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REGULATED RIVERS: RESEARCH & MANAGEMENT

Regul. Ri�ers: Res. Mgmt. 17: 637–650 (2001)

DOI: 10.1002/rrr.623

FLOW MANAGEMENT STRATEGIES TO CONTROL BLOOMS OF THECYANOBACTERIUM, ANABAENA CIRCINALIS, IN THE RIVER

MURRAY AT MORGAN, SOUTH AUSTRALIA

HOLGER R. MAIERa,*, MICHAEL D. BURCHb AND MYRIAM BORMANSc

a Centre for Applied Modelling in Water Engineering, Department of Ci�il and En�ironmental Engineering, Adelaide Uni�ersity,Adelaide, SA 5005, Australia

b Cooperati�e Research Centre for Water Quality and Treatment, Pri�ate Mail Bag 3, Salisbury, SA 5108, Australiac CSIRO Land and Water, GPO Box 1666, Canberra, ACT 2601, Australia

ABSTRACT

The Murray–Darling river system is highly regulated and is Australia’s major surface water resource. It is subject toblooms of the toxic cyanobacterium Anabaena circinalis, which present significant water quality problems. As a resultof these blooms, an algal management strategy has been developed for the Murray–Darling basin. One of the majorobjectives of the strategy is the development of flow management strategies for key reaches of the river system.Intensive studies in the Murrumbidgee River, Australia, have indicated that persistent thermal stratification is arequirement for blooms of this cyanobacterium to occur. In the lower Murray, mean wind speed was found to be themajor factor affecting the degree of thermal stratification under low flow conditions, which generally exist during themonths of December to March. In this paper, the effect of various flow management scenarios on the likelihood ofthe occurrence of blooms in the River Murray at Morgan, South Australia, are assessed. A frequency analysis iscarried out on 30 years of wind speed data to determine the probability of occurrence of persistent thermalstratification under a number of flow regimes. The scenarios evaluated include existing base flow conditions, alteredbase flow regimes, temporary releases from an upstream storage (Lake Victoria) and the temporary reduction of weirpool levels. The results obtained indicate that the dispersal of existing blooms by simultaneously reducing the weirpool levels at Locks 1–3 is the most effective and economical strategy for combating bloom formation by A. circinalisin the River Murray at Morgan. Copyright © 2001 John Wiley & Sons, Ltd.

KEY WORDS: Anabeana circinalis ; cyanobacterial bloom; flow management; River Murray; rivers; thermal stratification; windspeed

INTRODUCTION

Cyanobacteria or blue-green algae are photoautotrophic procaryotes, and are a naturally occurringcomponent of the freshwater phytoplankton community, along with the eukaryotic algae. Under certainfavourable conditions, usually a combination of stable warm weather and nutrient enrichment, cyanobac-teria can become dominant and form ‘blooms’ (Reynolds, 1987). These blooms have significant adverseeffects on water quality, as cyanobacteria can produce toxins and odours which affect drinking water, andblooms can impact on the use of water for recreational purposes (Chorus and Bartram, 1999).

Cyanobacterial blooms in rivers can be a major water quality problem as evidenced by blooms in thePotomac (Krogmann et al., 1986), Bure (Moss et al., 1984), Lot (Capblanc and Dauta, 1978) and Darling(Bowling and Baker, 1996) rivers. The toxic bloom of Anabaena circinalis in the Darling River in late 1991was the largest river bloom of cyanobacteria recorded anywhere in the world and caused the New SouthWales government to declare a state of emergency (MDBMC, 1994). The bloom extended for a distanceof 1000 km and occurred at a time of drought when the river had virtually stopped flowing (Bowling andBaker, 1996). Significant cyanobacterial blooms, which have been similar but not quite as extensive, have

* Correspondence to: Centre for Applied Modelling in Water Engineering, Department of Civil and Environmental Engineering,Adelaide University, Adelaide, SA 5005, Australia. E-mail: [email protected]

Copyright © 2001 John Wiley & Sons, Ltd.

Recei�ed 19 April 2000Re�ised 4 September 2000

Accepted 22 September 2000

H.R. MAIER ET AL.638

occurred in the River Murray in South Australia. These have been recorded intermittently since algalrecords commenced in 1947 and have in more recent times been identified as toxic. The blooms aregenerally dominated by A. circinalis and occur under low flow conditions in the warm summer months ofDecember to February (Burch et al., 1994). The blooms present significant water quality problemsassociated with toxins and odours and the intermittent and transient nature of their occurrence presentstreatment challenges for domestic water supply along the river. In addition the River Murray is a criticalurban water supply source for both the city of Adelaide and the northern towns of Port Pirie, PortAugusta and Whyalla, all of which are supplied by major pipeline systems (Figure 1).

