Optical and geochemical signatures suggest significant processing of dissolved organic matter transported across the terrestrial-aquatic interface within a boreal forest catchment

The terrestrial-aquatic interface is a hotspot for the mobilization and biogeochemical cycling of dissolved organic matter (DOM) en route to the aquatic environment. The boreal landscape, replete with water, contains large stores of soil organic matter. Therefore, understanding the fate of these stores is required to better ascertain climate feedbacks associated with this highly vulnerable ecoregion. However, the mechanisms driving the transformation and export of DOM within boreal landscapes remains poorly constrained. Here we assess the degree to which the optical (UV-VIS absorbance) characteristics of DOM and geochemical (e.g. iron) signatures of a small boreal headwater stream in western Newfoundland, Canada are linked to the hillslope hydrological processes within the catchment using samples collected both spatially and temporally from organic and mineral soil horizons and groundwaters in a forested hillslope. In addition, samples were collected from two stream locations including a downstream site dominated by steep hillslopes and an upstream site draining the uppermost, low relief pond and wetland dominated regions of this catchment. We use a multivariate statistical approach to evaluate the spatial variability in optical and geochemical features of water collected throughout the landscape and the upstream site. Despite the fact that more than 80% of the water provided to the downstream location is sourced from the hillslopes of the catchment, optical and geochemical features indicate a DOM signature more characteristic of waters draining from the upstream wetland regions. This suggests the water contributions measured from the landscape are distinct from the origins of DOM in the stream. Combined with the high mobilization of DOM from within the hillslope studied, these results suggest the landscape acts as a potential sink or hotspot for DOM remineralization en route to the aquatic environment. Furthermore, with increasing water contributions from the low relief wetland areas relative to steeper hillslope areas with decreasing snowpack and snowmelt, these results suggest climate change may enhance both DOM transport from and processing within these boreal landscapes.