Cell size versus taxonomic composition as determinants of UV-sensitivity in natural phytoplankton communities

Laurion, Isabelle, and Warwick F. Vincent

Limnol. Oceangr. 43(8), 1998, 1774-1779 | DOI: 10.4319/lo.1998.43.8.1774

ABSTRACT: Independent evidence from cultures, field studies, an: modeling suggests that cell size is a key factor determining the extent of ultraviolet radiation (UVR) damage of phytoplankton, with the greatest effects on small cells. We investigated this hypothesis in subarctic lake communities by measuring size-dependent photosynthesis under different UVR treatments. Lakewater was exposed to near-surface solar radiation in the presence or absence of UVA and UVB, and the phytoplankton was then fractionated into taxonomically distinct pico-, nano-, and microplankton size classes. The first part of the study was conducted in a large oligotrophic lake where UVA lowered photosynthesis on average by 32%, while UVB caused an additional 20% decrease. For the pooled data set, mean UVR inhibition of photosynthesis was lowest for the picoplankton and highest for microplankton, but the differences between size fractions were not significant. This photosynthetic assay was repeated in a shallow, mesotrophic coastal lake. This community was less inhibited by UVR (on average 10% inhibition); there was no difference in response between the pica- and nanoplankton fractions, whereas photosynthesis of the microplankton fraction was significantly enhanced under UVR. These results show that cell size is not a good index of UV sensitivity and that cyanobacteria-dominated picophytoplankton are less sensitive to the impacts of rising UVB in the polar and subpolar regions than would be predicted from relationships based only on cell size.

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