In response to the outbreaks of cyanobacterial blooms in the Murray–Darling river system, an algalmanagement strategy has been developed for the Murray–Darling basin (MDBMC, 1994). The goal ofthe strategy is ‘to reduce the frequency and intensity of algal blooms and other water quality problemsassociated with nutrient pollution in the Murray–Darling Basin through a framework of coordinatedplanning and management actions’ (MDBMC, 1994). Four key objectives have been identified in order tomeet the above goal. These include the reduction of nutrient concentrations, the improvement ofstream-flow regimes and flow management, an increase in the community’s awareness of the cyanobacte-rial problem and continued research in order to obtain better information and scientific knowledge aboutcyanobacteria (MDBMC, 1994).

The algal management strategy for the Murray–Darling Basin recognizes the need for site specificapproaches, and the objective of this paper is to investigate flow management options for controlling

Figure 1. The River Murray in South Australia and Morgan, the site for this study

Copyright © 2001 John Wiley & Sons, Ltd. Regul. Ri�ers: Res. Mgmt. 17: 637–650 (2001)

FLOW MANAGEMENT TO CONTROL BLOOMS 639

blooms of cyanobacteria, particularly A. circinalis, in the River Murray at Morgan, South Australia(Figure 1). The reach of the River Murray upstream of Morgan has been the site of recent majorcyanobacterial blooms (MDBMC, 1994). The occurrence of high numbers of cyanobacteria at Morgan isa significant water supply operational problem, as a water treatment plant is located there and waterabstracted at this point is delivered to the cities of Port Pirie, Port Augusta and Whyalla via theMorgan-Whyalla pipeline (Figure 1).

BACKGROUND

The Murray–Darling Ri�er system

The length of the Murray–Darling River system (Figure 1) is 5300 km, which is the fourth longest inthe world (Walker et al., 1992). Its catchment area is 1.073 million km2, which is sixth largest on theglobal scale (Thoms and Walker, 1993). The majority of the Murray–Darling Basin is arid or semi-aridand mean annual evaporation (1200 mm) generally exceeds mean annual rainfall (450 mm) (Thoms andWalker, 1993). As a result, the discharge is small (annual mean 318.3 m3/s), even by Australian standards(Walker et al., 1992). Despite this, the Murray–Darling system is the major source of irrigation water inAustralia and is the major source for stock and domestic water supply in South Australia (Davidson,1988).

The salient feature of semi-arid rivers, such as the Murray–Darling system, is their highly variabledischarge (Walker, 1992). As a result, they have been regulated for water resources purposes, particularlyfor agriculture (Walker and Thoms, 1993). Examples of semi-arid river systems include the Ebro inEurope, the Nile and Orange–Vaal in Africa, the Columbia and Colorado in North America and theGanges, Indus and Tigris–Euphrates in Asia (Walker, 1992). Of these, the Orange–Vaal system mostclosely resembles the Murray–Darling system (Walker, 1992).

The flow in the River Murray is controlled by four major storages, a series of 14 locks and weirs andfive barrages. The river is regulated in order to stabilize water levels for irrigation, water supply andnavigation purposes. The barrages stop seawater from travelling upstream during times of low flow. Thereare three major storages on the River Murray, namely: Lake Victoria (680 GL), Hume Reservoir (3040GL) and Dartmouth Reservoir (4000 GL). The fourth major storage is Menindee Lakes (1680 GL), whichis located on the Darling River. In addition to these four major storages, there are a number of smallerstorages on the Murray’s tributaries.

The flows in the Murray–Darling system are managed by the Murray–Darling Basin Commission. Aparliamentary agreement between the states determines how the water is divided between the states ofNew South Wales, Queensland (since 1992), Victoria and South Australia (Maheshwari et al., 1995). Aspart of the agreement, South Australia receives a minimum annual ‘entitlement flow’ of 1850 GL, whichis approximately 14% of the mean annual flow in the River Murray at the state borders (Jacobs, 1990).At times of low flow, water is released from Lake Victoria (Figure 1) so that entitlement flows are met.At present, ‘there is no legislative provision for environmental flow needs. . . ’ (Maheshwari et al., 1995).

The impact of river regulation on cyanobacterial blooms in the lower Murray is difficult to quantify,as the weirs were constructed from 1922–1935 and algal records were commenced in 1947. Cyanobacterialblooms are certainly not a new phenomenon. In 1830, explorer Charles Sturt noted that the water in theDarling River had a taste of vegetable decay as well as a slight tinge of green (Creagh, 1992). However,there have been some suggestions that the frequency and intensity of blue-green algal blooms hasincreased in recent years (MDBMC, 1994).

