ABSTRACT: The concentrations of filamentous diazotroph Trichodesmium spp., present as free trichomes and in colonial assemblages, were measured at approximately monthly intervals at Stn ALOHA (22*45'N, 158*00'W) between October 1989 and December 1992. The average abundance of filaments in the upper 45 m of the water column was highly variable ranging from 1.1 to 7.4 x 10⁴ trichomes m^-3 and from 0.02 to 1.4 x 10² colonies m^-3. Colonies were composed, on average, of 182 filaments accounting for 12% of total (free filament plus colonies) Trichodesmium biomass. Low densities of single trichomes were associated with, but not restricted to, deep mixing events and winter periods. During 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 assimilation and in the molar ratio of N:P of suspended particulate matter in the 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 photosynthetic carbon assimilation, 10% of the particulate nitrogen and 5% of the particulate phosphorus. We also estimate that Trichodesmium dinitrogen fixation accounted for, on average, at least 27% of the new production at this study site. These observations, combined with primary production experiments conducted on isolated colonies, suggest that phytoplankton production is enhanced due to the release of NH₄⁺ and dissolved organic nitrogen by Trichodesmium spp. during episodes of nitrogen fixation.

KEY WORDS: Trichodesmium . Nitrogen fixation . Nutrients . Productivity . Time-series

Published in MEPS Vol. 133 (1996) on March 28
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1996