Influence of nitrate availability and nutritional status on phytoplankton responses to UV-B radiation at three latitudes (Canada, Brazil and Argentina)

MLORENA LONGHI, SUZANNE ROY, GUSTAVO FERREYRA, IRENE SCHLOSS

MARINE ECOLOGY PROGRESS SERIES

INTER-RESEARCH

Año: 2005 p. 1 - 45

0171-8630

A test of the influence of nitrate enrichment on the response of natural phytoplankton populations to enhanced UV-B radiation was undertaken during a series of week-long mesocosms (1800 L) experiments that were carried out at three latitudes: in temperate Canada (Rimouski), tropical Brazil (Ubatuba) and temperate southern Argentina (Ushuaia). One mesocosm was submitted to ambient UV-B levels while the other received enhanced UV-B levels corresponding to a local 60% ozone depletion scenario. In two of these sites (Rimouski and Ushuaia) a phytoplankton bloom developed in these stirred mesocosms. To test for the influence of nitrate enrichment, seawater samples were incubated in UV-transparent plastic bags (4 L) floating at the surface of both mesocosms for 24 h. Half of these samples received nitrate, phosphate and silicate enrichments while for the other half nitrate was omitted. Responses in terms of photochemical yield (Fv/Fm, PAM fluorescence) and HPLC-determined pigments, differed among sites and through time. In Rimouski, enhanced UV-B significantly increased photoinhibition (decreased Fv/Fm) and reduced the net growth of fucoxanthin (marker pigment for diatoms, verified microscopically) in the surface bags during the bloom period, with no effect of nitrate. In the post bloom period, combined UV-B and nitrate stress increased photoinhibition but the net growth of fucoxanthin was not reduced (in fact it increased) likely because enhanced UV-B had a greater effect on the grazers. In Ubatuba, cells were affected by the high ambient light irradiances at surface and enhanced UV-B had no further effect, irrespective of their nutrient status. In Ushuaia, enhanced UV-B prevented all increases of fucoxanthin and Chl a, with much less effect on chlorophyll b (green algae). No nutrient stress was observed at this site. This strong and consistent negative response to enhanced UV-B is likely more related to the cold temperature prevailing in Ushuaia at the time of the experiment (slowing repair) than to the light conditions. Variable responses among these phytoplankton communities from different latitudes may be explained by their different taxonomic composition as well as by their nutritional condition, light history and local temperature. When compared with the responses seen in the stirred mesocosms themselves, these 24 h surface incubations showed more sensitivity to enhanced UV-B, indicating that mixing moderated the UV-B effects in all three experiments.