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).