Effects of UVR on microphytobenthos photosynthesis

Although benthic microalgal communities include the same major algal taxa as the pelagic communities, there is a crucial difference. The density of autotrophic, as well as heterotrophic microorganisms is several orders of magnitude higher in benthic communities, resulting in microbial mats or biofilms. These are characterized by steep physical, chemical and biological gradients, leading to a close spatial and temporal coupling of turnover processes within the mat system [149]. Thus, it can be expected that the responses to UVR of these communities are rather complex.

11.4.1 Short-term effects

Decreased photosynthetic rates (measured as 14C uptake, oxygen production, or chlorophyll fluorescence) appear to be the most frequently observed short-term effect for MBP, particularly at enhanced UV-B levels [15,47,57,64,65,68,120]. These results are, however, ecologically relevant only when realistic, moderate increases of UV-levels are used. Wulff et al. [68] found a 50% decrease in 14C-uptake of MPB on sand when UV-B was increased by 15% above ambient (23% when biologically weighted according to Cullen et al. [106]), though only under nutrient-depleted conditions. Using oxygen microsensors, Bebout and Garcia-Pichel [15] found a dramatic (50-90%) decrease in gross photosynthesis of the surface layers of a cyanobacterial mat (Microcoleus chthonoplastes) under moderate UV-B irradiances (0.35-0.79 W m-2). This decrease was also related to an active downward migration in response to UV-B. Non-invasive fluorescence measurements on natural diatom biofilms (dominated by Gyrosigma balticum) exposed to supplemented UV-B (7 and 15% above ambient) resulted in a sequence of responses, starting by significantly increased effective quantum yield ^ps ii (probably reflecting downward migration), followed by a reduction in maximum quantum yield of PS II (Fv/Fm) and minimal fluorescence (F0) [65].

Observed short-term responses of MPB photosynthesis to ambient UV levels are less clear-cut, when compared with supplemented UV-B, and appear to vary with substrate type and community density. For a muddy sediment, no significant effects of ambient UV-B on either carbon uptake or oxygen microprofiles of a diatom mat (dominated by large motile species) were found [46]. In a sandy sediment, on the other hand, both carbon uptake and allocation of a community dominated by small (mainly non-motile) diatoms and cyanobacteria decreased significantly under ambient UV-B, although only at the end of a 3-week experiment [47]. Finally, no effects of ambient UV-B on photosynthetic rates were observed in freshwater stream periphyton [150].

11.4.2 Long-term effects

Growth, measured as the accrual of biomass or chl-a, is the variable most often studied for long-term UV-B effects on MPB. The clearest negative effects of UV-B on MPB growth have been observed for periphyton colonizing artificial substrata [27,28,58,121,151]. However, in the majority of these experiments, significant negative effects on growth (30-100% decrease) were only found during the first few weeks. After this UV-inhibition phase, statistically significant negative effects disappeared, or were even reversed [25,26,58,121,152] (note that some of these studies excluded both UV-A and UV-B). In one case, the explanation for the reversed effect was the higher sensitivity of grazers than their prey [58,153, see Chapter 15]. When experimenting with already established periphyton communities, however, no detrimental effects were observed [150,154].

The effect of UV-B on the growth of natural, established MPB communities inhabiting marine sediments shows a different general pattern than the above-mentioned colonization experiments. In sediments, significant effects appear to be fewer, they are more frequently found for rate variables (photosynthesis, C-allocation) than for state variables (biomass, pigment and species composition), and they occur later during the experiment (after 1-2 weeks) (Figure 3) [29,47,64]. The delay of effects may partly be due to increasing nutrient limitation in the course of the experiments caused by the experimental set-up, particularly when working with sandy sediments, which are generally poorer in nutrients than fine sediments (see [47] and section 11.8.1). However, an intriguing question is why the observed effects on rate variables were not reflected more clearly in the state variables? Besides biological reasons, there could be methodological reasons. For example, if a deeper sediment layer than the actual layer affected by UV-B is sampled, there might be a "dilution" of effects (see further details in Wulffet al. [29]).

Peletier et al. [119] concluded, from laboratory experiments with diatom species isolated from intertidal sediments, that ambient (or even future increased) UV-B is unlikely to affect sediment-inhabiting MPB. Although it may appear that experiments on intact sediment MPB do at least partly support this conclusion [46], they do not fully rule out the role of (even ambient) UV-B as a controlling factor [47,65,68], Odmark et al. [47] found that, while the removal of UV-B created a response, moderate enhancement of UV-B had less obvious effects. This suggests that ambient UV-B can indeed be a factor exerting a selective pressure on MPB particularly in sandy sediments, whereas an increased UV-B exposure due to ozone depletion would not severely affect the type of MPB community studied. However, given that UV-B, at present level, is a selective force in MPB communities of sandy sediments, there is no a priori reason to assume that the communities should respond to a less degree to an increase in UV-B levels. Moreover, early successional growth phases of sediment communities are indeed, like periphytic communities, susceptible to moderately enhanced (15%) UV-B levels [68]. Epipelic communities on sediments of oligotrophy lakes have also shown a significant response to ambient UV-B [26],

11.4.3 Are UV-B effects on microphytobenthos habitat-specific?

The benthic habitats in the above-cited experiments differ in several aspects, such as the type of substratum, water movement and nutrient status. Also the community properties, such as the level of productivity and composition of the food webs (e.g., importance of grazers) vary greatly. Are UVR responses then habitat-specific? The answer appears to be yes, although similarities in responses also exist. When MPB on both hard and soft substrata (and phytoplankton) were studied simultaneously in alpine oligotrophic lakes in Canada [26,121], it was found that attached periphyton was affected by ambient UV-B, while epipelon of the sediment and phytoplankton remained unaffected. On the other hand, Vinebrook and Leavitt [154] found that ambient UVR had no effect at all on epilithon, while a significant stimulating effect was found for epipelon. This contradicting result was explained by the fact that established epilithon, and not early successional stages, were studied in the latter experiment. An indication of the importance of the substratum type was also found for sediment MPB in a microtidal brackish-water area in Sweden. More variables were affected by UV-B in the sandy than in the muddy sediment [46,47,64]. This suggests that muddy sediments may function as a refuge for MPB (shallow photic zone, dominance of motile diatoms), while in the sandy sediment UVR penetrates deeper and the MPB is dominated by small-sized attached species.

Among community properties, grazing pressure is obviously an important controlling factor for the susceptibility of the MPB to UV-B. Heavily grazed periphyton communities become thin and are thus more sensitive to UV-B [120]. On the other hand, if the grazers are more sensitive to UV-B than the algae, the algal growth will benefit from UV-B [153, Chapter 15].

On a larger geographical scale, climate, latitude/elevation, and general nutrient status of the ecosystem may explain differences in the UV-B responses of MPB. As discussed before, early colonization stages of MPB are more susceptible to UV-B than already established, thick communities. Thus, UV-B can be expected to be a more important controlling factor in for example a cold climate where colonization events are more frequent due to ice and scouring [121]. Similarly, freshwater periphyton in mid-latitudes [150,155] may be more resistant to current UV-B stress than periphyton communities of higher latitudes [26,58]. The combined effects of climate warming and increased input of dissolved organic matter (DOM) have been suggested to moderate the effects of U VR-increase in alpine oligotrophic lakes [26]. However, climate change in combination with acidification may also increase the exposure of organisms to UV-B, particularly in clear, shallow lakes and streams [156].

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