Indirect effects

There are several ways by which aquatic organisms may be indirectly influenced by UVR. UVR plays a major role in surface water chemistry that in numerous ways may aifect the aquatic biota. Breakdown or oxidation of complex macro-molecules, notably humus molecules, may both induce availability of organic substratum for microbial heterotrophs [61, Chapter 8]. This may also liberate mineral nutrients such as N and P, and in fact the effects on biogeochemical cycling of key elements like C, N and P in surface waters is one of the less recognized yet most important properties of UVR in many shallow waters. Further, UVR may also react with organic macromolecules and cause the release a number of potentially limiting, but also potentially harmful, metal species that may aifect aquatic biota. Finally UVR promotes the formation of a wide array of photoproducts in water (Chapter 8 and below). This set of important mechanisms, all of which may have profound effects on surface dwelling organisms, is one aspect that clearly is unique to aquatic organisms.

A range of food web effects may also be considered as indirect effects. Although direct UVR damage on particular organisms may be minor, there could nevertheless be pronounced effects if lower trophic levels are affected (bottom-up effects). Alternatively if competitors or predators suffer comparatively more from UVR, this could yield an apparent positive effect of UVR on other species or taxa [62]. While, especially, the effects of UVR on aquatic humus and heterotrophic bacteria as well as food web effects will be considered elsewhere, we shall here just briefly touch upon these indirect mechanisms.

12.3.1 ROSin water

Aquatic organisms may be exposed to ambient UV-induced ROS formed either in tissue or in surface waters by photon reaction with dissolved organic carbon [63]. The role of dissolved organic carbon (DOC) in surface waters is thus twofold; first of all DOC constitutes a highly efficient UV absorbent, and UV-attenuation in most aquatic environments is primarily a direct function of DOC [6]. Secondly, however, the trapping of high levels of energy in the upper few centimetres produces a number of biologically harmful photoproducts. They include excited triplet state DOM, solvated electrons, organic radical cations, superoxide, singlet oxygen, hydroxyl radicals and peroxy radicals (see Chapter 8). Since these compounds may only last nanoseconds, molecular probes have been used to quantify the rate of production (see review by Zafiriou et al. [64]). H202 is produced photochemically when UVR strikes DOM but is more long-lived since its decay is principally biological in most systems. In lakes, concentrations are much higher than in marine systems and may reach, during midday, in excess of 1000 nM, whence DOC concentrations may be a main determinant [63]. The highly toxic carbon monoxide is also generated in both fresh and marine waters when DOM is exposed to solar radiation.

The role of such indirect effects in ambient waters for the aquatic biota is not settled. Experiments with high UV-doses indicated negative effects on phyto-plankton but no effect on zooplankton (Daphnia) in water with high levels of DOC [65]. UV exposure experiments along a gradient of DOC (humus) with spectral distribution and doses close to natural outdoor maximum surface irradiation clearly suggested an overall net positive effect of DOC, and no detectable negative indirect effects either on phyto- or zooplankton [66]. This could suggest that UV mediated effects of radicals and strong oxidants in the water are minor compared with direct, intracellular damage. Borgeraas and Hessen [67] did, however, observe an increased catalase activity in Daphnia with increased DOC and increased UV, suggesting an enhanced need for peroxide detoxification. It should be recalled that most of the photo-products are very short-lived, and it is difficult to separate intracellular from extracellular effects during UV exposure even experimentally, since exposure to pre-irradiated water will not capture the effects of short-lived photoproducts. Knowledge of the biological role of this multitude of photoproducts is still scarce, but it is definitely a potential stressor that is not shared with terrestrial biota.

12.3.2 Food web effects

Obviously any organism, even those rather insensitive for UVR, may be positively or negatively affected to the extent that other members of the aquatic community are affected. Even harmful effects of UVR on a species may turn into net positive effects provided that the major competitor or predator is more adversely affected - and vice versa. UVR tolerant grazers of predators may suffer if preferred food organisms are vulnerable to UVR [62]. The complexity of most food webs does not allow for a clear judgement of such effects, yet there are some general observations indicating that such effects could be profound.

For zooplankton-phytoplankton interactions several different mechanisms could operate; first the phytoplankton productivity could be reduced, and there could be community shifts owing to different UV-susceptibility among different species or taxa [68-70] or cells could be morphologically or biochemically altered, causing reduced nutrient quality for the grazers [71-73]. This first set of mechanisms is related to direct effects on the autotrophs, and will not extensively be covered here. It should be emphasized, however, that there are a number of rather subtle indirect mechanisms that could affect various trophic levels via the food web. While both phytoplankton community structure [69,70] as well as cell membranes [71] may alter both ingestion and assimilation of algae, an even more important mechanism could be caused by the fact that, even at low dose rates, the total lipid content of some microalgae may be reduced [74] and this effect includes the polyunsaturated fatty acids (PUFAs) [71,75,76]. For zooplankton and fish larvae, the only source of these key fatty acids for membrane functioning, growth and development is dietary. Since they cannot synthesize them de novo, they must be obtained through prey organisms [e.g., 77,78].

Finally, the grazers themselves may be affected, causing reduced grazing pressure on phytoplankton. Such effects have rarely been studied, yet there is evidence that UVR may reduce grazing activity in marine [79] and freshwater [80] nanoflagellates. Laboratory experiments demonstrated a pronounced decline in grazing rates shortly after UVR exposure in Daphnia, and a gradual post-treatment recovery after several hours (Figure 5).

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