Summary and concluding remarks

Taken together, the results presented in section 15.2 suggest that the extinction of entire populations of basal species by enhanced UV-B levels seems improbable in established aquatic communities. In transparent and shallow aquatic ecosystems, UVR is undoubtedly a major force shaping the structure of pioneer communities. However, whether enhanced UV-B fluxes could offer a competitive advantage to tolerant species of phytoplankton in natural environments remains uncertain. A major effort is needed to understand the underlying physiological mechanisms resulting in the observed changes or lack of changes in community structure. Although there is probably no perfect experimental design to test the direct and indirect effects of UVR, ecological studies should resemble the conditions to which organisms are exposed to solar radiation. The contrasting results obtained with enclosure experiments and the highlighted methodological caveats call for extreme caution in extrapolating previous results on changes in species composition to natural environments. In connection to scenarios of shift in taxonomic composition, it has often been anticipated that a change in phytoplankton (or other community) species composition will have a major impact on higher trophic levels and cause altered patterns of trophic dynamics [7,8,105]. This Eltonian perspective of ecosystem functioning may not necessarily apply even under the worst-case scenario of population extinction. Analyses of food web studies where species have been removed, and predictions of the food-web theory, suggest that consequences for higher trophic levels will depend on both the functioning role of the species (e.g., a keystone species) and the complexity of the food web [106]. Thus, for example, the extinction of a species in a simple food web with few dominant species may have dramatic consequences for higher trophic levels (resulting potentially in other extinctions), while in a complex community the effects will be small. These predictions are further supported by the observation that linkage density in food webs increases with their size [107], and that regardless of the size of the food web there is a nearly constant ratio (~ 2 to 3) of prey to each predator species [108],

The existing information about indirect effects mediated by UVR on trophic interactions (section 15.3) suggest that positive feedbacks as observed in artificial flumes with benthic organisms and in laboratory studies with microorganisms are not the rule in natural systems. Certainly, more ecological studies are needed before we can consider them as important processes occurring in aquatic ecosystems. Particularly, a combination of autecological and synecological approaches could be fruitful in view of the large difference in species sensitivity observed. Assessments where entire components are considered as "black boxes" will mask the species' response. On the other hand, it can be anticipated that for planktonic groups with uncertain or difficult taxonomy this would be a difficult task. Regarding indirect effects of UVR on grazers mediated through algae, there is an urgent need to do experiments under more realistic UV exposure conditions considering the combined effect of UVR on grazers. A less explored interaction is when UVR acts together with predation as countervailing selective pressure on aquatic organisms that obtain protection through pigmentation but at the same time increase their conspicuousness to predators [109,110], The effect of UVR on food quality, particularly on polyunsaturated fatty acids are thought to play a major role in the food web of shallow and clear waters as these compounds are essential for a balanced growth in herbivores [111]. Consequently, studies considering the effect of changes in food quality and life history traits of invertebrates as affected by UVR are a promising research area. Finally, the effects of UVR on anti-predator behavioural responses as evidenced for amphibians [112] need to be investigated on different groups of organisms including the direct effect of UVR on chemical signals (e.g. kairomones) important in predator-prey relationships.

Although there is an increasing number of studies addressing the role of UVR on the mortality of symbiotic corals, our knowledge of U V effects on other types of symbioses in marine and freshwater systems is scarce (section 15.4). It seems reasonable to hypothesize that beside the well-established advantage of en-dosymbiosis for survival in nutrient-poor waters, symbionts, for instance, of protozoans, may also offer protection against UV damage by providing photo-protecting compounds such as mycosporine-like amino acids. The finding that symbiotic ciliates are less UV-sensitive than other ciliate species supports such assumptions. The study of the association between phototrophic endosymbionts and ciliates living in the illuminated zone of anoxic marine sandy sediments could be particularly interesting in this regard.

Information on the effects of UVR on parasites and diseases in aquatic systems is mainly restricted to viruses and Saprolegnia spp. As we have seen in section 15.5, the interaction among UVR, viruses, and their hosts is extremely complex including direct and indirect effects. The use of models may help to explore the response of this system under UV stress. For example, the time needed to intercept a host depends on the product of contact rate, host population density, and the inverse probability of infection per contact [90]. Results from a random encounter model predict that viruses of common (abundant) species may have an advantage by requiring less time to contact their host and consequently by receiving a lower UV dose in exposed habitats [113]. Consequently, viruses from bacteria should be less exposed to damaging UV irradiances than those of phytoplankton. However, host specificity may reduce the effective population size that could be infected. Obviously, this is a field where more research is needed to define the net response of the interaction.

The example of Saprolegnia-associated mortality on amphibians represents a good example of how other synergistic processes, like climatic warming may exacerbate negative effects of ambient UVR (see Chapter 17). Whereas secondary parasitic infection by Saprolegnia after UV exposure seems to be more common in captive fish, this parasite is responsible for a high mortality in natural populations of amphibians. Nevertheless, it remains to be established how sensitively Saprolegnia species react to increased UV-B fluxes.

This brief review clearly indicates that ecologists still have much to learn about the interactions of UVR in the functioning of aquatic ecosystems and, at the same time, much to contribute to this topic. Although it is obvious that the role of UVR on species interactions is now recognized by aquatic ecologists and considered essential for assessing the ecosystem response, our gap of knowledge is still large. One consequence of this situation is that many predictions about potential effects of enhanced UV-B fluxes on aquatic ecosystems remain only speculations. This must change rapidly in the near future, considering that scientific knowledge alone does not lead to political decisions, and that policy based on a weak scientific basis is doomed [2].

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