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The influence of nitrate enrichment on the response of natural phytoplankton populations to enhanced UV-B radiation was tested during a series of week-long mesocosm (1800 l) experiments carried out in Rimouski (Canada), Ubatuba (Brazil) and Ushuaia (southern Argentina). We set up 2 meso- cosms at each site, one was submitted to ambient UV-B levels while the other received enhanced UV-B levels corresponding to a local 60% ozone-depletion scenario, and both were continuously mixed. Sam- ples from the mesocosms were incubated in UV-transparent plastic bags (4 l) floating at the surface for 24 h. Half of these received nitrate, phosphate and silicate enrichments while for the other half nitrate was omitted. Responses in terms of photochemical yield (Fv/Fm, where Fv is variable fluorescence after dark relaxation and Fm is maximal fluorescence following short pulses of saturating white light, pulse ampli- tude-modulated (PAM) fluorescence) and high-performance liquid chromatography (HPLC) pigments, differed among sites and over time, particularly for Rimouski, where a large bloom took place in the meso- cosms. Enhanced UV-B significantly increased photoinhibition in the surface bags, while nitrate addition partly relieved this inhibition only during the post-bloom period. The net growth of fucoxanthin was reduced by enhanced UV-B except during the post-bloom period, when it increased, probably because grazing pressure was reduced under enhanced UV-B (strong decrease in ciliates). In Ubatuba, cells were affected by the high ambient-light irradiances at the surface, and enhanced UV-B had no further effect, irrespective of the nutrient status. At Ushuaia, enhanced UV-B prevented all increases of fucoxanthin and chl a, with much less effect on chl b (green algae), and no nutrient response was observed. This strong and consistent negative response to enhanced UV-B is likely to be related to the cold temperature prevailing at Ushuaia at the time of the experiment (slowing repair) as well as the local community composition. In contrast, no effects of enhanced UV-B were observed in the stirred mesocosms at the 3 sites, indicating that mixing moderated the UV-B effects in all 3 experiments.

Variable phytoplankton response to enhanced UV-B and nitrate addition in mesocosm experiments at three latitudes (Canada, Brazil and Argentina)