The effects of global climate change on species distributions and habitat selection are broadly documented for terrestrial and aquatic species (e.g., Vergés et al., 2014; Osland et al., 2021). Species distribution models that predict responses to climate change traditionally assume homogenous climate conditions along latitudinal boundaries; however, an increasing body of literature emphasizes the importance of heterogeneity within a species’ range (Thomas et al., 2001; Lembrechts et al., 2019). For example, differences in topography, forest cover, and urbanization in terrestrial habitats and wind, tides, and upwelling in aquatic environments can lead to variability in the microclimatic conditions a species experiences across its distribution (Doniol-Valcroze et al., 2007; Schofield et al., 2009; Lembrechts et al., 2019; Bosso et al., 2022; Müller et al., 2023). This variability has the potential to create microhabitats, which can be selectively used by organisms to improve reproductive success, increase access to prey, avoid predators, or thermoregulate (Doniol-Valcroze et al., 2007; Schofield et al., 2009; Petty et al., 2012; Searcy et al., 2013; Müller et al., 2023). Under changing climate conditions, microhabitats may function as microrefugia, defined as small areas with favorable environmental conditions in which some populations may persist while protected from the broader unfavorable landscape (Rull, 2009; Hannah et al., 2014; Lembrechts et al., 2019). A series of microrefugia may then act as stepping-stones to mediate species’ range shifts, but individual plasticity and local adaptations among populations will influence the extent of distribution shifts under future climate regimes (Hannah et al., 2014; DeMarche et al., 2018).

For migratory species, changing climate conditions not only influence spatial distribution, but also the phenology of migratory activities. For example, dozens of species of migratory birds have been documented changing their total speed of migration to arrive earlier at warmer breeding sites (Schmalijohann and Both, 2017). Changes in total migration speed are primarily linked to earlier departures from winter, non-breeding sites or shorter stop-over periods at refueling sites with abundant food resources (Seebacher and Post, 2015; Schmalijohann and Both, 2017). For these species, changes in migration phenology can have significant consequences on synchrony with favorable habitat conditions and food availability (Mayor et al., 2017). For example, as migratory bird species shift to earlier breeding and nesting dates, they exhibit mismatched timing to peak vegetation and associated insect biomass, reducing prey availability (Visser and Both, 2005; Moore, 2011; Mayor et al., 2017). Conversely, similar species or populations of the same species have not changed the timing of their migration and nesting activities in the face of climate change, illustrating that populations or individuals within the same or similar species may react differently to changing climate conditions (Visser and Both, 2005; Moore, 2011; Neate-Clegg and Tingley, 2022). Other species of birds and insects have ceased migrations in recent years and instead remain year-round at mid-latitude sites that were previously used as stop-over locations (Groepper et al., 2008; Pulito and Berthold, 2010; Moore, 2011; Satterfield et al., 2018). Warming temperatures and mild winters likely increase the duration of annual productivity and resource availability, allowing for year-round survival and migration cessation for some species (Pulito and Berthold, 2010; Moore, 2011). Experimental studies have shown that changes in migratory timing and distance as well as reductions in migratory behavior can occur over short time periods (< 15 years), demonstrating the potential for rapid response of migratory species to changing climate conditions (Pulito and Berthold, 2010).

In the United States (USA), the West Indian manatee (Trichechus manatus; hereafter manatee) is a vulnerable marine mammal species that is thought to comprise a single population of individuals employing various strategies of residency and movement. The species is known to use partial migration, including seasonal short and long-distance movements, making it an ideal study species to examine variation in migratory phenology under changing climate conditions (Deutsch et al., 2008; Cloyed et al., 2021). Manatee distribution within the USA is limited by physiological intolerance to sustained water temperatures < 20°C, traditionally restricting the species’ winter range to peninsular Florida (Irvine, 1983; Bossart et al., 2003). Manatees exposed to cold water temperatures for extended time periods experience cold stress syndrome, with symptoms including skin lesions, malnutrition, and decreased immune function (Bossart et al., 2003; Walsh et al., 2005; Martony et al., 2019). To survive sustained cold periods, manatees use microrefugia with favorable thermal conditions compared to the surrounding environment. For management purposes, such thermal microrefugia are categorized as primary sites, which are well-established to have high manatee use and water temperatures consistently > 20°C, or secondary sites, which have low or unpredictable manatee use and inconsistent water temperatures, especially during severe cold events (Valade et al., 2020). Types of thermal microrefugia typically used by manatees in peninsular Florida include warm water discharges created by industrial facilities, primarily power plants, or natural springs and passive thermal basins that retain heat without a constant inflow of warm water (Laist and Reynolds, 2005a, b; Laist et al., 2013; Valade et al., 2020). During warmer months, manatees disperse from winter refuge sites to access food and other resources throughout their range (Deutsch et al., 2003; Cummings et al., 2014; Hieb et al., 2017; Cloyed et al., 2021).

