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).