ESEL Paper Review_20140708

By Hong Guo

Mail: hongguo@gist.ac.kr

Phone: (+82) (0)10 82276568

1, Title and Author

Title: Management strategies for cyanobacterial blooms in an impounded lowland river

Journal: Regulated river: Research & Management

Authors:

Ian T. Webstera,b,*, Bradford S. Shermana, Myram Bormansa and Gary Jonesb

a CSIRO Land and Water, GPO BOX 1666, Canberra, ACT 2601,Australia

b CSIRO Land and Water, 80 Meiers Rd, Indooroopilly, QLD 4074, Australia

2. Summary of Paper

? 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 relationships between discharge, stratification, and growth of buoyant cyanobacteria in an impounded river section shall be referred to as the Discharge?Growth Hypothesis.

3. Results

? Flow speeds are a prime determinant of the degree of stratification in the weir pool, so that altering the timing and size of the discharge through the weir must be seen as a potential cyanobacterial management strategy.

? Measurements have demonstrated that Anabaena rise at rates between 0 and 5 m day1 (Reynolds et al., 1987). If an intermediate rate of rise of 3 m day1 is assumed, then it would take around 2 days for a population, which is initially spread through a 6 m water column, to accumulate into a surface layer if turbulent mixing has been suppressed by stratification. However, if the population were uniformly dispersed through the water column at sunrise, then during daylight hours the Anabaena population would not accumulate significantly into the near-surface euphotic zone where photosynthesis occurs.

? Nitrogen limitation was indicated occasionally during times of Aulacoseira dominance

before and after the Anabaena bloom. Unfortunately, the nutrient bioassays were not

repeated the following summer so it cannot be established whether nutrient limitation

played a role in the failure of the Anabaena population to reach problem proportions during

December 1994, which was also a 6-week period of persistent stratification.

? The degree of stratification within a water column is determined by the relative supply

rates of stratifying thermal energy and the supply of turbulent kinetic energy.

? For summer season, wind power is not important for generation of turbulent kinetic energy

at the bed but for determinant of the evaporation rate.

? Two factor for influence the stratification behavior of the weir pool

1. the temperature of the water flowing into the pool at its upstream end

2. the water depth within the pool

? Under low discharge conditions, cooler water from the river flows under the warmer

surface layer in the weir pool. The warmer the incoming water, the closer is its temperature

to that of the surface layer and the smaller is the degree of stratification.

? Water depth has two influences on critical discharges. When the depth of the weir pool is

reduced, its cross-sectional area is also reduced so that the average flow speed for a

particular discharge is increased. This increases the production of turbulent kinetic energy

responsible for mixing the water column.

? During periods of negative heat flux, the water column would tend to destratify even with

zero discharge.

? A simple strategy for minimizing the impact of an Anabaena bloom on water quality during

conditions of persistent stratification is to remove water for domestic consumption from

near the bottom of the weir pool where Anabaena concentrations are likely to be much

smaller than at the water surface. Of course, such a strategy would not be appropriate if

other measures involving mixing of the water column had been implemented.

? Increasing the discharge through the weir pool above a critical level is the best way of

preventing blooms, but such a strategy may prove to be unacceptable because of the volume

of water required to be sent downstream. Nonetheless, the data enable economic trade-off

analyses to be undertaken for an increased baseflow strategy. Withdrawal of water from

depth in the weir pool is a way of minimizing algal concentrations in water supply during

blooms of buoyant cyanobacteria such as Anabaena.

4. Contribution:

This research studies management strategies under the consideration of relationship between

the flow environment within the weir pool and algal growth, using physical, chemical and

biological data and give the model for prediction of the stratification. It would be greatly

helpful for the future studies.

5. Contact (Mail address): chencts@nus.edu.sg