Biomass and primary productivity of the cyanobacterium Trichodesmium spp. in the tropical N Atlantic ocean

Abstract

Primary production and standing crop, as chlorophyll- a (chl- a), of Trichodesmium 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 l −1 and depth integrated abundance of 91×10 6 trichomes m −2. Average surface densities were 292 and 222 trichomes l −1 and depth integrated abundance 21 and 8.6×10 6 trichomes m −2 for the April 1996 and October 1996 cruises, respectively. Total (phytoplankton plus Trichodesmium) chl- a standing crop and the percentage as Trichodesmium averaged 47 (62%), 22 (13%) and 30 (11%) mg chl- a m −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 m on the May–June 94 and October 96 cruises and at 40 m during the April 96 cruise. The average C:N ratio (atomic) of Trichodesmium was 6.5. Mean rates of total primary production ( Trichodesmium and other phytoplankton together) for May–June 1994, and April and October 1996 were 1080, 932 and 804 mg C m −2 d −1, and Trichodesmium accounted for an average of 47%, 7.9% and 11%, respectively, of the total primary production for each cruise. These primary production rates exceed those typically reported at oligotrophic open ocean sites. Trichodesmium C assimilation numbers (mg C fixed mg chl- a −1 h −1) were highest at the surface and were always lower than those of other phytoplankton. Average nitrogen demand, as calculated from the mean Trichodesmium C fixation from all three cruises, was about 40 mg N m −2 d −1, about 10% of concurrently determined rates of N 2 fixation.