Phosphorus fate and management on the Somerset Levels and Moors Ramsar ditch systems
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Fundamental to all life, phosphorus is an essential nutrient and, contrastingly, a significant threat to surface water biodiversity globally as one of the most common causes of eutrophication in surface waters worldwide. Freshwater wetland ditches afflicted by these conditions undergo a shift from primarily submerged aquatic vegetation to algae or duckweed dominance, leading to excessive shading and anoxic conditions. Phosphorus, from both point (e.g., wastewater treatment works) and diffuse (largely agricultural runoff) sources, is currently the central reason for failure in the majority of surface water bodies in England to meet required water quality guidelines. Historic data indicate that surface waters within the ditch systems of the Somerset Levels and Moors, a listed Ramsar site (no. 914) under the Ramsar Convention and designated Special Protection Area (SPA) under the Habitat Regulation 2017, are elevated in nutrients with potential for total phosphorus to be above the current Common Standards Monitoring Guidance target of <0.1 mg L-1 set out by the Joint Nature Conservation Committee in 2005. However, there is a general lack of up-to-date consistent monitoring data for the ditch systems and few comprehensive datasets are available, which cover an extended time period with good spatial coverage, leading to a lack of knowledge as to how the complex seasonal water flow paths and levels affect transport of phosphorus, from both point and diffuse sources, throughout the wetland ditch systems. This thesis pursues the closure of this knowledge gap through investigations conducted on West Sedgemoor, a designated site of special scientific interest and part of the Somerset Levels and Moors Ramsar site. Firstly, an investigation assessed the spatial distribution of sediment phosphorus storage in the ditch systems, as freshwater sediment acts as an internal source of legacy bound phosphorus that can induce production of algal and duckweed blooms beyond what may be expected from external loading of phosphorus alone. Elevated phosphorus concentrations in sediment were observed throughout the Moor up to 4,220 mg Kg-1, almost 10 times that which may be expected from background levels. The highest concentrations were generally observed at the more intensively farmed sites in the north of the moor, near key inlets and the outlet. Based upon their chemical and physical properties, clear distinction was observed between sites outside and within the Royal Society of the Protection of Birds nature reserve, using principal component analysis. Secondly, an investigation assessed the chemical speciation of sediment phosphorus in the ditch systems. Based upon their associations with different phosphorus species, clear distinction was observed between sites outside and within the Royal Society of the Protection of Birds nature reserve, using principal component analysis. Sites outside the nature reserve, typically wet and damp grassland used for arable use and grazing, were generally correlated to higher non-apatite inorganic phosphorus (associated with iron and aluminium minerology) and higher total phosphorus levels, associated with algal and duckweed blooms. Thirdly, an investigation assessed the seasonal variation in spatial distribution and chemical fractionation of surface water phosphorus, as well as surface biomass abundance and total phosphorus content in the ditch systems. Elevated phosphorus concentrations in the surface water were observed across the site, the highest being 1.88 mg L-1 during the summer, over 10 times the Common Standards Monitoring Guidance target of <0.1 mg L-1. Sites lacking hydrological flow connectivity with freshwater inputs, typically had lower surface water phosphorus concentrations than the rest of the moor. Summer and autumn were determined as the dominant duckweed growth seasons, in which an estimated 39 kg of phosphorus could be removed via duckweed biomass harvesting, per harvest period. The study has demonstrated that there is an undoubted need for practical management options to help mitigate phosphorus in eutrophic freshwater ditch systems. Evidence has been reviewed which demonstrates that appropriate and targeted ditch management practices can play a significant role in reducing both phosphorus load and legacy phosphorus concentrations. A wide variety of management options exist (e.g.; water level management; dredging, emergent macrophyte harvesting and channel widening (two-stage channels); algae/duckweed harvesting; and filter substrates), although some are best suited to particular environments and landscapes, some to accelerating recovery rate rather than initialising recovery, and data regarding the efficiency of certain approaches is rather limited. The development of the management options into functioning phosphorus mitigation solutions requires determination of likely costs, implementation timescales, maintenance requirements, and delivery mechanisms, at site specific level. Further studies are necessary to generate data useful to the development of mitigation schemes.
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