A seasonal study of the significance of N2 fixation by Trichodesmium spp. at the Bermuda Atlantic Time-series Study (BATS) site

A seasonal study measuring the rate of N-2 fixation by Trichodesmium was conducted on monthly cruises over a 2.5-yr period at the Bermuda Atlantic Time-series Study (BATS) site. The range in daily average N-2 fixation rates based on N-15(2) uptake measurements of single colonies was 0.03-0.74 and 0.04-0.80 nmol N colony-lh-l for puffs and tufts, respectively. Seasonal variations in aeolian dust deposition was not related to per colony N-2 fixation rates, but was correlated with colony abundance and hence with integrated rates of N-2 fixation, The iron content of the Trichodesmium colonies at the surface increased and the N:Fe ratio decreased, but the changes in the ratio was due to lower N preceding the dry-dust deposition increase in late summer. These patterns suggest a community level response rather than a physiological response to the change in aeolian dust flux. The molar C:N ratio of incorporation revealed a non-Redfield relationship and varied from 14-425, with highest values in summer. With the exception of nitrate and ammonium utilization during the spring bloom, incorporation of nitrate, ammonium, urea and an amino acid combination constituted less than 6% of the rate of N, fixation throughout the year. Average annual input to new production by N-2 fixation was 0.004 mol Nm(-2) yr(-1) over the 3 yr period based solely on in situ colony abundance. Including the contribution of single trichomes raises the annual average of N-2 fixation by Trichodesmium to 0.015 mol Nm(-2) yr(-1). This estimate is comparable to estimated net community production at BATS if a non-Redfield C:N fixation ratio is assumed. Nitrogen fixation was higher to the south of BATS (nearly 0.031 mol Nm(-2)yr(-1)) primarily due to increased colony abundance, as N-2 fixation per colony remained the same from 32 degreesN to 26 degreesN. These rates approach geochemical estimates of N, fixation for the North Atlantic, especially if N, fixation by trichomes is as important to the south as it is at the BATS site. (C) 2001 EIsevier Science Ltd. All rights reserved.