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

Abstract

Terrestrial ultraviolet radiation (UVR) is highly variable in both space and time, and phytoplankton in the mixed layer may be exposed at irregular intervals to significant daily doses. The influence of the natural UVR on phytoplankton dynamics was investigated in a small mesohumic lake, Lac Cromwell, in the Laurentian Hills by means of a time-intensive (about 60 days) daily study of the relationship between UVR flux and phytoplankton biomass. Following the onset of lake stratification, at which time the epilimnion became 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 throughout the stratified period, chlorophyll a concentration declined particularly rapidly (-65%) during a short-lived ozone-thinning period. There was a major shift in the community taxonomic composition during the same period from a typical diatom-chrysophyte spring bloom towards a dinoflagellate-dominated community that was followed by cyanobacteria. Here, we present evidence that the impact of the temporary increase in UVB was intensified by a concurrent lack of mixing, indicating that turbulence and thermal stratification are key components modulating UVB effects in lakes.