Different taxonomic assemblages and ecosystem dynamics
Among the six modules identified in the co-occurrences network, two
confirmed the link between transparency and low export. Module C was
related to the summer stratification community. At this time of the
year, new nutrient inputs in the ecosystem were scarce and taxa were
typical of a recycling community. It means that nutrients are recycled
by coprophagy, particle-feeding and throughout the trophic network which
helps to maintain zooplankton biomass in the upper mixed layer during
periods of intense stratification of the water column (Kiørboe, 1997).
Images analyses further revealed that the community tended to be more
transparent when module C was dominant which could be explained by the
presence of low carbon content organisms (Siphonophorae, Salpida). The
taxa in module C fit in these ecological niches. Oithona sp.
biomass is negatively correlated to the vertical flux of faecal pellets
carbon (Svensen & Nejstgaard, 2003) and they also are also known to
reduce the vertical flux of faecal pellets (González & Smetacek, 1994).Paracalanus aculeatus , a small copepod typical of low production
ecosystems which helps in nutrients recycling, was also present
(Paffenhöfer et al., 2006). The abundance of the recycling community and
the presence of organisms known to have a low carbon content explain the
reduced carbon export during summer stratification. Module C contained
five Gastropoda taxa, three of which do not have any shell reducing
their dry weight content (volume:C ratios 20 to 100 times those of
non-gelatinous organisms; Kiørboe, 2013) and contribute to the overall
transparency of the community (Alldredge, 1984). Found commonly in the
samples, Atlanta selvagensis has a transparent shell (de Vera &
Seapy, 2006). Along with Thliptodon sp., they are strong
recyclers which feed on other mesozooplankton, including Cymbuliasp. (personal communication of Amy Maas; Mifsud, 2001). Further
strengthening the link between opacity and low carbon export,Cymbulia sp. and Peracle reticulata are particle-feeders
associated with recycling communities. Module F, although relatively
less abundant than module C, was also negatively correlated with carbon
export. It included Doliolida, Larvacea or Bryozoa taxa whose centroids
are on the transparent and low export side of the morphospace (PC2
> 0, Supp. Figure III), further linking transparency and
recycling community. Again, it contained taxa indicating a recycling
community. For example, Halocypris sp. are predominantly
detritivores (Angel, 1983) and play a role in the recycling of organic
matter (Castellani & Edwards, 2017). In addition to having the same
transparency signal as module C, module F was also linked to a community
with more visible appendages (PC3 < 0, Supp. Figure III),e.g. , Bryozoa larvae can use their cilia to capture particles
(Strathmann, 2006).
Despite the above, an opacity-export signal could have been expected for
two other modules but was not measured in this study. Module A contained
almost exclusively taxa known to migrate, and using the co-occurrences
network, we identified these taxa potentially interacting (Berry &
Widder, 2014) that we would have missed otherwise. Euphausidae, except
for the genus Stylocheiron, , are in general strong migrators
(Brinton, 1962; Roe et al., 1984) and known to contribute to the diel
community changes at BATS station (Blanco-Bercial, 2020). Siphonophorae
and Salpidae also realise diel vertical migrations (Andersen et al.,
1992; Lüskow et al., 2019). DVM was shown for several genus of ostracods
(McHardy and Bary, 1965; Ursella et al., 2018; Kaeriyama and Ikeda,
2002). Even though DVM is a process known to significantly contribute to
the biological pump (Steinberg et al., 2008), no significant positive
correlation was found between carbon export and module A relative
abundances. There are several possible explanations for this
discrepancy. Euphausidae migration ranges are usually larger than 200
meters (Andersen & Sardou, 1992), meaning that sediment traps might
have missed their contribution to the overall export. A possible
explanation for DVM is the predator-evasion hypothesis, in which large
or pigmented organisms can hide at depth during the day to be less
visible while transparent organisms can stay in surface waters (Johnsen,
2014; Sha et al., 2021). However, no correlation was found between
migrators and opacity, which might be explained by the non-amplification
of certain taxa (see Quantitative property of metabarcoding). For
example, Amphipoda are common vertical migrators in the Sargasso Sea
(Steinberg et al., 2000). Individuals were part of the morphological
dataset and very opaque in a large majority (PC2 < 0, Supp.
Figure III), however they were not present at all in the molecular
dataset, leading to a potential DVM signal discrepancy. Finally, Module
D matched with the zooplankton bloom community when the peak of biomass
usually occurs (Madin et al., 2001; Steinberg et al., 2001). This
community was composed of organisms more elongated without visible
appendices, e.g. , Larvacea, Chaetognatha (PC3 > 0,
Supp. Figure III). Module D relative abundances were positively and
strongly correlated to carbon export; however, no opacity signal was
found. It might be due to the gelatinous plankton taxa it contained
(five Oikopleura sp., four Doliolidae and one Salpidae). Indeed,
by their ability to bloom and/or to produce high quantity of faecal
pellets, they can play an important role in carbon export (10-79% of
the 200-m POC flux for salps, Stone and Steinberg, 2016), but by being
highly transparent (PC2 > 0, Supp. Figure III; Johnsen and
Widder, 1998), their abundance might have skewed the expected
opacity-export signal.
Imaging data were obtained from formalin-fixed organisms through the
Zooscan process. Grey colour levels and shape might then not exactly
represent how organisms looked when alive, however, it is known that the
position of copepods preserved in formalin remain stable after an
initial shrinkage period (Sampei et al., 2009) which is much shorter (in
the order of days) than the time gap between sampling and scanning in
our study. Because more than 1500 organisms were scanned per size-class
and sample, we believe that shape-related morphological descriptors,
such as Size or Elongation (PC1 and PC3, Figure 1), were robust at the
scale of the community. Similarly, all the samples were stored under
exactly the same conditions, following the same protocols, and therefore
underwent the same potential degradation. There is still the fact that
fixation affects each taxonomic group differently (e.g.gelatinous vs. arthropods). The morphological descriptors linked to the
transparency of the organisms are however inter-comparable since we are
analysing trends and changes in the community, therefore the patterns
found would then be reliable.