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OCR for page 62
Chapter 4
ECOSYSTEMS AND THEIR COMPONENTS
SUMMARY
In order to predict the effects of increased levels of
W-B on natural and cultivated ecosystems, the nature of
the interactions of organisms with environmental vari
ables and the adaptations of organisms to changes in
nutrients, predation, climate, and light must first be
understood. Only then can dose-response relationships
for UV-B effects be established. Because these inter
actions are complex, mathematical models are often used
to express them and to facilitate data handling. The
usefulness of these models is limited, however, by the
information available. Thus, although data are available
on W-B effects on specific organisms and on W -B effects
on life cycle stages of organisms, the relationship
between the effects on the individual and the effects on
the population is not clear.
It appears that the yield of food from domestic
animals will not significantly decrease even with the
most extreme projections of ozone depletion. Economically
important cultivated crops may have reduced yields from
increased W-B levels. Further assessment is needed of
the organismal and cellular properties and adaptations in
plants and in most animals that modify the direct and
indirect effects of UV-B. Such research must be conducted
under carefully simulated field conditions. This capa-
bility exists in a number of laboratories; however, a
subtropical facility would be valuable. Other food,
fiber, and medicinal crops have received relatively
little attention and the effects of enhanced UV-B on them
cannot be assessed.
The potential impacts of increased W-B on natural
terrestrial ecosystems have received limited study. Any
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63
attempt at the present time to predict potential
consequences would be subject to large uncertainties.
Nevertheless, several physiological processes of plants
(e.g., leaf growth and photosynthesis) have been shown to
be adversely affected by W-B. Most of these responses
were determined in growth chambers or greenhouses where
visible radiation, WV-A, and other environmental factors
did not simulate ambient conditions. Since ambient
visible radiation and W-A ameliorate most, if not all,
of the deleterious effects of UV-B, most of this research
needs to be repeated under field conditions for
verification.
Terrestrial faunal ecosystems have received almost no
attention, and nothing can be said of their level of
susceptibility. In part, such neglect may be justified
because the few available studies indicate that in addi-
tion to possessing physiological and biochemical mecha-
nisms to alleviate the effects of radiation impinging on
the organism, some animals possess behavioral mechanisms
that lessen exposure to presumptively damaging radiation.
Marine faunal ecosystems have received some attention.
Both freshwater and marine systems have been examined in
several U.S. laboratories. It is, however, difficult to
assess the risk factor of increased W-B directly. In
part, the vertical mobility of aquatic creatures and the
fact that the dose changes exponentially with depth in
the sea make it difficult to measure the received dose as
a function of time, although upper and lower bounds can
be assigned. No assessment has been made of the con
sequences of the qualitative or quantitative changes in a
natural aquatic ecosystem exposed to enhanced levels of
W-B to determine whether one or more members of that
system are particularly sensitive to UV-B. Several
aquatic organisms have been exposed to W-B in laboratory
situations, and most of these studies have shown dele
terious effects on the organisms tested. However, as
with plants, the effects appear to be modified by visible
light and W-A. The research needs to be repeated under
field conditions for verification. If aquatic organisms
are generally susceptible to W -B damage in their natural
setting, the effects of increased solar W -B could have
profound consequences on the stability of the food chains
upon which the fish and shellfish used by humans depend.
A reduction in primary food organisms (for example,
algae) could drastically alter the protein supply for
large numbers of people throughout the world and have
obvious social and political consequences.
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64
INTRODUCTI ON
Studies involving several aquatic organisms, insects, and
terrestrial plants and animals exposed to various levels
of enhanced UV-B have shown a number of detrimental
effects on growth, reproduction, and physiological
processes. The capacity to tolerate increased levels of
UV-B through acclimation or repair of UV-induced damage
appears to be limited and is, to some degree, species
dependent. The value of most of these studies is in
question, however, because they have evaluated UV-B doses
either considerably beyond those expected to occur on the
basis of currently projected estimates of atmospheric
ozone reduction (approximately 7 percent) or under
experimental conditions with limited extrapolation
potential to natural conditions. The U.S. Environmental
Protection Agency is currently supporting limited
research programs on both terrestrial crop plants and
aquatic organisms, but the preponderance of projects
recommended by the NRC (1979a) to help resolve the great
uncertainties regarding the biological effects of
increased UV radiation have not been implemented.
