Monitoring program to support investigations of dilution
flows to control Cyanobacteria in the Torrens Lake
Prepared by:
Justin Brookes
The School of Earth and Environmental Sciences
The University of Adelaide
Dilution flows monitoring program
Background
Recurring cyanobacterial blooms in the Torrens Lake, Adelaide, compromise the
amenity of the lake. Long-term sustainable strategies to control cyanobacterial blooms
are sought in order to enhance water quality and improve the recreational and
ecological value of the lake.
One strategy that has been investigated is the use of controlled releases from upstream
catchment or from reuse water. This strategy relies upon the diluting flows flushing
the cyanobacteria from the system and thereby limiting the accumulation of high
biomass. There are several challenges with this strategy, not least of which is
achieving the desired dilution given the likely density difference between the inflow
and lake water.
In an assessment of the feasibility of dilution flows for cyanobacteria management in
the Torrens Lake, Brookes (2011) concluded that for a growth rate of 0.4 day-1,
which is a typical exponential growth rate of cyanobacteria in the Torrens Lake, a
diluting flow of at least 10% per day would be required to have noticeable impact on
the cyanobacteria population. With a starting cell concentration of 100 cells/mL, a
growth rate of 0.4 day-1 and a diluting flow of 10% the cell concentration after 20
days would be 74420 cells/mL, which is below the critical threshold for cell numbers.
Higher rates of growth may reach the threshold concentration in shorter timeframes.
A trial is proposed to determine how effective a flow release management strategy is
at controlling cyanobacteria in the Torrens Lake to below the threshold of 100,000
cells/mL. It is noted that even below this concentration the cyanobacteria can
accumulate at the surface and form unsightly blooms. Because there is uncertainty of
how well the inflows will mix the lake water and dilute the population it will be
necessary to monitor the inflows and their impact upon lake hydrodynamics and
cyanobacterial abundance. The criteria for the monitoring is to confidently know that
the flow dilution releases are stopping algal blooms and poor water quality episodes.
The aim of this work, as outlined by the Adelaide and MLR NRM Board and SA
Water, is to devise a release testing strategy to ensure flows are delivering benefits
and we can scientifically prove this.
The challenging aspect of this type of trial is that it is difficult to prove success (ie no
cyanobacterial blooms) but it is easy to prove failure (ie cyanobacterial numbers
exceed the threshold of 100,000 cells/mL). This is because there may be numerous
reasons, in addition to the diluting flows, why cyanobacteria do not reach problematic
concentrations. These may include nutrient-limitation, low inoculum, natural
washout, unsuitable hydrodynamic conditions and competition for resources by other
algae. However, there has been regular high abundance of Anabaena circinalis for the
past four years which can be used as a comparison (Figure 1).
Experimental Water release
The most suitable flow strategy to trial would be a relatively high flow commencing during the months of
December or January because this is when rapid cyanobacterial growth is observed in the lake (
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Dilution flows monitoring program
Figure 1). The actual commencement of supplemented flows would be triggered by
the presence of cyanobacteria at a concentration of 500 cells/mL.
Brookes (2011) concluded that a base flow of at least 6.5% dilution would be
recommended in summer but this may need to increase to over 10% when
cyanobacterial populations begin to show exponential growth. Summer rainfall to
supplement these flows would be advantageous to flush out populations and reset the
population to a low concentration. The caveat to this recommendation is that it is a
compromise between achieving adequate dilution and using appropriate volumes of
environmental water. Adequate dilution requires good mixing and export of a
proportion of the population downstream which is a function of inflow and lake
hydrodynamics.
Figure 1 Anabaena circinalis cell numbers in the Torrens Lake at the Weir .
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Dilution flows monitoring program
Monitoring and experimental program
Sites and sampling locations
The sampling sites used in this proposed monitoring program are the same as those
used in the regular monitoring program for algal counts by the City of Adelaide. This
will enable consistent datasets, comparison with historical records and a
representative coverage of the lake.
The sites are described in Table 1 and depicted in Figure 2. Not all parameters would
be monitored at all sites. Temperature sampling may be required in the river upstream
of the lake but this will be described in the section of hydrodynamics.
