Floating wetlands - Conclusion

The results of the investigations indicate strongly that the creation of areas of articial wetland, either naturally floating or utilizing engineered flotation, provide potential for ecological enhancement. The study also shows that extensive research and practical studies have been undertaken in this area. However, to date, little practical application has been made in the field of quarry restoration. A significant opportunity has therefore been identified which could greatly increase the ecological value of water-based quarry restorations. With careful thought and planning, quarry sites could contribute to achieving regional BAP habitat objectives.

The review of the published literature indicates that floating wetlands are widespread globally, and form a major component of some aquatic ecosystems, often supporting rich biodiversity. Floating wetlands may cover large areas of open water and are functionally important in wetland ecosystems and wetland economies. Natural or artificial floating islands are used by some indigenous peoples.

Fabricated vegetation rafts are being used for management of the water environment including, wildlife/fishery habitat, erosion control, nutrient sequestration and pollution remediation.

The vegetation of floating wetlands varies across the globe but there are some striking similarities. The floating wetland substrate typically consists of a thick organic mat, entwined with living roots, that rises and falls with the ambient water level. Rafts of floating vegetation are typically formed by invasion of key species from the grounded lake margin out into open water. Another method of formation is via the detachment of established vegetation and/or uplifting of buoyant masses of peat substrate.

Floating wetlands could potentially play a significant role in quarry restoration and further field trials are required. Planting of keystone raft-forming plants along sheltered shorelines of quarries may encourage natural development of floating fen. Reduced wave action and protection from grazing by waterfowl are likely to be essential. Within the UK, bobgean (Menyanthes trifoliata), marsh cinquefoil (Potentilla palustris) and bottle sedge (Carex rostrata) may promote initial raft formation, though it is unclear how well these species would perform in more nutrient-rich waters.   

Initial field trials were undertaken at Hatfield Moor nature reserve and Nosterfiled Quarry. These trials were intended to obtain a practical insight into the deployment of the rafts using existing ‘floating island’ technology and inform the planning of further field trials based on additional methods derived during the process of this project. Less than a single growing season was available for the project and longer duration trials will be required to assess the development of the raft forming vegetation. Suggestions are made as to possible applications to the quarry environment as a basis for further applied research. In particular, the potential use of low-cost and/or biodegradable floating platforms to establish auto-buoyant vegetation rafts needs further investigation.

The next stage of the process of determiing the feasibiltiy of adopting these measures is to scale-up the field studies which have been undertaken to date. It is recommendend that a test site is developed in which to trial the methodologies identified within this report in a realistic field-scale trial.

In practice, and in a quarry setting, the establishment of 'traditional', commerically available floating islands is not likely to be appropriate due to the high cost. Therefore emphasis must be placed upon the development of pragmatic, cost-effective and achievable habitat creation which is within the scope of the quarrying environment. It should be recognized that a trade-off must be made between creating ecologically valuable habitat and acceptable cost. This should be the focus of any subsequent field trials.

Recommendations

Based on the literature review of natural systems and discussions with floating wetland experts, quarry operators and nature conservationists a series of recommended field trials have been outlined. It is likely that a minimum of 3 growing seasons would be required to adequately monitor the development of the raft forming vegetation. Sourcing and propagation of appropriate plants would be required and assistance with installation of the necessary site infrastructure. The existing field sites are considered to provide ideal locations for extending the work but additional sites in other parts of the country are also required.

It is recommended that the following items represent a basis for further work.

• Establish stands of Typha angustifolia and Phragmites on site in a low bulk-density medium such as coir fibre within a textile ‘envelope’. Attempt this in-situ in very shallow water and then allow flooding to assess whether auto-buoyancy can be achieved. Also this approach should be tried on a thin loose substrate overlying an impermeable membrane allowing adequate time for the formation of an interlocking root system to support the plant top-growth before flooding.
• Construct a breakwater to create a sheltered area (1 Ha) of shallow (<2m) water on the south western margin of a quarry lake. This should incorporate some 100 meters of shoreline which may be subdivided into a series of experimental sections.
• Establish bobgean (Menyanthes trifoliata), marsh cinquefoil (Potentilla palustris), bottle sedge (Carex rostrata) and slender sedge (C. lasiocarpa) along the lake margins and monitor outward expansion. A constant water level or minimum fluctuation is required for this to be successful.
• Sectioned off areas of the shallows should be filled with loose semi buoyant fill such as brushwood, woodchips and loose peat, to provide buoyancy and support for establishing floating wetland vegetation (Figure 33).
• A sufficient volume of buoyant P.F.A. cenospheres should be purchased for use as in item 4 and also mixed with a suitable substrate as in item 1.
• Artificial attempts may be made to ‘seed’ the shallow areas with nuclei of pre-established plants. This may take the form of a ‘chequer board’ of artificially buoyant structures and fixed stakes. The maximum spreading width of the vegetation across the interstitial spaces of open water is unknown but a best estimate based on experiments in Louisiana would be less than 2 m. It is recommended that both commercially available rafts and the potentially cheaper DIY raft constructions are used. 
• Techniques used for river bank restoration could be trialed on the quarry lake margins. This would involve the use of pre-planted coir pallets or coir rolls along the shore or immediately off-shore on artificial floating rafts. Brush-wood bundles would be installed in the shallow water to trap silt and reduce circulation of colder water from the main lake.
• Where already present at a site the use of fast growing invasive species may be considered as a rapid means of generating natural floating support structures for colonisation by later (more important) wetland species.
• A detailed cost efficiency study should be undertaken of the different options tried, including nursery, construction, installation and management costs. This should also include an assessment of the additional mineral potentially released by the utilisation of the new techniques.

The pioneering experiments suggested may represent a world first in wetland habitat creation and potentially provide a cost effective method of creating high quality BAP habitat.