UVR competition and changes in species composition

In a pioneering study, Jokiel [6] observed that the UV-tolerant branching sponge Callyspongia diffusa replaced the UV-sensitive sponge Zygomycale par-ishi in shallow (< 3 m depth) reefs of Kaneohe Bay, Hawaii. He hypothesized that metabolic costs to obtain UV tolerance could place species at a competitive disadvantage in shaded environments, but on the other hand could offer a selective advantage in competition for space in sunlit areas. In a simple and elegant experiment, Jokiel [6] tested whether UV tolerance in C. diffusa offers a competitive advantage against Z. parishi in the presence of UVR. After 7 days, Z. parishi grew over C. diffusa in both experimental setups. However, while in the UV-shielded treatment the median tissue overgrowth was 7 mm, it was only 1 mm under full solar radiation. Within 2 months, Z. parishi overwhelmed C. diffusa in the UV-shielded treatment, but the latter species remained healthy in the UV-exposed one. Although this study did not test wavelength-specific effects of solar UVR on competition, it identified probably for the first time the importance of UVR as an environmental variable potentially affecting species competition in aquatic ecosystems.

One often expected effect of increased levels of incident UV-B radiation in aquatic ecosystems is a change in species composition particularly, of primary producers [7-10]. The rationale behind this hypothesis is based on the different sensitivity to UVR found among species of planktonic and benthic algal communities (see Chapter 11). Changes in species composition within a community are hypothesized to occur by replacement of UV-sensitive species by resistant ones, which occupy similar (trophic) niches [7,9,10]. A change in species composition can take place directly if the population of a UV-sensitive species does not survive to UVR levels above its tolerance threshold or indirectly if outcompeted by more tolerant species. There is strong evidence from several studies in marine and freshwater systems indicating that UVR plays a major role in shaping the structure of communities during the early colonization and succession of many aquatic habitats/ecosystems through selection against less UV-tolerant species [11-18]. However, could enhanced levels of incident UV-B radiation per se lead to the extinction of species in an established (mature) community? Or is it more probable that enhanced (ambient) UV-B levels could cause changes in community structure by an alteration in the population size of UV-sensitive and more resistant species as a consequence of different growth rates and competition [19]? Testing the first hypothesis (i.e., extinction) in natural communities is difficult because generally there is a lack of references to what to compare present population/community structure, particularly in places like Antarctica where enhanced UV-B levels have been experienced during the austral spring for more than 20 years. Thus, most studies, except those following a paleo-approach, have tested whether UV-B radiation offers a competitive advantage to tolerant species in long-term micro/mesocosm experiments where UV-B has been excluded and/or artificially enhanced.

Table 1 presents a summary of studies done to test the hypothesis of changes in taxonomic composition in phytoplankton (for which most information is available). The data to address this hypothesis are of uneven quality and the studies differ in experimental design, sophistication level, and statistic strength, so their interpretation in some cases is problematic. Thus, for example, when grazers were present but their abundance was not controlled or their food spectrum not assessed, their effect on changes in phytoplankton species composition will be difficult to discern from those potentially caused by UV-B radiation. Moreover, an additional limitation in the methodology used in exclusion experiments is the distinction between UV-B and UV-A effects. Separation between the effects of these wavebands is generally accomplished by the use of the polyester foil Mylar D (DuPont de Nemours & Co. Inc.). This material, however, cuts off only part of the biologically effective UV-B radiation. For example, when the transmittance of Mylar D (23 jum thickness, 50% transmittance at 316 nm) is multiplied by the solar spectrum for ~40°N latitude near summer solstice, it cuts off 60% of UV-B (<320 nm) or only 56% of the biologically effective radiation when the biological weighting function for Daphnia pulicaria mortality is used [20]. Furthermore, this value will change depending on the thickness of Mylar D used, which is seldom reported in the experimental design although it strongly affects the cut-off wavelength in the UV-B range.