Table 1 Sampling sites in the Torrens Lake
Site #
1
2
3
4
5
6
7
Site Description
Weir
Morphett St
Festival Theatre
King William Rd
Uni Footbridge
Frome Rd
Hackney Rd
Figure 2 Map of sampling sites in the Torrens Lake
Hydrodynamics
The premise of controlled upstream water releases to control cyanobacteria in the
Torrens Lake is that there is sufficient dilution and loss of cells downstream to
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Dilution flows monitoring program
overcome the growth and biomass expansion in the lake. Stratification will act to
decrease the efficiency of dilution but there is evidence from the ELCOM
hydrodynamic model that flows greater than 0.3 m/s (6.5%) dilution erodes
stratification particularly as the intrusion moves further into the lake (work by David
Lewis). This will act to make the lake more vulnerable to natural wind mixing and
complete mixing from nocturnal cooling. These processes will tend to make the
cyanobacterial population homogeneously mixed and dilution more efficient. If the
weir is allowed to be an over-top spill weir and not an underflow this will also
maximize cyanobacteria loss from the system.
Monitoring of the hydrodynamics during the experimental trial release should achieve
two objectives
1. Determine whether the inflow from the experimental water release achieves
mixing and efficient dilution
2. provide vertical profiles of temperature to validate the ELCOM modeling at
three sites in the Torrens Lake
Three thermistor chains should be installed in the lake with an additional thermistor
installed in the river upstream of the lake. The sites for the thermistor chain
installations would be at site 1,2, 3 and 6. The upstream thermistor would be installed
at site 7. If the thermistor chain is still operational at the meteorological station near
the weir then that would satisfy data collection of temperature for that site. It should
be checked for calibration in the field and recalibrated if thermistors are found to
diverge by more than 0.1C when suspended in the same water.
The other thermistor chains should contain 7 thermistors each spaced at depth 0,0.3,
0.6, 0.9, 1.2,1.5, 2 metres. Optical Stowaway thermistors should suffice. These has an
accuracy of about 0.1C. Two upstream thermistors should be installed at relatively
close proximity to each other to provide back up should one thermistor fail or be
stolen.
Data from other sources that is useful for this work is the routinely collected
temperature and nutrient concentrations of water in Kangaroo Creek Dam.
Depth profiles of water velocity would provide information on the flow and depth of
the instrusion. There are two ways that this could be undertaken. The first is using an
Acoustic Doppler current profiler (ADCP) permanently deployed on the bottom of the
lake. This measures water currents by exploiting the Doppler shift of sound waves
reflected from particles in the water. Velocity can be measured using Acoustic
Doppler devices as low as 0.001 m/s and up to over 2.5 m/s. The difficulty with a
bottom mounted ADCP in this application is that some depth is lost with the height of
the instrument and the blanking distance. Given this limitation the velocity in the
bottom 0.7m may not be measured, which is particularly problematic if the inflow
presents as an underflow.
A more appropriate method for measuring flow velocity and the depth of intrusion in
this case may be an intensive targeted filed campaign over two days. Velocity would
be measured with an ADV (Acoustic Doppler profiler) at the upstream inflow, at the
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Dilution flows monitoring program
point of insertion, and at the thermistor chain sites. It should be measured every 0.1m
over the depth of the water column, at 50 HZ and with a 120 second sampling period.
This sampling should occur every two hours from dawn until dusk for two days to
build up a picture of where the intrusion inserts and travels with varying inflow
temperatures and lake stratification.
Phytoplankton
There has been a routine monitoring program measuring phytoplankton, including the
cyanobacteria, weekly over the summer months. Algal counts are made for the
dominant species and the target species, Anabaena circinalis and Microcystis
aeruginosa, at eight sites (seven sites in the main lake and one site downstream of the
weir. This sampling should continue with similar frequency, which from the historical
record appears to be weekly or twice weekly on occasions when biomass is building
up.