Therefore the uncertainties still exist.
EFFECTS ON PLANTS
Research Difficulties
The inherent complexity of simulating a reasonable
representation of enhanced W-B regimes under ambient
field conditions has confined research almost exclusively
to greenhouses and growth chambers. However, the levels
of visible radiation and UV-A are considerably lower
under these experimental conditions than under ambient
field conditions. Recent evidence strongly suggests that
ambient field levels of visible radiation can substan-
tially reduce or even negate the damaging effects of UV-B
(Sisson and Caldwell 1976, Teramura et al. 1980). Thus
extrapolation of research conducted under low visible
radiation regimes to effects under field conditions would
be tenuous and would undoubtedly overestimate the
potentially deleterious effects of enhanced UV-B on
plants. Furthermore, the interactive effects of UV-B and
other environmental stresses (e.g., water, temperature,
air pollutants, and even WV-A) have not been adequately
addressed in the context of reduced ozone concentrations
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65
Nevertheless, existing evidence suggests that an increase
in terrestrial UV-B equivalent to that which would be
caused by a 7 percent reduction in atmospheric ozone
concentration could be potentially damaging to some higher
plants (Biggs and Kossuth 1978). Credible predictions
regarding the severity of damage to economically impor
tant crop plants or to natural plants are not yet feasi-
ble.
~ ~ v ~ ~ ~ ~ ~ & _ ~
-
This is even more true with regard to predictions
of species displacement or perturbation within the world's
natural ecosystems, because there is less information
available.
Although relatively few plants have been evaluated, a
wide range of sensitivity to UV-B radiation has been found
in crop plants (Biggs and Kossuth 1978) and agriculturally
derived varieties (cultivars) within single species
(Krizek 1978). This range of sensitivities may be due in
part to the differing capacity or efficiency of plants to
repair UV-B damage. Plants also appear to differ in their
capacity to attenuate UV-B before it is absorbed by the
target molecules.
Therefore information is needed
regarding repair processes, the limits and rates of
acclimation, and whether genetic control for acclimation
is already present or must be developed (an evolutionary
process) before the question of the effects of increased
W-B on plants can be addressed. This information is not
currently available.
Advances in Knowledge
NRC (1979a) addressed the applicable research to 1979 and
the limitations of existing light sources for simulating
increases in UV-B corresponding to those that would be
caused by reduced atmospheric ozone concentrations.
Although the Present discussion incorporates the results
_
~ _ _ ~ _ '___ ~ In HA. mean c; n-=
of some research cone DelC)L~ 171 7~ O.~vCL11-C~ &~ ~
1979 are emphasized. Unfortunately, little applicable
research has been done since then, because of the
research difficulties described above and a lack of
funding.
The variation among plant species in sensitivity to
W-B (Biggs et al. 1975, Biggs and Kossuth 1978, NRC
1979a) may be due, in part, to different acclimation
potentials.
-
An ability to tolerate even ambient W-B
levels appears to be induced by concomitant exposure to
W. For example, Bogenrieder and Klein (1977 ) found that
Rumex alpinus seedlings grown in an environment free of
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66
W-B displayed severely depressed photosynthetic rates
when exposed to ambient W -B levels. A similar response
was found in _. patientia again grown in a W-B-free
environment, but then exposed to enhanced levels of W -B
under ambient conditions (Sisson and Caldwell 1976).
Ambient levels of UV-B even killed some R. alpinus plants
after a 3-day exposure period, although all of the R.
patientia exposed to enhanced UV-B survived. As discussed
by Caldwell (1982), if acclimation to environments with
high intensities of UV is a phenotypic response already
available to the plant, the anticipated rate of atmos-
pheric ozone depletion would not be of concern. If,
however, acclimation involves genotypic changes that must
occur over a long period of time, the rate of ozone
depletion would be of considerable importance.
