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

We applied a high-sensitivity dual isotopic tracer technique (C-13:N-15) to measure N-2 fixation and primary production in the total phytoplanktonic community and in 3 size fractions (>10, <10 and <3 mu m) in oceanic waters around New Caledonia (Southwest Pacific). This region seemed to be favourable for diazotrophy, which is observed at significant rates over the year throughout both oceanic and lagoonal habitats. Nitrogen-fixation rates were high, but presented some spatial heterogeneity and high seasonal variability. Large phytoplankton (>10 mu m, i.e. Trichodesmium) often fixed the bulk of available nitrogen at very high rates (up to 1.8 nmol l(-1) h(-1)). Elevated N-15(2) accumulation (up to 0.83 nmol l(-1) h(-1)) was always observed in the <10 mu m fraction, representing a mean of 31 +/- 20% of total N-2 fixation, up to 92 % in the lagoon and 98 % in the oceanic region. Direct fixation was detected in the <10 mu m 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 N-15(2) was also detectable in the <3 mu m fraction, especially in surface samples. The rates of this nitrogen accumulation were generally very low (<0.17 nmol l(-1) h(-1)), representing similar to 10% of total fixation. However, in August 2002, this N-15 accumulation in the <3 mu m fraction contributed nearly 50% of the total nitrogen fixation. However, with the post-size -fractionation experiments it was not possible to distinguish direct N-2 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.