Taxa specific responsesevidence for in situ effects of UVR on pelagic metazoans

Over the past two decades there has been a rapidly growing body of literature on UV-effects. A major share of this literature deals with laboratory studies or otherwise artificial exposure or artificial conditions. For obvious reasons, laboratory experiments rarely mimic ambient conditions with regard to water temperature, oxygen or food. Even more troublesome is the fact that spectral qualities, recovery radiation, dose or dose-rate may differ from natural conditions. In particular, the principle of reciprocity of UVR (or lack of such) is commonly ignored or not tested for. Commonly, high intensity over short periods may generate a different pattern of damage than comparatively lower doses over long periods. Also, we are commonly faced with a long list of potential effects, this chapter being no exception, but there are comparatively few studies that really provide evidence for UVR effects in situ. One particular challenge is that while present-day UV may pose both stress and constraints on organism performance, these sub-lethal effects are not easily captured. Life is rarely optimal for organisms in nature, and such effects of UVR will work in parallel with sub-optimal access to food, sub-optimal water quality etc. Sometimes the sum of such sub-optimal parameters becomes lethal. Pelagic organisms have evolved under some UV-stress and hence should be able to tolerate this, but at some costs that can only be quantified by assessment of fitness in the absence of UVR in general or more specifically UV-B. Nevertheless, there are several observations of in situ effects of UVR, and these are particularly important for the final judgement of the ecological role of UVR.

12.5.1 Case studies of UVR and zooplankton

As stated above, the presence of a conspicuous and often costly pigmentation by carotenoids or melanin is strongly indicative of UV as a major evolutionary force. Similarly the vertical distribution and migration of zooplankton in lakes or ponds devoided of predators also supports this view. There is also evidence that geographical distribution in cases may be governed by the UV regimes. In fact Williamson et al. [86] provided evidence that macrozooplankton community structure in a set of lakes along a deglaciation chronosequence in Glacier Bay Alaska could be attributed to the UV attenuation in these lakes. Terrestrial succession in the watersheds of these lakes results in increasing DOC content over time. Due to the primary role of DOC in controlling UV attenuation in lakes, the oldest lakes supported more UV-susceptible species. Previous studies by the same author have demonstrated that UVR effects on zooplankton in general are strongly influenced by DOC-concentrations in lakes [87].

Several other studies have demonstrated detrimental effects of UVR in situ on a variety of zooplankton taxa [30, 60,87-90]. Clearly there are not only major differences among taxa and species, the same species may also show different susceptibility to UVR, suggesting that enhanced tolerance may be induced. Siebeck and Böhm [59] found that natural UVR strongly affected Daphnia species, but that individuals from a clear, alpine locality were far more tolerant compared with individuals from a large lowland lake. Correspondingly Hebert and Emery [16] found surface UVR to be lethal for transparent Daphnia, while melanized clones or morphotypes were largely unaffected. Zagarese et al. [60] reported a remarkable difference in UVR susceptibility even within the same genus of calanoid copepods, and while this study found Boeckella gracilipes to be sensitive for in situ exposure, Cabrera et al. [88] found that this species was highly tolerant in an alpine lake where other species were very sensitive. The use of a solar simulator calibrated to closely mimic natural exposure to UVR yielded intriguing differences among several cladocerans, an ostracod and an amphipod. The 96 h LD50 estimates ranged from 4.2 to 84.0 pW cm"2 [91], with the ostracod Cyprinotus incongruens as by far the most tolerant species. Somewhat surprisingly, the epineustonic Scapholebris kingii was most sensitive to UVR, with no difference between two color morphs.

There is also evidence based on in situ studies that there is a different spectral response among species. For example, Williamson et al. [87] found that while the cladocerans Daphnia and Diaphanosoma responded both to UV-A and UV-B, the sympatric copepod Diaptomus was affected by UV-B only. In one of the few reported studies on pelagic insects, Williamson et al. [92] found that the midge larvae Chaeoborus (which is highly transparent and normally shows a pronounced diurnal migration) not only was very sensitive to UVR in general, but that is responded mostly to UV-A. The fact that some species are very sensitive to UV-A suggests that organelles or other macromolecules other than DNA can be the main targets.

