Stream isotherm shifts from climate change and implications for distributions of ectothermic organisms

Stream ecosystems are especially vulnerable to climate warming because most aquatic organisms are ectothermic and live in dendritic networks that are easily fragmented. Many bioclimatic models predict significant range contractions in stream biotas, but subsequent biological assessments have rarely been done to determine the accuracy of these predictions. Assessments are difficult because model predictions are either untestable or so imprecise that definitive answers may not be obtained within timespans relevant for effective conservation. 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. A set of reference equations and formulas are provided for application to most streams. Example calculations for streams with lapse rates of 0.8 similar to degrees C/100 similar to m and long-term warming rates of 0.10.2 similar to degrees C similar to decade-1 indicate that isotherms shift upstream at 0.131.3 similar to km similar to decade-1 in steep streams (210% slope) and 1.325 similar to km similar to decade-1 in flat streams (0.11% slope). Used more generally with global scenarios, the equations predict isotherms shifted 1.543 similar to km in many streams during the 20th Century as air temperatures increased by 0.6 similar to degrees C and would shift another 5143 similar to km in the first half of the 21st Century if midrange projections of a 2 similar to degrees C air temperature increase occur. Variability analysis suggests that short-term variation associated with interannual stream temperature changes 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 should be prioritized to validate bioclimatic models and develop a better understanding about the effects of temperature increases on stream biotas.