Spatial and Temporal Distribution of Tricodesmium sp. at Station ALOHA: How Important Are They?

R. M. Letelier

Abstract

Abundance of Trichodesmium spp. colonies and single trichomes have been monitored at Sta. ALOHA since October 1989. Both morphologies present a vertical distribution with a distinct sub-surface biomass maximum located between 20 and 50 m depth that may account for up to 40% of the total chlorophyll a. Nevertheless, the contribution of Trichodesmium to the primary production in the euphoric zone estimated using H₁₄CO₃^-, is only 4-7%.

The temporal distribution of Trichdesmium during 1990 presents high variability when compared to the following years. During this period only two species were observed (T. thiebautii and T. erytraeum). T. contortum appears for the first time in 1991 and remains present throughout 1992. The presence of T. contortum is associated with an increase in the concentration of free trichomes in the mixed-layer and may be explained by an increase in the stability of the upper water collumn. During this period, nitrogen fixation by Trichodesmium may increase its importance as a source of new nitrogen to the euphoric zone and may be responsible for the observed shift in the elemental composition of the particulate organic matter.