One of the first studies on this topic was done by Worrest [21] with estuarine phytoplankton (Yaquina Bay, Oregon) exposed in small microcosms (15 L, depth: 0.30 m) to natural solar radiation of wavelengths >380 nm plus enhanced UV-B radiation. The phytoplankton dominated by diatoms changed after 4 weeks (sampling was done only at the beginning and the end of the experiment) with an apparent increase in the dominance of Chaetoceros sp. and a decrease of Skeletonema costatum. Similar findings have been reported in two studies with phytoplankton from the Gullmar Fjord, Sweden, exposed in small aquaria (18 L and 40 L, depth: 0.23-0.49 m) to artificial UVR or solar radiation plus enhanced UV-B [22,23]. In contrast, in two experiments done in the west coast of Sweden (Gullmar Fjord) with large enclosures (6 m3, depth: 3.5 m) shifts in phytoplankton species composition were not observed, even in the enhanced UV-B treatments [24]. In microcosm experiments using small containers (1 L) with phytoplankton cultures isolated from Seal Island (Antarctica), changes in taxonomic composition were observed only when exposed to high solar UVR fluxes typical

Table 1. Summary of studies using microcosms or mesocosms to investigate changes in taxonomic composition of phytoplankton caused by

UVR; see text for more details on each study

Container

Table 1. Summary of studies using microcosms or mesocosms to investigate changes in taxonomic composition of phytoplankton caused by

UVR; see text for more details on each study

Container

Habitat

Exposure conditions

volume (L)

Duration (d)

Changes

Comments

Reference

Estuarine

solar radiation + enhanced UV-B

15

28

yes

natural assemblage

[21]

Estuarine

artificial UVR

18

7

yes

natural assemblage

[22]

Estuarine

solar radiation + enhanced UV-B

40

10

yes

natural assemblage

[23]

Estuarine

solar radiation + enhanced UV-B

6000

8-11

no

natural assemblage

[24]

Marine

solar radiation

1

5-16

yes

mixed cultures

[25]

Marine

solar radiation

2

15

yes

natural assemblage

[26]

Marine

solar radiation

0.5

8

yes

6 co-occurring species

[27]

Freshwater

solar radiation

1000

16

no

natural assemblage

[29]

Freshwater

solar radiation

300

30

no

natural assemblage

[13]

Freshwater

solar radiation+enhanced UV-B

20000

44

no

natural assemblage

[30]

Freshwater

artificial UVR

600

56

no

natural assemblage

H 00

o on for the tropics, but not under ambient UV irradiances [25]. In another study with surface phytoplankton collected in Arthur Harbor, Antarctica, and exposed to solar radiation in 2 L flasks, important changes in taxonomic composition were observed already after 4 days [26]. In the presence of UYR, the original assemblage composition dominated by flagellates (their experiment #2) shifted by day 13 to the dominance of diatoms. Davidson et al. [27] performed competition experiments in nutrient-rich media using six co-occurring phytoplankton species isolated from the Southern Ocean that were exposed in small bags (0.5 L) to natural solar radiation in an outdoor tank. Their results indicated that overall growth and production by this artificial community was not affected by UVR. However, UV-B caused changes in the growth rate of some species. Thus, for example, growth rate of four diatom species did not change significantly but that of the flagellate stage of Phaeocystis antarctica decreased in the presence of UY-B. On the other hand, the growth of the colonial form of P. antarctica was enhanced when exposed to UV-B. Changes in species composition were elicited after 2 d exposure and by day 8 the proportion of the colonial form of P. antarctica increased mainly at the expense of Chaetoceros simplex, although extinction was not observed. These results contrasted with previous studies by Karentz [9] and Karentz and Spero [28] showing that growth of the colonial form of Phaeocystis sp. declined in the presence of UV-B radiation. In a field study at the marginal ice zone of the Bellinghausen Sea, Phaeocystis populations appeared to be negatively affected by increased levels of UV-B during the "ozone hole", but this did not offer a competitive advantage to co-occurring diatoms species [28].

