The importance of transparency in mesozooplankton communities
Transparency is a widespread, however largely understudied,
characteristic in oceanic zooplankton (Johnsen & Widder, 1998;
Orenstein et al., 2022). While most of the research on this
morphological characteristic have been done in relation to its
protective role from UV radiations or from visual predators
(e.g. , Brüsin et al., 2016; Hays et al., 1994; Sha et al., 2021),
here we showed that transparency might be a good parameter for assessing
carbon export intensity at the scale of the mesozooplankton community.
Indeed, while all transparency-related variables used were linearly
linked to carbon export intensity, none of the size-related parameters
were significantly correlated. This might be due to a constant size
variation in the BATS mesozooplankton community despite seasonal
dynamics among mesozooplankton size classes (Steinberg et al., 2012).
Moreover, differences between the three communities could be defined by
variations in overall transparency levels. Transparency gives
information about body structures and pigmentation links to
physiological functions (Orenstein et al., 2022). Hence, these
variations might reveal gut content or pigmentation as developed in
Vilgrain et al. (2021). Changes in community transparency also reflect
taxonomic variations in the community. For example, during the bloom
period, imaging data showed more individuals with folded appendages or
more elongated organisms. This might be explained by the presence of
chaetognaths and larvaceans, as represented on the positive side of PC3
axis (Supp. Figure III).
With copepods representing about 70% of the total scanned community in
this study well representing the community’s proportions (Deevey, 1971),
and knowing the high diversity of this group (probably around 400
species in the Sargasso Sea; Deevey and Brooks, 1977), inter-group
variations are likely to affect the overall community morphological
characteristics. For example, transparency variations can be linked to
the presence of migrating copepods, such as Pleuromamma sp.,
during the bloom period (Steinberg et al., 2000). These copepods migrate
more than a hundred thousand times their body length twice a day
(Goetze, 2011), and more generally diel vertical migrations vary between
dozens of metres to 2000 metres (Deevey & Brooks, 1977). To be
sufficiently efficient, they develop two systems for steady propulsion
and intensive escapes. The last one relies on a particular arrangement
of copepods’ muscles forming a lever system by being attached to
organisms’ exoskeleton (Kiørboe et al., 2010). Hence, it could be that
the tougher or quicker a copepod needs to be, the stronger its
exoskeleton must be to resist their muscles. This may explain why
organisms appeared more opaque during the bloom period to escape more
predators. Finally, a transparency-productivity relationship was found
for copepods in the Bay of Baffin, with a proportionally inverse
relationship between transparency and chlorophyll-aconcentrations (Vilgrain et al., 2021), which is quite similar to the
transparency-stratification and opacity-bloom dynamics found in this
study. Hence, transparency is related to the survival of an organism at
the individual scale (Martini et al., 2021) but it might also be a
carbon export intensity indicator at the entire community scale, as it
was suggested by Da Silva (2021).