Beneficial and Detrimental Effects of UV on Aquatic Organisms: Implications of Spectral Variation

Abstract

Abstract. Solar ultraviolet radiation (UVR) may have beneficial as well as detrimental effects on living systems. For example, UV-B radiation (280-320 nm) is generally damaging, while UV-A radiation (320-400 nm) may cause damage or stimulate beneficial photorepair of UV-B damage. The nature of both direct and indirect effects of UVR in nature depends on both the photon flux density and the spectral composition of the radiation incident on aquatic organisms across environmental UVR gradients in space (depth, transparency, elevation) and time (diel, seasonal, interannual). Here we use the common and widespread freshwater cladoceran Daphnia pulicaria as a model organism to demonstrate the potential importance of these wavelength-specific effects of UVR to the ecology of aquatic organisms. UVR-exposure experiments are used to manipulate both natural solar and artificial UVR sources to examine the beneficial as well as detrimental effects of different wavelengths of UVR. Changes in the spectral composition of solar radiation are also examined along several natural environmental gradients including diel gradients, depth gradients, and dissolved organic carbon (DOC) gradients. The implications of variation in the spectral composition of UVR for aquatic organisms are discussed.The first biological weighting function (BWF) for a freshwater cladoceran is presented here. It demonstrates that the shortest UV-B wavelengths in sunlight are potentially the most damaging per photon. However, due to the greater photon flux density of longer wavelength UVR in sunlight, the net potential damage to Daphnia in nature is greatest for the longer wavelength UV-B and shorter wavelength UV-A radiation in the 305-322 nm range. Overall the contribution of UV-B to the total mortality response of Daphnia exposed to full-spectrum solar radiation for 7 h on a sunny summer day is 64% while UV-A contributes 36%. The BWF for Daphnia is used with the transmission spectrum for Mylar D to demonstrate that Mylar D cuts out only about half of the damaging UVR in sunlight. Following exposure to damaging UV-B, Daphnia exhibits a dramatic increase in survival in the presence of longer wavelength UV-A and visible radiation due to the stimulation of photoenzymatic repair. We present data that demonstrate the importance of both atmospheric ozone and DOC in creating strong environmental gradients in the intensity (irradiance) and spectral composition of solar UVR in nature. The light-absorbing component of DOC, chromophoric dissolved organic matter (CDOM), is particularly important in creating depth refugia from damaging UV-B in freshwater ecosystems. CDOM may also cause intense variations in the ratio of potentially beneficial UV-A to detrimental UV-B radiation to which aquatic organisms are exposed. In addition to changes in atmospheric ozone, future changes in CDOM related to climate change or other environmental disturbances may substantially alter the underwater exposure of a variety of aquatic organisms to different wavelengths of solar UVR.

The effects of UVR irradiance and spectral composition on yellow perch (Perca flavescens) larvae survival

“…Filtering out only UV-B did not improve larval survival in our experiments, as also found by Williamson et al (2001) on yellow perch eggs. In contrast, filtering out both UV-B and UV-A greatly improved survival, even under the highest irradiance conditions (i.e., at a very shallow depth in clear water).…”

Spatio-seasonal variability of chromophoric dissolved organic matter absorption and responses to photobleaching in a large shallow temperate lake

“…It is one of the dominant colourforming constituents in lakes: it not only exerts a strong influence over the underwater light field and water-leaving radiance but it also has a number of important ecosystem functions. First of all, it absorbs light strongly in the ultraviolet (UV) spectrum, limiting the penetration of biologically damaging UV-B radiation that provides protection for phytoplankton and other primary producers (Hoge et al, 1995;Laurion et al, 2000;Zhang et al, 2007a;Williamson et al, 2001). In addition, CDOM can be remineralised by bacteria acting as a source of inorganic nutrients (Buchan et al, 2014), which is important for phytoplankton nutrition, thus fulfilling an important role in the development of phytoplankton blooms and lake metabolism more widely.…”

Damaging UV radiation and invertebrate predation: conflicting selective pressures for zooplankton vertical distribution in the water column of low DOC lakes

“…UVR that reaches the earth surface consists of UV-A (320-400 nm) and UV-B (280-320 nm), both of which can be potentially damaging to aquatic organisms. UV wavelengths of approximately 320 nm are the most biologically damaging to aquatic zooplankter Daphnia when one takes into account both the number of photons in incident sunlight and the lethal damage per photon (Williamson et al 2001a).…”