Longhi ML, Ferreyra G, Schloss I, Roy S
UV-B radiation, Nutrient status, phytoplankton, Fv/Fm, Pigments, Latitudinal variations ·, Nitrate availability, mesocosm, Quebec, Canada, Ushuaia, Brazil, Ubatuba, Argentina, 1.8 m3
The influence of nitrate enrichment on the response of natural phytoplankton populationsto enhanced UV-B radiation was tested during a series of week-long mesocosm (1800 l) experimentscarried out in Rimouski (Canada), Ubatuba (Brazil) and Ushuaia (southern Argentina). We set up 2 mesocosmsat each site, one was submitted to ambient UV-B levels while the other received enhanced UV-Blevels corresponding to a local 60% ozone-depletion scenario, and both were continuously mixed. Samplesfrom the mesocosms were incubated in UV-transparent plastic bags (4 l) floating at the surface for24 h. Half of these received nitrate, phosphate and silicate enrichments while for the other half nitrate wasomitted. Responses in terms of photochemical yield (Fv/Fm, where Fv is variable fluorescence after darkrelaxation and Fm is maximal fluorescence following short pulses of saturating white light, pulse amplitude-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 mesocosms.Enhanced UV-B significantly increased photoinhibition in the surface bags, while nitrate additionpartly relieved this inhibition only during the post-bloom period. The net growth of fucoxanthin wasreduced by enhanced UV-B except during the post-bloom period, when it increased, probably becausegrazing pressure was reduced under enhanced UV-B (strong decrease in ciliates). In Ubatuba, cells wereaffected 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 andchl a, with much less effect on chl b (green algae), and no nutrient response was observed. This strong andconsistent negative response to enhanced UV-B is likely to be related to the cold temperature prevailingat Ushuaia at the time of the experiment (slowing repair) as well as the local community composition. Incontrast, no effects of enhanced UV-B were observed in the stirred mesocosms at the 3 sites, indicatingthat mixing moderated the UV-B effects in all 3 experiments.