title: The importance of the relationship between scale and process in understanding long-term DOC dynamics.
abstract: Concentrations of dissolved organic carbon have increased in many, but not all, surface waters across acid impacted areas of Europe and North America over the last two decades. Over the last eight years several hypotheses have been put forward to explain these increases, but none are yet accepted universally. Research in this area appears to have reached a stalemate between those favouring declining atmospheric deposition, climate change or land management as the key driver of long-term DOC trends. While it is clear that many of these factors influence DOC dynamics in soil and stream waters, their effect varies over different temporal and spatial scales. We argue that regional differences in acid deposition loading may account for the apparent discrepancies between studies. DOC has shown strong monotonic increases in areas which have experienced strong downward trends in pollutant sulphur and/or seasalt deposition. Elsewhere climatic factors, that strongly influence seasonality, have also dominated inter-annual variability, and here long-term monotonic DOC trends are often difficult to detect. Furthermore, in areas receiving similar acid loadings, different catchment characteristics could have affected the site specific sensitivity to changes in acidity and therefore the magnitude of DOC release in response to changes in sulphur deposition. We suggest that confusion over these temporal and spatial scales of investigation has contributed unnecessarily to the disagreement over the main regional driver(s) of DOC trends, and that the data behind the majority of these studies is more compatible than is often conveyed.
authors.affiliation: Civil and Environmental Engineering, Imperial College London, South Kensington, London, UK. joanna.clark@imperial.ac.uk
authors.forename: J M
authors.initials: JM
authors.lastname: Clark
authors.forename: S H
authors.initials: SH
authors.lastname: Bottrell
authors.forename: C D
authors.initials: CD
authors.lastname: Evans
authors.forename: D T
authors.initials: DT
authors.lastname: Monteith
authors.forename: R
authors.initials: R
authors.lastname: Bartlett
authors.forename: R
authors.initials: R
authors.lastname: Rose
authors.forename: R J
authors.initials: RJ
authors.lastname: Newton
authors.forename: P J
authors.initials: PJ
authors.lastname: Chapman
mesh_terms: D015258:Acid Rain; D002244:Carbon / Q000032:analysis*; D005618:Fresh Water / Q000737:chemistry*; D007700:Kinetics; D011891:Rain; D013696:Temperature; D013995:Time; D014874:Water Pollutants, Chemical / Q000032:analysis*; D014877:Water Pollution, Chemical / Q000706:statistics & numerical data
chemical_list: D015258:Acid Rain; D014874:Water Pollutants, Chemical; D002244:Carbon