Abstract
Ultraviolet radiation (UV) is a crucial abiotic stressor that can have severe impacts on biota residing in the upper euphotic zone, especially if UV stress coincides with other stressors such as extreme sea surface temperatures (SSTs). Exposure-dependent effects of UV exposure have been described for a broad range of marine taxa and ecosystems such as coral reefs, yet little is known about the magnitude and seasonality of UV exposure in natural waters. In the present study, we determined how daily exposure of UV-B and UV-A varies seasonally along the water column of a reef system in the central Red Sea, and identified periods when damaging UV levels are likely to coincide with episodes of extreme SST, both presently and in the future. Between July 2016 and September 2018, UV spectroradiometer profiles were recorded fortnightly at a pelagic site adjacent to a mid-shore reef off the Saudi Arabian Red Sea coast, while atmospheric UV-B and UV-A irradiances were measured in 10-minute intervals. Additionally, we quantified the concentration of chlorophyll-a (Chl-a) and absorption by chromophoric dissolved organic matter (aCDOM) in the reef as well as the surrounding waters. Biologically effective optical depths (Z10%) ranged from 6.3–12.9 m (UV-B) and 14.4–27.3 m (UV-A), with the highest UV transparency being observed in late summer when photodegradation of dissolved organic matter (DOM) was most intense and the concentration and molecular weight of CDOM were at their lowest. Incident UV peaked a few weeks prior to this later summer maximum in UV transparency. Consequently, organisms living close to the water surface experienced their most intense UV exposure in May/June, while the timing of maximum UV exposure for biota below ~ 2–4 m coincided with the annual peak in water transparency and water temperature, i.e., in July/August. However, SSTs in the Red Sea are increasing at a rapid rate due to climate change, with the consequence that extreme temperatures are occurring earlier in the year and may eventually coincide with extreme UV radiation in shallower areas of the reef. This development could have potentially detrimental effects on highly sensitive, immotile reef biota such as reef-building corals.
Highlights
Ultraviolet radiation (UV: 200–400 nm) is a crucial constituent of solar radiation that is further subdivided into three spectral bands: UV-A (315–400 nm), UV-B (280–315 nm), and UV-C (200–280 nm)
Whereas maximum midday UV-B and UV-Ery irradiances were recorded in May in both years, the monthly means peaked in June 2017 (UVB: 2.24 ± 0.18 W m−2; UV-Ery: 0.30 ± 0.02) and in April the following year (UV-B: 2.2 ± 0.16 W m−2; UV-Ery: 0.30±), which was the same time when we recorded the highest mean values of midday UV-A irradiances in each year: 64.7 ± 3.8 W m−2 in June 2017 and 65.0 ± 4.0 W m−2 in April 2018 (Table 1)
We identified that the central Red Sea coast exhibits exceptionally high transparency to UV radiation compared to other coastal waters due to negligible inputs of terrestrial chromophoric dissolved organic matter (CDOM)
Summary
Ultraviolet radiation (UV: 200–400 nm) is a crucial constituent of solar radiation that is further subdivided into three spectral bands: UV-A (315–400 nm), UV-B (280–315 nm), and UV-C (200–280 nm). The extent of both direct and indirect UV effects depends on the time of day as well as the exposure, i.e., the intensity and duration of solar UV exposure, which in turn are determined by latitude, season and atmospheric conditions such as cloud cover and pollution (Pfeifer et al, 2006). Due to its proximity to the equator and low cloud cover, the Red Sea region receives intense solar and UV radiation (Khogali and Albar, 1992; McKenzie et al, 2007; Acker et al, 2008; Smyth, 2011). In the Sahara and Arabian Peninsula, aerosols are primarily composed of mineral dust originating from the desert (Hsu et al, 2004), with dust storms frequently passing over the Red Sea, causing reduced visibility and high attenuation of UV-B radiation in the atmosphere (Kalenderski et al, 2013; Prakash et al, 2015)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
