5. Summary and Conclusions

Small, slow sinking particles are usually under-sampled by canonical particle collection methods and thus have been poorly characterized. Hence, little is known about their origins, biogeochemical composition, and role in the ocean biogeochemical cycles.
Our study shows that suspended and sinking particles were small and similarly sized. Unexpectedly, sinking particles were not characterized by a higher contribution of ballast materials (bSi, lSi and PIC), which would have justified their sinking behavior. However, they displayed low TEP-to-POC ratios in comparison to suspended particles. This observation suggests that high TEP contribution may have increased the residence time of suspended POC and thus reduced sinking particle fluxes impacting the efficiency of the biological carbon pump. Hence, we suggest that the composition of POC (i.e., presence of gels like TEP) should be routinely measured to assess how it affects the sinking of particles.
Our data together with findings from the field campaign hypothesize that the pool of small sinking particles in the mesopelagic formed via fragmentation of sinking fecal pellets, and zooplankton-mediated repackaging of suspended particles into fecal pellets. A smaller contribution to the small sinking particulate pool may have been provided via the disaggregation of low-density suspended aggregates. Such mechanisms may be especially important in low productivity systems during late summer months.
A comparison between particle fluxes calculated from the MSC with fluxes measured with drifting sediment traps and Thorium-234, suggests that small slow sinking particles exceeded the contribution of rare, large sinking particles to POC flux in the upper mesopelagic. Our data suggests that such flux of small sinking particles tends to be underestimated by trap and Thorium approaches. Potentially this flux of small sinking particles may contribute towards resolving the budget discrepancies between sinking flux and metabolic carbon requirements in the mesopelagic (Burd et al., 2010).