A seasonal study of the significance of N-2 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 co nducted on monthly cruises over a 2.5-yr period at the Bermuda Atlantic Tim e-series Study (BATS) site. The range in daily average N-2 fixation rates b ased 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 vari ations in aeolian dust deposition was not related to per colony N-2 fixatio n rates, but was correlated with colony abundance and hence with integrated rates of N-2 fixation, The iron content of the Trichodesmium colonies at t he surface increased and the N:Fe ratio decreased, but the changes in the r atio was due to lower N preceding the dry-dust deposition increase in late summer. These patterns suggest a community level response rather than a phy siological response to the change in aeolian dust flux. The molar C:N ratio of incorporation revealed a non-Redfield relationship and varied from 14-4 25, with highest values in summer. With the exception of nitrate and ammoni um 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 trichome s raises the annual average of N-2 fixation by Trichodesmium to 0.015 mol N m(-2) yr(-1). This estimate is comparable to estimated net community produc tion at BATS if a non-Redfield C:N fixation ratio is assumed. Nitrogen fixa tion was higher to the south of BATS (nearly 0.031 mol Nm(-2)yr(-1)) primar ily 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) 20 01 EIsevier Science Ltd. All rights reserved.