Perhaps the most comprehensive studies on marine Zooplankton and fish were undertaken in a series of experiments where in situ studies on eggs and embryos of atlantic cod (Gadus morhua) and eggs and adult copepods (Calanus finmar-chicus) were combined with experimental studies [2-4,93-95]. One key conclusion from these comparative studies was that the copepod was far more susceptible to UVR than eggs or fry of cod. Both were strongly dependent on Kd, determined by levels of DOC (Figure 6). Eggs of Calanus finmarchicus and Atlantic cod were incubated under the sun, with and without the UV-B and/or UV-A wavebands. UV-exposed eggs exhibited a lower percent hatching compared to those protected from UVR: UVR had a strong negative impact on C. finmarchicus eggs. Further, the percent hatching in UV-B-exposed eggs was not significantly lower than that in eggs exposed to UV-A only, and, under natural solar radiation, UV-A appeared to be more detrimental to C. finmarchicus embryos than was UV-B. The strong effect of UV-A is simply an effect of higher absolute levels of UV-A compared to UV-B. A highly wavelength-dependent mortality was found for C. finmarchicus, with the strongest effects occurring under exposures to wavelengths below 312 nm. At the shorter wavelengths (<305 nm) UV-B-induced mortality was strongly dose-dependent, but not significantly influenced by dose-rate.

The BWFs derived for UV-B-induced mortality in C. finmarchicus were simi

Figure 6. (Top) Kd vs. modeled survival of Atlantic cod (Gadus morhua) embryos exposed to UVR in a mixed water column. (Middle) Kd at 305 nm vs. modelled survival of Calanus finmarchicus embryos exposed to UVR in a mixed water column. (Bottom) Dissolved organic carbon (DOC) vs. diffuse attenuation coefficient (Kd) at 305 nm from field measurements in the estuary and Gulf of St. Lawrence, Canada. The straight line is the regression, the curved lines are the 95% confidence intervals. [Modified from Browman and Vetter 19, with permission from the Estuarine Research Federation.]

Figure 6. (Top) Kd vs. modeled survival of Atlantic cod (Gadus morhua) embryos exposed to UVR in a mixed water column. (Middle) Kd at 305 nm vs. modelled survival of Calanus finmarchicus embryos exposed to UVR in a mixed water column. (Bottom) Dissolved organic carbon (DOC) vs. diffuse attenuation coefficient (Kd) at 305 nm from field measurements in the estuary and Gulf of St. Lawrence, Canada. The straight line is the regression, the curved lines are the 95% confidence intervals. [Modified from Browman and Vetter 19, with permission from the Estuarine Research Federation.]

lar in shape to the action spectrum of naked DNA. Further, the wavelength-dependence of DNA damage was similar to that for the mortality effect. These observations suggest that UV-induced mortality in C. finmarchicus was a direct result of DNA damage. It was concluded that UVR could cause egg mortality as high as 32% under natural conditions [4]. A model that included the BWFs, vertical mixing of eggs, meteorological and hydrographic conditions, and ozone depletion indicated that UV-induced mortality in the C. finmarchicus egg population could be as high as 51% [94]. These values are certainly maximum estimates, but nevertheless points to a potential susceptibility of this key species in northern marine waters. Also, for marine zooplankton species it must be assumed that UVR susceptibility varies over a wide range. For instance zoea larvae of the American lobster (Homarus americanus) were virtually insensitive to natural UVR [96].

While most attention has been devoted to the effects of UVR on crustacean zooplankton (and a few observations on rotifers), there are other members of the pelagic community and evidence exists that pelagic opistobranchs may suffer direct damage under natural conditions [97].

Most of these studies have addressed direct effects and notably mortality, but there are also some reports on more subtle and sublethal effects of UVR on zooplankton such as developmental anomalies in nauplii [3]. Such effects have also been observed in crab larvae and euphausides [53] and juvenile cladocerans (Hessen unpublished) under artificial UVR. Also, general reproductive problems such as decreased number of offspring or skewed sex ratios have commonly been reported under experimental conditions (see Zagarese and Williamson [98] for review), suggesting that severe mutations or distorted embryogenesis probably is an ontogenetic bottleneck with regard to UVR.

