Influence of ultraviolet-B radiation, stratospheric ozone variability, andthermal stratification on the phytoplankton biomass dynamics in a mesohumic lake

Terrestrial ultraviolet radiation (UVR) is highly variable in both space an d time, and phytoplankton in the mixed layer may be exposed at irregular in tervals to significant daily doses. The influence of the natural UVR on phy toplankton dynamics was investigated in a small mesohumic lake, Lac Cromwel l, in the Laurentian Hills by means of a time-intensive (about 60 days) dai ly study of the relationship between UVR flux and phytoplankton biomass. Fo llowing the onset of lake stratification, at which time the epilimnion beca me shallower than 2.5 m, the study revealed a strong negative relationship between ultraviolet-B radiation (UVB) and algal biomass at the surface (r(2 ) = 0.61) and at 1 m (r(2) = 0.38). Although this relationship held through out the stratified period, chlorophyll a concentration declined particularl y rapidly (-65%) during a short-lived ozone-thinning period. There was a ma jor shift in the community taxonomic composition during the same period fro m a typical diatom-chrysophyte spring bloom towards a dinoflagellate-domina ted community that was followed by cyanobacteria. Here, we present evidence that the impact of the temporary increase in UVB was intensified by a conc urrent lack of mixing, indicating that turbulence and thermal stratificatio n are key components modulating UVB effects in lakes.