Between the 1940s and 1970s, newly built power plants in peninsular Florida provided artificial warm water refuge, acting as thermal microrefugia for manatees in areas where the species had not previously occurred. These microrefugia allowed for stepping-stone movements to previously unused sites at more northern latitudes (Laist and Reynolds, 2005a). Correspondingly, sightings of manatees in northern Florida and states west and north of Florida also increased (Fertl et al., 2005; Cummings et al., 2014; Hieb et al., 2017), and manatees are now documented as regular seasonal visitors to these areas (Hieb et al., 2017; Cloyed et al., 2021). Few studies, however, have examined migration and habitat selection in these regions (Deutsch et al., 2003; Cloyed et al., 2021; Cloyed et al., 2022). In particular, no studies have attempted to identify thermal microrefugia outside of Florida that may affect the phenology of seasonal manatee migrations. Understanding range-wide movements under current and future climate conditions will be essential to guide management and recovery efforts for thermally constrained species like manatees.

In this study, manatees serve as a model for thermally constrained migratory species whose distributions are influenced by natural and anthropogenic variation in thermal conditions and for which climate change is likely to further affect the species’ migration patterns and distribution. We examined how thermal microrefugia combined with climate driven increases in regional water temperatures outside of Florida may affect the timing of manatee occurrence and duration of habitat use in the northern Gulf of Mexico (nGOM), a temperate region at the traditional northwestern range margin for manatees. Historically, most manatees were thought to migrate to the nGOM during spring (Apr–May) and depart in the autumn (Sep–Oct), spending the colder months of the year (historically Nov–Mar) in peninsular Florida when nGOM water temperatures drop below 20°C (Hieb et al., 2017; Cloyed et al., 2021). We used aerial thermographic imaging to identify potential warm-water sites that could act as thermal microrefugia for manatees during unfavorable winter conditions and drive changes in the timing of migratory movements. We then defined the timing of potential manatee use of these and nearby sites using citizen-sourced manatee sightings and stranding response data. To relate use of thermal refugia to regional and longer-term water temperatures, we compared temporal distributions of manatee sightings to nGOM air and sea surface temperatures on a decadal scale.

We found that West Indian manatees are increasingly using waters at a northwestern migratory endpoint in the northern Gulf of Mexico (nGOM) during colder months of the year. Increased sightings during Nov and Dec in recent years suggest that some manatees are changing their migratory phenology and extending the duration of seasonal habitat use in the nGOM, which may be facilitated by use of thermal microrefugia. While we did not find any sites in the study area that were consistently warm enough to support high manatee use (constituting primary thermal refugia), we identified microrefugia that may act as secondary thermal refuge sites. These secondary thermal refugia may be at least suitable to allow manatees to withstand mild cold periods for relatively short durations and delay migrations to primary refuge sites in peninsular Florida. For example, manatees along the Atlantic coast of Florida have delayed migration to primary winter refugia when able to access warm effluents farther north or when ambient temperatures remain warm (Deutsch et al., 2003; Cloyed et al., 2022). Cold season sightings have also been documented with increasing frequency in states at the range margins such as Mississippi, Louisiana, Texas, South Carolina, and North Carolina (Leslie, 2017; Carmichael et al., 2021). Additional study is needed throughout these areas to identify potential warm water sources that may be used by manatees as thermal microrefugia and further alter migratory phenology, dispersal patterns, and habitat use.

The effects of climate change-associated increasing sea surface temperatures on prolonged use of habitats at range margins remains uncertain (Cummings et al., 2014; Cloyed et al., 2022). As water temperatures increase under predicted climate change scenarios, it has been suggested that conditions may become more favorable for manatees to overwinter in areas outside of Florida (Cloyed et al., 2021). Promisingly, we found that despite an increase in cold season manatee sightings in recent years, manatee stranding numbers remained constant, suggesting that most manatees sighted during colder months survived instead of stranding. The contribution of microrefugia to this success is unclear. Our finding that minimum temperatures during the cold season have remained low, despite higher temperatures during key periods of movement, suggests that manatees staying in the nGOM at the end of the season (Jan–Feb) remain at risk of cold stress-related mortality, even if using microrefugia. Recent studies propose that extreme cold events linked to Artic warming are increasing in frequency, especially in mid-latitude regions (Cohen et al., 2021; Michelfelder and Pilotte, 2022) and could pose a risk to animals like manatees that may shift migratory phenology in response to climate change. Our finding of a marginal increase in temperature variation, with some time periods having very low temperatures during the cold season in the nGOM in more recent years, is sufficient to raise concern for increased risk of unexpected, drastic temperature decreases that could lead to increased manatee mortalities. Such mortality was previously seen due to unusually cold temperatures in Florida during 2009–2011 (Hardy et al., 2019). Further study of increasing regional water temperatures and potential for extreme cold events will be important to understand the potential for year-round residency and long-term survival of manatees in the nGOM and other marginal sites.

