ABSTRACT: The long-term effects of natural and enhanced ultraviolet-B (UVB) exposure on picocyanobacteria, size-fractionated chlorophyll a (chl a), primary production (photosynthesis-irradiance parameters) and grazing activity on phytoplankton were investigated over a 20 d period in a mesotrophic lake in central Ontario, Canada. Large volume enclosures (20000 l) open to the sediments were installed in the littoral zone and sampled at 6 d intervals. Four radiation treatments were run: UVB-screened giving 0% incident UVB, natural ambient (100% UVB), and 2 lamp-enhanced treatments giving 112% biologically weighted UVB (range of 109 to 126% depending on cloud cover) and 118% biologically weighted UVB (114 to 138%). The light utilization efficiency of the phytoplankton (a) decreased under enhanced UVB, but there was no significant decrease in biomass (chl a or cell counts). Populations of the nanoflagellate Ochromonas sp. significantly increased under enhanced UVB; after 20 d the cell concentration of this species was ca 3.6 times higher relative to the natural UVB control. However, differences between treatments in picocyanobacteria, <2 µm chl a, other nanoplanktonic groups, maximum photosynthetic rates, sensitivity to photoinhibition and grazing activity were generally small relative to the variability between duplicate enclosures. These results indicate a low impact of natural and increased levels of incident UVB on the inshore plankton community in this type of lake environment.

KEY WORDS: Ultraviolet-B · Photosynthesis · Phytoplankton · Enclosure · Temperate lake · Ochromonas · Grazing · Mixotroph

Published in AME Vol. 16, No. 2 (1998) on November 27
ISSN: 0948-3055. Copyright © Inter-Research, Oldendorf/Luhe, 1998