Climate Velocity in Streams: What Does It Mean for Fish?

Climate velocity is the rate at which temperature isotherms shift within streams. Species distributions must track an isotherm that delimits thermally suitable habitat as they move upstream with climate warming. Here, we develop the equations for calculating isotherm shift rates (ISRs) in streams that can be used to represent historic or future warming scenarios and be calibrated to individual streams using local measurements of stream temperature and slope. Example calculations for streams with lapse rates of 0.8 °C/100 m and long-term warming rates of 0.1–0.2 °C decade indicate that isotherms shift upstream at 0.13–1.3 km decade in steep streams (2–10% slope) and 1.3–25 km decade in flat streams (0.1–1% slope). Variability analysis suggests that short-term inter-annual variation will mask long-term isotherm shifts for several decades in most locations, so extended biological monitoring efforts are required to document anticipated distribution shifts. Resampling of historical sites could yield estimates of biological responses in the short term and test bioclimatic model predictions to develop a better understanding about the effects of temperature increases on stream biotas. For a copy of this manuscript, visit the TreeSearch website (http://www.treesearch.fs.fed.us/pubs/42640).