Thermal microrefugia documented in Alabama waters align with the various types of refugia used by manatees in their winter range in peninsular Florida. Approximately, 65–75% of manatees use warm water discharges from power plants or natural springs as primary refugia (Laist and Reynolds, 2005a; Laist and Reynolds, 2005b; Laist et al., 2013; Valade et al., 2020). In Alabama, a power plant and two natural springs were identified as warm water anomalies, with manatee use documented at the spring sites but not the power plant outfall, during the study period. An additional discharge site identified at a wastewater treatment plant outfall was used by a distressed manatee that was later recovered deceased. Manatees in Florida have also been documented to use wastewater treatment plant outfalls, which are not artificially heated, but are often slightly warmer than surrounding ambient water temperatures (Laist and Reynolds, 2005b). Potential passive thermal basins identified in Alabama included multiple freshwater inflow or run-off sites and dredged channels. Two of these freshwater inflow sites observed in Alabama were documented as cool water microhabitats during summer surveys but may conversely be warmer than surrounding waters during colder months as is the case for some thermal microrefugia in Florida that maintain constant temperature year-round (Laist et al., 2013). In peninsular Florida, passive thermal basins with the highest manatee use are dredged canals where lighter, freshwater flows over heavier, saline water, creating a strong vertical halocline that effectively traps warm water on the bottom (Stith et al., 2011; Stith et al., 2012). A similar mechanism may occur in Alabama at dredged channels identified via thermal imaging. Temperature data collected at one of these sites during response to a distressed manatee in Dec 2017 found a salinity gradient of 0.9–12.6 and temperature gradient of 12.2–15.9°C from surface to bottom over a 4m depth. While temperatures at many of the anomalies identified in this study were < 20°C, the occurrence of animals at these locations suggests they are capable of detecting these thermal differences, and manatees used the various types of available microrefugia in Alabama just as they do in Florida waters.

The ability of manatees to successfully use thermal microrefugia in the nGOM in the future will depend on specific attributes that dictate the availability and suitability of these microhabitats. For example, the Barry Steam Plant was the most consistently documented thermal anomaly detected during seasonal aerial surveys, but warm water discharge at this site is episodic, based on the community’s energy use. Maximum temperatures where the plant discharges into the Mobile River during Dec and Mar were ~22–27°C, above the thermal threshold for manatees, but lower discharge rates at the plant in Jan resulted in greater mixing with ambient water, yielding a maximum water temperature of ~12°C. The location of the power plant at the northern extent of most documented manatee sightings in Alabama may also limit manatee use. Flow rates at natural springs and freshwater inflow sites will change with precipitation, discharge patterns, and human activities that can also affect local hydrology (Wang et al., 2014). Groundwater-fed springs may be more likely than other sources to maintain consistent temperatures, but reductions in groundwater flow from natural or human causes could limit the volume and surface area of warmwater available to manatees at this and other springs, as has been shown in Homosassa Springs, FL, a natural spring heavily used by manatees as a primary thermal refuge (Valade et al., 2020; Chen, 2021). Similarly, dredged channels with temperature inversions linked to haloclines depend on specific hydrology to maintain warm water conditions (Stith et al., 2011; Stith et al., 2012; Decker et al., 2013). Knowledge of the various factors that may affect the volume and consistency of warm water sources, particularly at range margins, will be critical to defining the suitability and availability of thermal microrefugia in the future.

