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Drone‐based characterization of intertidal spring cold‐water plume dynamics

Thermal infrared remote sensing from satellites and aircraft has been frequently applied to characterize marine and freshwater thermal regimes, including detection of marine heat waves (Holbrook et al., 2019; Oliver et al., 2018) and mapping thermal heterogeneity in rivers (Dugdale et al., 2019; Fullerton et al., 2018; O'Sullivan et al., 2019; Torgersen et al., 1999). The recent emergence of thermal remote sensing using small unoccupied aircraft vehicles (sUAVs or drones) has greatly reduced the costs and expanded the possibilities for local thermal investigations (e.g., Dugdale et al., 2019; Fitch et al., 2018). Thermal drift and other issues associated with inexpensive thermal sensors often complicate the quantitative interpretation of such thermal data (Kelly et al., 2019; Mesas-Carrascosa et al., 2018). Nonetheless, sUAV-based thermal sensing has been shown to be an impactful tool with which to study groundwatersurface water interactions and thermal anomalies in rivers (Dugdale et al., 2019) or along marine coastlines (Lee et al., 2016). Cold-water anomalies (plumes) in rivers can provide critical thermal refuges for poikilotherms (Torgersen et al., 2012; Wilbur et al., 2020), including many anadromous, cold-water fish. For organisms that behaviourally thermoregulate or physiologically modulate their thermal tolerance, thermal plumes may influence their survivability by affecting their metabolic processes and by lowering the probability of exceeding critical thermal thresholds (Morash, Speers-Roesch, Andrew, & Currie, 2020; Torgersen et al., 2012). Only a few river reaches have been thermally mapped to delineate cold-water plumes, and, with rare exceptions (Dugdale et al., 2013), such river aerial thermal surveys have only been recorded at a single point in time. These single snapshots may obfuscate important temporal cold-water plume dynamics, particularly in coastal reaches where hydraulic and thermal interactions between groundwater and surface water are tidally influenced (Lee et al., 2016; LeRoux et al., 2021). The lack of temporal data for coldwater plumes has limited our understanding of the mechanisms that control their morphology and dynamics and how these conditions may impact cold-water species. In this study, we used repeated sUAV-based thermal images to assess cold-water plume dynamics at the mouths of intertidal springs within a coastal lagoon ecosystem that is threatened by rising water temperatures (DFO (Fisheries and Oceans Canada), 2020). We consider this transitional coastal system given the foci of most previous thermal remote sensing studies on either freshwater or marine environments. Thermal refuges in transitional coastal waters (e.g., lagoons and estuaries) may be critical, as these corridors are physiologically taxing for anadromous fish due to sharp gradients in temperature and salinity (Thorstad et al., 2012). This study specifically examines how the longshore tidal current direction and water stage impact the size and shape of groundwater-sourced cold-water plumes in coastal waters.