Seasonal controls on DOC dynamics in nested upland catchments in NE Scotland

Spatial and temporal variability of hydrological responses affecting surface water dissolved organic carbon (DOC) concentrations are important for determining upscaling patterns of DOC export within larger catchments. Annual and intra-annual variations in DOC concentrations and fluxes were assessed over 2 years at 12 sites (3.40-1837 km(2)) within the River Dee basin in NE Scotland. Mean annual DOC fluxes, primarily correlated with catchment soil coverage, ranged from 3.41 to 9.48 g m(-2) yr(-1). Periods of seasonal (summer-autumn and winter-spring) DOC concentrations (production) were delineated and related to discharge. Although antecedent temperature mainly determined the timing of switchover between periods of high DOC in the summer-autumn and low DOC in winter-spring, inter-annual variability of export within the same season was largely dependent on its associated water flux. DOC fluxes ranged from 1.39 to 4.80 g m(-2) season(-1) during summer-autumn and 1.43 to 4.15 g m(-2) season(-1) in winter-spring. Relationships between DOC areal fluxes and catchment scale indicated that mainstem fluxes reflect the averaging of highly heterogeneous inputs from contrasting headwater catchments, leading to convergent DOC fluxes at catchment sizes of ca 100 km2. However, during summer-autumn periods, in contrast to winter-spring, longitudinal mainstem DOC fluxes continue to decrease, most likely because of increasing biological processes. This highlights the importance of considering seasonal as well as annual changes in DOC fluxes with catchment scale. This study increases our understanding of the temporal variability of DOC upscaling patterns reflecting cumulative changes across different catchment scales and aids modelling of carbon budgets at different stages of riverine systems. Copyright (C) 2010 John Wiley & Sons, Ltd.