ROLE OF TRICHODESMIUM SPP IN THE PRODUCTIVITY OF THE SUBTROPICAL NORTH PACIFIC-OCEAN

The concentrations of filamentous diazotroph Trichodesmium spp., prese nt as free trichomes and in colonial assemblages, were measured at app roximately monthly intervals at Stn ALOHA (22 degrees 45' N, 158 degre es 00' W) between October 1989 and December 1992. The average abundanc e of filaments in the upper 45 m of the water column was highly variab le ranging from 1.1 to 7.4 x 10(4) trichomes m(-3) and from 0.02 to 1. 4 x 10(2) colonies m(-3). Colonies were composed, on average, of 182 f ilaments accounting for 12% of total (free filament plus colonies) Tri chodesmium biomass. Low densities of single trichomes were associated with, but not restricted to, deep mixing events and winter periods. Du ring 1991 and 1992 the concentration of Trichodesmium spp. present in the water column increased relative to the pre 1991 observations. This increase coincided with increases in photosynthetic carbon assimilati on and in the molar ratio of N:P of suspended particulate matter in th e upper 45 m of the water column. However, the change in Trichodesmium biomass alone does not account for the change observed in autotrophic carbon assimilation and elemental biomass composition. Trichodesmium spp. comprised, on average, 18% of the chlorophyll a, 4% of the photos ynthetic carbon assimilation, 10% of the particulate nitrogen and 5% o f the particulate phosphorus. We also estimate that Trichodesmium dini trogen fixation accounted for, on average, at least 27% of the new pro duction at this study site. These observations, combined with primary production experiments conducted on isolated colonies, suggest that ph ytoplankton production is enhanced due to the release of NH4+ and diss olved organic nitrogen by Trichodesmium spp. during episodes of nitrog en fixation.