Synchrony in catchment stream colour levels is driven by both local and regional climate

Abstract. Streams draining upland catchments mobilise significant loads of carbon from terrestrial reservoirs to downstream freshwater and marine aquatic ecosystems and ultimately, via a range of biotic and abiotic processes, to the atmosphere. There are increasing concerns over the long-term stability of terrestrial carbon stores in blanket peatland catchments as a result of anthropogenic pressures and climate change. We analysed sub-annual and inter-annual changes in river water colour (a reliable proxy measurement of dissolved organic carbon (DOC)) using six years of weekly data (2011 to 2016) from three contiguous river sub-catchments (Black, Glenamong and Srahrevagh) in a blanket peatland catchment system in western Ireland, assessing differences in catchment characteristics and in the drivers of temporal change. General additive mixed modelling was used to identify the principle environmental drivers controlling changes in colour concentration in the rivers, while wavelet cross correlation analysis was used to identify common frequencies. Although at 130 mg PtCo L−1, the colour levels in the Srahrevagh (the subcatchment with lower rainfall and higher forest cover) were almost 50 % higher than those from the Black and Glenamong, 95 and 84 mg PtCo L−1 respectively. The decomposition of the colour datasets revealed similar multi-annual, annual, and event-based (random component) trends, illustrating that environmental drivers operated synchronously at each of these temporal scales, and also spatially within the same catchment. For the Black and its nested Srahrevagh catchment, soil temperature, soil moisture deficit and the weekly North Atlantic Oscillation (NAO) explained 54 % and 58 % of the deviance in colour respectively. In the Glenamong, which had steeper topography and a higher percentage of peat intersected by streams, soil temperature, log of discharge and the NAO explained 66 % of the colour concentrations. Cross-wavelet time-series analysis between river colour and each environmental driver revealed a significant high common power relationship at an annual time step. Each each relationship however, varied in phase, further highlighting the complexity of the mechanisms driving river colour in the sub-catchments. The estimated mean annual DOC loads for the Black and Glenamong rivers to Lough Feeagh were 15 t C km2 yr−1 and 14.7 t C km2 yr−1 respectively, although the export values displayed significant inter-annual variation. The results of the study highlight the interaction of catchment conditions and regional meteorological drivers of aquatic carbon export and, therefore the vulnerability of blanket peatland carbon stores to changes in temperature and precipitation. These changes are presently being observed and are predicted to become increasingly extreme and variable.