The effect of high levels of visible and ultra-violet radiation on the photosynthesis of phytoplankton from a freshwater lake

Abstract

Rates of oxygen production of a natural population of phytoplankton from a freshwater lake were measured in situ at mid-summer under near-maximal irradiances for PAR (400-700 nm), UV-B (280-315 nm) and UV-A (320-400 nm), The radiation climate was altered by incubation at different depths, and hence different PAR, and by using flasks which cut-off different amounts of UV-radiation: quartz flasks transmitted all wavelengths >280 nm, glass flasks had a 50 % transmission at 309 nm and glass flasks with a plastic shield had 50 % transmission at 354 nm. At 0.01 and 0.11 m, rates of oxygen consumption in the light were greater than dark respiration in all flasks, but were particularly high in the full UV treatment, possibly as a result of photochemical degradation of dissolved organic carbon. Gross oxygen production was achieved in all flask types at depths of 0.41, 0.53 and 1.1 m but rates were lowest under the full UV-spectrum. An ANOVA showed a significant variation in rate of gross photosynthesis with depth and type of flask. A multiple-regression against received flux of PAR, UV.9 and UV-B explained 81 % of the variation in gross photosynthesis but co-variation of irradiance with depth prevented the effect of the different wavebands to be separated statistically. The biological weighting function of CULLEN et al (1992) for the inhibition of photosynthesis by ultraviolet radiation for a marine phytoplankter, explained 80% of the variation in our data but we could not distinguish between two scenarios with different sensitivity to UV radiation. Extrapolation to in-situ conditions suggested that UV-A radiation was more inhibitory than UV-B radiation.