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We applied a high-sensitivity dual isotopic tracer technique ( 13 C: 15 N) to measure N2 fix- ation and primary production in the total phytoplanktonic community and in 3 size fractions (>10, 10 µm, i.e. Trichodesmium) often fixed the bulk of available nitrogen at very high rates (up to 1.8 nmol l -1 h -1 ). Elevated 15 N2 accumulation (up to 0.83 nmol l -1 h -1 ) was always observed in the <10 µm fraction, representing a mean of 31 ± 20% of total N2 fixation, up to 92% in the lagoon and 98% in the oceanic region. Direct fixation was detected in the <10 µm fraction during the day as well as during the night in the New Caledonia lagoon, indicating that unicellular nanoplanktonic cyanobacteria could be a significant source of new nitrogen. Some accumulation of 15 N2 was also detectable in the <3 µm fraction, especially in surface samples. The rates of this nitrogen accumulation were generally very low (<0.17 nmol l -1 h -1 ), repre- senting ~10% of total fixation. However, in August 2002, this 15 N accumulation in the <3 µm fraction contributed nearly 50% of the total nitrogen fixation. However, with the post-size-fractionation experiments it was not possible to distinguish direct N2 fixation from picoplanktonic assimilation of organic compounds released by large cyanobacteria. Nevertheless, the results demonstrate a close coupling between larger diazotrophs and picoplanktonic populations, and show that new nitrogen could rapidly be provided for the pelagic microbial food web.

marine ecology. progress series

Seasonal nitrogen fixation and primary production in the Southwest Pacific: nanoplankton diazotrophy and transfer of nitrogen to picoplankton organisms