Flow and cyanobacteria

It is widely recognized that flow is the dominant variable in rivers (Walker and Thoms, 1993; Roos andPieterse, 1996). This particularly applies to semi-arid environments, where the degree of variability of the



flow is high (e.g. Poff and Ward, 1989). Several studies have shown that flow is a key or controllingvariable for the development of cyanobacterial blooms in Australian rivers (Hotzel and Croome, 1994;Sherman et al., 1998). Low flows are considered to stabilize the water environment, increase lightavailability, create longer retention times and allow the release of nutrients from sediments (MDBMC,1994). There has also been speculation that increased flows may merely dilute the concentrations ofcyanobacteria or transport them downstream (Sherman and Webster, 1997). However, until recently,the actual mechanism by which flow controlled the size of cyanobacteria populations was not known.

Intensive studies in the Murrumbidgee River have indicated that flow was the dominant factorcontrolling the degree of thermal stratification (Sherman and Webster, 1997; Sherman et al., 1998).Stratification in turn is a necessary condition for the growth and development of cyanobacterialblooms in rivers (Sherman et al., 1998). This is because under stratified conditions turbulent mixing isreduced and positively buoyant cells of Anabaena can accumulate and grow in the surface layers in afavourable light climate. For sufficiently large discharges stratification can be either prevented oreliminated by turbulence generated by the flow over the bottom. Consequently, it is possible topropose a range of flow control strategies and options for destroying thermal stratification, therebylimiting cyanobacterial growth (Sherman and Webster, 1997).

The onset of stratification is not a function of river flow alone, but also depends on solar radiationand wind speed. An investigation into the relationship between temperature stratification, dischargeand meteorological conditions in the lower River Murray in South Australia found significant differ-ences to the Murrumbidgee River (Bormans et al., 1997). It was found that under entitlement flowconditions (velocity=0.04–0.06 m/s), which are generally experienced in summer, wind rather thanflow is the dominant variable affecting the degree of thermal stratification. One of the reasons for thisis that the River Murray is much wider than the Murrumbidgee River and is therefore much moreexposed. Under entitlement flow, the daily average critical wind speed below which permanent thermalstratification occurs was found to be 1.2 m/s in summer near Swan Reach. Typical daily average windspeeds of 3 m/s were observed in summer at that site suggesting a low probability of stratification atthat time of the year under entitlement flow.

Increased flow results in a reduction in the critical wind speed required for persistent thermalstratification, which is less likely to occur, especially for sustained periods. As a result of the highlyregulated nature of the River Murray in South Australia, a number of options exist for altering theflow regime of the river. It is important to distinguish between flow management to help disperse anestablished bloom and the flow required to maintain a healthy river system (MDBMC, 1994). Theformer include the temporary release of additional flow from upstream storages or the temporaryreduction in weir pool levels. The latter includes the sustained release of additional flow from up-stream storages.

In this paper, the effectiveness of a number of flow management scenarios for controlling blooms ofthe cyanobacterium A. circinalis in the River Murray at Morgan, South Australia, is assessed. Thescenarios considered include a number of different base flow regimes as well as short-term increases invelocity as a result of additional releases from Lake Victoria, and the temporary reduction in weirpool levels at Locks 1–3. The scenarios are evaluated for the period December to March, as histori-cally, cyanobacterial blooms have occurred during these months. Separate analyses are carried out foreach month because entitlement flows vary from month to month (Q=7000 ML/day in December,Q=7000 ML/day in January, Q=6900 ML/day in February and Q=6000 ML/day in March). Thecritical wind speeds which allow for the development of persistent thermal stratification are calculatedfor each of the flow management options. The probabilities of the sustained occurrence of thesecritical wind speeds are then calculated based on historical wind records to provide a measure of riskassociated with the occurrence of cyanobacterial blooms for the various flow management optionsinvestigated. The economic and environmental impact of the various options, as well as the applicabil-ity to similar river systems, are also discussed.



