Benthic ecology is the plants and organisms living at the lowest level of a body of water, on the bed surface or in the sub-surface sediment layers. With the large tidal range in the areas that Tidal Lagoon Power (TLP) are developing, some of the benthic ecology will be submersed all of the time, and some will be submersed for part of the time with the rise and fall of the tides, and this variation in conditions generates different ecological characteristics. We term these subtidal benthic ecology and intertidal benthic ecology respectively. The plants and organisms living in this zone generally provide food for bottom-dwelling fish or foraging birds.

As part of the development of any of our proposed lagoons, TLP would undertake detailed characterisation surveys of the subtidal and intertidal benthic ecology present at the site of the lagoon and the surrounding water body. For the subtidal aspect, this would involve collecting samples from a boat with a ‘grab’ that is lowered to the bed of the water body to collect a sample of the sediment present. For the intertidal, this could involve walkover surveys to identify species present on the surface, or core samples for organisms that live below the surface such as worms. The benthic ecology composition is highly dependent on the sediment type present and so we refer to any sediment type data available also, including from sampling and analysis for particle size distribution and also from geophysical surveys such as Side scan sonar. TLP would then monitor for impacts on these species and habitats whilst the lagoon is being built and once it is operational, repeatedly sampling key areas over a number of years.

TLP are currently undertaking an extensive characterisation survey programme of the benthic ecology within the Severn Estuary to inform the development of Tidal Lagoon Cardiff and any further lagoons proposed within the Severn, and are also progressing the monitoring of benthic ecology within Swansea Bay as part of the Adaptive Environmental Management Plan (AEMP) for Swansea Bay Tidal Lagoon. The characterisation surveys within the Severn Estuary include the collection of an extensive set of intertidal cores across the soft sediments of the estuary. Each core is supported by a corresponding quadrat sample (pictured below). The extent and condition of a variety of key intertidal habitats within the estuary, such as saltmarsh, seagrass, sabellaria reef and hard substrates/macroalgae are also being characterised in more detail, being visited on foot or by hovercraft where access is difficult. Subtidal grab samples from a range of locations within the estuary will also be collected over a range of sediment and habitat types.

Dwarf eelgrass

Dwarf eelgrass (Zostera noltii) on muddy gravel in the Severn Estuary


Sabellaria reef
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