Impact of ultraviolet radiation on marine crustacean zooplankton and ichthyoplankton: a synthesis of results from the estuary and Gulf of St. Lawrence, Canada

The objectives of the research program reported upon here were (1) to measu re ambient levels of UV radiation and determine which variables most strong ly affected its attenuation in the waters of the estuary and Gulf of St. La wrence, Canada; and (2) to investigate the potential direct impacts of UV r adiation on species of crustacean zooplankton and fish whose early life sta ges are planktonic. In this geographic region, productivity-determining bio physical interactions occur in the upper 0 to 30 m of the water column. Mea surements of the diffuse attenuation coefficients forultraviolet-B radiatio n (UV-B, 280 to 320 nm) at various locations in this region indicated maxim um 10% depths (the depth to which 10% of the surface energy penetrates at a given wavelength) of 3 to 4 m at a wavelength of 310 nm. Organisms residin g in this layer-including the eggs and larvae of Calanus finmarchicus and A tlantic cod Gadus morhua- are exposed to biologically damaging levels of UV radiation. As a result of these physical and biological characteristics, t his system offered a relevant opportunity to assess the impacts of UV on su barctic marine ecosystem. Eggs of C. finmarchicus were incubated under the sun, with and without the W-B and/or UV-A (320 to 400 nm) wavebands. UV-exp osed eggs exhibited low percent hatching compared to those protected from U V: UV radiation had a strong negative impact on C. finmarchicus eggs. Furth er, percent hatching in UV-B-exposed eggs was not significantly lower than that in eggs exposed to UV-A only: under natural sunlight, UV-A radiation a ppeared to be more detrimental to C. finmarchicus embryos than was UV-B. In analogous experiments with Atlantic cod eggs, exposure to UV-B produced a significant negative effect. However, W-A had no negative effect on cod egg s. Additional experiments using a solar simulator (SS) revealed high wavele ngth-dependent mortality in both C, finmarchicus and cod embryos exposed to UV. The strongest effects occurred under exposures to wavelengths below 31 2 nm. At the shorter wavelengths (<305 nm) W-B-induced mortality was strong ly dose-dependent, but (for both C, finmarchicus and cod) not significantly influenced by dose-rate. Thus, at least within the Limits of the exposures under which the biological weighting functions (BWFs) were generated, reci procity held. The BWFs derived for UV-B-induced mortality in C. finmarchicu s and cod eggs were similar in shape to the action spectrum for UV-B effect s on naked DNA. Further, the wavelength-dependence of DNA damage was simila r to that for the mortality effect. These observations suggest that W-induc ed mortality in C, finmarchicus and cod eggs is a direct result of DNA dama ge. There was no evidence of a detrimental effect of UV-A radiation in thes e SS-derived results. A mathematical model that includes the BWFs, vertical mixing of eggs, meteorological and hydrographic conditions, and ozone depl etion, indicates that UV-induced mortality in the C, finmarchicus egg popul ation could be as high as 32.5%, while the impact on the cod egg population was no more than 1.2%. Variability in cloud cover, water transparency (and the variables that affect it), and vertical distribution and displacement of planktonic organisms within the mixed layer can all have a greater effec t on the flux of W-B radiation to which they are exposed than will ozone la yer depletion at these latitudes. Our observations indicate that C. finmarc hicus and cod eggs present in the first meter of the water column (likely o nly a small percentage of the total egg populations) are susceptible to UV radiation. However, although exposure to UV can negatively impact crustacean zooplankt on and ichthyoplankton populations, these direct effects are Likely minimal within the context of all the other environmental factors that produce the very high levels of mortality typically observed in their planktonic early Life stages. The impact of indirect effects - which may well be of much gr eater import - has yet to be evaluated.