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Ecosystems

Freshwater

The succession of periphytic and limnic algal communities is controlled by a complex array of external conditions, stress factors and interspecies influences (Rai et al., 1996). Freshwater ecosystems have a high turnover and the success of an individual species is difficult to predict but the development of general patterns of community structure follows defined routes (Biggs, 1996). Even though transparency for solar UV-B is considerably lower than in oceanic waters, increased solar UV-B is an additional stress factor which may change species composition and biomass productivity (Williamson, 1995, 1996; Häder and Häder, 1997; Piazena and Häder, 1997). The interaction of UV-B and heavy-metal concentrations resulted in synergistic inhibition of nutrient uptake, enzyme activity, carbon fixation, ATP synthesis, and oxygen evolution in a number of phytoplankton species (Rai et al., 1996; Rai and Rai, 1997). Sixty-seven freshwater species of algae (Chlorophyta and Chromophyta) were screened in an experiment to determine their UV-B sensitivity (Xiong et al., 1996). The algae were selected to represent different ecosystems ranging from high-altitude lakes to thermal springs. The most sensitive species lost 3050% of their oxygen-evolving capacity during a 2-h UV-B exposure (2 Wm-2). Many UV-B resistant species were found in high mountain locations. They often have solid cell walls encrusted with sporopollenin. In another experiment the effects of solar UV-B on growth and species composition were studied in an exclusion experiment in a high-altitude mountain lake (Halac et al., 1997). In this study no significant differences were found between the control (full sunlight) and the UV-B depleted enclosure. However, it should be mentioned that UV-A also has been found to affect growth and photosynthesis (Kim and Watanabe, 1994). In other organisms UV-A had a beneficial effect, partially counteracting UV-B inhibition (Quesada, 1995). In addition to the primary producers, the significance of heterotrophic picoplankton in freshwater ecosystems needs to be taken into account (Sommaruga and Robarts, 1997).

    The results of an experiment by Bothwell et al. (1994) reinforce the view that predictions of responses by ecosystems to elevated UV-B exposure should not be based solely on single-species assessments. As reported, greater algal growth occurred in an artificial stream under UV-B exposure than in the control, after some lag time. The explanation of this surprising (at that time) result was that the grazers, larval chironomids, were more sensitive to UV-B radiation than their food, the algae.

The Antarctic Aquatic Ecosystem.

Productivity in the Southern Ocean is characterized by large scale spatial and temporal variability (Sullivan et al., 1993; Arrigo, 1994: Smith et al., 1998). This makes it difficult to filter out UV-B specific effects from other variable environmental effects (Neale et al., 1998a), or to estimate the impact on single species or whole phytoplankton communities (Karentz and Spero, 1995; Davidson et al., 1996). Especially at high latitudes, variability in solar elevation, cloud cover, deep vertical mixing and the cover of ice and snow significantly confound field results of UV-B effects on phytoplankton and the consequent interpretation of these results. With increasingly complete observations, recent estimates of the effect of 50% ozone reduction on integral water column productivity are relatively consistent, <5% (Boucher and Prezelin, 1996) and 0.7-8.5% (depending on BWF, assumed mixing regime and cloudiness, Neale et al., 1998b), with earlier estimates (6%, Smith et al., 1992).

    Observations by many workers, which vary greatly in both time and space, show convincing evidence of UV-B damage to phytoplankton, but in order to determine long-term effects acclimation and adaptation phenomena (Villafane et al., 1995; Lesser, 1996; Helbling et al., 1996) as well as other factors (Neale at al., 1998a) need to be assessed. Several models have been developed (Arrigo, 1994; Behrenfeld et al., 1994; Broucher and Prezelin, 1996; Neale at al., 1998a) to permit estimate of ecosystem productivity loss based on short-term observations. While it has long been known that vertical mixing is a major complication in attempting to quantify UV-B effects on phytoplankton, only recently have the interactive effects of ozone depletion and vertical mixing on photosynthesis of Antarctic phytoplankton been modeled (Neale et al., 1998a). Field results of these workers (Neale et al., 1998b), in agreement with others (Smith et al., 1992; Helbling et al., 1994; Vernet et al., 1994), clearly demonstrate that photosynthesis of Antarctic phytoplankton is inhibited by ambient UV during incubation in fixed containers. The difficulty comes in the generalization of these experimental results to Antarctic waters where mixing significantly alters the exposure of phytoplankton to UV-B. To estimate this environmental influence, Neale and coworkers (Neale et al., 1998a) have developed a model of UV-influenced phytoplankton during vertical mixing. They find that near-surface UV strongly inhibits photosynthesis under all modeled conditions and that inhibition of photosynthesis can be enhanced or decreased by vertical mixing, dependent upon the depth of the mixed layer. Further, they show that an abrupt 50% reduction in stratospheric ozone could, as a worst case, lower daily integrated water column photosynthesis by as much a 8.5%. Note, that this modeling result is consistent with the results of Smith and coworkers who specifically targeted the marginal ice zone (MIZ), where meltwater provides stability and minimizes vertical mixing, for their studies. However, Neale and coworkers also note that inhibition associated with realistic environmental variability can have a stronger influence on integrated water column photosynthesis than UV-B effects: vertical mixing by about ±37%, measured variable sensitivity of phytoplankton to UV about ±46%, and cloudiness about ±15%. These workers conclude "that ozone depletion can inhibit primary productivity in open waters of the Antarctic, but that natural variability in exposures of phytoplankton to UV, associated with vertical mixing and cloud cover, has a major role in either enhancing or diminishing the impact on water column photosynthesis". They also note that "regardless of these natural interactions, UV is a significant environmental stressor, and its effects are enhanced by ozone depletion".