METHOD

The mixing criterion (R) developed by Bormans and Webster (1997) was used to determine the conditionsunder which thermal stratification will occur in the River Murray at Morgan. The same criterion was usedby Bormans and Webster (1997) and Bormans et al. (1997) to investigate the stratification potential of theMurrumbidgee River and the lower River Murray, respectively, and is given by:

R=U3

H�

Qnet−2QI

KdH� �g

�Cp

, (1)

where U is the depth-averaged velocity, H is the water depth, Qnet is the net surface heat flux into thewater column, QI is the net shortwave radiation, Kd is the light attenuation coefficient, � is the thermalexpansion coefficient (2×10−4 °C), g is the gravitational acceleration (9.81 m/s2), � is the density ofwater (1000 kg/m3) and Cp is the specific heat capacity of water (4180 J/kg/°C). The parameter R is onlyrelevant when the factor in parenthesis in Equation (1) is positive; otherwise the water column is losingheat, and stratification will not build up even under low discharge. In order to use Equation (1) as apredictive tool, an estimate of Qnet in terms of easily available parameters is needed. Qnet can be expressedas the sum of the radiative (short- and long-wave radiation), evaporative and sensible heat fluxes asfollows:

Qnet=QI+Qlw+Qs+Qe (2)

where QI is the net short-wave radiation which is attenuated exponentially with depth by turbidity andwater colour, Qlw is the net long-wave radiation, Qs is the sensible heat flux and Qe is the latent heat fluxdue to evaporation. The relationship between Qnet and QI can be determined as a function of wind speedusing typical empirical formulae used by Bormans and Webster (1998). The mixing criterion is based onthe relative rates of supply of thermal energy (provided by solar heating) and turbulent energy (providedby wind blowing over the surface and flow over the bottom). It is therefore applicable to any riveraffected by solar radiation, flow and wind.

For each of the flow scenarios considered, average monthly velocities (U) and water depths (H) atMorgan were calculated using the River Murray Hydraulic Model (Water Studies, 1992). Model outputsare dependent upon the operation and effects of the regulating structures on the river, including LakeVictoria, the 10 weirs, the five barrages, and upon inflows and outflows to the river, abstractions,evaporation and evapotranspiration.

The calculations of the remaining parameters in Equation (1) (Qnet and QI) utilize the results of anintensive field study carried out near Swan Reach (Figure 1) in the summer of 1996 (Bormans et al., 1997).A value of Kd=2.2 m−1 was measured as part of this study when the critical wind speed of 1.2 m/s wasobserved under entitlement flow conditions and was thus used in this research. It should be noted that thisstudy site is located approximately 150 km downstream of Morgan. However, it is reasonable to assumethat the results of the above field study are representative of typical average summer conditions in thelower Murray, including Morgan (Bormans et al., 1997). Averaged monthly values for QI can becalculated theoretically. However, this calculation assumes the absence of clouds. The average of themeasured values of QI obtained by Bormans et al. (1997) was 86% of the theoretical values calculatedunder the assumption of cloudless skies. Consequently, the theoretical average values of QI for December,January, February and March were multiplied by a factor of 0.86 in this study. The resulting values ofQI for the above months were 312, 304, 272 and 217 W/m2, respectively. The relationship between Qnet,QI and wind speeds was determined with the aid of the empirical equations given in Bormans and Webster(1998) and the field data obtained by Bormans et al. (1997). Examples of these relationships are given inBormans et al. (1997).

Bormans and Webster (1997) found that the transition between well mixed and stratified conditions inrivers occurs at a value of around R=45000, with a stratified water column at R�45000 and a wellmixed water column at R�45000. This condition was used to calculate critical wind speeds for a rangeof velocities (i.e. U) resulting from the following flow regimes:



1. Existing entitlement (base) flow conditions.2. A reduction in base flows by 10% and 20%, which is a situation which could be envisaged under

exceptional conditions of drought in the eastern states of Australia (New South Wales and Victoria).3. Additional release of water from Lake Victoria. The maximum amount of water that can be released

from Lake Victoria is 10000 ML/day (Water Studies, 1992). However, during summer entitlementflow conditions, the amount released per day does not exceed 7000 ML/day under current operatingrules (Water Studies, 1992). Both scenarios were investigated in this paper. It should be noted that themaximum rate of change of outflow from Lake Victoria is 2500 ML/day (Water Studies, 1992).Consequently, it would take 3 and 4 days to reach maximum releases of 7000 and 10000 ML/day,respectively. The other variable that needs to be considered is the duration of the additional release.Preliminary trials indicated that the magnitude of the flow peak is dependent upon the length of therelease period. There was an increase in the flow peak up to a release period of 7 days (excluding the3 or 4 days needed to reach the target flows and an equivalent time to return to the baseline flow),after which there was little increase in flow peak with an increase in release time. Consequently, arelease time of 7 days was chosen. Another point of note is that the additional amount of water thatcan be released from Lake Victoria at any point in time is determined by the quantity of water alreadybeing released to meet entitlement (base) flow conditions. The above scenarios represent the casewhere no release from Lake Victoria is required to meet entitlement flow conditions and thusrepresents the best possible scenarios in terms of the potential for disrupting persistent thermalstratification.

