In addition to ozone and clouds, numerous other factors can influence the amount of UVB exposure received by an organism. Latitude affects the intensity of sunlight through solar zenith angle (greatest at the poles) and thickness of the atmosphere (greatest at the equator). In terrestrial habitats, UVB doses are modified by canopy layers and availability of shaded microenvironments. In aquatic ecosystems, dissolved substances and particulate matter in the water column usually eliminate biologically harmful intensities of UVB within the upper 20 m. Vertical mixing will further determine actual exposures of planktonic organisms.
Temporal exposure factors of seasons and photoperiod are also defined by latitude, and local weather will affect the intensity of UVB on scales of minutes or days or longer. At latitudes below 60°, changes in ozone concentration occurred gradually over two decades and were accompanied by measurable increases in UVB. Over Antarctica, large predictable springtime declines of ozone cause substantial short-term increases in UVB radiation.
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