The Arctic Aquatic Ecosystem

Though being in a similar situation of increasing UV-B stress as the Antarctic aquatic ecosystems, the Arctic differs in many respects from its antipode (Weiler and Penhale, 1994; Wängberg et al., 1996). The Arctic ocean is a nearly closed water mass with limited water exchange with the Atlantic and Pacific oceans. It represents 25% of the global continental shelf and receives about 10% of the world river discharge. This considerable freshwater inflow causes pronounced stratification year round and is responsible for high concentrations of particulate and dissolved organic carbon (POC and DOC), which strongly affect the penetration of solar UV into the water column. The plumes of major rivers can be traced several hundred kilometers (Burenkov, 1993). Another difference between the Arctic and the Antarctic is the greater importance of macroalgae in the Arctic. The Arctic aquatic ecosystem is one of the most productive ecosystems on earth and is a source of fish and crustaceans for human consumption. Both endemic and migratory species breed and reproduce in this ocean in spring and early summer, at a time when recorded increases in UV-B radiation are maximal. Productivity in the Arctic ocean has been reported to be higher and more heterogeneous than in the Antarctic ocean (Springer and McRoy, 1993). In the Bering Sea, the sea edge communities contribute about 4050% of the total productivity. Because of the shallow water and the prominent stratification of the water layer the phytoplankton may experience relatively high levels of solar UV-B. In addition, many economically important fish (e.g., herring, pollock, cod and salmon) spawn in shallow waters where they are exposed to this increased solar UV-B radiation when ozone is depleted. Many of the eggs and early larval stages are found at or near the surface. It is possible, given the general relationships between primary and fish production, that reduced productivity of fish and other marine crops would afffect no only humans in the region but also natural predators (orrers, seals, foxes, ice bears). However, further careful analysis is necessary to quantify UV-B related phytoplankton inhibitation and possible affects on the flow of energy to higher trophic levels. Currently we cannot accurately estimate if ozone-related impacts will, or will not, influence fish and other important marine crops.

    The high concentrations of humic substances, which tend to be strong absorbers of UV-B radiation, may alter the underwater light penetration significantly (Wängberg et al., 1996). On the other hand, UV-B is known to photochemically attack humic substances altering the absorptive nature of the water column and leading to faster uptake by bacteria and heterotrophic nanoflagellates (Wängberg et al., 1996). The problem is more complicated and not well understood since UV-B has been found to be more detrimental for small phytoplankton organisms (Karentz et al., 1994) and even more so for the bacterioplankton (Herndl, 1997). In contrast, a recent study of size fractionated phytoplankton in a lake indicated that cells larger than 2 µm were twice as sensitive to solar UV-B than smaller cells (Milot-Roy and Vincent, 1994). The Arctic ocean is often nutrient limited, especially with respect to the inorganic nutrients such as nitrogen and phosphorus. The nitrogen cycle governs the primary productivity of the marine ecosystems. The same is true for the oligotrophic lakes and streams. Nitrogen and phosphorus uptake are UV-B sensitive (Döhler, 1992) which may augment the UV-B sensitivity of Arctic phytoplankton communities. Low doses of UV-B increase the uptake of phosphate, which is probably used for DNA repair, while it impairs the uptake at higher doses. All these effects have an impact on the biogeochemical cycles.


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