4. The simultaneous reduction in the weir pool levels at Locks 1, 2 and 3 by 300, 400, 450, 500 and 600mm. It should be noted that the maximum reduction that would be considered desirable/acceptablefrom an operational viewpoint is 300 mm (B. Erdmann, personal communication). Preliminary trialscarried out indicated that the maximum increase in velocity was independent of the length of thereduction in weir pool levels.

In order to obtain an indication of the probability of occurrence of the various critical wind speedscalculated using the above procedure, and hence the likelihood of thermal stratification and theoccurrence of cyanobacterial blooms, a frequency analysis (McCuen, 1998) was carried out on a long-termrecord of wind speed data. No long-term records were available for Morgan, but wind speed data forMurray Bridge were available from 1966 to 1996. A comparison of the wind speed data from MurrayBridge with those obtained from a floating weather station near Swan Reach indicated a high degree ofcorrelation. This suggests that the wind speed data at Murray Bridge are representative for the lowerMurray (Bormans et al., 1997). Consequently, it is considered acceptable to use the wind speed data fromMurray Bridge to assess the potential of thermal stratification at Morgan.

Two-day and 7-day moving averages of the wind speeds were obtained. Thermal stratification persistingfor 2 days can form ‘blooms’ of cyanobacteria caused by the accumulation of cyanobacterial cells in thethermally buoyant layer as a result of re-distribution. Stratification persisting for 7 days represents a muchmore serious problem, as it allows for growth of cyanobacteria in an optimal light climate. Theprobability analysis was carried out on the annual series of the minimum values for the 2- and 7-dayaverages. Separate series were generated for each of the 4 months considered. The probability distribu-tions tried include normal, log-normal and gamma distributions.

RESULTS

For the range of probability distributions evaluated, the normal distribution resulted in the best fit to thewind data. A truncated normal distribution was used, as the probability of occurrence of negative windspeeds has to equal zero. Typical cumulative probability plots are shown in Figure 2.

For existing entitlement flow conditions, there is a significant difference in the probabilities that meanwind speeds which will allow thermal stratification to persist for 2 days will occur in each of the months



Figure 2. Cumulative probability plots for 2- and 7-day averaged minimum monthly wind speeds for the months of January andFebruary

investigated (Table I). The highest probability occurs in January (44.7%), followed by December (38.9%),February (22.4%) and March (18.1%). The probabilities that the mean wind speeds required for thermalstratification persist for 7 days, given current entitlement flow conditions, are much lower. They rangefrom 1.74% in December to 0.65% in January (Table II). The 2- and 7-day probabilities are not affectedsignificantly by reductions in base flows by 10% and 20%. They are increased by less than 1% in each ofthe months investigated (Tables I and II).

The reductions in probabilities of stratification resulting from additional releases of 7000 and 10000ML/day from Lake Victoria are shown in Tables III and IV. For the 2-day case, a release of an additional10000 ML/day results in a reduction in probabilities from 38.9% to 30.9% in December, from 44.7% to32.7% in January, from 22.4% to 14.4% in February and from 18.1% to 11.8% in March (Table III). Forthe 7-day case, a release of an additional 10000 ML/day results in reduction in probabilities from 1.74%



Table I. Probabilities (%) that mean wind speeds resulting in thermal stratification willpersist for 2 days under various flow regimes (current entitlement flow, 90% of currententitlement flow and 80% of current entitlement flow) in each of the months ofDecember–March

MonthFlow regime

December MarchJanuary February

Entitlement 18.138.9 44.7 22.490% Entitlement 39.2 18.445.2 22.880% Entitlement 39.4 45.5 18.623.0

Table II. Probabilities (%) that mean wind speeds resulting in thermal stratification willpersist for 7 days under various flow regimes (current entitlement flow, 90% of currententitlement flow and 80% of current entitlement flow) in each of the months ofDecember–March

Flow regime Month

December January February March

Entitlement 1.74 1.170.65 1.0090% Entitlement 1.77 1.190.67 1.0380% Entitlement 1.79 1.210.68 1.05

Table III. Probabilities (%) that mean wind speeds resulting in thermal stratification willpersist for 2 days after the release of additional water from Lake Victoria in each of themonths of December–March

Lake Victoria additional Monthrelease

March(ML/day) December January February

0 38.9 18.144.7 22.47000 34.3 37.9 14.417.7

10 000 30.9 32.7 11.814.4

Table IV. Probabilities (%) that mean wind speeds resulting in thermal stratification willpersist for 7 days after the release of additional water from Lake Victoria in each of themonths of December–March

Lake Victoria additional Monthrelease(ML/day) December MarchJanuary February

0 1.171.74 0.65 1.007000 1.38 0.880.46 0.73

10 000 0.691.14 0.35 0.55



to 1.14% in December, from 0.65% to 0.35% in January, from 1.00% to 0.55% in February and from1.17% to 0.69% in March (Table IV).

