Ultraviolet radiation (UVR) is a key environmental variable shaping the evolution of life, largely due to the detrimental effects of high UVR on biological structures like DNA (Cockell, 2000; Cockell & Blaustein, 2001). Although UVR exposure has wide-ranging effects on organisms and biological processes, not all wavelengths of UV reach the Earth’s surface. The sun emits UV in three major wavelength groups: UVA (315–400 nm), UVB (280–315 nm) and UVC (280–315 nm). Stratospheric ozone (O3) absorbs short-wave UVC and most of UVB, but not UVA. Although the majority of UVB is absorbed by the stratospheric ozone column, biologically significant levels of solar UVB reach the Earth’s surface. Incident UVR varies spatiotemporally (Bais et al. 2015), driven by multiple interacting factors such as ozone column length, solar angle, cloud and canopy cover, and surface reflectance (albedo) (Koepke et al. 2002; Belmont et al. 2009). Altitude and latitude are important variables influencing incident UVR levels in aquatic environments (Körner 2003; Diamond et al. 2005; Wang et al. 2014). UVB can increase by 8–10% per kilometre ASL (Madronich et al. 1995; Pfeifer et al. 2006; Dahlback et al. 2007), is lowest at the poles and highest at the equator (Caldwell et al. 1980; Barnes et al. 1987; Gehrke 1998). UVR can penetrate significantly into aquatic systems, particularly those with clear water and little overhanging vegetation (Xenopoulos and Schindler 2001). Dissolved organic matter (DOM) is an important factor determining the penetrance of UVR in most freshwater systems (Kirk 1994; Morris et al. 1995). DOM concentrations can vary substantially across waterbodies (Schindler et al. 1992; Morris et al. 1995; Williamson et al. 1996; Gergel et al. 1999), resulting in different UVR penetrance across environments. Aquatic habitats in montane ecosystems are typically associated with higher UVR penetrance due to lower aquatic DOM concentrations (Häder et al. 2007).
In addition to abiotic drivers of global UVR patterns, anthropogenic factors have influenced levels of UVR reaching the Earth. As a result of stratospheric ozone depletion by the emission of industrial chemicals such as chlorofluorocarbons, UVB levels have increased by 2–6% in some areas since last century(Lemus-Deschamps and Makin 2012) and will likely remain high throughout the 21st century (Montzka et al. 2018). Though the effects of ozone depletion on UVR reaching the Earth’s surface have been largely mitigated by the Montreal Protocol (World Meteorological Organization 2022), UVR is predicted to change significantly in freshwater ecosystems because of climate change, as changes such as cloud cover, extreme weather events and vegetation responses modify incident UVR (Williamson et al. 2014; Bais et al. 2018; Barnes et al. 2019; McKenzie et al. 2020). This is important because UVR is a critical determinant of health in freshwater organisms (Peng et al. 2017), and has been implicated in the global amphibian extinction crisis.
Globally, amphibians face one of the highest extinction risks of any vertebrate clade (Houlahan et al. 2000; Hof et al. 2011). Over the past few decades amphibian extinction rates exceeded and will continue to exceed background rates by over four orders of magnitude (McCallum 2007; Alroy 2015). Many of these declines were classed as enigmatic (Stuart et al. 2004), but now most of these, including those in Australia, have been largely linked to the emergence and spread of the pathogenic amphibian chytrid fungus Batrachochytrium dendrobatidis (Bd) (Berger et al. 1998; Young et al. 2001; Gillespie et al. 2020). The links between environmental drivers of amphibian declines and disease is an active area of research, and multiple abiotic factors have been implicated with amphibian disease occurrence and severity including temperature, rainfall, and changing levels of UVR (Carey 1993; Broomhall et al. 2000; Kiesecker et al. 2001). In Australia, amphibian declines have primarily occurred across a significant latitudinal gradient along the eastern coastline, with a disproportionately high number of declines found at higher altitudes > 400 m above sea level (ASL) associated with cooler temperatures and higher levels of ultraviolet radiation (UVR; Biodiversity Group, 1999; Blaustein & Wake, 1990; Bradford, 1991; Kiesecker et al., 2001; McDonald & Alford, 1999; Richards et al., 1993; Young et al., 2001).
