The Silbersee, a 7 ha recreational lake near Stuhr, Germany, is popular as a swimming lake for the town of Stuhr and the surrounding area, but is also used for fishing and general relaxation. It is approximately 40 years old and was formed from a gravel pit used for the construction of the A1 Motorway, which passes directly to the north of the lake. It is bordered on the southwestern side by a camping area, and two beaches with lawns for sunbathing areas are located on the northern and southern shores of the lake. The lake is surrounded by trees, and fields used for agriculture extend from the southern and eastern shores of the lake. It has a maximum depth of 8 m.
In 2006, blue green algae were a yearly nuisance and a cause of frequent swimming bans at the lake. The sediment contained a very high loading of nutrients caused by direct inputs, fish and flooding. Phosphorus in the sediment was measured to be approximately 1,000 mg P kg DW with about 40% of this being bio-available P. The Institut Dr Nowak, based in Germany, was commissioned by the community of Stuhr to effect a permanent reduction in phosphate levels in the lake and thereby reduce the incidence of blue green algal blooms. In order to achieve this aim, 21.5 tonnes of the in situ phosphate binder Phoslock were applied to the lake in November 2006. Phoslock is a modified bentonite clay which immobilises phosphorus in both water and sediments and was developed by the Australian Commonwealth Scientific and Industrial Research Organisation (CSIRO) and commercialised by Phoslock Water Solutions of Sydney, Australia.
Binding phosphorus
Before Phoslock was applied to the lake, nutrient levels had been reduced through the removal of deep water. During 2005, approximately 18 kg of phosphorus had been removed by pumping up and discharging water from the deepest parts of the lake. It was calculated, however, that nutrient levels could only be expected to drop very slowly through hypolimnetic discharge, due to the large quantities of phosphate that had accumulated in the sediment over many years. A permanent reduction of the phosphate level to the degree required would have therefore taken many years. Removing the deep water was also not possible as it would have caused a reduction in the water level, particularly in the summer, and this would have affected recreational users of the lake. Dryness in the fringe zones could also have affected the environment and destroyed the habitats of birds, fish and insects.
In the middle of November 2006, 21.5 tonnes of Phoslock were applied to the lake in order to bind the phosphorus in the sediment and water column and minimize the release of sediment P that could otherwise be used in biological processes. In so doing, it was anticipated that the intensive blooms of blue green algae that had occurred in previous years could be prevented.
Before the application of Phoslock, the water of the lake was visibly stratified and anaerobic conditions had formed below 5 m. Immediately prior to the application, however, the destratification process had commenced as a result of strong winds – by the time the application started, the Silbersee was completely mixed. Aerobic conditions extended to the bottom of the lake at 7 m and the pH stabilised at a value of 7.6.
This created favourable conditions for the removal of phosphate because a large part of total phosphorus existed as orthophosphate and was distributed evenly over the entire water column. Upon applying Phoslock, it was therefore possible to achieve the maximum phosphate binding efficiency during the slow settling phase through the water column.
Phoslock was applied to the surface of the lake from a motorized barge carrying a venturi mixing system. In situ water was pumped into the mixer where it was mixed with Phoslock and then sprayed evenly over the water surface of the lake, except for the shallow areas near the bathing areas.
Results
The lake’s water was monitored regularly following the application of Phoslock. One month after the application, orthophosphate in the water column had fallen below detection levels. Total phosphorus concentrations in the lake’s water were also reduced by 80% and over the past two and half years have remained around 30 μg P/L, even during the summer and early autumn periods when anoxic conditions developed in the lake. Before the application of Phoslock, the total phosphorus concentrations of around 1,500 mg/L had been recorded in the hypolimnion during these periods.
As well as this, sediment testing following the application indicated a very stable situation in relation to phosphorus. As a result, it is not expected that the bound phosphate will be re-released from the sediment.
Total lanthanum concentrations in the lake water were measured at 100 μg/L following the application. These dropped during the monitoring period to 4 μg/L. Despite the very windy period after the application of Phoslock, with winds of more than 9 Bft, no resuspension of the material in the lake was observed, indicating that both lanthanum and phosphate have been deposited solidly in the sediment.
Within a month of the application of 21.5 t of Phoslock to the Silbersee, 100% of ortho-phosphate and 80% of total phosphorus was removed from the water column, and since the application, ortho-phosphate levels have remained below detection limits and total phosphorus levels have remained constant at around 30 μg/L, despite the rise in temperature in the water column, the stratification of the lake and the development of anoxic conditions in the hypolimniom. This shows that the re-release of phosphorus from the sediment has been almost completely prevented. In relation to its phosphorous content, the trophic status of the lake changed from a strongly eutrophic to a mesotrophic condition within a period of two months.
There were no effects on the flora and fauna in the Silbersee through the use of lanthanum. In fact, the application did not result in the complete disappearance of blue green algal blooms from the lake – but the blooms that did occur were minor and it was not necessary to close the lake to swimming.
However, anoxic conditions did develop in the hypolimnion of the lake during the summers of 2007 and 2008 and this situation is likely to continue in the future. Bacteria in the sediment first use dissolved oxygen in the water for the decomposition of organic materials. When oxygen is no longer available, they switch their respiratory processes to nitrate. This leads to a reduction in nitrogen levels with the result that nitrogen becomes the limiting nutrient for primary production. This in turn limits the growth of algae such as green algae and diatoms – but blue green algae, on the other hand, are able to fix nitrogen from the atmosphere and are not dependent on the availability of nitrogen in water, which is present in the form of ammonium or nitrate. Under such conditions, blue green algae have enormous advantages over other forms of algae and are able to form huge blooms.
Ongoing control of the phosphorus load is therefore very important. When the phosphorus pool decreases, the microbial decomposition of organic substances also decreases. This in turn results in shorter anoxic periods and a deterioration in the conditions in which blue green algae can grow. In the Silbersee, there are already signs that this is occurring. After the temperature of the water at the surface of the lake dropped in autumn 2007 and 2008, oxygen reappeared in the hypolimniom within a short period of time. Before the application of Phoslock in 2005, this process took much longer. In other words, the biological oxygen demand (BOD) was lower in 2007 and 2008 than in previous years. The reduction in BOD can only be attributed to a significant reduction in the phosphorus load.
The future
Since the application of Phoslock at the Silbersee lake, more applications have been undertaken on another six lakes in Germany and the Netherlands with comparable results. A substantial number of large and small applications are planned across Europe for 2009.