The nutrients in the lake come from upstream in the catchment. The source of nutrients can be from soil erosion or from agricultural fertilisers that have made their way into the creek. Once the nutrients enter the lake they stay there until they are consumed, exported or deposited in the mud.
What methods have been used elsewhere to reduce nutrient loads?
High nutrient concentrations have been remediated in lakes in other parts of the world by:
Removal of nutrients at their source (prevention of sediments and fertiliser entering the lake from the farm-land along the creek). This is the reason that the BBCAG has advocated and facilitated fencing and revegetating the Fiery Creek upstream of the lake. Giving the creek a buffer of vegetation and preventing stock access have been shown to result in less nutrient loss from the land, and better water quality in the stream. This should be continued, it just makes sense…..keep the nutrients on the land, use them for food production, and keep the lake clean.
Filtration of the lake water through a nutrient uptake system. Artificial wetland systems are often used in sewerage developments, as plants are very efficient at removing nutrients from water. Natural wetlands can do this too, and are likely to have done it for Lake Bolac in the past, when its catchment was a mosaic of swampy wetlands. Creation of a wetland system that can filter water from the catchment is an expensive idea, and might be the solution we are looking for.
Removal of the deposited nutrient-rich sediment on the lake bed. This sounds like a good idea, but it would require the lake to be dry, and given the area of the lake, would be prohibitively expensive. Also, where would you put all the nutrient rich soil? When the lake dries the wind does remove some of the nutrient-rich soil and deposits it in and over the lunette.
In situ removal of nutrients by water plants other than algae. Given the area of the lake, the extent of ‘weed beds’ required would be extensive, and so far there has been limited natural re-establishment of these.