Spatio-temporal variations of N2 fixation rates and abundance of N2-fixers in the north Western Pacific

Abstract

The availability of nitrogen is important in regulating biological productivity and deepwater nitrate has long been considered the major source of new nitrogen supporting primary production in oligotrophic regions of the open ocean. Recent studies have shown that biological N2-fixation has a critical role in supporting oceanic new production. However, limited information is available to help understand the nitrogen cycle in the NW Pacific. To help understand characteristics of N2-fixation, N2-fixation rates, abundance and diversity of nitrogen fixers in the different water masses of the NW Pacific were examined during October 2007 and June 2008 cruises. N2-fixation rates were 49.1~2,156.6 μmolN m-2d-1 in June and 20.9~171.7μmolN m-2 d-1 in October showing much variation among different water masses. The average rates for the warm pool, Kuroshio, and East China Sea were 913.8, 403.8, and 105.9 μmolN m-2d-1 in June and 107.8, 38.9, 40.9 μmolN m-2d-1 in October. Relatively high N2-fixation rates observed in June for warm pool were mostly due to high concentrations of Crocosphaera (upto 3.5x103 cells/ml). However, Crocosphaera abundances in warm pool were relatively low in October, and unobserved in the East China Sea in both cruises. For the Kuroshio currents in June, the high N2-fixation rates were mainly affected by Trichodesimum and dinoflagellates with nitrogen fixing endosymbionts. The results showed that N2-fixation rates in the warm pool are very high, and the main contributor is Crocosphaera. The analyses also reflected variation of N2-fixation rates, diazotroph diversity and abundance in different water masses of the NW Pacific.