Increased UVR is hypothesised to influence amphibian populations by having negative effects on eggs and tadpoles because these life stages are often diurnal, are typically laid during spring and summer when UVR levels are highest, and because these life history stages have a limited ability to avoid UVR (Blaustein et al. 2003; Lundsgaard et al. 2023). Environmental temperature is a significant determinant of UVR-associated impacts in amphibian larvae. Temperature has profound effects on biochemical reaction rates and physiological function, particularly for ectotherms where physiological rates are directly related to organismal performance (Huey and Kingsolver 1989; Angilletta 2009). The negative effects of UVR on amphibian health are compounded when UVR exposures occurs at low temperature (Kiesecker and Blaustein 1995; van Uitregt et al. 2007; Bancroft et al. 2008a; Alton and Franklin 2012; Morison et al. 2019; Lundsgaard et al. 2020; Hird et al. 2022). This was hypothesised to be caused by the depressive effects of temperature on photoenzymatic DNA repair rates (Morison et al. 2019; Hird et al. 2022). While studies using sensors to characterise thermal microenvironments experienced by organisms are numerous (Humphreys 1978; Stelzner and Hausfater 1986; Jimenez et al. 2008; Pincebourde et al. 2016), fewer studies have attempted to build a comparable picture of the UVR microenvironment in freshwater ecosystems (Sommaruga and Psenner 1997; Bukaveckas and Robbins-Forbes 2000; Markager and Vincent 2000; Laurion et al. 2000). An understanding of the UVR microenvironment in freshwater ecosystems and the UVR doses that amphibians experience in situ is lacking (Licht 2003).
A mechanistic basis for the spatiotemporal correlation between high altitude amphibian declines and anthropogenic increases in UVR remains unclear. Data from mesocosm and laboratory-based studies have shown that the larvae and embryos of many ectotherms are highly sensitive to UVR (Alton and Franklin 2017; Peng et al. 2017; Downie et al. 2023), yet we lack an understanding of the range and magnitude of biologically relevant UVR exposures within freshwater ecosystems. Although many juvenile and adult amphibian life stages are nocturnal, most aquatic larval stages are diurnal and may actively seek out sunlight to thermoregulate (selecting preferred water temperatures), potentially exposing them to significant UVR doses (Brattstrom 1979; Bradford 1984; Wollmuth and Crawshaw 1988; Ultsch et al. 1999; Bancroft et al. 2008b). Likewise, embryos may receive significant UVR doses if oviposition sites receive significant sun exposure. Understanding ecologically relevant UVR doses in amphibian habitats is challenging due to the highly variable nature of UVR attenuation. Satellite estimation of UVR only gives broad scale UVR measurements and are prone to error (Bais et al. 2015). Because individual animals experience climate at fine spatial scales, climate heterogeneity ultimately determines the microclimates that organisms will experience (Pincebourde et al. 2016). For this reason, understanding the UVR that amphibians may receive in nature requires fine scale temporal and spatial measurements of UVR.
Considering that UVR is a critical determinant of health in freshwater organisms, understanding how UVR will change and interact with other environmental variables is critical to predict how climate change will impact freshwater ecosystems. There is a critical need to develop novel and feasible ways to quantify ecologically realistic UVR exposure regimes in freshwater ecosystems to accurately monitor changing UVR in the future. In this study, we used bespoke low cost UVR and temperature loggers, handheld spectroradiometry, and UVR dosimeters to generate a unique dataset that characterises the UV environment in two amphibian habitats in southeast Queensland (Australia) over the amphibian breeding period. We hypothesised that traditional monitoring techniques would provide course estimates of natural UVR exposure regimes compared with fine-scale UVR measurements obtained with UVR data loggers. Furthermore, we expected traditional techniques would miss high UVR exposures potentially harmful to aquatic life. These data will be important for understanding the relevance of past and future laboratory-based studies investigating how freshwater organisms respond to UVR, and ultimately predicting how aquatic ecosystems will respond to global climate change.