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).