Abstract. Diel vertical migration (DVM) can enhance the vertical
flux of carbon (C), and so contributes to the functioning of the biological
pump in the ocean. The magnitude and efficiency of this active transport of
C may depend on the size and taxonomic structure of the migrant zooplankton.
However, the impact that a variable community structure can have on
zooplankton-mediated downward C flux has not been properly addressed. This
taxonomic effect may become critically important in highly productive
eastern boundary upwelling systems (EBUSs), where high levels of zooplankton
biomass are found in the coastal zone and are composed by a diverse community
with variable DVM behavior. In these systems, presence of a subsurface
oxygen minimum zone (OMZ) can impose an additional constraint to vertical
migration and so influence the downward C export. Here, we address these
issues based on a vertically stratified zooplankton sampling at three
stations off northern Chile (20–30∘ S) during
November–December 2015. Automated analysis of zooplankton composition and
taxa-structured biomass allowed us to estimate daily migrant biomass by taxa
and their amplitude of migration. We found that a higher biomass aggregates
above the oxycline, associated with more oxygenated surface waters and this
was more evident upon a more intense OMZ. Some taxonomic groups, however,
were found closely associated with the OMZ. Most taxa were able to perform
DVM in the upwelling zone withstanding severe hypoxia. Also, strong
migrants, such as eucalanid copepods and euphausiids, can exhibit a large
migration amplitude (∼500 m), remaining either temporarily or
permanently within the core of the OMZ and thus contributing to the release
of C below the thermocline. Our estimates of DVM-mediated C flux suggested
that a mean migrant biomass of ca. 958 mg C m−2 d−1 may contribute
with about 71.3 mg C m−2 d−1 to the OMZ system through respiration,
mortality and C excretion at depth, accounting for ca. 4 % of the net
primary production, and so implies the existence of an efficient mechanism
to incorporate freshly produced C into the OMZ. This downward C flux
mediated by zooplankton is however spatially variable and mostly dependent
on the taxonomic structure due to variable migration amplitude and DVM
behavior.
Distribution of Euphausia mucronata at the upwelling area of Peninsula Mejillones, northern Chile: The influence of the oxygen minimum layer