A series of three outdoor mesocosm experiments was undertakenin Rimouski (Canada), Ubatuba (Brazil) and Ushuaia(southern Argentina) to examine the effects of lamp-enhancedUV-B (280-320 nm) on phytoplankton communities isolatedfrom seawater at each site. Detailed pigment compositionwas used to identify these communities. Each experiment comparedthree replicated UV-B treatments, consisting of naturalsunlight conditions (NUVB), low-level UV-B enhancementcorresponding to local 30% ozone depletion (LUVB) andhigh-level enhancement corresponding to 60% ozone depletion(HUVB). Each mesocosm (ca 2 m deep) was mixed continuously(turnover time, ca 1.3 h) and samples were obtained daily over7-10 days. In Rimouski a large diatom bloom occurred duringthe first week. Repeated-measures analysis of variance (RMANOVA),with time as the repeated factor, showed slight butstatistically significant increases in the chlorophyll (Chl) a levelwith the HUVB treatment, which were especially obvious overthe last 3 days of the experiment. A large decrease in grazers(ciliates) that was observed concurrently with this treatment isthe most likely explanation for the increase in Chl a level. Thelack of negative effect on algal biomass by enhanced UV-B isattributed to the mixing inside the mesocosms and to therelatively low UV-B penetration. In Ubatuba levels of mostpigments decreased over time, particularly fucoxanthin, Chl c3and alloxanthin. The FW-ANOVA showed no effect of theUV-B treatments, except for Chl c3, which had significantlylower concentrations under natural UVB conditions, indicatingthat enhanced UV-B directly or indirectly favored Chl c3 algae(likely prymnesiophytes). Although particulate organic carbonconcentration was significantly larger during HUVB treatmentthan during the other treatments, Chl a was unaffected,suggesting that enhanced UV-B favored heterotrophs. Lack ofalgal growth during this experiment was attributed to lownutrient concentrations (which were the lowest of the threesites), high irradiances (which were the highest noon incidentphotosynthetically available radiation and UV of the threesites) and UV-B penetration down to the bottom of themesocosms. In Ushuaia a small bloom took place over the first5 days. The RM-ANOVA showed no overall effect of the UV-Btreatments for any of the pigments examined but on the last 3days of the experiment several green algae-type pigments, suchas Chl b and siphonein, showed increased concentrations underthe HUVB treatment. UV-B enhancement hence favored greenalgae, as seen from the stronger increase over time in the ratioof Chl b to Chl a associated with the HUVB treatment. UV-Benhancement also seemed to cause a slight decrease in physiologicalcondition, because the relative concentration of chlorophyllidea and some pheophorbides that may be the productof dying algae increased during the HUVB treatments inUbatuba and particularly in Ushuaia (where UV-B also penetratedto the bottom of mesocosms). For all three sites changesin phytoplankton biomass due to the UV-B treatments wereminor, even though UV-B enhancement was important. Thisstudy indicates that effects of enhanced UV-B on the communitystructure of both phytoplankton and their grazers arepotentially more important than effects on overall algalbiomass.
