Relative UVB exposure

2360 8P

6730 BP

6730 BP

10700 yr BP

10700 yr BP

Figure 4. Changes in UVR exposure (shaded) and total algal abundance (line) in Big (a), Burnell (b), and Valentine lakes (c), British Columbia, Canada [27], UVR exposure and total algal abundance as in Figure 3, except for scale change. Approximate chronological control of Burnell and Valentine lakes was provided by volcanic ash and basal clay-organic contact layers only, and by 14 accelerator mass spectrometric (AMS) determinations of 14C activity and sediment age in Big Lake [27, not shown here]. Comparison among lakes demonstrates that algal biomass is low when UVR penetration is great, and that such periods occur immediately following deglaciation in all cases (see also Figure 3).

[Figure modified from 27.]

irradiance regimes in lakes [11].

In a similar study, Saulnier-Talbot et al. [25] used a diatom-based paleo-optical approach to estimate past depths of UVR penetration in a coastal basin (Lake Kachishayoot) following its isolation from the marine waters of Hudson Bay in subarctic Québec, Canada. Prior comparisons of optical environments in coastal systems have revealed that shifts from marine to freshwater conditions are accompanied by increased DOC, changes in C-specific UVR attenuation and declines in UVR penetration [39]. Consistent with these modern analyses, abrupt increases in diatom-inferred DOC concentrations and water color coincided with the retreat of postglacial marine waters and arrival of spruce trees within the local catchment [25]. Their analyses also revealed large changes in the underwater irradiance environment over the course of the post-glacial period, from extremely high UVR exposure after the initial formation of the lake and its isolation from the sea, to an order-of-magnitude lower exposure following development of spruce forests in the catchment. Interestingly, the use of additional macrofossil markers allowed investigators to show that UVR penetration remained high even following development of alternative DOC sources such as Sphagnum mats. These results further support the hypothesis that development of local conifer populations represents the critical step altering spectral irradiance characteristics of northern lakes.

16.4.3 Historical changes in polar UVR flux

High latitude aquatic ecosystems may be particularly susceptible to UVR both because of extremely low concentrations of photo-protective DOM and because of natural and anthropogenic mechanisms that lead to stratospheric ozone depletion [21]. For example, in the Antarctic, strong westerly circulation each winter causes a circumpolar vortex that isolates part of the stratosphere, allowing it to cool and subsequently form thin high clouds that contain chlorine (CI) and bromine. These elevated concentrations of active CI, mainly derived from chlorofluorocarbons (CFCs), are known to catalyze the reaction of ozone to molecular oxygen (203 + 2C10 -»• 302 + 2C10), leading to the spring ozone hole. Thereafter, the stratosphere warms and the polar vortex breaks up, allowing the 03-depleted stratosphere to mix with mid-latitude air and replenish polar 03 concentrations. Depletion of stratospheric ozone is currently estimated at 4-6% per decade over northern mid-latitudes [33,113], and is expected to increase fluxes of UV-B at least until the middle of the 21st century [34].

Despite sophisticated understanding of the anthropogenic mechanisms that regulate ozone depletion, little is known of the causes or magnitude of natural variations in UVR flux in polar regions. Fortunately, several factors may make high latitude lakes particularly amenable to reconstruction of past irradiance regimes. First, many lakes have no terrestrial source of UVR-absorbing DOM within their catchments [e.g., 21,114], therefore post-glacial changes in UVR exposure would be expected to arise solely from changes in solar production, atmospheric transmission or lake depth. Second, as a consequence of their shallow nature and low thermal capacity, ponds and shallow lakes are ice-free for part of the summer thereby allowing biological responses to changes in irra-diance environments. Finally, these small systems have often accumulated sediments and fossils continuously throughout the Holocene, particularly in more moderate climates of coastal regions [115,116]. In particular, diatoms and pigments are often well preserved, probably because the sites are frozen completely and biological inactive for much of the year. As a consequence of these factors, investigators have recently begun exploiting polar sedimentary deposits to better understand past variability in UVR environments.

Surveys of shallow lakes and ponds in eastern Antarctica have revealed that modern sedimentary environments preserve fossil pigments and that the abundance of photo-protective compounds increases as a function of algal exposure to UVR (Hodgson et al. unpublished). For example, the Larsemann Hills region (69°23' S, 76°53' E) is the second largest of four major ice-free oases on the east coast of Antarctica and contains more than 150 lakes and ponds. Minimal cloud cover, very transparent waters (Kduv_B 0.21-0.35 m-1; [117]), and a 2-3 month ice-free season allow UV-B penetration beyond maximum lake depth and select for filamentous cyanobacteria mats [118] that produce both MAAs and scytonemin to reduce UVR impacts [119]. A survey of 70 lakes and ponds has revealed that photo-protective compounds are abundant in benthic communities of shallower lakes (< 6 m depth). Although MAAs were found to be degraded in the first few centimetres of sedimentary deposits, scytonemin was well-preserved for 1000s of years, as oxidized (yellow-green), reduced (red) forms and partly degraded forms [105].

