Linking Seasonal and Spatial Stream Carbon Dynamics to Landscape Characteristics in Selected Watersheds on the Olympic Peninsula

Abstract

Understanding the factors that affect freshwater export of terrestrially derived carbon is key to creating a comprehensive model of stream ecology and to developing an accurate carbon budget. Though efforts have been made to quantify carbon in Pacific Northwest forests, little is known about the carbon in their freshwater systems. To begin informing this knowledge gap, we collected dissolved organic carbon (DOC) and water quality data along the stream networks of four small, fish-bearing watersheds in the Olympic Experimental State Forest on the Olympic Peninsula, WA during the summer and fall of 2018. Conditional reference random forest models were used to explore how landscape characteristics and climatic variables affect the spatial and temporal variability of carbon composition and water quality parameters. We found that slope-related variables and precipitation were the primary drivers of carbon export. The strengths and magnitudes of these relationships were different for the summer and fall. We also identified two pools of different carbon composition that were present in three of the four study watersheds. The results of this study give us a first look at the drivers of carbon export and the quantity and quality of carbon being exported through freshwater systems. Our work also advises on the spatial and temporal considerations of stream carbon monitoring. We identify three key questions to pursue in future studies that will improve our understanding of stream carbon on the Olympic Peninsula and allow us to monitor it going forward. Our results indicate that future research should explore seasonal variability, hyporheic influences, and management impacts on carbon dynamics.