The underwater UV environment

Solar radiation is both absorbed and scattered as it penetrates through water (see Chapter 3). As a result, downwelling irradiance decreases with depth, with shorter and longer wavelengths attenuating more rapidly than the wavelength of peak transmission (which is generally found from 470-550 nm). While particulates and the water itself contribute somewhat to the attenuation of UVR, absorption by DOC is one of the primary factors regulating variations in UV attenuation in aquatic ecosystems [4,5], In systems with high DOC, UVR is attenuated rapidly while, in systems with low DOC, UVR can penetrate deeply into the water column. In 25% of lakes in several regions of North America, 1% attenuation depths (the depth to which 1 % of surface irradiance penetrates) were estimated to be greater than 4 m for 320 nm and greater than 10 m for 380 nm

[21]. In the clearest ocean waters, 1% attenuation depths are estimated to be 50 m for 320 nm and approximately 200 m for 380 nm (Figure 1) [22], and there is evidence that increased levels of UV-B are entering the oceans as stratospheric

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Wavelength (nm)

Figure 1. Percent of surface irradiance present at depth in the clearest ocean waters. Percent surface irradiance was determined using diffuse attenuation coefficients derived from Smith and Baker [22]. Measurements of irradiance were taken with a submersible spectoradiometer in the Sargasso Sea and the Central Equatorial Pacific.

10 m

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Wavelength (nm)

Figure 1. Percent of surface irradiance present at depth in the clearest ocean waters. Percent surface irradiance was determined using diffuse attenuation coefficients derived from Smith and Baker [22]. Measurements of irradiance were taken with a submersible spectoradiometer in the Sargasso Sea and the Central Equatorial Pacific.

ozone decreases [23]. Thus, biologically relevant UVR is present at considerable depths in many freshwater and marine ecosystems. UV-sensitive organisms may avoid depths at which damaging wavelengths are present but may seek depths at which potentially beneficial wavelengths (i.e., used for photorepair and UV vision) are present.

In addition to DOC, other factors influence the depth to which UVR penetrates, including season, latitude, sea state, time of day, cloud cover, and turbidity (Chapters 2 and 3). Light intensity and spectral composition are both affected by each of these factors, creating potential "light niches". For example, relative quantities of UVR are greater during crepuscular periods (i.e. dawn and dusk) than daylight hours (Figure 2) due to the increasing proportion of high-UV skylight in the total irradiance [24]. Many species of larval fish that possess UV vision feed primarily during crepuscular periods [24-26]. These twilight hours may provide an "optical foraging niche" for fish predators with UV vision, enhancing target-background contrast. Indeed, near the surface of the ocean, up to 40% of the photons in the horizontal and downward directed lines of sight are UV-A [27], potentially silhouetting prey (Figure 3).

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