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

Published in MEPS Vol. 199 (2000) on June 26
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 2000