Introduction: enhanced UV-B radiation in natural ecosystems as an added perturbation due to ozone depletion.

This issue of the journal contains 16 articles in a symposiumin-print about the effects of UV radiation (UVR) on natural ecosystems. These articles result from a 5 year multidisciplinary project on UVR and UV effects on natural systems carried out by a network of 17 principal investigators from 13 institutions in 5 countries. The research project’s aim was to test the effect of UVR on natural systems, including marine, freshwater and coastal environments, on a continental scale in the Americas. The symposiumin-print presents results of field sampling and field and laboratory experiments to document UVR effects in natural gradients, both latitudinal and altitudinal, and test hypotheses on what controls UVR exposure and UVR effects, such as dissolved organic carbon loading in lakes and the evolution of trophic dynamics in marine systems. The symposium-in-print is a unique contribution because of its multidisciplinary scope, the geographical scale of the experiments and the diversity of systems studied. The discovery of the Antarctic ozone hole in 1986 brought the public’s attention to the degradation of the ozone layer by anthropogenic contamination of the stratosphere (1). In one of the most successful stories of environmental international policy, the Montreal Protocol was implemented in 1990 to control emissions of chlorofluorocarbons and other gases creating chlorine and bromine derivatives that interact with ozone to produce two molecules of oxygen (2). Ozone absorbs radiation strongly in the UV and the presence of ozone and oxygen in the atmosphere results in the absorption of nearly all solar radiation with a wavelength of ,290 nm. As a result, virtually no UV-C radiation (200–280 nm) reaches the Earth’s surface, UV-B radiation (280–320 nm) is significantly absorbed (mostly by ozone) and only a small fraction (,3%) of UV-A radiation (320–390 nm) is absorbed by ozone. The effect of UVR on natural ecosystems is function of several biotic and abiotic factors that interact directly with the ecosystem components and alter the relationship among those components (3). Additionally, the system provides feedback mechanisms that either increase or decrease the direct effect of UVR. The articles in the symposium-in-print are based on the premise that natural ecosystems have dealt with UVR as a natural stress factor since the beginning of life on Earth (4) and the present increase in UVR due to stratospheric ozone decrease is an added perturbation to the system. Incident UVR depends on geometric factors such as Sun-Earth distance and solar zenith angle; UVR can be modified in the atmosphere by gases, aerosols and clouds and is further a function of altitude and surface albedo (5). In the water, UVR is strongly affected by dissolved organic matter that alters its transmission to depth because of absorption and scattering. Net damage by UVR is a balance between damage and repair (6). Damage is dependent on radiation exposure or amount of UVR absorbed by the organism or system. At the organism level the damage can be avoided, screened or repaired (7). At the system level deleterious effects at the population level can be ameliorated by differential damage to a predator thus decreasing predation pressure (8). In addition community composition can be altered because of differential damage to certain components. For example, damage to large cells in planktonic communities favors a microbial loop and small cells rather than net plankton and larger grazers. The research presented in the symposium-in-print focuses on the main pathways of net UVR damage on diverse natural ecosystems, emphasizing common processes along environmental gradients.