Influence of hydrology and seasonality on DOC exports from three contrasting upland catchments

Abstract

Variation in dissolved organic carbon (DOC) concentrations of surface waters is a consequence of process changes in the surrounding terrestrial environment, both within annual cycles and over the longer term. Long-term records (1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006) of DOC concentrations at six catchments (0.44-10.0 km 2 ) across a climatic transect in Scotland were investigated for intra-annual relationships to evaluate potential long-term seasonal patterns. The intra-annual mode of DOC export contrasted markedly between catchments and appeared dependent on their hydrological characteristics. Catchments in wetter Central Scotland with high rainfall-runoV ratios, short transit times and well-connected responsive soils show a distinct annual periodicity in DOC concentrations throughout the long-term datasets. Increased DOC concentrations occurred between June and November with correspondingly lower DOC concentrations from December to May. This appears unrelated to discharge, and is dependent mainly on higher temperatures driving biological activity, increasing decomposition of available organic matter and solubility of DOC. The drier eastern catchments have lower rainfall-runoV ratios, longer transit times and annual drying-wetting regimes linked to changing connectivity of soils. These are characterised by seasonal DOC concentration-discharge relationships with an autumnal Xush of DOC. Temperature inXuences the availability of organic matter for DOC transport producing a high DOC concentration-discharge relationship in summer/autumn and low DOC concentration-discharge relationship in winter/spring. These two distinct modes of seasonal DOC transport have important implications for understanding changes in DOC concentrations and export brought about by climate change (temperature and precipitation) and modelling of aquatic carbon losses from soil-types under diVerent hydrological regimes.

Long-term Trends of Organic Carbon Concentrations in Freshwaters: Strengths and Weaknesses of Existing Evidence

“…However, this would have little sense for many reasons. First, in general: (i) studies cover very different temporal periods; (ii) the number of systems included in each individual study is very different, ranging from one intermittent stream [88] to a major comparison of 705 systems [66]; 33 of the published studies cover less than 10 systems and only 9 more than 100. Curiously, if all trends were pooled together, the existence of four earlier studies [64,66,71,73] covering a huge number of lakes in Canada and Scandinavia (1324 in total), where about 90% of the systems in each study showed no statistically significant trend, would probably give an overwhelming majority for the "non trend" category.…”

“…Non statistically increases and decreases are considered "no trends"; d Values only given when trend statistically significant, except when mixed in the original publication (in this case, a cautionary note is added). When value in italics, original value in other units; e [88] includes data from two streams (Loch Ard Burn 2 and Allt a'Mharcaidh) but refers to this study for DOC long term trend values; f Data from one of the catchments (Langtjern) further treated in [89] by empirical regression analysis and a process-based model; g Original units: μmol C L −1 y −1 ; h [17] show exactly the same results. They cite [9] as a source; i Original units: μeq C L −1 y −1 ; j Probably slopes for non statistically significant trends included.…”

“…Nevertheless, this remains unproved and the few studies where, directly or indirectly, the question of the quality of the organic matter present has been addressed, do not seem to support this hypothesis. For instance, Dawson et al [88] showed a significant change in the relationship between UV absorbance and DOC over 22 years at two upland moorland catchments in Scotland; despite increases in long-term DOC concentrations, their analysis suggests that the proportion of hydrophobic material declined. Erlandsson et al [46] have reported on Swedish rivers where DOC and absorbance (420 nm) were measured between 1987 and 2004 and found that there was a significant increase in the absorbance/DOC ratio in 19 of the 21 rivers considered.…”

Modelling landscape controls on dissolved organic carbon sources and fluxes to streams

“…This release is strongly dependent on both soil temperature and moisture availability with consequent seasonal differences in periods of high and low DOC production in summer/autumn and winter/spring, respectively (Dawson et al 2008;Winterdahl et al 2011a;Peterson and Lajtha 2013). Additionally, water fluxes are of critical importance, and hydrological connection is needed to transport DOC through the landscape and into streams (Boyer et al 1996;Dawson et al 2008).…”

“…Additionally, water fluxes are of critical importance, and hydrological connection is needed to transport DOC through the landscape and into streams (Boyer et al 1996;Dawson et al 2008). DOC is generated across the landscape, but areas with the highest soil carbon density are most important (Dawson et al 2011).…”