The Tidal Thames is a recovering ecosystem of great ecological importance and the Estuary supports a diverse flora; rich populations of invertebrates; 121 species of fish and many internationally important aquatic birds.
In 1957 however the River Thames was declared biologically dead with water quality so poor that it could not sustain life. Since then the river has undergone a massive transformation, water quality has improved, and wildlife has returned. Nevertheless long stretches of concrete flood defense walls on the Thames are still preventing plant growth and the invertebrates that thrive in healthy riverbank habitats.
Projects such as Love the Lea have begun to address this with Green Walls, which provides grid like structures containing native plant species mounted to embankment just above and below the mean high tide line. These help support species that currently have little or no habitat on the Thames, boosting the ecological value of the river in central London, while preserving the flood defense function of the hard surfacing.
Green Walls project at North Woolwich by Love the Lea. Photo courtesy Thames21.
Further upriver in the urban and industrial reach that flows through inner city boroughs such as Lambeth, the little habitat that exists is placed under considerable stress from the rise and fall of the tide, wash from river traffic and the changing composition of fresh and saltwater. Riverside redevelopment has meant the loss of supportive environments such as the historic Lambeth Marshes, leaving only vertical and hard flood defense walls, creeks, docks, inlets and other artificial structures. Invertebrate diversity along this stretch is the lowest of the tidal river.
There can be no invertebrates, fish or birds however without the micro organisms that support the food chain. Lambeth Floating Marsh is an experiment in the creation of more places in which these organisms can thrive together with the invertebrates that feed on them. While reed beds can fulfill such a function and their use is now widespread, there is little opportunity to locate these in the shored up urban reaches of the Thames. For this project long basket structures have been constructed along the sides of a Dutch barge, which floats at high tide and then rests on the foreshore when the tide recedes. These baskets have been planted with an assortment of reeds in deep, narrow baskets that afford some protection when the barge is afloat.
Reeds are known to increase biodiversity on river margins.
Reeds typically grow in a wet environment on soil base. To recreate a constantly wet environment alongside the barge would require a means of retaining the soil and water when the tide is out, which can be done by lining the baskets with impermeable sheets filled with mud so that some water is retained when the tide recedes. A similar approach has worked successfully in an adjacent dock where a small derelict rowing boat has been used to house a reed bed.
On this project however a different method is being tried out with the baskets instead being lined with nylon mesh, then filled with gravel. While only affording minimal water retention, this approach offers an alternative habitat for those organisms that do not necessarily burrow in mud. Identification has been made of microorganisms already present in the river water and monitoring will continue to find out more about how they thrive in this new ‘floating marsh’. In addition there will be opportunity to learn about their interaction with the root systems of the plants that grow in the bed and observe how this environment changes over time.
Sources
Thames Estuary Partnership Biodiversity Action Group (2005) Tidal Thames Habitat Action Plan, London: Thames Estuary Partnership. Download the plan: www.thamesweb.com/projects-introduction/80-projects/other-projects/75-habitat-action-plan
Love the Lea (2012) Green Walls, Available at: http://www.thames21.org.uk/project/green-walls/greenwalls4-2/ (Accessed: 14th August 2015).