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

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 (1987–2006) of DOC concentrations at six catchments (0.44–10.0 km²) 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–runoff 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–runoff 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 flush of DOC. Temperature influences 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 different hydrological regimes.