A seasonal study of the significance of N-2 fixation by Trichodesmium spp. at the Bermuda Atlantic Time-series Study (BATS) site
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A seasonal study measuring the rate of N2 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 N2 fixation rates based on 15N2 uptake measurements of single colonies was 0.03-0.74 and 0.04-0.80 nmol N colony-1 h-1 for puffs and tufts, respectively. Seasonal variations in aeolian dust deposition was not related to per colony N2 fixation rates, but was correlated with colony abundance and hence with integrated rates of N2 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 N2 fixation throughout the year. Average annual input to new production by N2 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 N2 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 N2 fixation per colony remained the same from 32N to 26N. These rates approach geochemical estimates of N2 fixation for the North Atlantic, especially if N2 fixation by trichomes is as important to the south as it is at the BATS site. 2001 Elsevier Science Ltd.