There are several factors to consider when determining the frequency of sampling
including cost, availability of samplers and the potential cyanobacterial population
growth between sampling. With a 10% dilution rate and 0.4/day growth rate the
population could grow from 1000 cells/mL to 10,000 cells/mL in seven days (days 714 in Figure 3) or from 10,000 to 104,000 in seven days. Given the potential rapid
growth from fairly low levels to exceeding the threshold (1000 cells/mL to >100,000
cells/mL in 14 days at a 0.4/day growth rate) it would be prudent to sample twice
weekly once cyanobacteria counts exceeded 1,000 cells/mL.
Population size (cells/mL)
Popuation growth with 10% dilution
1E+09
100000000
10000000
1000000
100000
10000
1000
100
10
1
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0
5
10
Time(days)
15
20
0.8
Figure 3 Population size of algae growing at eight different growth rates but with
a daily dilution rate of 10%
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Dilution flows monitoring program
Modelling
Modelling enables different management scenarios to be compared as lakes are
always changing and rarely are replicate lakes available for manipulation and
comparison. Hydrodynamic modeling of the actual controlled release trials would
enable a direct comparison of what hydrodynamics would be expected with no inflow
with that which occurred during the inflow. Lake water and inflow water mixing and
residence times can be determined using tracers put into the model.
Quality input data is required for future modeling. This includes the full suite of
meteorological data, inflow and outflow volumes and temperature.
Data requirements are:
 Short wave radiation
 Long-wave radiation
 Relative humidity
 Air temperature
 Wind speed and direction
 Inflow volume
 Inflow temperature
 Outflow volume including extraction
These data are available from the meteorological station on the lake near the weir but
it is critical that data are downloaded regularly and archived.
Summary of sampling design
Table 2 Summary of sampling design
Site
Temperature
Velocity
1
Continuous
measurement with
Thermistor chain 10
minute intervals
2
Thermistor chain
3
Thermistor chain
2 day
intensive
sampling
every two
hours
2 day
intensive
sampling
every two
hours
2 day
intensive
sampling
every two
hours
4
Cell
counts
Twice
Weekly
Nutrients
Twice
Weekly
Twice
Weekly
Weekly
Twice
Weekly
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Dilution flows monitoring program
5
6
Thermistor chain
7
1 thermistor
7b
1 thermistor
2 day
intensive
sampling
every two
hours
2 day
intensive
sampling
every two
hours
Twice
Weekly
Twice
Weekly
Twice
Weekly
Weekly
Cost estimate
Component
Thermistors
Measurement
4 strings with 7
thermistors @$300
each
(optical
Stowaways Tidbit)
Phytoplankton
Cell counts at
Sampling costs
monitoring/Cell
seven sites twice a (counts @
counts
week
$128.50)
Cell counts cost
per sample x 14
samples per week
by ~12 weeks
Nutrient
Weekly at three
3 sites x 12 weeks
monitoring
sites
x $102.90 per
sample
Velocity
2 days continuous 7 people days
monitoring and
profiling with two @$1500 (2x2 for
analysis of data
people and boat
measurement and
3 days analysis and
reporting
Model validation
4 days @1500
Comparing
thermistor data
with appropriate
model output
Sampling and work 4 hours x 2 people 16 hr x 12 weeks x
use
x twice weekly for $150/hr
12 weeks
Field work for
sensor deployment
and maintenance
Cost
8400
21588
5400
10,500
$6000
28800
$4000
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Dilution flows monitoring program
Data analysis and
reporting
Total (indicative
only)
15 days @$1500
$22500
$107188
Note cost could be reduced by counting seven sites weekly and three sites
Costs are indicative costs only. Suppliers of these services may have price variations
Nutrient analysis
TP $24
FRP $17
NOx $17.50
TKN $23.80
NH3 $20.60
Total for five nutrients $102.90
Cell counts
Total direct costs $128.50
Partial count (cyanobacteria and dominants only) $72.10
References
Brookes, J.D. (2011) The feasibility of dilution flows for cyanobacterial management
in the Torrens Lake. Report to the Mount Lofty Ranges NRM Board. June 2011.
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