Abstract
Two lowland catchments in the U.K. were sampled throughout 2010–11 to investigate the dominant controls on dissolved organic matter quantity and composition. The catchments had marked differences in terms of nutrient status, land cover and contrasting lithologies resulting in differences in the dominant flow pathways (groundwater vs. surface water dominated). The Upper Wylye is a chalk stream with a baseflow index of 0.98, draining a catchment dominated by intensive agricultural production. Millersford Brook is a lowland peat catchment with a baseflow index of 0.43, draining a semi-natural catchment with heather moorland and coniferous forest. Samples were collected weekly between October 2010 and September 2011 from eleven sampling locations. Samples were analysed to determine dissolved organic carbon, nitrogen and phosphorus fractions with DOM composition evaluated via the DOC:DON ratio, DOC:DOP ratio, specific UV absorption at 254nm, absorbance ratio (a250:a365) and the spectral slope parameter between 350 and 400nm (S350–400). Significant differences were observed in all determinands between the catchments, over time, and spatially along nutrient enrichment and geoclimatic gradients. Seasonal variation in preferential flow pathways mobilising groundwater-derived DOM were identified as likely controls on the delivery of DOM in the permeable chalk dominated catchment. Steeper S350–400 values and elevated a250:a365 ratios in this catchment suggest material of a lower bulk aromatic C content and molecular weight delivered during the winter months when compared to the summer. DOC:DON ratios were markedly lower in the chalk catchment than the peatland catchment, reflecting the paucity of organic matter within the mineral soils of the chalk landscape, and higher fertiliser application rates. This manuscript highlights that DOM composition varies according to catchment landscape character and hydrological function.
Highlights
The pool of dissolved organic matter (DOM) in aquatic ecosystems is dynamic, consisting of a wide range of compounds with differing chemical structures and reactivities
This paper presents the outcomes from a research programme undertaken to determine the relative importance of DOM as a component of C, N and P pools from land to stream in lowland catchments, while assessing the composition of DOM in each system as this varies in relation to the land cover and hydrological function of each catchment over the annual cycle
Bound nitrogen and phosphorus fractions differ in their relative contribution to total N and total P concentrations recorded in the two catchments
Summary
The pool of dissolved organic matter (DOM) in aquatic ecosystems is dynamic, consisting of a wide range of compounds with differing chemical structures and reactivities. As a result DOM has a multifaceted role in the biogeochemistry of aquatic ecosystems, impacting upon ecosystem production (Lindell et al, 1996), complexation of trace metals (Christensen et al, 1996; Brooks et al, 2007) and the mobilisation of pollutants (Aiken et al, 2011; Deb and Shukla, 2011) In both freshwater and marine systems DOM acts as a major global carbon reservoir (Battin et al, 2008; Waeles et al, 2013) while containing significant concentrations of organically bound nutrients such as dissolved organic nitrogen (DON) and phosphorus (DOP). DOP has received less attention to date, but comprises a significant proportion of the total P flux to freshwaters, in intensively farmed and heavily populated catchments (e.g. Prior and Johnes, 2002) and in coastal and oceanic systems (e.g. Karl and Björkman, 2002; Ruttenberg and Dyhrman, 2012)
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