Mass sedimentation of picoplankton embedded in organic aggregates

During a survey cruise crossing the Subtropical Front over the Chatham Rise east of New Zealand, we made the first observation in High Nutrient Low Ch lorophyll waters of massive sedimentation of picoplankton embedded in large (>0.5 mm diam.) organic aggregates (OAs) sensu Riley (1963). We estimate 9 .3 mg C m(-2) yr(-1) of prokaryotic picoplankton biomass alone may be trans ported below the euphotic zone via this mechanism. Using confocal microscop y, we made direct observations of picoplankton within undisrupted individua l OAs, collected in sediment traps fitted with acrylimide gels, which large ly conserved particle structure. Prokaryotic picoplankton autofluorescence was well-preserved and concentrations were extremely high within large rapi dly sedimenting aggregates, ranging from 1.06 X 10(8) ml(-1) in the 120 m s ediment traps in subantarctic waters to 7.5 X 10(6) ml(-1) at 550 m in subt ropical waters, yielding Enrichment Factors of 10(3)-10(5) relative to pico plankton concentrations in the water column. Aggregate picoplankton concent rations showed a well-constrained exponential decline with depth, which we speculate may represent an estimate of protozoan grazing rate within the ag gregates. Picoplankton were found within heterotrophic flagellates, within copepod fecal pellets, and within organic matrices, all of which were incor porated in OAs. The key event of picoplankton incorporation into initial pa rticles occurs in the upper water column, very likely through grazing with low assimilation efficiency. Once OAs are formed, their changing porosity a nd reduction in picoplankton cell numbers via heterotrophy is likely to be a key factor mediating picoplankton carbon fluxes in moderately productive ecosystems, and in determining the overall particle structure of sedimentin g OAs.