This article is from the Ozone Depletion: UV Radiation and its Effects FAQ, by Robert Parson firstname.lastname@example.org with numerous contributions by others.
Again, generally harmful but hard to quantify. Seawater is
surprisingly transparent to UV-B. In clear waters radiation at 315
nm is attenuated by only 14% per meter depth. [Jerlov]. Many marine
creatures live in surface waters, and they have evolved a variety
of methods to cope with UV: some simply swim to lower depths, some
develop protective coatings, while some work at night to repair the
damage done during the day. Often these natural mechanisms are
triggered by _visible_ light intensities, in which case they
might not protect against an increase in the _ratio_ of UV to visible
light. Also, if a photosynthesizing organism protects itself by
staying at lower depths, it will get less visible light and produce
less oxygen. An increase in UV-B can thus affect an ecosystem
without necessarily killing off individual organisms.
Many experiments have been carried out to determine the
response of various marine creatures to UV radiation; as with land
plants the effects vary a great deal from one species to another,
and it is not possible to draw general conclusions at this stage.
[Holm-Hansen et al.] We can assume that organisms that live in tropical
waters are safe, since there is little or no ozone depletion there, and
that organisms that are capable of living in the tropics are probably
safe from ozone depletion at high latitudes since background UV
intensitiesat high latitudes are always low. (One must be careful
with the second inference if the organism's natural defenses are
stimulated by visible light.) The problems arise with organisms
that have adapted to the naturally low UV levels of polar regions.
In this case, we have a natural laboratory for studying UV
effects: the Antarctic Ozone hole. (Part III of the FAQ discusses
the hole in detail.) The outer parts of the hole extend far out
into the ocean, beyond the pack ice, and these waters get
springtime UV-B doses equal to or greater than what is
seen in a normal antarctic summer. [Frederick and Alberts] [Smith
et al.]. The UV in shallow surface waters is effectively even
higher, because the sea ice is more transparent in spring than in
summer. There has been speculation that this UV could cause a
population collapse in the marine phytoplankton, the microscopic
plants that comprise the base of the food chain. Even if the plankton
are not killed, their photosynthetic production could be reduced.
Laboratory experiments show that UV-A and UV-B do indeed inhibit
phytoplankton photosynthesis. [Cullen and Neale] [Cullen et al.]
In one field study, [Smith et al.]. measured the photosynthetic
productivity of the phytoplankton in the "marginal ice zone" (MIZ),
the layer of relatively fresh meltwater that lies over saltier
deep water. Since the outer boundary of the ozone hole is
relatively sharp and fluctuates from day to day, they were able to
compare photosynthesis inside and outside the hole, and to
correlate photosynthetic yield with shipboard UV measurements.
They concluded that the UV-B increase brought about an overall
decrease of 6-12% in phytoplankton productivity. Since the "hole"
lasts for about 10-12 weeks, this corresponds to an overall decrease
of 2-4% for the year. The natural variability in phytoplankton
productivity from year to year is estimated to be about + or - 25%,
so the _immediate_ effects of the ozone hole, while real, are far
from catastrophic. To quote from [Smith et al.]: "Our estimated
loss of 7 x 10^12 g of carbon per year is about three orders
of magnitude smaller than estimates of _global_ phytoplankton
production and thus is not likely to be significant in this
context. On the other hand, we find that the O3-induced loss to a
natural community of phytoplankton in the MIZ is measurable and the
subsequent ecological consequences of the magnitude and timing of
this early spring loss remain to be determined." It appears, then,
that overall loss in productivity is not large.
The cumulative effects on the marine community are not known. The
ozone hole first became large enough to expose marine life to large
UV increases in 1987, and [Smith et al.] carried out their survey in
1990. Ecological consequences - the displacement of UV-sensitive
species by UV-tolerant ones - are likely to be more important than
a decline in overall productivity, although they are poorly
understood at present. [McMinn et al.] have examined the relative
abundance of four common phytoplankton species in sediment cores from
the fjords of the Vestfold hills on the Antarctic coast. They conclude
that compositional changes over the past 20 years (which should include
effects due to the ozone hole) cannot be distinguished from long-term
natural fluctuations. Apparently thick coastal ice protects the
phytoplankton in these regions from the effects of increased UVB;
moreover, these phytoplankton bloom after the seasonal hole has closed.
McMinn et al. emphasize that these conditions do not apply to ice-edge
and sea-ice communities.
For a general review, see [Holm-Hansen et al.]