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Title: Environmental factors regulating N2 fixation by Trichodesmium spp. in the Sargasso Sea
Author: Orcutt, K. M.
Awarding Body: University of Wales Swansea
Current Institution: Swansea University
Date of Award: 1999
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A seasonal study measuring N2 fixation, primary production and biomass of Trichodesmium spp. was conducted on a monthly basis for over two years at the Bermuda Atlantic Time-series Study (BATS) site. Due to seasonal changes in hydrography and meteorology, the BATS site offers a unique opportunity to study the environmental factors regulating N2 fixation by Trichodesmium. Nitrogen fixation was measured using the 15N2 technique and the acetylene reduction assay simultaneously. The geometric mean of the molar ratio between the two techniques was 3:1 but varied by a factor of 40. Measurements of 15N2 incorporation was conducted on single colonies and appeared to be the more sensitive method. Rates of N2 fixation per colony ranged from 0.01-17 ng Ncol-1 H-1. Nitrogen fixation rates were compared to primary production, incorporation of organic and inorganic nitrogen and the iron content of the colonies. The seasonal pattern of N2 fixation was highly correlated to aeolian dust deposition data collected from the AEROCE tower in Bermuda. Single trichomes of Trichodesmium spp. were enumerated and found to develop at the nutricline. Peak abundance of trichomes preceded maximal abundance of colonies in the surface waters at the BATS site in later summer and fall. An annual integrated N2 fixation rate, from the upper 140 m of the water column, was 0.024 mol N m-2 y-1 using an average in situ depth profile of colony distribution, enumerated trichomes and corrected for the effect of reduced light on the rate of N2 fixation. High rates of ectoenzyme activity associated with Trichodesmium colonies showed that the cyanobacteria are also important in regenerated production. The primary environmental factors regulating N2 fixation by Trichodesmium appeared in the following sequence throughout the year: nitrate intrusion during vertical mixing of the water column, increased surface seawater temperature, low wind speed causing stratification and atmospheric deposition of iron. The annual contribution of N2 fixation by Trichodesmium spp. to new production was of the same order or magnitude as recent refined model estimates for the North Atlantic Ocean.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available