Experimental test of the effect of ultraviolet-B radiation in a planktoniccommunity

Ultraviolet-B (UVB, 280-320 nm) radiation is a natural component of sunligh t that harms organisms and disturbs natural communities in surface waters. A natural planktonic assemblage of organisms (<240 mu m) was studied in a m esocosm experiment for 7 d under varying conditions of UVB radiation: UVB e xcluded, natural radiation, and UVB enhanced at two different levels. The d ynamics of several populations at different trophic levels comprising heter otrophic bacteria (<1 mu m), heterotrophic flagellates (2-10 mu m), small p hytoplankton (<5 mu m), large phyto plankton (5-20 mu m), and ciliates (15- 35 mu m) were monitored during the experiment. Enhanced UVB provoked a sign ificant decrease in the number of ciliates (66%) and large phytoplankton (6 3%) relative to natural WE conditions. The severe effects of UVB radiation on ciliates and large phytoplankton communities shown here would strongly l imit upward transfer of mass and energy. The decline of predator abundance (ciliates) under UVB stress relative to natural conditions resulted in a po sitive feedback between enhanced UVB radiation and prey abundances, shown b y increased abundances of bacteria (49%), heterotrophic flagellates (up to 300%), and small phytoplankton (41%). Similarly, with respect to carbon par titioning, the decrease in ciliate and diatom carbon biomass (64 and 56%, r espectively) under enhanced WE exposure was balanced by an increase in the carbon biomass of heterotrophic bacteria (48%), heterotrophic flagellates ( 126%), and autotrophic flagellates (162%). As a manifestation of enhanced U VB at the community level, the ecosystem develops toward a microbial food w eb in preference to an herbivorous food web. Thus, enhanced UVB radiation c an change the structure and dynamics of the pelagic food web.