The simultaneous reduction in weir pool levels at Locks 1–3 can result in significant reductions in theprobabilities that the critical wind speeds will persist for 2 and 7 days (Tables V and VI). A reduction inweir pool levels by 300 mm has a slightly bigger impact than a release of an additional 10000 ML/dayfrom Lake Victoria. For the 2-day case, a reduction in weir pool levels by 600 mm in December andJanuary and by 500 mm in February and March ensures that thermal stratification is disrupted (Table V).For the 7-day case, a reduction in weir pool level by 500 mm is sufficient to ensure disruption of thermalstratification in all of the months investigated (Table VI).

DISCUSSION

Under current entitlement flow conditions, there is little risk of sustained blooms of A. circinalis in theRiver Murray at Morgan. The probability that wind speeds resulting in thermal stratification will persistfor 7 days is less than 2%, which corresponds to an average recurrence interval of 50 years. Stratificationfor this length of time is needed for substantial growth of A. circinalis to occur. Baker et al. (2000) foundthat populations of Anabaena spp. would double in size every 4–5 days in this reach of the river.However, there is a relatively high probability that thermal stratification will persist for 2 days, rangingfrom 44.7% (1 in 2.2 year event) in January to 18.1% (1 in 5.5 year event) in March. This can result in‘blooms’ of A. circinalis in particular, as a result of the re-distribution of the cells already in the watercolumn, resulting in surface scums. Such blooms can represent significant problems for water resourcesmanagers.

Additional releases from Lake Victoria could be made on a continuous basis (e.g. increase in base flow)or to disperse existing thermal stratification. The results obtained indicate that the maximum flow that

Table V. Probabilities (%) that mean wind speeds resulting in thermal stratification willpersist for 2 days after simultaneous drops in weir pool levels at Locks 1–3 of 300, 400,450, 500 and 600 mm in each of the months of December–March

MonthWeir pool leveldrop

December(mm) January February March

38.9 44.7 22.4 18.1011.013.230.029.5300

20.1 7.7400 7.222.0450 9.3 5.1 1.5 1.8

00.52.5500 00600 0 00

Table VI. Probabilities (%) that mean wind speeds resulting in thermal stratification willpersist for 7 days after simultaneous drops in weir pool levels at Locks 1–3 of 300, 400,450, 500 and 600 mm in each of the months of December–March

Weir pool level Monthdrop

MarchJanuary(mm) FebruaryDecember

0.651.74 1.170 1.000.630.500.301.05300

0.260.15 0.380.644000.09450 0.18 0.02 0.04

0 0 0500 0



can be released from Lake Victoria (10000 ML/day) is insufficient to ensure disruption of persistentthermal stratification (Tables III and IV). For the 2-day case, the largest reduction in probability, from44.7% (1 in 2.2 year event) to 32.7% (1 in 3.1 year event), occurs in January. For the 7-day case, thelargest probability of stratification, which occurs in December, is reduced from 1.74% (1 in 57 year event)to 1.14% (1 in 88 year event). The costs associated with the ad hoc release of an additional 10000 ML/dayfor an extended period would be approximately AU$15.5 million per month. These figures are based ona unit cost of AU$50/ML (P. Harvey, personal communication). This is the approximate price paid forthe occasional (one-off) trade of water rights between irrigation users within the Murray–Darling basin,and is a reasonable market model for costing water for these algal control scenarios. The cost for aone-off release of 10000 ML/day for 7 days is AU$5 million, including the water that needs to be releasedto gradually increase and decrease the outflows from Lake Victoria to the desired level in accordance withthe current operating policies (see the ‘Results’ section).

By taking into account that thermal stratification which persists for 2 days has probabilities ofoccurrence ranging from 18.1% (1 in 5.5 year event) to 44.7% (1 in 2.2 year event), and, more importantly,that thermal stratification which persists for 7 days has probabilities of occurrence ranging from 0.65% (1in 154 year event) to 1.74% (1 in 57 year event) over the 4-month period considered, it appears as thoughtemporary releases of water from Lake Victoria are the better option. However, another issue that needsto be considered is that there is approximately a 10 day lag between the release of water from LakeVictoria and the arrival of the resulting flow peak at Morgan. This makes this method unsuitable forresponding to existing problem conditions. One way to overcome this particular difficulty is to forecastAnabaena concentrations for at least 10 days in advance, and to start releasing water from Lake Victoriaif the predicted concentrations of Anabaena exceed a pre-determined critical level. An existing artificialneural network model, which is capable of forecasting concentrations of Anabaena up to 4 weeks inadvance (Maier and Dandy, 1997; Maier et al., 1998), could be used for this purpose. However, thisapproach does not overcome the problem that the maximum release of water from Lake Victoria is notguaranteed to prevent the formation of persistent thermal stratification. In addition, the maximumamount of water that can be released from Lake Victoria at any particular point in time is a function ofhow much water has already been released to meet entitlement flow conditions (see the ‘Results’ section).Consequently, the amount of water that is available for additional release might be substantially less than10000 ML/day.

