[WATER] Cultivation of biological filters for the improvement of reservoir water quality and biodiversity

Eutrophication of freshwaters is a serious problem in many places worldwide, causing marked changes in the biota. N&P enrichment of the Norfolk Broads saw a shift from a clear water system dominated by charophytes, macrophytes and a diverse invertebrate fauna in the 1940s, to one dominated by phytoplankton and an impoverished invertebrate fauna by the 1980s. Eutrophication-driven biodiversity loss is a concern in many UK reservoirs which are important sites for conservation (SSSIs and SACs). Furthermore, the European Water Framework Directive (WFD) demands good ecological status of all European surface waters by 2015. Eutrophic reservoirs also present a considerable problem for water purification, supply and consumption. Algae can block microstrainers and sand filters, reducing throughput of water and sometimes requiring the plant to be taken out of service. The smallest algal cells can pass through the filters, and decompose in the distribution pipes. Some breakdown products, notably mucopolysaccharides, chelate with iron and aluminium that is added to the treatment, leading to increased metal levels passing to the supply. Fungi and invertebrates can feed on the resultant biofilms, leading to taste and odour problems. Cyanobacterial blooms can produce toxins (e.g. microcystin) that pose a risk to human health. The primary routes to nutrient removal include a) dredging of sediment and dumping on land to remove sediment-locked phosphorus; b) planting of enlarged reedbeds; c) direct stripping of algae through microfiltration; d) chemical dosing (e.g. iron or copper sulphate) to strip phosphorus through coagulation. All of these techniques are expensive, many are environmentally harmful, and most are unreliable in their performance. Recent innovations have shown that harvesting filter-feeding organisms such as the blue mussel (Mytilus edulis) that feed on phytoplankton may be a sustainable method for producing food of high nutritional value while simultaneously recycling nutrients from sea to land. Simple nutrient budgets for N and P suggests that mussel farming could offset the need for some sewage treatment plants with marine outfalls (Lindahl et al., 2005). The aim of this project is to test whether the broadscale cultivation of filter-feeding biota in UK reservoirs may offer a similarly efficient, cost-effective tool for improving reservoir water quality for potable supply and to enhance biodiversity. Qualitative observations in a number of UK reservoirs suggest increasing abundance of filter-feeders in recent years has driven improved water quality. The project will focus on the contributions that sponges, bryozoans and invasive bivalves (zebra mussels) can make to reservoir management. By working in collaboration with Anglian Water's (AW) Innovation team, the student will investigate the identity, growth rates, biomass and nutrient content (N&P) of filter-feeders that naturally attach to different settlement rigs. To account for thermal stratification and the effects of UV radiation, depth patterns will also be investigated. To investigate the effects of faecal and pseudofaecal deposition, sediment and macroinvertebrate communities will be compared between paired replicates beneath rigs and control sites. Close collaboration with AW at three reservoirs known to contain many zebra mussels will ensure that rig design is optimal. The student will visit a marine mussel cultivation programme and liaise with feedstuff and fertiliser manufacturers to identify possible end-use of harvested material. AW will also train the student in invertebrate and algal collection and identification. The project will finish with a cost-benefit analysis, considering harvest frequency, possible revenue or disposal costs, N&P budgets compared with alternatives (e.g. chemical phosphate stripping), ecological benefit and design options. A risk assessment will be made relating to the spreading of non-native species.