Characterising Watershed Coherence in Stream Dissolved Organic Carbon Across Boreal Shield Forested Catchments

Abstract:

The wide range of forested landscapes in boreal environments store and cycle substantial amounts of carbon, although the capacity of these systems to act as either a carbon sink or source is uncertain under a changing climate. In particular, within forested landscapes, interfaces between aquatic and terrestrial boundaries have been identified as key controls on ecosystem-scale carbon dynamics due to the disproportionately high processing rates at these locations. Short-term temporal dynamics at these interfaces may constitute the majority of annual turnover and transport of dissolved organic carbon (DOC), a substantial term in the carbon cycle. Coherence is a framework of examining if adjacent spatial units change synchronously or asynchronously through time which has been widely applied in the context of lentic hydrochemistry. The objective of this research is to determine the extent of DOC export synchrony in these forested environments, and to what extent is this synchrony varied by season. A secondary objective is to assess whether the concept of coherence can be applied in the watershed context by examining patterns of hydrochemistry at the stream outlet. To address this research need, three existing long-term ecological research sites (Experimental Lakes Area, Turkey Lakes Watershed Study, and Dorset Environmental Science Centre, with 29,256 streamwater DOC samples across 127 site-years; annual DOC means of 2.1, 3.9, and 8.4 mg L^-1) were selected to investigate patterns of spatial and temporal coherence both within-sites and across-sites. Seasonality in coherence was consistent across sites, where coherence was lowest in spring and greatest in winter. Further, coherence in stream DOC was related to discharge, however, the effects of discharge appear to be driven largely by season. This research has important implications for determining which factors contribute to synchronous watershed behaviour, and which factors may contribute to the timing of watershed-scale deviations from landscape-level patterns.