1.9.1 Particulate Organic Carbon (POC)
Export pathways of POC are predominantly within the fluvial environment and according to studies, the amount of POC exported varies according to the state of the catchment. Some studies have measured POC to represent 10-15% of total organic carbon flux. Dinsmore et al. (2010) found that POC represents a small portion of peatland carbon flux; POC was the
smallest portion of C export within the fluvial output (after, DOC, DIC), also smaller than CO2
atmospheric losses. Peatland soil erosion is an important driver for the production of POC; the correlation between soil erosion and POC is evident in bare and eroded catchments, in which POC represented up to 80% of fluvial exports according to studies by Evans et al. (2006), Pawson et al. (2008), and Worrall et al. (2011). Peatlands revegetation can effectively control erosion and sediment flux (Evans et al. 2006). According to Moors for the Future intervention and ecological restoration efforts can reduce erosion and cut POC losses by up 95% within two years.
1.9.2 Dissolved Organic Carbon (DOC)
DOC is a general term describing a wide range of molecules from simple acids and sugars to complex humic substances with large molecular weights (Moore 1998). DOC is identified as carbon that is capable of passing through a 0.45µm syringe filter (Tranvik 1998). Compounds of DOC are formed in various stages of decomposition ranging from acids to complex humic substances (Wallage et al. 2006). Studies report DOC plays a major role in fluvial carbon export (Dawson et al., 2002). Studies are biased towards aquatic fluxes of DOC (Clark et al. 2008, Hope et al. 1996, McDowell and Likens 1988).
32 Surface waters DOM concentration and speciation is dependent on import, washout, indigenous primary production and processes. Internal system loss can be incurred due to abiotic mineralization (particularly photo-oxidation), microbial mineralization and flocculation
followed by sedimentation (Tranvik 1998).Broad climatic and site factors have been identified
as key factors influencing DOC concentrations (van den Berg et al. 2012). For example drought years were linked to observations of lower DOC concentration by Clark et al. (2005). There are several mechanisms which increase DOC production at varying time scales. These mechanisms can be divided into abiotic and biotic factors related to the following: a) abiotic such as increased air temperatures (Freeman et al. 2001) and severe drought events (Clark et al. 2009, Neff and Hooper 2002, Worrall and Burt 2004), potentially associated with climate change (Dinsmore et al. 2013, Frolking et al. 2006, Worrall et al. 2003b); changes in soil pH (Clark et al. 2008, Scott et al. 1998); changes in water flow volume and nature; increases in atmospheric
concentrations of CO2, and changes in atmospheric deposition and eutrophication (Freeman
et al. 2004). b) Biotic, historic vegetation type, which controls the physical and geochemical characteristics of the peat mass (Brown et al. 2014); vegetation cover and composition (Armstrong et al. 2012, Neff and Hooper 2002), land management (Clutterbuck and Yallop 2010, Yallop and Clutterbuck 2009). All these mentioned factors are enhanced by anthropogenic activities and local land management (Freeman et al. 2004, Mitchell et al. 2008, Wallage and Holden 2010).
The impact of land management upon DOC concentrations in soil pore water has been investigated for a number of land management types on peatlands, including: prescribed burning (Clay et al., 2009a), drainage (Gibson et al., 2009), deforestation (Glatzel et al., 2003), afforestation (Jandl et al., 2007), and grazing (Ward et al., 2007). There is little research on the long term effects of bare peat restoration or comparative heather management methods on DOC.
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1.9.2.1 DOC and water quality
Dissolved organic carbon is mobile, non-fixed OM (Whitbread 1994). At high concentrations DOC is considered a pollutant which can lead to biological contamination and is linked to water taste, odour (Volk et al. 2002) and discoloration (Butcher et al. 1995). DOC influences river water quality through the transport of complex metals and nutrients and its effect on acidity and pH (Driscoll et al. 1989, Driscoll et al. 1994). Microbial energy and nutrient supply, light absorbance and photochemistry in surface waters can be influenced by DOC (Evans et al. 2005). DOC represents a significant challenge to water supply companies who may have to remove DOC to meet drinking water quality standards (Dinsmore et al. 2013). DOC is costly to remove. In some cases incomplete removal can result in the formation of disinfectant by- products (DBP’s) such as carcinogenic trihalomethane (Condie et al. 1983, Gough et al. 2014, Volk et al. 2002). Water colour is therefore a primary precursor for trihalomethanes and other DBPs (Reckhow et al. 1990). Trihalomethane concentrations in drinking water are limited by law in the UK under the Water Supply (Water Quality) Regulations (2010). The regulation specifies a maximum total trihalomethanes concentration of 100 μg L-1 (Gough et al. 2014, Hsu et al. 2001).
Studies over the past five decades on terrestrial surface waters report a significant rise in DOC concentrations (Evans et al. 2005, Monteith et al. 2007, Worrall and Burt 2007) in the UK (Freeman et al. 2001), central Europe (Hejzlar et al 2003), and the USA (Skjelvale et al., 2001, Driscoll et al., 2003). Trends have been attributed to increasing temperatures due to climate change. However, other studies have not been able to sufficiently explain DOC from temperature variation alone. The observed trends have given rise to the interest to conduct further research into water quality and carbon flux within a peatland system. This is of interest to water companies which own land (e.g. in the Goyt Valley) which constituted the catchment feeding into reservoirs for the following reasons: The UK is an estimated annual net sink of
34 0.32 MtC/yr (Holden et al. 2007), these upland regions source 70% of the UKs drinking water (Burt et al. 1997). The distribution of organic-rich peat, present in these upland regions, is closely associated with increased DOC, POC (Hope et al. 1997) and water colour (Butcher et al., 1995).
1.9.2.2 DOC components
As disused earlier, DOC variation is influenced by microbes, frequency and intensity of rainfall, pH and sulphate (Scott et al. 1998). Composition of DOC can vary from fulvic, lignin derived, relatively lower molecular weight molecules to the humic relatively higher molecular weight associated with darker water colours (Carlsen et al. 2000). The degree of peat humification is related to initial peat breakdown and decomposition (Whitbread 1994). Specific UV absorbance (SUVA), which is the UV absorbance of a water sample at a given wavelength , is
related to DOC (Weishaar et al. 2003). A proxy for the degree of sample humification is E4/E6
(discussed in General methodology section).
Table 1.2: Characteristics of DOC Humic and fulvic components (Carlsen et al. 2000, Chen et al. 1977, Thomsen et al. 2002).
Humic Fulvic
Present in deeper peat Lignin derived
Lower E4/E6 Higher E4/E6
above 11
Macro Molecular/ heterogeneous Lower molecular weight (MW)
Chemical structure:
More aromatic Phenolic units
More acid groups Carboxylic structures
Polyelectrolytes Less aromatics and O-alkyl C
Carbolic and phenolic function groups More carboxyl carbon
Higher C:N ratio due to
protein/peptides
Low aromatics
Large aliphatic structures
Environmental Consequence:
Can effect water sorption capacity
towards pollutants
Increase water colour
Better for bioactivity (Chen et al. 1977)(above
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