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

Abstract

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. 5There 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 10 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 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 15 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 20 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 km 2 yr -1 and 14.7 t C km 2 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 25 carbon stores to changes in temperature and precipitation. These changes are presently being observed and are predicted to become increasingly extreme and variable.

Late summer peak inpCO2corresponds with catchment export of DOC in a temperate, humic lake

“…We hypothesised that the lake was net heterotrophic and supersaturated with CO 2 due to high year-round inputs of coloured allochthonous OC from the catchment, which would both stimulate ecosystem respiration and restrict net ecosystem production. We also hypothesised that pCO 2 in the lake would peak during late summer and early autumn, responding to the regular, strong, annual peak in DOC concentrations observed in the main catchment streams entering the lake in previous studies (Ryder et al 2014, Doyle et al 2019.…”

“…We assumed that the high rainfall regime experienced at Lough Feeagh results in the lake being continuously "topped up" with allochthonous C from the surrounding catchment throughout the year, which can be observed by the relatively high loading of DOC to the lake (Fig. 2b-c; Ryder et al 2014, Doyle et al 2019. We suggest that this almost constant and regular OC supply allows pCO 2 in Feeagh to climb even during the main period of thermal stratification.…”

“…Land cover in the Burrishoole catchment was reported by Doyle et al (2019), using CORINE data, as comprising 52% blanket peat, 15% forestry, and the remaining 33% transitional woodland and scrub, natural grasslands, and agricultural land. The catchment top soils are predominantly humic, with blanket peatlands covering upland slopes and high C content soils including poorly drained gleys, podzols, and alluvial soils on the valley floors (May and Place 2005).…”

“…Lough Feeagh's geographic setting within a peatland-dominated catchment and temperate maritime climate zone on the west coast of Ireland makes it an interesting case study from a C dynamics perspective. Previous studies suggest that OC is stored in abundance in the surrounding catchment soils and continuously supplied to the lake, driven mainly by the catchment's hydrologic regime (e.g., Ryder et al 2014, Doyle et al 2019. Within-lake OC processing is not disrupted during the winter because the relatively warm winters experienced locally prevent ice-over.…”

Characterizing ventilation events in an anoxic coastal basin: Observed dynamics and the role of climatic drivers

“…However, as increased runoff also increased the depth of the surface layer, the renewal time decreased slowly with increasing freshwater input. Given the fast transit time of freshwater through the system, Furnace likely has a limited capacity to sequester quantities of dissolved organic carbon exported from the Burrishoole watershed, which drains a blanket peatland catchment (Doyle et al 2019).…”