Preliminary analysis of 14C-dated Holocene cores suggests both that UVR flux was greater prior to ~4000 yr BP and that historical variability in UVR exposure was expressed on a variety of temporal scales (Figure 5). In this study, scytonemin and its derivatives were summed and expressed as a ratio with algal carotenoids, an index which has been shown to be linearly related to the depth of UVR penetration when calculated using similar compounds [24,26]. In addition, scytonemin was expressed as a function of both total xanthophyll carotenoids, compounds which disperse excess heat energy during photosynthesis [120], and total chls to roughly estimate the proportion of metabolic effort expended by cells on photo-protection versus production. Although the magnitude of historical change varied among indices, in all cases photo-protective pigments were relatively more abundant in sediments older than ~4000 years, particularly compared with those deposited between ~2000 and ~700 yr BP. While the absence of modern increases in UVR exposure suggests that recent ozone depletion has not greatly altered natural variation in UVR flux, the low resolution of these core analyses may preclude the detection of very recent events. However, regardless of the chronological resolution of the core, it appears likely that biotic exposure to UVR has varied by at least 400% during the past 12 000 years, with ancient levels possibly reflecting increased solar production arising from long-term changes in planetary orbit. Further paleoecological research is being conducted to determine whether changes in exposure arise from variation in UVR flux to the lake surface, or from changes in lake characteristics (transpar past UV-B exposure in Antarctica

depth (cm)

Figure 5. Historical change in past UVR exposure at Larsemann Hills, Antarctica (69°23'S, 76°53'E), during the past 12 500 years [Hodgson et al. unpublished data], UVR exposure estimated as in Figure 3, using ratios of scytonemin : total chlorophylls (- - -), scytonemin: xanthophyll carotenoids (---) and scytonemin: total carotenoids (solid line).

Similar indices have been shown to be linearly related to the depth of UV-B penetration (as 1% surface irradiance) based on whole-lake experiments [24,26]. All pigment and derivative concentrations were quantified using high-performance liquid chromatography and mass spectrometry [68]. Sediment age was estimated from three accelerator mass spectrometric determinations of 14C activity. Analyses suggest that algal exposure to UVR was at least two-fold greater prior to ~ 4000 yr BP than at present, and that UVR exposure varied ~400% during the Holocene.

depth (cm)

Figure 5. Historical change in past UVR exposure at Larsemann Hills, Antarctica (69°23'S, 76°53'E), during the past 12 500 years [Hodgson et al. unpublished data], UVR exposure estimated as in Figure 3, using ratios of scytonemin : total chlorophylls (- - -), scytonemin: xanthophyll carotenoids (---) and scytonemin: total carotenoids (solid line).

Similar indices have been shown to be linearly related to the depth of UV-B penetration (as 1% surface irradiance) based on whole-lake experiments [24,26]. All pigment and derivative concentrations were quantified using high-performance liquid chromatography and mass spectrometry [68]. Sediment age was estimated from three accelerator mass spectrometric determinations of 14C activity. Analyses suggest that algal exposure to UVR was at least two-fold greater prior to ~ 4000 yr BP than at present, and that UVR exposure varied ~400% during the Holocene.

ency, depth, ice cover).

Unpublished analyses of fossil pigments in Arctic ponds also suggests that UVR flux declined after ~4000 yr BP (Figure 6; Leavitt et al. unpublished data). Further, comparison among fossil markers suggest that variance in UVR exposure arose from changes in atmospheric processes rather than from variation in lake properties. Our analysis is based on a 150 cm (of 223 total) sedimentary sequence recovered from Col Pond on Cape Herschel, Ellesmere Island, Canada (78°37'N, 74°42'W), a primary reference region for high Arctic research [115]. Col Pond occupies the central plateau of Cape Herschel, is the first pond to thaw, and is the least nutrient-rich of local sites. Further, Col Pond has been subject to previous biological [114] and paleoecological analyses [115] which show that diatom communities had been stable for at least 4000 years, but that community composition had altered during the past ~150 years due to human activity. Because diatom composition is extremely sensitive to both changes in lake depth [reviewed in 121] and DOC [reviewed in 79], the absence of community vari-

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