Northern hemisphere stratospheric ozone
depletion raises concern whether increased
ultraviolet radiation (UVR) will adversely affect
primary production in Norwegian kelp forests.
The Arctic ozone hole associated with the polar
stratospheric vortex is periodically located over
northern Scandinavia. The result has been
several occurrences in the late 1990s and
2000 of record low spring ozone concentrations
over northern Norway, compared to two decades
prior. In addition, warm Atlantic currents along
the coast of Norway ameliorate seawater
temperatures, discourage winter ice formation,
and allow the dominant kelp, Laminaria
hyperborea (Gunn.) Foslie, to thrive in the upper
subtidal up to mean low water (MLW).
Consequently, L. hyperborea forests are
potentially susceptible to increases in incident
UVB during ozone depletion events. Using an
experimental and modeling approach, the
principle objective of this research was to
quantify how continued ozone loss over
Scandinavia will affect primary production in L.
hyperborea kelp forests.


Quantitative predictions of UVR effects on kelp
forest production require knowledge of spectral
sunlight, its attenuation in marine waters, and
the formulation of a high-resolution biological
weighting function (BWF) that reflects the
wavelength dependency of UV photoinhibition.
First, a full spectrum irradiance model is
presented which describes the temporal and
spectral character of underwater light in a
Laminaria hyperborea kelp forest at Finnoy,
Norway (62.8 N, 6.5 E). The
measurement-validated irradiance model was
formulated based on a solar radiation transfer
model, measured attenuation coefficients at the
kelp forest, and astronomical and simulated
tides. The least canopy exposure to UVR
occurred when spring high waters coincided with
solar noon, and the greatest UVR exposure
occurred during neap low waters at noon. Tides
altered daily photosynthetically active radiation
(PAR) available for photosynthesis by < 9%
whereas overcast skies reduced PAR by as
much as 70%. The irradiance model was then
combined with a BWF to produce a daylight- and
depth-integrated production model.
Photoinhibitory BWFs were determined
experimentally for L. hyperborea collected near
the island of Finnoy. Both oxygen evolution and
13C-uptake were measured concurrently using
kelp from high light (0 m from MLW) and low
light (10 m from MLW) environments, and
BWFs were fit to each of the four data sets.
Sensitivity of daylight- and depth-integrated (0 -
20 m) photosynthesis was assessed by
manipulating ozone concentrations (100 - 600
D.U.), tide height and timing, and clear and
overcast skies. In the worst simulated case,
when neap low tide occurred at solar noon with
clear skies and severe ozone depletion (100
D.U.), daily kelp production near the surface
declined 7.9% compared to values for the same
tide and 600 D.U. ozone. In contrast, ozone
depletion was inconsequential in limiting
production for the depth-integrated kelp forest,
but overcast weather conditions decreased total
daily forest production by 20%. Thus, whereas
tide structure and overhead ozone affect diurnal
patterns and magnitude of photosynthesis in
shallow-water kelp, the kelp forest as a whole
responds to available PAR. These model
simulations demonstrate that ozone depletion
will have negligible impact on kelp forest
production. Coastal waters shield the subtidal
population from UVR, that is biologically effective
UVR penetrates < 3 m, even when low neap
tides occur at noon. Cloud cover has a much
greater affect than potential ozone depletion on
daily kelp forest carbon production. It is further
speculated that kelp exudates (e.g.,
phlorotannins) may alter the optical properties of
the water in the UVR waveband and potentially
create UV-refuges for other organisms in the
coastal community.

lanny@thalassia.marsci.uga.edu