| Popis: |
Dissolved organic carbon (DOC) has received increased prominence in drinking water research, as concentrations in surface waters have increased in many parts of northern Europe and North America in recent decades. This presents a problem, since DOC gives colour, taste and odour to drinking water and can form potentially harmful by-products on disinfection. This research explored these issues through a series of experiments focussing on DOC flux from peat soils and the litter layer under different climate conditions. The aim of the work was to assess the likely impact of changes in temperature, precipitation and drought frequency and severity as well as changes to vegetation species diversity on DOC production and drinking water treatment. This will determine whether common treatment processes (coagulation/flocculation) are able to cope with potential changes in DOC quality, and therefore whether new treatment processes may be required as an adaptation to climate change. Through assessment of the impact of changes in species diversity and the effect of drought, this work also aims to build the evidence base for current catchment management schemes which have sought to support Sphagnum dominance and raise peatland water-tables. The experimental work included the optimisation of coagulation/flocculation for DOC removal from common peatland sources as well as grassland species which are currently encroaching on the uplands. The role of temperature, rainfall and drought in controlling DOC production from litter and soils was considered, as well as the abiotic role of temperature in DOC solubilisation. The findings were also tested in the field using a litterbag study across an altitudinal gradient which provided a natural range of temperature and rainfall. The environmental persistence of DOC from different sources was also examined. The research indicates that the type of vegetation is significant in controlling the amount, treatability and environmental persistence of the DOC produced. Temperature and rainfall influence DOC quantity and quality, however the effect of these climatic variables is much smaller than for the source of the DOC. The decomposition of litter and resulting DOC production are controlled primarily by biotic factors such as the litter’s ratio of carbon to nitrogen, which indicates nutrient availability to the decomposer community. Common peatland species such as Sphagnum and Calluna vulgaris are adapted to nutrient poor environments and therefore have high C:N ratios and produce low amounts of DOC per unit weight when compared to grassland species. The invasive grassland species, such as Molinia caerulea and Juncus effusus, produce high amounts of DOC per unit weight and large amounts of above-ground biomass which decays readily due to its low C:N ratio. A new conceptual model of carbon cycling in peatlands is proposed which shows that climate change may decrease the overall size of the litter carbon pool by facilitating the encroachment of grassland species which decompose rapidly. These species will increase the seasonality of DOC flux due to their cycles of growth and produce DOC which is more persistent and harder to treat than common peatland species. Drought has been highlighted as major problem for drinking water treatment in peatland catchments as it causes large increases in DOC production from peat soils whilst also decreasing its treatability. Taken together these results suggest the secondary effects of climate change, namely vegetative change, may be more important for DOC production that changes in temperature and rainfall. Catchment management schemes which support Sphagnum and increase resilience to drought are justified as Sphagnum produces DOC readily removed by coagulation/flocculation and drought can cause an increase in the amount of DOC from peat soils and produce DOC which is also harder to remove by standard treatment processes. |
