Cassington lies adjacent to the A40 in the Thames Valley approximately 4km north-west of Oxford city centre. The site is an extension to 70 hectares of existing traditional open water restorations that are used for fishing but with no general public access. The site lies within 500 metres of two wet meadow SSSI’s (Cassington Meadows and Pixey & Yarnton Meads).
Restoration / Priority Habitats
Worton Bird Group
The original restoration plan was for more open water, but concerns were raised from the local Kidlington airfield, as Cassington is within the 13km flight safeguard zone of this site. A new plan was put forward that featured a reedbed restoration for this final stage of the extraction. The site has a 20-year aftercare condition imposed on it in addition to the statutory five-year aftercare.
Topsoil and subsoil were stored in discrete bunds around the site ready for use within the restoration. The bunds were seeded to prevent erosion and the slopes graded to prevent slumping. This image shows that despite the low nature of the bunds, anaerobic conditions quickly established within the centre of the bund, and this should be born in mind when creating bunds.
Habitat design and creation
The reedbed habitat was designed in conjunction with the RSPB, and many important features of reedbed design are present:
- Final spring water level will be approximately 800mm average depth.
- Subsoil was used in the landforming of the beds to reduce fertility.
- An area of vertical sand cliff has been retained on the northern side (facing south) for the benefit of sand martins.
- The landforming of the beds has included provision of many small peninsulas and scalloping of the edges.
- The surface of the beds was roughed up and made more uneven during the final stages of restoration.
- Channels and deep-water areas feature fish refuges such as over-deepening of discrete areas and bundles of brash from nearby tree felling operations.
- Annual weed growth on the exposed islands will be controlled in the first year by raising water levels, before water levels are dropped and reed is planted.
A small area of the site had been restored to a paddock on the landowners request, but the lower areas of this have been landscaped to include a small number of small and shallow ponds in conjunction with Pond Conservation.
The wider landscape
The site sits within the Thames Valley and is close to the Lower Windrush Valley, another area of extensive sand and gravel extraction. The Thames floodplain and its tributaries remain important for biodiversity despite the highly modified nature of the landscape. It remains a key area for breeding and wintering wetland birds – the area is part of the Upper Thames Clay Vales Character Area, and was previously an RSPB ‘Futurescape’ area.
The aim is for the reedbed to cover 75% of the area of the waterbody, so as to reduce open water (thereby reducing the possibility of large bird flocks creating a risk of bird strike) and to provide quality habitat for priority conservation species such as Eurasian bittern (Botaurus stellaris).
Nine islands were profiled in 2010, and inoculated with reed in 2011 and 2013, using plugs. The planted areas of the islands were fenced to limit grazing damage to establishing reed. Improvements and repairs to fencing were undertaken in 2016 and subsequently reed has established well inside (but not outside) the fenced areas. In autumn 2018 several of the fenced areas were extended to provide protection for an expansion of reed from the core fenced areas. There is limited reed growth in these extended areas and extremely limited growth outside of fenced sections. Continued maintenance and extension of the reed protection, plus further reed planting, will be needed to ensure the reedbed expands in size towards 75% cover.
Reed establishment has been affected by the following factors:
- Herbivory. The fence enclosures have been repaired and enhanced to ensure that the structures are solid and the mesh size is small enough to keep out wildfowl that will be inclined to eat the new reed growth. Coot grazing is believed to have been a significant issue and has hampered reed establishment outside of fenced areas, and within fenced areas where damage has occurred to structures.
- Water level fluctuation. The lagoon is served by one small outflow pipe with a simple water control but there is no active control of water levels on site. Water supply is via a connection south to the adjacent water body and onward to the River Thames, plus via ground water and rainfall. Water levels on the River Thames are a key driver of site water depth and are outside the control of site management. This has meant the site has not always had optimal water levels to promote reed growth.
- Water quality. Water testing results show high water quality throughout most of the site. Nitrate and phosphate levels are low, but not growth limiting for reeds.
Reed establishment has been hampered primarily through the damage caused by coot grazing. Where coot, or geese, numbers are high careful monitoring is needed to establish if they are causing an issue to reed establishment. Cages or fenced areas are crucial to protect new reed planting and allow establishment. Once reed has matured somewhat the fencing can be removed and used elsewhere. This can require a significant investment in time and funds to achieve establishment.
Additionally, on sites where there are no water control structures it is not always possible to achieve optimal conditions for reed planting or establishment, which can further slow reed take up.