Several studies have recently addressed the question
of acclimation by investigating leaf epidermal trans-
mittance of UV-B. Wellman (1974) has shown that UV-B-
absorbing pigments are synthesized in some species in
response to UV levels equivalent to existing ambient
fluxes. Leaf epidermal extracts (containing flavonoids
and other related pigments) from several plants increased
their absorbance at 380 nm after exposure to W-B
(Robberecht and Caldwell 1978). Extracts from squash
(Cucurbita pepo) leaves exposed to three levels of UV-B
increased their absorbance as dose increased (Sisson
1981). Although absorbance by these extracted pigments
increased substantially with increasing W-B dose rate,
photosynthesis and leaf growth were repressed at the
higher radiation level. Even though W-B apparently
induced a synthesis of leaf pigments, the attenuation
appeared to be insufficient to protect leaf growth
processes and the photosynthetic apparatus completely.
Nevertheless, this response would be of significant value
in alleviating the potentially deleterious effects of any
increase in UV-B at the earth's surface.
The intensity of biologically effective W-B (NRC
1979a) increases by a factor of more than 7 from the
arctic (70°N) to the equator (Caldwell et al. 1980). The
increase experienced in moving through this latitudinal
gradient greatly exceeds the increase expected from
atmospheric ozone reduction at temperate latitudes. The
leaf epidermal transmittance of W-B of several plants
along this gradient was evaluated by Robberecht et al.
(1980). The calculated mean effective UV-B dose
transmitted by the epidermis to the physiologically
active mesophyll cells was found to be similar along this
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67
gradient even though the ambient levels of UV increased
substantially. Thus along this latitudinal gradient
plants are apparently coping with ambient W through
acclimation processes that include a reduction in
epidermal transmittance as intensity increases. Whether
plants will be able to cope with additional UV-B along
this gradient by increasing the attenuation properties of
the cuticle, epidermis, or mesophyll cell structure and
function is not known.
In attempting to evaluate the potential impacts on
higher plants, it is important to know whether photo-
synthesis and other physiological processes have a
reciprocal relationship (i.e., damage is cumulative and
dose dependent) with UV-B damage. Reciprocity has been
demonstrated in isolated chloroplasts (Jones and Kok
1966) and for photosynthesis in a sensitive plant exposed
to UV-B over a 50-day period (Sisson and Caldwell 1977).
Trocine et al. (1981) demonstrated that W -B damage is
cumulative in two of three seagrasses tested. They
suggested that the differential degree of UV-B
sensitivity within these species was a function of
epidermal cell wall thickness and associated trans-
mittance properties. Teramura et al. (1980) demonstrated
that reciprocity also applies for photosynthesis in
soybeans; the damaging effect of W-B was shown to be
more deleterious when visible radiation was low. In
these studies, the apparent dependence of response on the
total dose rather than on the dose rate suggests that
repair of damage within the photosynthetic apparatus may
not involve nucleic acid repair systems, which are dose
rate dependent (Caldwell 1982). Although the reduction
in photosynthesis is less pronounced at high visible
radiation levels (Sisson and Caldwell 1976, Teramura et
al. 1980), suggesting either photoprotection or photo-
repair (see Chapter 3) of the photosynthetic apparatus,
the particular repair mechanism(s) involved has not been
determined.
Caldwell et al. (1980) studied leaf inclination as an
avoidance mechanism for reducing the solar W radiation
loads on plant leaves. However, at temperate latitudes
40 percent to 75 percent of solar UV-B is in the nondirect
sunlight that reaches the leaves, which substantially
reduces the effectiveness of leaf inclination as an
avoidance mechanism. Their calculations indicated that
even vertical foliage received at least 70 percent of the
daily effective UV-B. Consequently, a breeding program
directed at developing crop plants of economic importance
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68
with canopies to avoid direct-beam W radiation may be of
little use.
Several sites of inhibition within the photosynthetic
apparatus have been determined (see review by Caldwell
1982). Inhibition of electron transport associated with
photosystem II (Brandle et al. 1977, Yamashita and Butler
1968) disruption of thylakoid membranes and other struc-
tural components of the chloroplast (Brandle et al. 1977,
Mantai et al. 1970), and inactivation of photosystem I
(Okada et al. 1976) have been demonstrated after exposure
to UV. Recently, Vu et al. (1981, 1982) demonstrated
partial inhibition of carboxylating enzyme activity.
