Impact of Ozone Depletion on Primary Productivity

Several studies have suggested that a depletion of the ozone layer could lead to a reduction in the primary production of aquatic ecosystems due to an increase in UV-B radiation (Hader, 1997; Hader et al., 1998). Smith et al., (1992) estimated a 6% -12% reduction in the primary production in the marginal ice of the Southern Ocean for a reduction in the stratospheric ozone from 300 DU - 200 DU. Other studies have indicated that an ozone depletion from 300 DU -150 DU in Antarctica would lead to a reduction in the primary production of < 3.8% (Holm-Hansen et al., 1993b), and between 8.5% and 0.7% under clear skies (6.5% and 0.8% under cloudy skies).

Many marine organisms are sensitive to UV radiation. The extent to which these marine organisms will be able to adapt to expected increases in UV exposure is uncertain due to a sparcity of measurements. Increased levels of UV-B radiation may impact phytoplankton communities by: (1) initiating changes in cell size and taxonomic structure, (2) reducing the productivity, (3) influencing the protein content, dry weight, and pigment concentration, (4) inducing chloroplast damage, and (5) directly affecting the proteins of the photosynthetic apparatus.

To explore the potential impact of ozone depletions, one may use a simple model, in which marine photosynthesis (or primary production, ignoring respiratory losses)

is parameterized as follows (Cullen et al., 1992; Neal et al., 1998):

where Ps is the maximum photosynthesis in the absence of UV radiation inhibition, and Ip is the photosynthetically utilizable radiation (PUR):

Here, I (A) is the mean intensity (scalar irradiance), and a' (A) the normalized algal absorption spectrum. In Eq. (9.23), Is is the PUR saturation level, and 1/(1 +1') describes the inhibition caused by UV radiation (UVR), where I is given by:

Here, s(A) is the action spectrum for UVR-induced inhibition of photosynthesis.

The mean intensity I (A) that drives and inhibits primary photosynthesis depends on several environmental factors including the total ozone column amount, solar elevation, and depth in the ocean, as well as the presence of clouds, ice, and snow. Employing the radiative transfer model described above for the coupled atmosphere-sea ice-ocean system, and assuming total ozone column amounts of 400 DU and 200 DU, Hamre et al. (2008) computed I(A) at the bottom of the snow-covered sea ice and at various depths of open water, and used the resulting I (A)-values in Eq. (9.23) to calculate the primary productivity. The results of this study may be summarized as follows:

• an ozone depletion increases not only harmful UVR, but also beneficial PUR;

• at high latitudes, the benefits of increased PUR for phytoplankton under sea ice and below a certain depth in the ocean dominates over the damage caused by increased UVR;

• a large fraction of the primary production in the polar regions is caused by ice algae growing in environments well protected from UVR;

• the primary production in the polar regions could increase by as much as 1% for a 50% ozone depletion.

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