Primary production and standing crop, as chlorophyll-a (chl-a), of Trichodesmiurn spp., and other phytoplankton as well as the abundance and depth distribution of Trichodesmium were measured on three cruises to the tropical North Atlantic Ocean. Trichodesmium abundance was greatest on a cruise in May-June 1994, with average surface densities of 2250 trichomes 1(-1) and depth integrated abundance of 91 X 10(6) trichomes m(-2). Average surface densities were 292 and 222 trichomes 1(-1) and depth integrated abundance 21 and 8.6 x 10(6) trichomes m(-2) for the April 1996 and October 1996 cruises, respectively. Total (phytoplankton plus Trichodcsmium) chl-a standing crop and the percentage as Trichodesmium averaged 47 (62%), 22 (13%) and 30 (11%) mg chl-am(-2) for May June 1994 and April and October 1996. On the May-June 1994 and April and October 1996 cruises 89%, 93% and 92% of the trichomes were in colonies, and the remainder occurred as free trichomes. Peak abundances of Trichodesmium were generally in the upper water column, with an average biomass maximum at 12 in on the May-June 94 and October 96 cruises and at 40 in during the April 96 cruise. The average C:N ratio (atomic) of Trichodesmium was 6.5.
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