Effects of Extreme Drought on the Organic Carbon Dynamics and Hydroecology of Intermittent, Salmon-bearing Streams

Abstract:

Intermittent streams—streams that become disconnected during the dry season—serve as important habitat for anadromous fish (coho salmon and steelhead trout) in California. Though their adaptation to the hydrologic regime may give salmonids a competitive advantage, they remain vulnerable to excessive stream drying due to drought and climate change. Because of their habitat provisioning services and their vulnerability, intermittent streams are increasingly popular management targets for conservation, restoration, and mitigation activities. Nonetheless, their hydrologic and biogeochemical dynamics, and how those dynamics affect fish, remain poorly understood. To elucidate how seasonal and drought-induced stream drying mediate carbon-oxygen dynamics and fish survival, we conducted a paired watershed study in adjacent streams—one intermittent and one perennial—that began in 2012 and continued through the extreme drought of 2014-2015. Pre-drying recruitment and post-drying survival surveys of salmonids were paired with continuous and spot measurements of oxygen, conductivity, and stream temperature in four stream reaches and biweekly sampling of dissolved organic carbon (DOC) through the drydown. DOC samples were analyzed for their specific UV absorbance and fluorescence, and an EEM-PARAFAC analysis was performed to identify fluorescent components that could be used as tracers of origin and biogeochemical processing.

Previous analyses revealed that coho and steelhead utilize different parts of stream pools during the drydown, with the survival of coho being most limited by dissolved oxygen and that of steelhead being most strongly determined by pool geometry. Here we tested the hypothesis that hyporheic flow during the period of intermittency sustains high dissolved oxygen in pools with high coho survival rates. Conductivity data and a DOC tracer analysis (based on the EEM-PARAFAC approach) confirmed this hypothesis and revealed that when hyporheic inflows shut down, pools develop a distinctly microbial DOC signal, which coincides with rapid oxygen depletion. Results suggest that summertime baseflow augmentation, together with preservation of the geomorphic complexity of streams, are likely to be the most effective measures to mitigate future drying extremes.