Lakeside is a man made aquifer fed lake of 22 Ha and has no through flow of water. This standing water creates an unmixed and heavily stratified lake. Previous reports showed that the lake was subject to toxic blue green algal blooms and low Dissolved Oxygen (DO) in the deeper waters (11m), which coupled with a high temperature drop through the water column, meant that the lake was struggling to support the nature reserve and aquatic flora and fauna.
Aquarius Marine Group Ltd was asked to install an AQUAERATION system on the bed of the lake in winter of 2006. They were turned on at the end of May 2007. Figure 4 shows the AQUAERATORS distributed throughout 'Lakeside' and are marked by buoys. The 4 AQUAERATORS create a highly turbulent plume of water/air which entrains more water as it moves through the water column. In 'Lakeside' the AQUAERATORS were situated at a depth of about 10m. With an inflow of air of 11l/s the plume at the surface has a flow of 4.5tonnes/sec.
AMG Ltd undertook a detailed survey of the lake by using a YSI multi parameter 6600 sampling Sonde, which accurately takes readings of temperature, pH, dissolved oxygen, depth, turbidity, chlorophyll A, conductivity and fluorescence. The Sonde was lowered into the water at a steady pace and then brought back to the surface and repeated at 4 positions throughout the lake.
Data was collected before the AQUAERATORS were switched on (May 2007), after they were switched on (June 2007) and in winter when turnover had occurred and the AQUAERATORS were turned off (October 2007)
The collected temperature data shows that in summer months, before the AQUAERATION System was installed, the lake was prone to thermal stratification. The top meter of the water was reaching temperatures of almost 17ºC with the bottom waters only 8.5ºC. The thermocline is the area in the graph with the shallowest curve. The shallower the curve, the greater the change in temperature with depth. The thermocline at 'Lakeside' is at a depth of about 8m, the temperature at this level drops from 13ºC at 7.5m to 10.5ºC at 8.5m.
After the AQUAERATOR System was switched on in June, the temperature profile is much more constant through the water column and is higher in the lake in June. The surface temperature is about 0.3ºC higher than May while the bottom waters are about 7ºC higher, showing destratification was achieved.
In the winter months the water turns over, creating a similar profile to June when the AQUAERATION system is in use, with an overall temperature in October about 3ºC lower.
In the May readings, the pH remains about 8.5 until a depth of about 7m. At this depth it drops from 8.3 (at 6.5m) to 7.6 (at 8.5m). From ~9m the pH remains roughly constant at about 7.5.
In June, after the AQUAERATORS were switched on, the pH change is much less and does not happen so high up the water column. The pH change occurs at ~ 10m with a drop from 8.5 (at 9.5m) to 8.0 (at 11.5m) and a slight decrease at the surface.
In October, during the turnover, the pH is lower throughout the lake. There is very little change in the pH through the water depth and a slight decrease is shown in the surface waters.
Analysis of Results
The June results show that the AQUAERATION system works very well, the temperature profile shows that there is very little difference between the surface and bottom temperature (only a 2º decrease over the 12m), proving destratification. The plume of well mixed water/air going through the lake aerates the whole of the water column, increasing the bottom water Dissolved Oxygen (DO) from 8.5% to 47.2% (0.99mg/l to 4.68mg/l). There is a decrease in the DO in the very bottom of the lake caused by respiring bacteria feeding on dead organic matter. The aeration of the lake also evens out its pH. Although respiration is still occurring, the presence of O2 from the aeration means that any acid (H+) created, is oxygenated to form water (H2O), keeping the pH level. The surface pH decrease in the top 1m is caused by atmospheric CO2 depositing on the surface of the water; the decrease of the bottom 2m is caused by the bacteria on the sediment.
The AQUAERATORS also helped to prevent the build up of algal blooms. High concentrations of chlorophyll A were present at various points through the water column in May, in June these had gone. The high concentrations of chlorophyll A 1-2m from the surface is probably caused by higher concentrations of sunlight compared to May. The turbulent nature of the AQUAERATOR system helps to prevent algal blooms from occurring, without affecting the amount of photosynthesising biota in the lake. A report by the World Health Organisation states that harmful cyanobacteria cannot easily live in water with a flow greater than 1m/hour. The AQUAERATORS create flow of 4.5tonnes/sec, which inhibits the cyanobacterial growth, whilst encouraging the growth of much less harmful bacteria like diatoms. The rotating header bridge scene above shows the large area affected by a single AQUAERATOR, situated in a depth of about 10m.
In autumn, the colder surface waters create natural convection currents, enabling the AQUAERATORS to be turned off. The temperature is stable through the depth of the lake and the mixing ensures that the DO and pH remain constant. The top 1m has a lower pH due to CO2 depositing in the surface waters. Chlorophyll A is high and constant through the water column with the surface water having little or none due to the wind disrupting the algae on the surface.
'Lakeside' Development in Doncaster is a small aquifer fed lake. The lake has no through flow of water and will be used for recreational purposes all through the year.
Aquarius Marine Group Ltd was asked to install an AQUAERATION system of 4 AQUAERATORS in winter of 2006. Water measurements were taken in May which showed that the lake was stratified. The thermal stratification of the lake decreased the dissolved oxygen in the bottom waters and decreased the pH. The AQUAERATION system was turned on in June. Measurements taken while the AQUAERATORS were turned on showed that the lake was no longer stratified, thus scientifically proven by the higher concentrations of dissolved oxygen in the bottom waters and there was less of a decrease in the pH. The Chlorophyll A measurements showed a more even distribution of algae, helping to prevent the build up of algal blooms. Measurements were then taken in October when turnover had occurred. The even water temperature meant that the dissolved oxygen and pH also remained constant through out the water column.
This proved that the AQUAERATION system destratifies lakes, improves the water quality of bottom waters, can help to prevent algal blooms and enable compliance with the new EU Water Framework Directive . The way the AQUAERATOR is designed, means that it does not disrupt the recreational uses of the lake as the top of the AQUAERATOR is still about 8m from the surface