Air temperatures overpredict changes to stream fish assemblages with climate warming compared to water temperatures

Studies predicting how the distribution of aquatic organisms will shift
with climate change often use projected increases in air temperature or
water temperature. However, the assumed correlations between water
temperature change and air temperature change can be problematic,
especially for mountainous, high elevation streams. Using stream fish
assemblage data from 1,442 surveys across a mountain - plains gradient
(Wyoming, USA; 1990-2018), we compared the responsiveness of thermal
guilds, native status groups, and assemblage structure to projected
climate warming from generalized air temperature models and
stream-specific water temperature models. Air temperature models
consistently predicted greater range shift differences between warm-water
and cold-water species, with air temperatures predicting greater increases
in occurrence and greater range expansions for warm-water species. The
“over-prediction” of warm-water species expansions resulted in air
temperature models predicting higher rates of novel species combinations,
greater increases in local species richness, and higher magnitudes of
biotic homogenization compared with water temperature models. Despite
differences in model predictions for warm-water species, both air and
water temperature models predicted that three cold-water species would
exhibit similar decreases in occurrence (decline of 1.0% and 1.8% of sites
per 1 °C warming, respectively) and similar range contractions (16.6 and
21.5 m elevation loss per 1 °C warming, respectively). The
“over-prediction” for warm-water species is partially attributable to
water temperatures warming at slower rates than air temperatures because
local, stream-scale factors (e.g., riparian cover, groundwater inputs)
buffer high elevation streams from rising air temperatures. Our study
provides the first comparison of how inferences about climate-induced
biotic change at the species- and assemblage-levels differ when modeling
with generalized air temperatures versus stream-specific water
temperatures. We recommend future studies use stream-specific water
temperature models, especially for mountainous, high elevation streams, to
avoid the “over-prediction” of biotic changes observed from air
temperature variables.