12.5.2 Case studies of UVR and vertebrates

There are also several studies that have attempted to verify the potential effects on fish under natural conditions, covering a wide range of effects, from cellular effects such as DNA-damage to skin lesions, cataracts or spawning failure and death [3,99-103], To these add a large number of experiments with artificial radiation. The vulnerability among fish to UVR expressed as sun-burn skin lesions has long been recognized [104]. Such effects have been observed in a variety of fishes in shallow waters [e.g., 105-107], yet some of these observations have been made on fish in captivity and may thus not represent natural conditions. Such effects may expose the fish to pathogen invasions as UVR also depresses the immune system in fish [108,109]. Lens damage (cataract) has also been observed in fish exposed to in situ UVR, but is apparently confined to very shallow waters [110].

A study on UVR susceptibility in cod eggs was undertaken in parallel with the exposure of Calanus finmarchicus reported above [2,4,111]. While the BWFs for cod eggs resembled that of naked DNA, and UVR effects were indeed observed under natural solar intensities, it was nevertheless concluded that cod eggs and embryos were far less susceptible to natural UVR compared with the tested copepods. While a model evaluation suggested that a more than 50% increase in damage could occur under realistic ozone depletion scenarios for Calanus, this was almost negligible for cod eggs or embryos. This does not imply that UVR is irrelevant for cod recruitment, but suggests that indirect mechanisms could be superimposed on the direct effects.

Cod eggs and larvae are damaged by UV-A and UV-B and possess the typical means of repairing DNA damage via photorepair [2,19]. While cod eggs and larvae clearly carry out photorepair, it was found that the capacity for repair would not be adequate for full repair before the onset of new damage on the following day. This low capacity for photorepair can lead to a greater multi-day accumulation of DNA damage than currently observed for temperate fishes [43], There are clearly species differences in rates of DNA damage and repair. These differences may account for why some larvae appear to conform to dose reciprocity while others do not. In the northern anchovy, Engraulis mordax, a baseline level of CPDs remains after the first exposure but in general CPD levels do not accumulate over many days [43]. Anchovy larvae do not obey dose-reciprocity relationships [100,101]. Cod, with a more limited capacity for photorepair, accumulate damage over multiple days and adhere more closely to dose reciprocity under the conditions tested [4].

It is important to point out that variability in cloud cover, water quality, and vertical distribution and displacement within the mixed layer (Chapter 4) are all likely to have a greater effect on the flux of UV-B radiation to which the eggs of zooplankton and fishes are exposed than will ozone layer depletion at these latitudes. Thus, although UV-B radiation can have negative impacts (direct effects) on pelagic animals, it must be viewed as only one amongst many environmental factors that produce the mortality typically observed in the planktonic early life stages of these organisms. For fish species whose early life stages are distributed throughout the mixed layer, it seems most likely that UV-B would represent only a minor source of direct mortality for the population, but sublethal UVR might for instance work in concert with pathogens due to immunosuppression.

With the exception of amphibians, that at least may spend part of their life history in surface water, there is little information on in situ effects of UVR. One could expect that, in particular, populations of polar pinnipeds could suffer both from retinal damage and immunosuppression as a result of an increased number of incidents with low ozone, but this has not yet been confirmed. Amphibians, however, have been the subject of particular interest, since their populations have suffered widespread declines and extinctions in recent decades, and UVR has been suggested as one of the potential causes [36]. It is demonstrated that UVR can cause a variety of damage in natural populations such as egg mortality, retinal damage and altered patterns of distribution [112]. As for other species, the UVR sensitivity is widely different among species. Apparently UVR may work in concert with a range of other ambient factors (drainage, toxins, pathogens) [113], and this is probably how UVR works in general.

Can strong conclusions now be drawn as to the effect of UVR on pelagic metazoans? For some species yes but for most species no. This is still a field where most work is yet to be done, and where, probably, the major effects are rather subtle and sub-lethal. Judging from the general knowledge on UVR and autotrophs, it is reasonable to believe that present day UVR strongly modifies not only productivity and community composition, but also food web interactions. We still know little about adaptations to UVR and their costs, but like the structuring eifect of past competition, present day pelagic food webs correspondingly bear the footprints of the "ghost of UVR in the past".

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