Effects of ultraviolet-B radiation on two arctic microbial food webs

The impact of UVB radiation on arctic planktonic microb~al communities was examined in 2 shipboard experiments conducted on the northeast coast of Greenland, using water from a sea-ice meltwater pond and a land pond. The experiments were designed as a preliminary investigation of the interaction of UVB and grazer effects on microbial communities, in order to evaluate the net effect of UVB on intact communities. We used Mylar bags to remove the natural levels of UVB, and a variation of the Landry-Hassett type dilution design to manipulate grazer abundance. The experiments allowed UVB effects to be scaled relative to grazing intensity and to growth in the absence of grazing. UVB effects on ciliates and rotifers were highly species-specific and ranged from no growth rate enhancement with the removal of UVB, to strong enhancement, and even to higher numbers in the presence, rather than absence, of UVB. Removal of UVB had little or no effect on heterotrophic flagellates. Nanoand picoplanktonic algae were suppressed by UVB in only 1 experiment, and bacteria showed no enhancement in the absence of UVB in either experiment. When UVB effects occurred in nanoor picoautotrophs or HNF, they were substantially smaller than the mortality produced through grazing. It would appear that at current UVB levels, community-level deleterious effects of UVB on arctic microbial lood webs can be difficult to discern, due to the species-specificity of the effects.