THE CONTRIBUTION OF MICROORGANISMS TO PARTICULATE CARBON AND NITROGENIN SURFACE WATERS OF THE SARGASSO SEA NEAR BERMUDA

Seawater samples were collected from the euphotic zone of the Sargasso Sea near Bermuda in August of 1989 and March-April of 1990. Microbial population abundances, chlorophyll concentration, particulate carbon and particulate nitrogen were measured. Calculations were performed to establish the relative and absolute importance of the various microbi al assemblages. The choice of conversion factors (g C and N cell(-1), or g C and N mu m(-3)) for the microbial populations dramatically affe cted the estimation of ''living'' and ''detrital'' particulate materia l in the samples, and the relative importance of the various microbial groups. Averaged over all samples on either of the two cruises, micro bial biomass constituted a greater proportion of the total particulate carbon and nitrogen during March-April (55% and 63%, respectively), t han during August (approximate to 24% and 30%, respectively) using ''c onstrained'' conversion factors that were derived. Accordingly, detrit al material constituted the bulk of the particulate material during Au gust, but was similar to the amount of microbial biomass during March- April. The bacterial assemblage constituted the largest single pool of microbial carbon (35%) and nitrogen (45%) in the water, and a signifi cant fraction of the total particulate carbon (approximate to 10-20%) and nitrogen (approximate to 15-30%). Phototrophic nanoplankton (micro algae 2-20 mu m in size) were second in overall biomass, and often dom inated the microbial biomass in the deep chlorophyll maxima that were present during both cruises. The results temper recent assertions conc erning the overwhelming importance of bacterial biomass in the oligotr ophic Sargasso Sea but still support a major role for these microorgan isms in the open ocean as repositories for carbon and nutrients.