Within-reach temperature heterogeneity is limited in a southern Appalachian stream network, southeastern USA

Abstract

Stream temperature monitoring networks, combined with geospatial data and statistical modeling, facilitate temperature mapping at the spatial resolution of inter-confluence stream reaches and at continental spatial extents. Comparatively few monitoring efforts quantify spatiotemporal temperature heterogeneity within inter-confluence stream reaches. We measured within-reach temperature heterogeneity for six study reaches in a southern Appalachian stream network, southeastern USA. Within each reach, we logged temperature at 27 monitoring points every 15 min for three days during early summer and three days during late summer of 2018. Analysis of variance indicated that daily minimum, mean, and maximum water temperature metrics vary more among reaches than within reaches. For example, the difference in daily mean temperature from the warmest to coldest reach was 8.7 °C, whereas within-reach variation never exceeded 2.3 °C for any of the three daily temperature metrics. Mixed effects models indicated that most variation in the three daily water temperature metrics can be explained by reach-resolution covariates including elevation, watershed area, and forest cover. The limited within-reach temperature heterogeneity was driven by longitudinal position along the reach but surprisingly not by riparian canopy gaps, vertical stratification, or other factors. Our findings verify temperature mapping at the resolution of inter-confluence reaches effectively captures temperature gradients and indicate that thermally-sensitive organisms have limited access to fine-scale refuge in southern Appalachian streams. Generally, within-reach temperature heterogeneity should be quantified in more geographic and physiographic contexts to understand natural and anthropogenic drivers of this variation, validate reach-resolution temperature mapping, and forecast climate change refuges.