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Socioeconomic
Data and Applications Center
Environmental Effects of Ozone
Depletion 1998 Assessment |
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
30–50% 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 40–50% 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.