The simultaneous reduction in weir pool levels at Locks 1–3 has the greatest potential for disruptingthermal stratification. A reduction by 300 mm, which is considered to be acceptable from an operationalpoint of view, has an effect that is similar to that of a release of an additional 10000 ML/day from LakeVictoria (see Tables III, IV, V and VI). However, the advantages of this method are that there are nooperational constraints, that there is no considerable delay between the reduction in weir pool levels andthe time peak velocities are reached and that the associated costs are negligible. In addition, weir poollevels can be lowered by larger amounts (e.g. 500–600 mm) in emergency situations, ensuring thedisruption of thermal stratification, regardless of wind speed (see Tables V and VI). The only drawbacksof this approach are that there is a large drop in water level throughout the reach (Figure 3) and thatthere is a large reduction in flow velocity after the restoration of the original weir pool levels (Figure 4).These reduced velocities persist for a number of days, and represent an increased risk of thermalstratification, and hence cyanobacterial bloom development. As mentioned in the ‘Results’ section, thepeak velocities reached are independent of how long weir pool levels are reduced. Consequently, thelength of time during which water levels and velocities are reduced can be minimized by restoring weirpool levels after a short period of time (e.g. 1 or 2 days).

The reductions in water levels needed to ensure thermal stratification is disrupted (e.g. 500–600 mm)are significantly greater than the daily fluctuations of �50 and �200 mm currently experiencedupstream and downstream of the weirs (Thoms and Walker, 1993). Water level reductions of thismagnitude could leave irrigation intakes stranded and might result in difficulties for river navigation. Inthe lower Murray, water level fluctuations can also have a significant effect on the plants and animals inthe littoral zone (Walker et al., 1994; Maheshwari et al., 1995). The littoral zone is particularly important



Figure 3. Simulation of changes in water depth at Morgan for the scenario involving a reduction in weir heights of 0.3 m for aperiod of 5 days

Figure 4. Simulation of velocity changes at Morgan for the scenario involving a reduction in weir heights of 0.3 m for a period of5 days

to the ecology of the Murray, as it contains the majority of the biodiversity associated with the river(Walker et al., 1992). Although the distribution and abundance of some plants can be affected byshort-term fluctuations (Maheshwari et al., 1995), it is unlikely that a reduction in water level of 500–600mm for a period of 1–2 days associated with a 1 in 2 year event will have a significant impact. This isparticularly so as the emergent plants, which can be subject to daily fluctuations, generally occur in theuppermost 0.5 m, and the submergent plants occur down to a depth of 2 m (Walker et al., 1992; Walkerand Thoms, 1993). However, this needs to be investigated more fully.

The flow management strategies proposed here, which are based on the disruption of favourablepersistent thermal stratification, are principally for the control of A. circinalis, which is the most commonbloom-forming cyanobacterium in the lower River Murray in South Australia. The cyanobacterialcomponent of the phytoplankton in this part of the river consists of a diverse assemblage which includesA. circinalis, A. flos-aquae, A. perturbata var. tumida, A. aphanizomenoides, A. planktonica, Aphani-zomenon issatschenkoi, Anabaenopsis elenkinii, Planktothrix perornata var. attenuata, Cylindrospermopsisraciborskii, Pseudanabaena spp., Planktolyngbya subtilis, Microcystis aeruginosa, with rare occurrences ofNodularia spumigena (Baker, 1999). Of this range A. circinalis is the only species that has been recordedin significant surface scums in the main river channel, whereas A. flos-aquae and A. perturbata var. tumidaoccasionally form scums in wetlands associated with the river (P. Baker, personal communication).Interestingly these Anabaena species are all capable of forming relatively large clumps of aggregatedfilaments which is consistent with their capacity to rise rapidly to the surface under stable conditions. This



is a function of enhanced floating velocity associated with larger-sized colonies or aggregations offilaments of buoyant cyanobacteria (Oliver and Ganf, 2000). It follows that the majority of other speciesfound in this assemblage, which exist as small solitary filaments (excluding M. aeruginosa), are notparticularly favoured by the predominantly mixed conditions in the lower River Murray, nor can theytake significant advantage of the occasional periods of persistent stratification to form blooms. It is alsoreasonable to assume that the growth of these species would be adversely affected by a flushing orelevated flow-velocity strategy that would result in both dilution and deep turbulent mixing out of theshallow euphotic zone.

