Linking fish assemblages and spatiotemporal thermal heterogeneity in a river-floodplain landscape using high-resolution airborne thermal infrared remote sensing and in-situ measurements

Abstract

River floodplains are heterogeneous landscapes composed of a mosaic of aquatic and terrestrial habitats. While flow has frequently been considered as a master variable that controls aquatic biodiversity and species behavior in river floodplains, little is known about thermal heterogeneity and its effect on aquatic biota in complex landscapes. However, quantifying thermal patch dynamics at the river floodplain scale remains a challenging task. In this study, we applied airborne thermal infrared (TIR) imagery to characterize spatial thermal heterogeneity under high (March 2010) and mean (July 2010) flow conditions in a 347.5ha lowland river floodplain (Oder River, Germany). Concurrently, we electro-fished all major floodplain water bodies (main channel, side channel, and permanently connected ponds) to identify the composition of associated fish assemblages. In addition, we deployed temperature loggers from March to July across the entire range of floodplain water bodies to assess seasonal temperature patterns at 20min intervals. Under both flow conditions, the TIR imagery revealed a complex mosaic of thermal patches across the floodplain. Cumulative degree-days and average and maximum temperatures were the main variables that thermally differentiated individual water bodies. The water body types delineated based on spatial and seasonal thermal signatures also contained distinct fish assemblages. Distinct temperature gradients at the floodplain scale and both within and across distinct water body types were resolved with airborne thermal data, providing a fully spatial and concurrently available temperature mosaic. A delay in response of fish distribution to a strong thermal gradient recorded in spring, that has never been predicted or observed elsewhere, could be assessed by the use of TIR imagery. This study demonstrates the potential of airborne remotely sensed TIR imagery as a non-invasive method for detecting and quantifying spatial heterogeneity and ecologically relevant temperature gradients in complex landscapes, thereby facilitating the “up-scaling” of ecosystem processes to the landscape scale and expanding current understanding of fish behavior.