecosystem metabolism was strongly net heterotrophic upstream and closer to balanced downstream. Phytoplankton production dominated ecosystem production throughout the entire reach and was likely to have been phosphorus limited throughout the study. Additionally, phytoplankton biomass was limited by short residence times at the upstream sites and nitrogen limited downstream in summer, despite an increase in turbidity. Both production and respiration rates were generally lower in winter, as expected, owing to lower temperatures.

A mixing criterion for turbid rivers

Abstract A criterion for the formation/destruction of thermal stratification in turbid rivers is developed and tested against an extensive data set in a weir pool on the Murrumbidgee River, Australia. The criterion estimates whether a river section would stratify or not, subject to a prescribed heat flux through the water surface and a prescribed river flow. It can be determined from readily accessible parameters, including solar radiation, turbidity, wind speed and river discharge. It is suggested that this simple mixing criterion be used as a flow management tool in a variety of water quality applications in rivers.

Setting Flow Levels for Controlling Cyanobacterial Blooms in Tropical Weir Pools

ABSTRACT Toxic cyanobacterial blooms occur frequently in water supply weir pools and storages in tropical Australia and elsewhere. They flourish under warm stratified conditions and their amelioration and management is an ongoing problem. We investigated bloom occurrence and growth in 3 separate water storages on the Fitzroy River in Queensland, Australia. We show that there is an opportunity to reduce its prevalence by manipulation of the riverine flow to remove the stratification, and to raise the turbidity through the resuspension of the finely divided bottom sediments. These effects are not due solely to flushing the weirs but to “resetting” the optical and mixing depths so that conditions are inimical to buoyant cyanobacteria. The persistence of the sediment particles in the water column, and the size of the discharge are the two most important factors in determining the effectiveness of the procedure.