Thus UV might be a nearly universal inhibitor of compo-
nent reactions within the photosynthetic apparatus, as
well as deleterious to its structural integrity. However,
these studies were conducted in growth chambers or green-
houses with correspondingly low visible radiation levels.
In order to predict real-life effects of increases in
solar UV, the studies would need to be repeated under
ambient conditions where UV-B can be adequately
supplemented.
Since physiological responses of plants to UV are
highly wavelength dependent, an appropriate action
spectrum for weighting heterochromatic UV-B becomes
necessary for expressing effective dose, determining
threshold levels for damage, and developing predictive
dose-response relationships. The inconsistency in
conclusions that can be arrived at by using different
action spectra has been thoroughly addressed elsewhere
(Caldwell 1982, NRC 1979a). Although action spectra are
useful for a prediction, their utility may be reduced
because of apparent synergisms arising from interactions
due to different wavelengths (Elkind et al. 1978, Elkind
and Han 1978) (see Chapter 3). It is not known whether
this synergistic effect is a general phenomenon within
plant physiological processes. Attempts to develop new
action spectra should therefore incorporate simultaneous
investigations into potential synergistic action among
the wavelengths involved. This will facilitate accuracy
in dose-response relationships and make them more useful
in predicting the consequences of increased intensities
of UV-B at the earth's surface.
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69
EFFECTS ON DOMESTIC ANIMALS
Among the domestic animals that are necessary for the
maintenance of our food supply, only one breed of cattle
appears to show deleterious effects from exposure to
solar W. The white-faced Hereford shows
some suscepti
bility to injury and disease of the eye. Kopecky et al.
(1979) have shown that W-B is a probable causal factor
in cancer eye (bovine ocular squamous cell carcinoma) and
enhances the onset of infectious bovine keratoconjunc-
tivitis (IBK), or pinkeye, in these cattle. It is
questionable, however, whether the level of injury is
serious enough to warrant extensive further investigation.
Ladds and Entwistle (1977) reported on squamous cell
cancers occurring on the ears and nose of sheep. They
found that incidence in tropical Queensland, Australia,
was greater than that found in temperate areas in earlier
studies (Lloyd 1961). Increasing incidence with
advancing age was also demonstrated. The authors suggest
that squamous cell cancer in sheep would provide a good
model for studies of skin cancer in humans.
EFFECTS ON AQUATIC ORGANISMS
Research Difficulties
Measurement of the effects of enhanced W -B on aquatic
organisms presents difficult experimental problems. It
is possible and desirable to assess effects of W-B on
individual organisms under controlled laboratory condi-
tions. These experiments can at least establish which
organisms may be sensitive to enhanced UV-B. It is in
determining the relationship of the dose received by the
organism in the laboratory to that received by the
organism in its natural ecosystem that uncertainties
occur, because of the many variables in natural systems.
First, position of organisms in the water column is
conditioned by UV-B, by light other than W -B. by
nutrient availability, by the nature of turbulent mixing
of the waters by wind, and, in shallower regions, by
tidal frictional forces along the bottom. - ~
are seasonal and annual variations in species composi-
tions, in larval developmental stages, in predation
success, and in the physical transport, turbidity, and
pigment-absorptive characteristics of coastal and
estuarine waters where major food chain productivity
Second, there
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70
occurs. The large statistical variance associated with
the reproductive success of organisms that produce large
numbers of spawn (up to 108 per adult female) makes it
difficult to assess the effects of small changes in
environmental factors (e.g., a 10 percent change in UV-B
levels) with any degree of statistical significance.
m is would be particularly difficult without precise base
line data regarding the effects of present levels of UV-B.
It is important to distinguish between assessing the
possible effects of enhanced UV-B on individuals of a
species, which can best be accomplished in the laboratory,
and assessing the effects of damage to individuals on the
populations of those individuals in the natural ecosystem,
which requires extensive field studies.
