Synchrony in catchment stream colour levels is driven by both local and regional climate
Authors: Doyle Brian C. , de Eyto Elvira , Dillane Mary , Poole Russell , McCarthy Valerie , Ryder Elizabeth , Jennings Eleanor 
Published: March 2019

Abstract. Streams draining upland catchments carry large quantities of carbon from
terrestrial stocks to downstream freshwater and marine ecosystems. Here it
either enters long-term storage in sediments or enters the atmosphere as
gaseous carbon through a combination of biotic and abiotic processes. There
are, however, 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 6 years of weekly data, from 2011 to 2016. This
time-series dataset was gathered from three contiguous river sub-catchments,
the Black, the Glenamong and the Srahrevagh, in a blanket peatland catchment
system in western Ireland, and it was used to identify the drivers that best
explained observed temporal change in river colour. The data were also used
to estimate annual DOC loads from each catchment. General additive mixed
modelling was used to identify the principle environmental drivers of water
colour in the rivers, while wavelet cross-correlation analysis was used to
identify common frequencies in correlations. At 130 mg Pt Co L−1,
the mean colour levels in the Srahrevagh (the sub-catchment with lowest rainfall and higher forest cover) were almost
50 % higher than those from the Black and Glenamong, at 95 and
84 mg Pt Co 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. For both the Black and its
nested Srahrevagh catchment, three variables (soil temperature, soil moisture
deficit, SMD, and the weekly North Atlantic Oscillation, NAO) combined to
explain 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, the log of stream 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 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 similar at 15.0 and 14.7 t C km−2 yr−1
respectively. The important role of past and current precipitation and, in
particular, temperature emphasises the vulnerability of blanket peatland
carbon stores to projected climate change and highlights the interaction of
local and regional climate in controlling aquatic carbon export. Our results
show that water colour (and hence DOC) concentrations can vary considerably
between neighbouring catchments and also that regional-scale climatic drivers
control the trends in intra- and inter-annual flux of DOC through the system.
The combination of locally determined concentrations and regionally
controlled fluxes produces aquatic DOC loads that vary over both the annual
cycle and over multiple years.