The influence of geomorphic and landscape characteristics on stream temperature and streamwater sensitivity to air temperature in the coastal temperate rainforest of Southeast Alaska

Abstract

Climate warming is projected to increase the regional air temperature in southeast Alaska and alter precipitation patterns and storage, with potentially important implications for the region's aquatic resources. Streamwater temperature is controlled by energy inputs from the atmosphere and surrounding environment that are modified by a watershed's geomorphic and landcover characteristics. The climate-landcover relationships that influence stream temperature have not been comprehensively evaluated in southeast Alaskan watersheds. Thus, improving our understanding of current streamwater thermal regimes is critical to better assess how these regimes may be altered by climate change on a regional scale. In this study, seasonal streamwater thermal regimes in forty-seven watersheds across southeast Alaska were evaluated, and the influence of watershed geomorphic and landscape characteristics on stream temperature and streamwater sensitivity to air temperature was assessed. Stream temperatures were measured during the 2015 water year and analyzed for winter and summer seasons. Mean summer stream temperature ranged from 4.0°C to 17.2°C, while mean winter stream temperature were less variable across the region, ranging from 0.5°C to 3.5°C. Maximum weekly average temperatures (MWAT) ranged from 4.3°C to 21.5°C. Regression and time series analyses revealed that lower latitude, low gradient watersheds with higher lake coverage experienced warmer maximum and average summer stream temperatures and were more sensitive to air temperature fluctuations compared to higher latitude watersheds with high gradients during the summer. Winter mean stream temperature was warmer in higher gradient watersheds with greater forest and lake coverage. Moreover, higher latitude watersheds with steep gradients were less sensitive to changes in air temperature relative to low gradient / low latitude watersheds during the winter. Findings from this study demonstrate thermal regimes and air sensitivity are moderated by watershed geomorphology and landcover to create streamwater thermal heterogeneity across the coastal temperate rainforest of southeast Alaska. Results presented herein demonstrate that streamwater sensitivity to air temperature fluctuations are moderated by watershed geomorphology, and should be considered as a framework for predicting thermal regimes to assess relative watershed thermal response to climate change. This information, in turn, is important for quantifying the likely magnitude and spatial extent of climate-driven thermal impacts on Pacific salmon during their freshwater life history stages in southeast Alaska.