WHAT THE SCIENCE SAYS
Spring-fed rivers may also represent important coldwater refuges for coldwater fishes in the face of climate change. Lusardi et al. (2016) recently examined differences between spring-fed and runoff rivers in northern California. The authors studied Hat Creek, Rising River, Fall River (spring-fed rivers) and compared environmental conditions (production of stream invertebrates, water temperature, flow) to adjacent runoff rivers (Castle Creek, the McCloud River above spring sources, and the south fork Sacramento River) over a period of four seasons. Although the study did not examine how salmonids grow in these different systems (this is the subject of current research), they did find large differences in several of the habitat variables examined, with important implications for how trout (and salmon, historically) used these ecosystems. Specifically, spring-fed rivers supported stream invertebrate densities (i.e., fish food) that were 7 to 16 times greater than those associated with runoff rivers, indicating that food was plentiful for foraging fishes in spring-fed rivers. Spring-fed systems in this study also exhibited less water temperature and flow variability than runoff rivers. In short, spring-fed rivers were generally warmer during winter and cooler during summer, lacked high flow events during winter, but exhibited higher low flows during summer.
Current climate predictions indicate the onset of earlier snowmelt runoff, higher magnitude winter flood events, prolonged periods of low flow particularly during summer and fall, and strong declines in thermal habitat for coldwater fishes in California and throughout the greater western United States.
The results of Lusardi et al. (2016) and Jeffres et al. (in review), suggest that spring-
fed rivers may be uniquely resilient to such changes in climate because they are less dependent (at least in the short-term) on snowmelt runoff and more dependent on springs. The dependency on springs means that these ecosystems are better able to moderate temperature and flow fluctuations. In addition, coldwater fishes would likely be able to metabolically compensate for slight increases in water temperature with the abundant food resources associated with spring-fed rivers.
Despite spring-fed rivers being somewhat rare in the larger network of stream types in California, they are incredibly important from an ecological perspective. They are productive ecosystems, are likely resilient to climate change, and, due to their inherent environmental differences compared with other stream types, are strong contributors to trout and salmon diversity. Improving trout and salmon diversity is key to enabling these emblematic fishes to adapt to a changing landscape in order to promote long-term species persistence.
Dr. Robert Lusardi is the California Trout-UC Davis Wild and Coldwater Fish Scientist.
Jeffres, C. A., Nichols, A. L., Lusardi, R. A., Deas, M. L., Mount, J. F., Moyle, P. B., and R. A. Dahlgren. Geologic subsidies drive high productivity in a volcanic spring-fed stream. Submitted to Frontiers in Ecology and the Environment.
Lusardi, R. A., M. T. Bogan, P. B. Moyle, and R. A. Dahlgren. 2016. Environment shapes invertebrate assemblage structure differences between volcanic spring-fed and runoff rivers in northern California. Freshwater Science 35:1010-1022.