While temperatures are an important determinant of microrefugia use by manatees at their range margins, other factors are likely to be equally influential in determining when and where manatees occur during colder months. Heavily used habitats in Alabama during both warm and cold seasons align with locations of submerged aquatic vegetation (SAV), freshwater access, and shallow water known to be preferred by manatees (Hieb et al., 2017; Cloyed et al., 2022). As regional temperatures cool during autumn and winter, manatees may preferentially remain in these resource abundant areas and delay migration to winter refuge sites where resources are more limited (Deutsch et al., 2003; Haase et al., 2019; Littles et al., 2019; Cloyed et al., 2022). While the availability of thermal microrefugia necessarily enhances manatees’ ability to alter migration phenology, the temperature thresholds at which individual manatees depart warm season foraging habitats and travel to primary winter refugia are also likely dependent on behavioral or physiological variation among manatees (Deutsch et al., 2003). Larger, older individuals may be better able to withstand colder temperatures due to smaller surface area to volume ratios that better sustain homeostasis (Buergelt et al., 1984; Chirachevin, 2017) as well as more experience in managing cold temperatures (Buergelt et al., 1984; Deutsch et al., 2003). The severity of cold fronts in terms of the magnitude of temperature variation and cold duration may also influence whether manatees initiate long distance migrations to primary warm water sites or seek closer, less favorable secondary refugia (Deutsch et al., 2003; Hardy et al., 2019). At least some manatees in the nGOM may follow similar strategies to manatees along the Atlantic coast of Florida, using warm water microhabitats as temporary stepping-stones to withstand cold fronts during autumn and early winter as they travel to primary refuge sites farther south (Reid et al., 1991; Deutsch et al., 2003; Cloyed et al., 2021). This approach may be particularly beneficial if primary refugia in Florida reach carrying capacity and become increasingly resource limited (Littles et al., 2019). To best take advantage of resources in warm season habitats, manatees may take a combined approach of using microhabitats for foraging and thermal microrefugia along migration pathways, with exact strategies dependent on environmental variation and individual plasticity.

While methods used in our study were successful at identifying some thermal anomalies, there are potential limitations to using point-in-time thermal imaging methods alone. For example, during field responses to cold-stressed manatees, we often identified warm water sources not detected during aerial surveys. Thermal cameras were effective at capturing differences in surface water temperature, but anomalies at bottom depths were not always detected if temperature signals dissipated before reaching surface waters. These thermal differences were only observed by directly collecting environmental data from surface to the bottom of the water column. For this reason, use of a vertical profiling system that captures water quality data at various depths is useful to supplement thermal imaging data. In addition, the specific conditions during aerial surveys also likely influenced detection of thermal anomalies, highlighting the importance of higher-resolution, year-round monitoring to help identify potential refuge sites. Thermal microrefugia at some locations also may be ephemeral, existing only under certain conditions that are yet to be defined, and may not be detected using point-in-time sampling methods. In some cases, temperature differences between warm water sources and ambient waters were minimal and not captured as thermal anomalies during our surveys, but higher manatee occurrence at these sites during the cold season indicates manatees are selecting these sites for use, potentially as thermal microrefugia. More work is needed to understand the capability of manatees to detect subtle temperature differences, but Deutsch et al. (2003) found that tagged manatee movements were influenced by temperature changes of only 2.0–3.5°C. In addition to thermal imaging and manatee occurrence data used here, other data such as bathymetry and hydrology of local waterways may be useful in detecting potential warm water sources. Citizen-sourced data and local knowledge can also be informative to supplement traditional scientific data collection in understudied regions (Hieb et al., 2017; Bélisle et al., 2018). While our study focuses on manatees in the nGOM, our approach can be transferred to other species and systems and can complement other types of data collection to better understand changes in species distribution and habitat use.

Thermally dependent species are facing rapid changes in dispersal, migration, and habitat use patterns as climate change and other anthropogenically driven factors increasingly pressure these species to adapt to new conditions. Manatees are an informative case study of changing migratory strategies, potentially linked to microrefugia use and warming temperatures. Predicting manatee use of thermal microrefugia outside of Florida and changes in migratory phenology relative to long-term temperature variation will lead to enhanced understanding of range-wide habitat use during colder months of the year and has the potential to inform management actions to conserve these habitats. As primary refugia within traditional manatee habitats face threats from human impacts and potential for reaching carrying capacity, microhabitats at range margins may be increasingly utilized by manatees. Our study provides novel insight for a thermally constrained migratory marine mammal that may also be used as a model for other thermally constrained species expanding into higher latitudes. To effectively manage and conserve these species and their habitats, future studies should consider local microhabitat use within regional and global scales of temperature change, while also considering individual and population level variation in species responses.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Ethical review and approval was not required for the animal study because this study uses only citizen-sourced and stranding response data. Citizen-sourced data are non-invasive and do not require approval by an animal ethics committee. Stranding response data were collected under U.S. Fish and Wildlife Service permits LOAFC770191-H and MA66525C.

EH wrote the main draft of the manuscript. EH and CC analyzed the data. KD, AG, and RC contributed to the collection and analysis of data and participated in writing and editing the manuscript. RC conceived of the project. All authors contributed to the article and approved the submitted version.