Impact of natural ultraviolet radiation on rates of photosynthesis and on specific marine phytoplankton species

Natural phytoplankton populations from both Antarctic and tropical waters were exposed to solar radiation to determine the effects of ultraviolet radiation (UVR) on rates of photosynthesis. Radiation in the UV-A region (320 to 400 nm) was responsible for over 50 % of the total inhibition due to UVR, with less than 50 % due to UV-B (280 to 320 nm). Wavelengths c305 nm, which is the spectral region most enhanced under conditions of low ozone concentrations in the atmosphere, accounted for only 15 to 20% of the total inhibition due to UV-B radiation. Under high-light conditions on sunny days, photosynthetic rates were increased 200 to 300 % by screening off all UVR below 378 nm. When the average UVR (295 to 385 nm) during the incubation period was below a threshold value of 5 to 10 W m-', there was no significant depression of photosynthetic rates. Microscopic examinations of a phytoplankton population which was allowed to grow for 5 d under high solar irradiance indicated that UVR is more inhibitory to microplankton than to nanoplankton, and that it induced the formation of resting spores in diatom species of the genus Chaetoceros. Phytoplankton from tropical waters showed marked resistance to UVR as compared to Antarctic phytoplankton. Under the highest light conditions an increase in photosynthetic rates of 10 to 20 % was recorded in some experiments by screening off all UVR c 3 7 8 nm, but when data from all experiments with samples from surface waters in the tropics were analyzed, there was no significant difference between samples with or without natural solar UVR. Phytoplankton samples from below the pycnocline in tropical waters, however, were very sensitive to solar radiation, suggesting that the resistance shown by phytoplankton from surface waters reflects a photoadaptive process.