Ultimately the issue of 
-related UV-B increases must be
assessed with respect to the direct impact on natural populations.
Smith, Prezelin and co-workers
[Ray Smith Prezelin macintyre 1992,
Prezelin Boucher Smith 1992 daytime]
directly measured the increase in and penetration of UV-B radiation
into Antarctic waters and provided the first conclusive evidence of
a direct 
-related effect on a natural population.
Making use of the extreme change in ozone associated with the hole,
which creates a sharp gradient (or ``front'') in incident UV-B analogous
to an atmospheric or oceanographic front
[Ray Smith ozone aquatic environment 1989, Ray Smith Baker stratospheric
ozone phytoplankton Oceanography 1989],
they made comparative studies of the impact of UV-B on phytoplankton
in the marginal ice zone (MIZ) of the Southern Ocean.
The MIZ was selected for their study because it is, like the ozone hole,
a spring phenomenon and because the physical conditions of water column
stability that give rise to enhanced productivity within the MIZ also
promote conditions for maximum exposure of phytoplankton to UV-B.
Also, production within the marginal ice zone is estimated to contribute
significantly to the overall production of the Southern Ocean and to
be a significant element in Antarctic spring-time ecology.
Their results indicate a minimum of 6 to 12 percent reduction in MIZ primary
production associated with 
depletion within the ozone hole.
Figure 2 shows average values for in situ phytoplankton production
versus depth in the MIZ of the Southern Ocean.
Results show a comparison of productivity inside the ozone hole
(stratospheric ozone less than 200 Dobson Units,
)
with productivity outside the hole
(stratospheric ozone levels greater than 300 DU).
Higher UV-B levels (inside the hole) are consistently associated with
reduced (left hand curve) levels of production.
That there is less UV-B inhibition at the surface as compared to deeper
has sparked interest in possible photoregulatory interactions of UVR
on phytoplankton.
The causes of depth-related variations in UV-B inhibition are consistent
with the hypothesis
[Ray Smith Prezelin macintyre 1992, Prezelin Boucher Smith 1994]
that the regulation of UV-B damage to cell vitality is initiated through
UV-A, but not UV-B, photoreceptors (see Fig. 6, Smith et al., 1992).
Under this hypothesis, phytoplankton are cued to short-term (minutes to a
few hours) changes in UV-A flux as an index of changing total UVR thus
leaving the phytoplankton unable to respond favorably to significant
elevations in the magnitude of the ratio of UV-B to UV-A.
Alternatively, Cullen has suggested the possibility that relatively
less UV-B inhibition was observed at the surface because UV-A treatments
were strongly inhibited and leaving proportionally less ``target''
for the UV-B + UV-A treatment to inhibit.
Further studies on photoregulaton are needed to clarify this issue.
The work by Smith and co-workers was done in stratified waters of the MIZ,
where the fixed-depth incubations
of 7 to 12 hours should be appropriate for quantifying UV effects.
In waters where vertical mixing is active, similar experiments would yield
artifactual overestimates of photoinhibition
[Marra movement 1978, Cullen Neale 1993, Cullen Lewis 1994].
It is also important to recognize that their research in the MIZ,
by making use of UV-B variability associated with the ozone-hole and by
comparing phytoplankton UV-B inhibition inside to outside the hole,
are in effect making use of a very large-scale human-induced experiment
on natural populations and thus their results are largely independent of
various modeling assumptions and methodological issues.