Streams are Nature's Chemistry Lab! Or, Freshwater Carbon Dynamics in the Coastal Temperate Rainforest
Freshwater streams are biogeochemical catalysts, transporting and transforming carbon between terrestrial, marine, and atmospheric stocks. Streams in British Columbia’s humid, organic-rich Pacific Coastal Temperate Rainforest (PCTR) deliver globally significant amounts of dissolved organic carbon (DOC) to the ocean, which affects in-stream primary production and provides a major energy source for coastal marine ecosystems. High stream DOC concentrations have implications for climate change as well, as a portion of stream DOC is mineralized to CO2 and off-gassed to the atmosphere. This study has focused on determining the key processes that control the delivery of soil-derived DOC to streams during storm events, to predict how stream DOC will be affected by future climate change. We have assessed the temporal variation of stream DOC across four PCTR watersheds over two years with continuous, automated, in-situ sensors. Through a combination of univariate, multivariate, and structural equation models, we have explored temporal trends in hysteresis between DOC and discharge and tested previous process-based hysteresis models. For the most part, our results support the model of clockwise hysteresis caused by flushing during drier, warmer periods, and anticlockwise hysteresis caused by dilution during wetter, colder periods. However, our results also show high DOC responsivity during periods with higher monthly antecedent soil moisture and lower temperatures, which indicates a lack of DOC dilution during wetter periods and departs from previous models. The PCTR’s high soil organic matter content and thick organic soil layer may be what allows DOC responsivity even during saturated conditions. According to our model, in high-carbon regions such as the PCTR stream DOC concentration increases during cold, wet periods, which likely indicates that high soil organic matter is overriding the influence of hydrologic factors, and that anticlockwise hysteresis does not always mean a dilution of DOC.