Biological weighting of ultraviolet (280-400 nm) induced mortality in marine zooplankton and fish. I. Atlantic cod (Gadus morhua) eggs

In the Gulf of St. Lawrence, Canada, productivity-determining biophysical i nteractions occur in the upper 0 to 30 m of the water column. The eggs and larvae of several commercially important marine invertebrates and fishes (e .g. Gadus morhua L.) are found in this layer. Measurements of the diffuse a ttenuation coefficients for ultraviolet-B radiation (280 to 320 nm, UV-B) a t various locations in this geographic region indicated maximum 10% depths (the depth to which 10% of the surface energy penetrates at a given wavelen gth) of 3 to 4 m at a wavelength of 310 nm. This represents a significant p ercentage of the summer mixed-layer water column: organisms residing in thi s layer are exposed to UV-B radiation. Laboratory experiments using a Xenon -arc-lamp based solar simulator revealed that cod embryos exposed to UV-B e xhibited high wavelength-dependent mortality. The strongest effects occurre d under exposures to wavelengths below 312 nm. This susceptibility was also dependent upon developmental stage; mortality was particularly high during gastrulation. At the shorter wavelengths (<305 nm) UV-B-induced mortality was strongly dose-dependent, and not significantly influenced by dose-rate. The biological weighting function (BWF) derived for UV-B-induced mortality in cod eggs is similar to that reported for naked DNA - suggesting that th e mortality is a direct result of DNA damage. There was no evidence of a de trimental effect of ultraviolet-a radiation (320 to 400 nm). Calculations b ased upon the BWF indicate that, under current noon surface irradiance, 50% of cod eggs located at or very near (within 10 cm) the ocean surface will be dead after 42 h of exposure. Under solar spectral irradiance simulating a 20% decrease in ozone layer thickness, this time drops to 32 h. These are first-order estimates based upon surface irradiance taken at a time of day during which the values would be maximal. Nonetheless, they illustrate the relative changes in UV-B impacts that will result from ozone layer depleti ons expected over the coming decades. It is also important to point out tha t variability in cloud cover, water quality, and vertical distribution and displacement of cod eggs and larvae within the mixed layer, can all have a greater effect on the flux of UV-B radiation to which fish eggs are exposed than will ozone layer depletion at these latitudes.