Habitats on the Tidal Thames
The habitats of the tidal Thames provide a variety of functions to both the animal species of the river and estuary and the people living along its length.
The species found within an ecosystem is determined by the habitats found within it. Habitats provide shelter, food and water, space and the potential to reproduce for the species it supports. In addition, the presence of natural habitats provides benefits to humans such as carbon sequestration, flood defence and local amenity values.
As an estuary, habitats in the tidal Thames are associated with marine and freshwater environments.
Freshwater Tidal Gravels
Freshwater gravel habitats occur in the upper reaches of the tidal Thames and are important habitats for spawning fish, many fish species require gravel substrates to lay their eggs.
Mudflats & Sandbanks
Mudflats and sandbanks are the most extensive habitat in the tidal Thames. They are formed of fine sediments and are exposed at low tide and fully submerged at high tide. These mudflats provide important habitat for small invertebrates, such as the tentacled lagoon worm (A. romijni), and the birds which feed on them. Mudflats are typically found in the marine and brackish sections of the estuary.
Coastal saltmarsh is found in the marine sections of the estuary where salinity levels are higher and where currents are slower, allowing for salt tolerant plants to become established on the sediments. The height of saltmarshes is between mean high-water neaps (MHWN) and mean high-water springs (MHWS), and saltmarsh plants can tolerate being submerged at high tide.
The vegetation and natural creeks within saltmarshes provide shelter and food sources for juvenile fish and provide important roost habitats for birds. These habitats are at risk from the sea level rise and solid flood defence walls. Flood defence walls stop natural retreat and causes the habitat to become ‘squeezed’ between rising sea levels and the solid structure. This causes erosion of the saltmarsh at the water’s edge and a reduction in saltmarsh extent.
Seagrasses are the only flowering plants able to live completely within seawater and pollinate whilst submerged. The beds are found at the edges of marine areas of an estuary where the water level is shallow enough to allow the required light wavelengths to reach the plants for photosynthesis.
Seagrass beds provide shelter for juvenile fish species and seahorse species whilst their root systems provide habitats for small invertebrates. These root systems help to stabilise sediments, providing a form of natural defence. In addition, it is estimated that seagrass beds are responsible for 15% of the ocean’s total carbon sequestration and for this reason they have the potential to play an important role in mitigating climate change.
Reedbeds are found in the transitional areas between aquatic and terrestrial environments, where there are no solid flood defences to interrupt the natural transition. Reedbeds are usually dominated by the common reed (Phragmites australis) which grows up to two metres tall. Reedbeds provide habitat for birds and invertebrates, for example, reed bunting, sedge warbler and reed warbler, who use the habitat for breeding. The common reed dies back slightly in winter but provide shelter against predation for a variety of species. Reedbeds also provide habitat for feeding on fish at high tide.
Reedbeds also act as a natural filter of pollutants, such as run off from agricultural fields and roads, that may negatively affect the water quality of the wider water body.
Man-made structures such as buildings, flood defence walls and jetties can be enhanced to provide benefits to the natural environment. One method of enhancing these structures is to create artificial habitats in areas that do not affect their operational use and maintenance.
Green Roofs and Walls
Green roofs and walls can be installed as an alternative to traditional materials, they provide multiple benefits, for example, urban habitats, improving local air quality, providing natural insultation, reducing running costs and reducing surface water flooding. When considering the use of artificial habitats, it is important to balance the needs of the structure with the desire for environmental improvements.
The Estuary Edges initiative provides guidance and cases studies on how the solid flood defences of the tidal Thames can be softened through the incorporation of engineered habitats. The guidance includes design principles for managed realignments, intertidal vegetated terraces and options for vertical walls with previous case studies on each topic and advice on safety and navigation, archaeology and heritage, geomorphology, sustainability and adaptability, wildlife and local community benefits.