Spatial and temporal variation of the zooplankton community
The spatio-temporal structure of the zooplankton community was
investigated at both the family (mean abundances; SIMPER) and
ASV-species (Bray Curtis, Jaccard, weighted and unweighted Unifrac
distance metrics; PCoA) taxonomic levels. Over the spatial scales
examined, the results of the PCoAs obtained using the genetic
information from all stations supported a significant differentiation of
the zooplankton community between regions with low/high oxygen
concentrations, warm/cold temperatures, and east/west longitudes (Fig. 7
from A to F; PERMANOVA; pseudo-f: > 5.088; p <
0.0004). Notably, the regional differences in zooplankton community
structure were consistent with ecoregion partitioning based on the
aforementioned environmental and geographic variables (Fig. 3A and 3B;
PERMANOVA; pseudo-f: > 2.346; p < 0.0002). In
addition, higher zooplankton alpha-diversity was found at stations with
low mean oxygen concentrations and low water temperatures (Southern
ecoregion; Fig. 8A and 8B). Nevertheless, this increase was only
statistically supported for zooplankton communities detected with the
use of 18S rRNA (average Shannon diversity: N = 4.69, S = 4.93, and Y =
4.15; ANOVA; F = 13.54, p = 0.0001). Similar results were also observed
when the datasets from each expedition were individually analyzed
(Supplementary Information Fig. S5 and Fig. S6). Once again, zooplankton
community structure varied regionally and was mainly associated with
local changes in oxygen and/or local temperature. Finally, the empirical
north-south spatial boundary around the 22 °N parallel was statistically
supported for XIXIMI-04 and XIXIMI-06, whereas an alternative oxygen
and/or temperature water column configuration was observed for XIXIMI-05
(Fig. 6). Specifically, during this early summer expedition, warm waters
formed at a south-west boundary, resulting in a south-west/north-east
regional partitioning of the GoM (Fig. 6).
Given that significant differences in community structure were observed,
we implemented a SIMPER analysis in order to determine the contribution
of each taxa to the dissimilarity among the groups of samples. The same
analysis was conducted for both genetic markers; nevertheless, the
discrepancy in taxa abundance among the markers hampered an exhaustive
comparison between zooplankton communities detected with 18S rRNA and
COI, which was outside the scope of this study. As a result, we cannot
propose an unambiguous pattern for all the taxa, but we observed that
Centropagidae, Salpidae, Enoploteuthidae, Sagittidae, and Pyrosomatidae,
among others, were mainly detected at stations with lower than average
water temperatures and relatively low oxygen concentrations, which were
found in the western GoM. Accordingly, those taxa primarily contributed
to the spatial segregation of the southern ecoregion. On the contrary,
other taxa, such as Scaridae, Calanidae, and Processidae, mainly
contributed to making the Yucatan bio-ecoregion distinct, as they were
mostly detected in warm waters with high concentrations of oxygen in the
eastern GoM. The taxa contribution (cutoff at 1%) for 18S and COI is
reported in Supplementary Information Table S5.
Over the temporal scales analyzed, the dynamics of the zooplankton
communities exhibited similar patterns to those mentioned above. In this
regard, the PERMANOVA based on all distance metrics revealed significant
differences between all three sampling periods (pseudo-f: >
2.177; p < 0.0002; Fig. 9); nevertheless, those variations
where not accompanied by significant changes in zooplankton diversity.
Once again, we observed that differences in zooplankton community
structure may reflect seasonal environmental conditions, which may
consequently promote changes in the abundance of certain taxa, such as
Mysidae, Hormathiidae, and Euphausiidae, among others. These taxa were
mainly detected in spring (XIXIMI-05) when significantly higher oxygen
concentrations were detected. Likewise, during the summer expeditions
(XIXIMI-04 and XIXIMI-06), we observed an increase in the abundance of
Euphausiidae, Calanidae, and Hormathiidae, which suggests that high
water temperatures may be positively associated with these taxa
(Supplementary Information Table S6).