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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.dsr.2009.09.005 Byline: Fernando Gonzalez Taboada, Ricardo Gonzalez Gil, Juan Hofer, Sonia Gonzalez, Ricardo Anadon Keywords: Diazotroph; Nitrogen cycle; Size structure; North Atlantic subtropical gyre; Trichodesmium Abstract: Trichodesmium spp. population structure was studied based on a transect in the eastern subtropical North Atlantic in October 2006, covering a gradient from near-temperate conditions up to fully subtropical, oligotrophic waters. Trichodesmium spp. trichomes were counted and measured, distinguishing free trichomes from those forming macroscopic colonies ('puffs' or 'tufts'). Moreover, for both free and colonial individuals, abundance and size structure variation were quantified from single trichome-level data. Nitrogen fixation rates were estimated using an empirical approach based on abundance observations and a heuristic approach to incorporate variation in trichome size and environmental temperature. A gradual change towards greater sizes and increased abundances was observed as physical conditions became subtropical. An extreme response was detected at one station (29.8[degrees]W, 26.3[degrees]N), in which both in situ and remote sensing data revealed the presence of a Trichodesmium bloom. Despite the presence of a mesoscale anticyclonic eddy, a close examination of prior surface conditions using altimeter data indicated that the bloom was related to the advection of southern waters. In general, our results highlighted the importance of free trichomes in Trichodesmium spp. populations in this part of the North Atlantic. Furthermore, they suggested a possible role of both size and population structure on the wide range of N.sub.2 fixation estimates currently available in the literature. Author Affiliation: Area de Ecologia. Dpto. Biologia de Organismos y Sistemas de la Universidad de Oviedo, C/ Valentin Andres Alvarez s/n, E33071, Oviedo, Asturias, EspaA[+ or -]a Article History: Received 27 March 2009; Revised 12 September 2009; Accepted 25 September 2009