We examlned the effects of solar ultrav~olet ~a d~a t~o n (UVR) and phosphorus enrichment on m~c r o b~a l food web structure In a subarct~c o l~g o t r o p h~c lake (Lac a I Eau C l a~r e Canada l a t~t u d e 56"10 N long~tude 74"30 W) uslng repl~cated, 20 l polyethylene contamers Incubated In sltu The expenments were run for between 6 and 9 d at the surface of the lake at 3 tlmes dunng the summer open water perlod Chlorophyll a (chl a ) production of the > 2 pm fraction of the phytoplankton

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... mmunlty was stimulated by P t h~s response was reduced on average by 31 % in the presence of U\/-A and by an addit~onal 29% in the presence of UV-B Conversely the <2 pm chl a fraction declined \mth P enrichment but to a lesser extent In the presence of UV-B Cell counts of plcocyanobacteria hhowed a decrease under P ennchment w~t h s~gnificantly greater declines Induced by UV-A and add~tionally by UV-B The nanoflagellate community Increased In cell concentrat~on In response to P a d d i t~o n but only in the absence of UV-A Bacter~al cell concentrat~ons In the P enrichments remained close to or slightly above (up to 20%) the init~al values but sharply d e c l~n e d In the treatments without P a d d~t i o n the posltive effect of enrichment was not influenced by UV-A or by UV-B T h~s diverse array of responses unplies that shlfts In underwater UVR caused by changes In spectral penetrat~on through the atmosphere (ozone deplet~on) or water column (changes In d~ssolved organlc carbon) \v111 influence mlcrob~al t r o p h~c structure as well as productiv~ty w l t h~n s u b a r c t~c lakes Such effects, however w~l l be lim~ted to aquatic environments that are naturally exposed to h~g h UVR such as shallow water columns or surface waters d u r~n g dlurnal s t r a t~f~c d t~o n KEY WORDS: M~c r o b~a l food webs . Phosphorus. Polar. Subarctic lakes UV