MEPS 199:69-81 (2000)  -  doi:10.3354/meps199069

Effects of ultraviolet B radiation on simultaneous carbon and nitrogen transport rates by estuarine phytoplankton during a week-long mesocosm study

Laure Mousseau1,*, Michel Gosselin1, Maurice Levasseur2, Serge Demers1, Juliette Fauchot1, Suzanne Roy1, Piedad Zulema Villegas1, Behzad Mostajir1

1Institut des sciences de la mer de Rimouski (ISMER), Université du Québec à Rimouski, 310 allée des Ursulines, Rimouski, Québec G5L 3A1, Canada
2Institut Maurice-Lamontagne, Ministère des Pêches et des Océans, CP 1000, Mont-Joli, Québec G5L 3Z4, Canada
*Present address: Observatoire Océanologique, BP 28, 06230 Villefranche-sur-Mer, France. E-mail:

ABSTRACT: The effect of UV B radiation on photosynthesis and nitrogen uptake by an estuarine phytoplankton community was investigated during a week long experiment, conducted in 8 mesocosms under varying conditions of UV-B radiation: reduced UV-B, natural radiation, and 2 levels of enhanced UV-B. Twice a day, dissolved inorganic carbon (13C) and total dissolved nitrogen (15N) transport rates were estimated simultaneously from in situ incubations. Irrespective of the treatment, phytoplankton biomass (chlorophyll a) and primary production increased over the first 3 d. Subsequently, nitrate and silicate depletion resulted in a decrease in algal biomass and productivity. Enhanced UV-B radiation was deleterious to chlorophyll a specific transport rates of C and N when compared to reduced and natural UV B. The C:N transport ratios, as well as the POC:PON ratios, were generally not affected by enhanced or reduced UV-B. In the enhanced UV-B treatments, carbon transport rates were often significantly higher in the afternoon than in the morning, suggesting that phytoplankton exposed to UV B developed photoprotective mechanisms against UV radiation on a daily basis. A shift in the algal community assemblage from diatoms (>10 µm) to small flagellates (5-10 µm) was observed during the study. Small flagellates were less sensitive to the UV B treatments than diatoms, whose abundance decreased under reduced and enhanced UV B. Results from this study suggest that UV B exposure on a daily basis could change the chlorophyll a specific transport rates of C and N and alter the structure of the phytoplankton community.

KEY WORDS: Mesocosms · Nitrogen transport · Photoinhibition · Photosynthesis · Phytoplankton · St. Lawrence Estuary · UV B radiation

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