Advances in Knowledge
The population ecology and dynamics of aquatic food
chains--the anchovy, striped bass, herring, shellfish,
crustaceans--are just beginning to be studied by
interdisciplinary teams of physical hydrographers,
phytoplankton-, zooplankton-, and fish-biologists, and
biochemists and physiologists. This ecosystem research
is expensive because it requires time on board ships and
large numbers of personnel. But until the capability is
developed for predictions of specific food chain effects
under the range of present physical, chemical, and
biological interactions, the inclusion of the UV-B
variable (unless the effects of UV-B are catastrophic)
will not lead to statistically significant conclusions.
There is strong experimental evidence that current
levels of W-B in surface waters depress near-surface
productivity of organisms at the base of food chains
(primary productivity) in marine waters (Calkins and
Thordardottir 1980, Lorenzen 1979, Smith et al. 1980,
Steemann Nielsen 1964). The measurement of the penetra-
tion of UV-B below the surface and the estimation of
doses to aquatic organisms are very complex. The
penetration of UV-B into waters with low transparency has
not been as well documented as it has for clear waters
(Smith and Baker 1981). Turbidity and blue-light-
absorbing material (gelbstoff) in waters that have a high
organic matter content severely limit UV-B penetration.
This fact, the turbulent mixing of surface waters by
wind, the ability of organisms to adjust their positions
in the water column, and the 24-hour rhythms of vertical
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71
migration observed for many marine species all add to the
difficulty of estimating the dose of W-B to which aquatic
organisms are exposed. Because of these difficulties,
the possible effects of predicted increases in W-B (due
to ozone depletion) on phytoplankton populations within
the entire photic zone are at present unknown.
Attempts have been made to maintain captured phyto-
plankton populations in tanks and to document species
composition changes and changes in primary productivity
resulting from enhanced W-B (Worrest et al. 1978,
198la,b). While these experiments show statistically
significant effects of enhanced W -B. the difficulties of
mimicking natural mixing conditions, the visible and W-A
photic regimes, and the nutrient and predation conditions
of the real world make extrapolation of these data to
populations in the natural ecosystem difficult. Predic-
tion of the effects on food chains for which phytoplankton
serve as food sources is even more difficult. These
experimental difficulties have not yet been overcome,
and, except for the preliminary studies by Worrest et al.
(1981a), have not been attempted on any concerted level.
Recently, it has been reported (Hunter et al. 1979)
that the W -B threshold for lesions and for retardation
of growth in anchovy larvae can be reached after exposure
during a 4-day period to W-B intensities of 760 joules
per square meter (J m~2) (DNA effective dose). These
intensities are equivalent to what would be expected at
the surface of clear ocean water if stratospheric ozone
concentrations were reduced by 25 percent. Current W -B
levels just below the sea surface were estimated to be
413 J m~2 over a 4-day period. From Smith and Baker
(1979) the absorption coefficient for W-B can be
calculated to be approximately 0.2 m~l. This means,
for example, that at 5 m below the surface the W -B
intensity is reduced to 37 percent of the surface
intensity. These data indicate a marginal effect that
depends strongly on the vertical distribution of larvae.
The particular value of this study is that it attempts to
relate in situ measurements of W-B in seawater to
physiological effects on a sensitive stage in the life
cycle of a commercially important marine species. This
physiological study, combined with sampling for the
vertical distributions of larvae, physical hydrographic
measurements of water mixing patterns, and further
measurements of in situ W-B doses, could serve as a
model for investigating the effects of W-B on specific
food chains in the marine ecosystem. At the least, the
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72
research including photoreactivation effects would provide
data from which predictions could be made about the
significance of enhanced levels of W -B on the anchovy
populations.
A photorepair mechanism for W -B lesions in anchovy
larvae has been reported by Kaupp and Hunter (1981). The
amount of light required to activate photorepair mecha-
nisms fully was less than 10 percent of that available
from the sun on a clear day even in March. Thus the
authors concluded that even with increased W-B,
sufficient light exists in the sea to ensure photorepair
of W damage in anchovy larvae.
: it, .. _ ~ ~ · -~ . . . . . .
This report again
~''u~ur~eS one a~rr~cu~es Inherent in studying the
effects of enhanced W-B in the laboratory where the
effects of other wavelength regions of the solar spectrum
cannot easily be considered.
