BNG Partnership

Case Study:

Sanctuary Lake

Location: Dorset, UK
Area: 4.6 hectares
Habitat: Freshwater lake, lowland meadow
Number of Trees: 5,000

Project Focus: Biodiversity Net Gain
1.2-hectare lake
Extensive lake profiling
Installation of drainage systems
Aquatic marginal planting
End to end project

BNG UPLIFT – 265%

BNG Partnership

Site History

The site was a low diversity grassland located in rural North Dorset, primarily used for grazing cattle. Although the fields were resown with clover leys regularly, the site was very wet and could only be grazed for a couple of months each year. It is located adjacent to a SSSI, which informed design decisions to expand habitat for butterflies, and the wet ground and topology made a lake a good choice. The centre point of the design is a large 1.2-hectare lake, with aquatic marginal planting, new trees, and a surrounding wildflower meadow. A detailed ecology report was completed prior to works beginning as part of the planning process, which highlighted that there were no records of red-listed species such as great-crested newts or water voles in the area.

BNG Partnership

Project Design

The 1.2-hectare lake was carefully designed to provide a multitude of microhabitats with shallow shelves for amphibians, deeper areas and gravel shallows for fish, islands and mud scrapes for waterbirds, and clay bars. Around the lake there are reeds and marginal aquatic plants to provide habitat for invertebrates, amphibians, mammals, and birds, then a larger lowland wildflower meadow beyond that. Scattered across the meadow there is riparian woodland planting with nectar-rich trees such as bird cherry. The design of the site was informed by the topography of the land, local nature priorities, and research into the site history. Click image to expand.

BNG Partnership

Project Implementation

The ambitious dig was completed in autumn 2021 and required two diggers working across 40 days to excavate and sculpt the lake. The lake was seeded with invertebrates including copepods and other zooplankton after filling using an innovative technique to jumpstart the establishment of the lake ecosystem. The trees and marginal aquatic plants were planted in 2023 and are already flourishing. A total of 3 hectares of wildflower meadow has been sown, providing a rich source of nectar for pollinating insects, including local priority species such as the marsh fritillary butterfly. For more information about the lake dig process, please see our YouTube channel.

BNG Partnership

Project Results

The project is predicted to deliver a BNG uplift of 24 units (265%) according to the DEFRA Biodiversity Metric. We have already noted dramatic increases in dragonfly and damselfly populations, and the appearance of red-listed species such as water voles and slow worms, demonstrating the real-world success of our habitat work. In addition to the planted wildflower species, we are finding natural wildflower regeneration in the meadow area of species such as Dorset notable corky-fruited water dropwort. In addition to habitat level monitoring, required for BNG, we are using eDNA to assess biodiversity increases against the baseline, and traditional surveying methods such as ocular surveys and camera trapping.