The techniques described in this paper may have the potential to be used in other lowland regulatedriver systems. As discussed in the ‘The Murray–Darling River system’ section, the Murray–Darling riversystem is most similar to the Orange–Vaal system in southern Africa (Cambray et al., 1986). In the VaalRiver, there have been signs recently that increasing total phosphorus concentrations are causing a shiftin algal assemblages from diatoms and green algae to one dominated by cyanobacteria (Roos andPieterse, 1996). Consequently, some of the techniques discussed in this paper might be applicable at somestage in the future.

Another river which is similar to the lower Murray is the River Ebro, which is one of the main riversin Spain (see Munoz and Prat, 1989). In the Ebro, diatoms and green algae tend to dominate, althoughsporadical maxima of cyanobacteria (up to 9800 cells/mL) have been recorded in autumn (Sabater andMunoz, 1990). There have been no recorded blooms of cyanobacteria and high levels of turbulence arecited as the most likely reason for this (Sabater and Munoz, 1990). This appears reasonable, as thevelocities in the lower Ebro range from 0.2 to 0.3 m/s (Sabater and Munoz, 1990; Munoz and Prat, 1994),which are much higher than those in the lower Murray (0.055–0.063 m/s). However, the mean annualflow in the River Ebro has been reduced by 29% during this century, and this trend is likely to continuein the future as a result of increasing water use in the basin (Ibanez and Prat, 1996). Consequently, therewill be an increased likelihood of persistent thermal stratification, and hence cyanobacterial blooms, as aresult of a reduction in velocities. It should be noted however, that the increased likelihood of blooms willalso be dependent upon whether the types of cyanobacteria resident in the river can take advantage of thestable conditions associated with persistent stratification. Notwithstanding this, appropriate flow manage-ment strategies can be developed to deal with this increasing risk by using the techniques discussed in thispaper.

CONCLUSIONS

Under existing entitlement flow conditions, the likelihood that sustained blooms of the cyanobacterium A.circinalis will occur in the River Murray at Morgan is very small. This is because the probability thatmean wind speeds that are small enough to cause thermal stratification, which is a necessary condition forA. circinalis to bloom, will persist for 7 days is less than 2% (1 in 50 year event). However, the chancesthat blooms (i.e. surface scums) of cyanobacteria, particularly some Anabaena species, will occur for shortperiods of time as a result of the re-distribution of cells in the water column under thermally stratifiedconditions is much higher. The probability that mean wind speeds resulting in thermal stratification willpersist for 2 days is 38.9% (1 in 2.6 year event) in December, 44.7% (1 in 2.2 year event) in January, 22.4%(1 in 4.5 year event) in February and 18.1% (1 in 5.5 year event) in March.

These probabilities can be reduced by increasing base flows. However, the costs associated with thisoption are very high. For example, the approximate cost of releasing an additional 10000 ML/day fromLake Victoria, which reduces the probability that mean wind speeds resulting in thermal stratification willpersist for 2 and 7 days by up to one half, is AU$15.5 million per month. Flow management techniquesaimed specifically at dispersing existing blooms provide a more economical alternative. The simultaneousreduction of weir pool levels at Locks 1–3 was found to be the cheapest and most effective option. Areduction in weir pool levels by 300 mm has a similar effect to releasing an additional 10000 ML/dayfrom Lake Victoria. Reductions by 500–600 mm ensure disruption of thermal stratification, regardless of



wind speed. However, the above procedure should only be used in emergency situations, as there is areduction in water levels for a 1- or 2-day period, which could leave irrigation intakes stranded, posedifficulties for river navigation and have adverse effects on the plants in the littoral zone. In addition,there is a reduction in flow velocities for a few days after weir pool levels have been restored, whichincreases the likelihood of thermal stratification.

ACKNOWLEDGEMENTS

Brenton Erdmann of SA Water provided valuable advice on the operation of the River Murray in relationto the development of flow management scenarios, and Martin Lambert from the Department of Civiland Environmental Engineering of the University of Adelaide valuable advice on the frequency analysisof the wind data. This work was carried out as part of a larger project investigating algal blooms in theRiver Murray which was funded jointly by Environment Australia through the National River HealthProgram (managed by the Land and Water Resources Research and Development Corporation), theSouth Australian Water Corporation, and the CRC for Water Quality and Treatment.

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