A number of studies of W effects on aquatic systems
(Calkins and Thordardottir 1980, Karanas et al. 1981,
Smith and Baker 1980, Smith et al. 1980, Thomson et al.
1980, Worrest et al. 1980) and on underwater penetration
and characterization of W (Green and Miller 1975; Green
et al. 1980; Smith and Baker 1979, 1981) have been made
recently. The phytoplankton studies of Worrest et al.
(1978, 1981a,b) and the anchovy and physical optics
studies of Hunter et al. (1979) have been used as
examples.
RESEARCH RECOMMENDATIONS
Prediction of the possible effects of increased solar
UV-B on economically important biological organisms, such
as specific crop species, and on selected ecosystems is
not currently possible for two reasons.
First, two major
areas ot research that may have potential for improving
predictive capability and that were proposed bv NRC
(1979a) have not been implemented.
These are (1) studies
AL one elects or ennancea UV-B conducted in the presence
of the natural photoperiodic intensities of the complete
solar spectrum, and (2) the development of a low-latitude,
W-B-transmitting greenhouse facility where specific
higher-latitude species of plants or specific aquatic
organisms could be subjected to lower-latitude W-B
intensities while other conditions such as spectral
intensities and temperatures were maintained close to
higher-latitude ranges. Second, in the absence of
meticulous attention to the need for careful simulation
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73
of environmental parameters implicit in the research
areas described above, most of the research data reported
appear to lack predictive capability. Our assessment
therefore is essentially the same as that given in detail
in previous reports (NRC 1979a, SRI 1980, 1981), and the
research recommendations largely repeat those in the NRC
report (1979a).
The following list of research recommendations is not
exhaustive. It has been limited to those that should
receive attention first, but it is not organized
according to priority.
1. Most of the current experimental data on plants
was gathered under greenhouse or growth chamber conditions
where visible light and W-A levels were considerably
lower than plants would experience under ambient field
conditions. The absence of ambient levels of visible
light and W-A appears to increase substantially the
susceptibility of plants to damage by the W levels
tested. Thus economically important crop plants need to
be evaluated in a field situation where enhanced levels
of W -B approximating those predicted to occur under
reduced atmospheric ozone conditions are simulated.
These experimental conditions might best be obtained
in a low-latitude (subtropical), minimal-cloud-cover,
multiuser facility. Productivity, photobiological, and
physiological studies should be conducted simultaneously.
During these studies, W levels incident upon plant
surfaces should be carefully monitored. Before the data
from such a facility could provide the most precise and
unequivocal results, it would be necessary to develop
instrumentation, data manipulation, and environmental
regulating techniques that would enable the facility to
simulate the changing ambient conditions of the higher-
latitude location whose plants are under study. Without
this capability, the data generated would have limited
usefulness for predicting the effects of enhanced W-B.
Priority should be given to screening for W-B-
sensitive species to look for adverse effects on
productivity and to study the mechanisms of W -B stress.
2. Sensitive phases of plant growth need to be
determined. For example, is the reproductive stage
especially sensitive to an increase in W -B?
3. Dose-response curves and threshold levels for
reduced crop yields are currently known for few plant
species under ambient field conditions. Important crop
plants, selected native plants, representative forage
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74
plants, and economically and ecologically important
forest species need to be similarly evaluated.
4. Virtually nothing is now known about the inter-
actions of W and other known stress factors. The
interaction of W with such factors as temperature, water
stress, and air pollutants needs to be addressed.
5. Studies of the effects of W on aquatic
ecosystems must be approached on two levels: (1) the
effects on individuals under controlled conditions where
all environmental factors are reproduced and W -B levels
are varied, and (2) integration of laboratory dose-
response data into ecosystem studies to establish
possible effects among populations or specific food
chains. Only in this way can it be determined whether a
reduction in stratospheric ozone concentration will
significantly affect aquatic populations. The inter-
disciplinary approach used in the Hunter et al. (1979)
anchovy study (described earlier) to assess W -B damage
to food chains, together with the specific laboratory
measurements, should serve as a model for future research
proposals.
Representative terms from entire chapter:
field conditions
