Site Description
Conningbrook Lake is a mature gravel pit that covers an area of approximately 15 hectares and water depths up to 5m. The lake is used for a range of amenity uses. These include recreational coarse angling and a range of contact water sports including bathing during organised Triathlon events.
Management Issues
The lake displayed a range of management issues that impacted on it’s amenity function and water quality status. These included:
- Excessive growth of Canadian Pondweed which interfered with amenity use and for which costly manual cutting proved ineffective;
- Development of potentially toxic blue-green algae (cyanobacteria) that led to routine closure of the lake during the summer months; and
- Accumulations of floc organic sediments in the deepest areas of the lake, which lead to the development of low dissolved oxygen concentrations. Additionally, this encouraged potential internal remobilisation of nutrients that were likely to contribute to excessive plant growth and algae blooms.
A structure rehabilitation strategy was developed for the lake that involved:
- A treatment with powered chalk to consolidate the floc sediments;
- Applications of lake dye to inhibit the growth of Canadian Pondweed and to mimic the light regime of a deep-water lake. This ameliorates the development of blue-green algae when combined with a relatively vigorous ISS-Flowthrough aeration / mixing system; and
- Installation of an extensive diffuser-based aeration system (system design shown below) to create a vigorously mixed system. This system was designed to inhibit the development of blue-green algae. Furthermore, this system will prevent low dissolved oxygen concentrations developing towards the lake bed.
The Results
Canadian Pond weed was successfully controlled by the use of lake dye with residual plant growth restrict to the lake margins and shallowest parts at the southern end of the lake. This provided the local authority with large saving in terms of aquatic plant management costs.
Deployment of the ISS-Flowthrough aeration system, comprising of 37 diffusers has resulted to date in successful control of the blue-green algae blooms with no requirement for closure of the lake since the system has been in operation. Routine monitoring of the lake is undertaken for the presence of blue green algae. In the first year of operation (2018) some blue-algae were recorded in the southern end of the lake at the start of the summer. During 2019, no blue algae were recorded in the lake across all sampling locations.