Stable Instability: Exploring the role of Arctic stream-lake chains on watershed-scale carbon and nutrients

Lakes are integral parts of many surface-water networks, where their increased hydrological residence time and distinct chemical conditions can influence landscape-scale carbon and nutrient balance. Understanding lake-stream interactions is particularly important in Arctic watersheds, which are experiencing accelerated climatic change. With this study, we explored how lakes influence biogeochemical transport and transformation on the North Slope of Alaska. We analyzed data from two highly-studied subwatersheds in a small (66.9 km2), low-gradient headwater network that drains directly into Toolik Lake and eventually the Arctic Ocean. For three decades (1992-2020), the inlet and outlet of each lake were sampled monthly from June to August. First, from these long-term data, we quantified temporal synchrony (i.e., temporal covariance) and spatial persistence (i.e., between-event correlations). While this dataset is expansive, we focused primarily on reactive solutes including dissolved organic carbon (DOC) and nitrate (NO3). When synchrony and persistence were assessed across the entire watershed, solutes were generally spatially persistent (Pearson's correlation: 0.52-0.88), indicating that signals were largely preserved between each sampling event. However, when we compared signals between the subcatchments, we found greater instability and resulting asynchrony in the lake-dense network. Our results indicate that the density of lakes within the network is an important control on the spatial distribution of water chemistry patterns, as solute concentrations can be "reshuffled" as water flows into and out of the stream-lake sequence. Then, to further assess the biogeochemical influence of a stream-lake chain across variable hydrologic conditions, we deployed high-frequency in-situ DOC and NO3 sensors at the inlet and outlet of Lake-I8. With these temporally-intense data, we found that storm events increase NO3 and buffer DOC exports at the watershed scale relative to baseflow conditions. Results from this study underscore that the density of lake chains, which may increase or decrease as climate change progresses, may be an important control of the stability and synchrony of biogeochemical processing within Arctic river networks.