MEPS 179:291-296 (1999)  -  doi:10.3354/meps179291

Nitrogen isotopic fractionation during a simulated diatom spring bloom: importance of N-starvation in controlling fractionation

Nathalie A. Waser1,*, Zhiming Yu2, Kedong Yin1,**, Bente Nielsen1, Paul J. Harrison1, David H. Turpin3, Stephen E. Calvert1

1Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada
2Institute of Oceanology, Chinese Academy of Sciences, Qingdao, Shandong 266071, China
3Department of Biology, Queen's University, Kingston, Ontario, K7L 3N6, Canada
**Present address: Department of Biology, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

ABSTRACT: N isotope fractionation ε was first determined during ambient NO3- depletion in a simulated diatom spring bloom. After 48 h of N-starvation, NH4+ was resupplied to the diatoms in small pulses to simulate grazer-produced N and then ε was determined. Large variations in ε values were observed: from 2.0-3.6 to 14-0o/oo during NO3- and NH4+ uptake, respectively. This is the first study reporting an ε value as low as 0 to 2o/oo for NH4+ uptake and we suggest that greater N demand after N-starvation may have drastically reduced NH3 efflux out of the cells. Thus the N status of the phytoplankton and not the ambient NH4+ concentration may be the important factor controlling ε, because, when N-starvation increased, ε values for NH4+ uptake decreased within 30 h. This study may thus have important implications for interpreting the δ15N of particulate N in nutrient-depleted regimes in temperate coastal oceans.

KEY WORDS: N isotope fractionation · 15N/14N · Nitrate uptake · Ammonium uptake · Diatoms · Spring bloom

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