Results from an experiment with 1 m3 enclosures (depth: 0.95 m) in a transparent alpine lake from the Austrian Alps indicated no significant differences in species composition after 16 days between the UV-B-shielded and -exposed treatments [29]. Although there were important changes in the proportion of co-occurring species, for example, a decrease in the chrysophyte Chromulina sp. and an increase in the chlorophyte Dyctiosphaerium sp., the change in dominant species was not caused by UV-B radiation. In another alpine system (Pipit Lake, Canada), no changes in species composition in phytoplankton assemblages, consisting mainly of picocyanobacteria, chrysophytes, cryptophytes, and dino-flagellates, were observed during a 30 days enclosure (0.3 m3, depth: 0.7 m) experiment where UV-B was excluded [13]. Experiments with large enclosures (20 m3, depth: 1 m) placed in the littoral zone of mesotrophic Jack's Lake, Canada, showed no evidence for collapse of specific phytoplankton populations or any large-scale taxonomic shift under ambient, UV-B-excluded, or -enhanced treatments [30]. In another long-term experiment (8 weeks) with indoor microcosms (600 L) receiving artificial UVR, no effects of UV-B radiation on species composition, abundance or biovolume of phytoplankton (and other planktonic and benthic communities) was observed [31].

Certainly, a small database as the one presented above may lead to generalizations that are not correct. However, three factors that appear to explain the contrasting results of these studies are the variations in UVR transparency of the water in which the experiments are done, the prior exposure regimes of the species in their place of origin, and the size of the experimental container used in the studies. The growth rate of phytoplankton species originating from sunlit habitats appears to be less or not at all affected when exposed to UV-B radiation [25,32, see also 33 for a review on other photosynthetic organisms]. Thus, exclusion of UV-B radiation or even its enhancement may not offer a competitive advantage to species already adapted to high solar UV-B irradiances. Beside the obvious disadvantages of the enclosure approach, like, for instance, the elimination of advection and diffusion, the size of the enclosure has a major effect on natural avoidance mechanisms, such as vertical displacement, and on the characteristics of the radiation field experienced by the organisms. Thus, in small-sized enclosures, organisms are exposed to a uniform field of UV radiation due to the short path-length that solar radiation needs to travel before reaching an algal cell [24]. This situation, however, differs largely from natural conditions, particularly in turbid waters (e.g., estuaries) where the water column is characterized by a strong gradient of UV irradiance and spectral characteristics. Together with the absence of mixing that may minimize the UV effect (see Chapter 4) and the long-term (days to weeks) exposure, it is not surprising that significant shifts in species composition caused by UV-B have generally been observed in experiments with small enclosures.

Examples of studies with natural communities of benthic microalgae or with communities of periphyton growing on artificial substrates for long periods are less common. However, in contrast to the dramatic changes observed when pioneer species colonizing new substrates are exposed to UVR, these studies suggest that neither ambient nor enhanced UV-B radiation significantly affects algal species composition [34-37].

Finally, an alternative approach to test the hypothesis of change in species composition has been to look at changes in the dominance of algal species that remain preserved in the sediments, for instance diatoms. In this approach, the main advantage is that the 'historical' reference or initial assemblage structure can be reconstructed in most cases (see Chapter 16). On the other hand, it may be difficult to isolate the effect of UVR from other environmental changes, except when recognition of present UV-sensitive species with a long sediment record is possible. Results by McMinn et al. [38] showed that changes during 20 years (~1971 to 1991) in the relative abundance of diatom taxa analysed in three sediment cores from anoxic basins in Vestfold Hills, Antarctica, were not distinguishable from long-term natural variability. However, as the authors acknowledged, the study was done in a coastal area where a thick ice-cover is present at time of phytoplankton growth and therefore it was not representative of the zone affected by the ozone reduction (see also [39] for other critics on this study). Nevertheless, this approach remains an interesting alternative to explore.

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