Assessing Stream Thermal Heterogeneity and Cold-Water Patches from UAV-Based Imagery: A Matter of Classification Methods and Metrics

Abstract

Understanding stream thermal heterogeneity patterns is crucial to assess and manage river resilience in light of climate change. The dual acquisition of high-resolution thermal infrared (TIR) and red–green–blue-band (RGB) imagery from unmanned aerial vehicles (UAVs) allows for the identification and characterization of thermally differentiated patches (e.g., cold-water patches—CWPs). However, a lack of harmonized CWP classification metrics (patch size and temperature thresholds) makes comparisons across studies almost impossible. Based on an existing dual UAV imagery dataset (River Ovens, Australia), we present a semi-automatic supervised approach to classify key riverscape habitats and associated thermal properties at a pixel-scale accuracy, based on spectral properties. We selected five morphologically representative reaches to (i) illustrate and test our combined classification and thermal heterogeneity assessment method, (ii) assess the changes in CWP numbers and distribution with different metric definitions, and (iii) model how climatic predictions will affect thermal habitat suitability and connectivity of a cold-adapted fish species. Our method was successfully tested, showing mean thermal differences between shaded and sun-exposed fluvial mesohabitats of up to 0.62 °C. CWP metric definitions substantially changed the number and distance between identified CWPs, and they were strongly dependent on reach morphology. Warmer scenarios illustrated a decrease in suitable fish habitats, but reach-scale morphological complexity helped sustain such habitats. Overall, this study demonstrates the importance of method and metric definitions to enable spatio-temporal comparisons between stream thermal heterogeneity studies.

Land Use Change Influences Ecosystem Function in Headwater Streams of the Lowland Amazon Basin

“…We observed only marginally significant differences in litterfall biomass between land uses despite documented differences in riparian forest communities and micro-climate conditions between forest and cropland watersheds [40,42] (Table S6). Because mean litterfall biomass was higher in forested watersheds than cropland watersheds (Table S6), it is possible that high variation and relatively low replication (n = 10 streams, 3 litter traps within each stream) hindered detection of significant differences in this study.…”

“…Forests in the region are evergreen, semi-deciduous, seasonally dry transitional forests containing a mixture of tree species from ombrophilous rainforests (i.e., high annual rainfall) in the north and the woody Cerrados (tropical savannas; lower annual rainfall) in the south of the Xingu River Basin [52]. Riparian forests in cropland areas differ from those in intact forests, however, having lower diversity of trees and seedling/samplings and more opportunistic pioneer species [40,42]. The landscape at Tanguro Ranch is broadly level, with wide interfluves that grade to streams with less than 65 m in elevation change and channel slopes between 0.3 • and 1.9 • [33].…”

“…Because of the large influence of streamside vegetation on headwater stream function, policies that protect riparian forest buffers to mitigate land use effects have been a widely used restoration and conservation tool [31,39]. Riparian forest buffers provide shade and modulate stream temperatures [24,40] and potentially can reduce transport of agricultural nutrients to streams [39]. However, riparian forests do not completely mitigate land change effects.…”

Environmental threats and conservation implications for Atlantic salmon and brown trout during their critical freshwater phases of spawning, egg development and juvenile emergence

“…The negative impacts of missing longitudinal connectivity and habitat functionality due to structural deficits on riverine fish are well documented and continue to be the major challenge for the future of freshwater conservation (Geist, 2011, 2015; Grill et al, 2015; Grill et al, 2019). Analogously to other cold‐water adapted species, access to cold‐water patches and refugia during extended hot periods becomes a crucial factor of population resilience in the light of global warming (Kuhn et al, 2021). For Atlantic salmon and brown trout, both factors are considered key management objectives to re‐establish self‐sustainable populations in fresh water (Birnie‐Gauvin et al, 2017; ICES, 2019a,2019b,2019c).…”

“…The predicted increase in the average temperature in the next years is suspected to facilitate the spread of parasites (Bruneaux et al, 2017), invasive species (Bean, 2020) and increase the toxic effects of pollutants (Dar et al, 2020) with negative consequences for all life stages of Atlantic salmon and brown trout. From the perspective of research and management, it is thus particularly important to (i) understand and predict the effects of climate change on habitat suitability for both species; (ii) identify and ensure access to cold‐water refugia as a key to improving population resilience (Kuhn et al, 2021); and (iii) understand and manage the interactions with other stressors, especially those that are also temperature dependent.…”