Relationships between environmental conditions and the
zooplankton community
In order to evaluate the extent to which physical environmental
variables may shape the spatial and temporal structure of the
zooplankton community in the GoM, the relationships between
environmental data and taxa-abundance matrices were evaluated through
DistLM. These analyses were conducted using both the genetic information
of each cruise and in a comprehensive analysis in which all stations
(from all cruises) were included. For the latter, the results of the
DistLM marginal tests indicated that each of the environmental
predictors (when considered in isolation) explained a fraction of the
variation observed in the zooplankton community structure (ranging from
2% up to 8.5%; Table 1A and 1B). However, the sequential tests showed
that up to 18% of zooplankton diversity may be explained by the
synergistic combination of DO, temperature, and longitude for both loci
(Table 1C and 1D; R2 < 0.19, P <
0.0004). Moreover, for only 18S, an additional 1.68% of zooplankton
variability may be explained by salinity (Table 1C). Similar results
were also obtained with the individual cruise datasets (Supplementary
Information Table S5).
The relationship between environmental parameters and abundance at the
family level detected with both loci was visualized with a dbRDA plot.
The first axis captured up to 43.1% of the fitted variability and 8.4%
of the total variation among both loci and was mainly correlated with
two hydrographic variables (oxygen and temperature; Fig. 3A and 3B). The
second axis captured up to 35.2% of the fitted variability and
~6% of the total variation, and the strongest
correlation was with longitude. Notably, zooplankton abundance scaled
negatively with DO, temperature, and longitude for the majority of
stations, indicating that a higher degree of complexity in the
zooplankton community could be present in waters that are relatively
cold and that have low dissolved oxygen concentrations, which were
located in the western portion of the GoM.
In order to further evaluate this result and explore the potential
differences in community structure between stations, we clustered each
station considering its hydrographic features and geographic position
(Fig. 4). Our finding suggests the presence of three different
“ecoregions” in the deep water region of the southern GoM. The first
ecoregion mostly included stations off the eastern coast of the Yucatan
platform (sampling line Y, hereafter Y) located at < 86.30 °W
(Fig. 5 and Fig. 6), which were mainly characterized by high oxygen
concentrations and temperatures. Moreover, among the western stations of
the GoM (> 86.30 °W), an additional latitudinal boundary
was observed in the proximity of the 22 °N parallel (Fig. 6). Indeed,
the northern stations (sampling lines: A, B, and C; hereafter N)
generally presented higher concentrations of oxygen and higher water
temperatures than those of the southern stations (sampling lines: D, E,
F, G, H, and J; hereafter S). However, the proposed partition was less
clear for the stations located along Lines C, D, and E, which suggests
that environmental differences are most notable between the extreme
southern and northern sectors of the surveyed area and less marked in
between (Fig. 6).
Overall, the hydrographic measurements followed trends that are expected
with seasonal change; salinity and temperature were higher during summer
(and highest in August-September 2017 cruise XIXIMI-06) while oxygen,
fluorescence, and density were higher in late spring (June 2016 cruise
XIXIMI-05). However, no significant inter-annual differences were
observed with regard to hydrographic predictors with the exception of
dissolved oxygen, which was significantly higher in late spring
(XIXIMI-05) compared to that in summer (XIXIMI-04 and XIXIMI-06; one-way
ANOVA; F = 9.97, p < 0.0002). Additionally, we observed
significant differences in the environmental variables among the N, S,
and Y ecoregions (PERMANOVA; pseudo-f > 4.7623; p =
0.0002).