Climate Related Shifts in Hydrology Can Impact Source and Delivery of Terrestrial Dissolved Organic Matter to Aquatic Systems in a Mesic Boreal Forest Watershed

Abstract:

Boreal systems in Atlantic Canada are experiencing changes in watershed hydrology. Such changes are expected to significantly alter the timing and delivery of dissolved organic matter (DOM) from these carbon-rich landscapes to headwater systems. Here, we use lignin phenol (LP) biomarkers to understand vascular plant sources and landscape processing of DOM en route from boreal forest hillslopes to a small headwater stream in Newfoundland, Canada. Dissolved LPs were measured within various soil zones of forested and harvested hillslopes and compared with an adjacent stream during two periods of expected hillslope-stream connectivity (spring snowmelt and fall storms). The dissolved LP signatures reveal clear source-related differences in DOM composition between forested and harvested plots. Furthermore, the LPs in organic horizon water, delivered either to the stream via runoff or vertically to deeper mineral horizons, depict seasonally distinct diagenetic signatures suggesting soluble DOM mobilized during the spring snowmelt is more degraded in comparison to the fall storm period. However, the LP diagenetic signatures become highly variable within the deeper soil horizons highlighting the varying degrees of processing and environmental conditions that control landscape DOM dynamics and influence the composition of DOM delivered to the aquatic environment. As a comparison, stream LP signatures are most similar to the hillslope during the spring snowmelt period, which is consistent with water source apportionments indicating greater hillslope-stream connectivity to surficial horizons during this time. This contrasts with the fall period where hillslope contributions are primarily from lateral subsurface flow through mineral horizons and when contributions from shallow wetland areas may be more important. These results indicate the timing and source of DOM delivered to the aquatic environment will be impacted by shifting watershed hydrology that is linked to climate change trends in these boreal landscapes.