Laura Sánchez-Velasco

The genus Symphurus is a group of demersal fish with high diversity and taxonomic challenges during their early development. This study addressed the morphological and molecular identification of Symphurus larvae in La Bahía de la Paz and the entrance to the Gulf of California. Zooplankton samples were collected during the day and night at 58 stations in October 2022. Based on morphological characteristics such as meristics and pigmentation, seven Symphurus morphotypes were pre‐classified and genetically analysed using the mitochondrial genes cytochrome c oxidase subunit I (COI) and 16S rRNA (16S). Genetic analysis identified four morphotypes: three of them corresponded to a single species, Symphurus williamsi , with 100% similarity in the 16S gene, while the other morphotype was identified as Symphurus chabanaudi , with 100% similarity in both markers (COI and 16S). The pigmentation patterns described after molecular identification were consistent and allowed differentiation between the two species. S. williamsi presented three to five strongly stellate melanophores on the dorsal side and five to six melanophores on the ventral side during the preflexion to postflexion stages, while S. chabanaudi had three prominent melanophores in the central part of the notochord (lateral line), as well as a series of small melanophores positioned on the dorsal and ventral sides. S. williamsi larvae were located throughout the entire entrance to the Gulf of California, mainly in Transitional Water, while Symphurus chabanaudi was collected only in La Bahía de la Paz in Gulf of California Water. These results demonstrated that the integration of genetic and morphological tools is fundamental in taxonomic studies of ichthyoplankton, particularly for the identification of species in early developmental stages that exhibit high morphological similarity. In addition, water masses might be a signal of spawning habitat of different species.

This study aimed to identify species of marine ichthyoplankton communities from the entrance of the Gulf of California, using morphological and DNA metabarcoding methods. Twenty‐eight zooplankton samples were collected by vertical trawls in the study area, from 2 to 8 March 2016. A total of 742 marine fish larvae and 333 fish eggs were separated and morphologically identified to the most precise taxonomic level or typified, respectively. Subsequently, larvae and eggs were pooled by sampling station for DNA extraction. A 12S amplicon library (167–185 bp) was constructed and sent for high‐throughput sequencing on an Illumina HiSeq (2X150 bp) platform. The Obitools bioinformatic pipeline was implemented for assembly and quality control of amplicon sequences. Ecotag and BLAST searches were performed for taxonomic assignment of amplicons (similarity ≥97%) when compared with 12S fish sequences from GenBank. Metabarcoding results showed a similar number of orders, families and genera to the morphological method. However, at the species level, the DNA metabarcoding method revealed 17 species that were not recognized by the morphological method, indicating that the DNA method was more precise and contributed to the knowledge of the species richness of fishes that spawn in the study area. Furthermore, our presence/absence results aligned largely with the more common ichthyoplankton species found in most oceanographic surveys in the region. A comparison with a previous study performed in the same oceanographic survey indicates concordance with the spatial distribution of Benthosema panamense , which was delimited by temperature. However, differences with the spatial distribution of Diaphus pacificus were observed, likely because this species was concealed within unidentified eggs. In conclusion, DNA metabarcoding is a valuable complement to traditional morphological identification in ichthyoplankton research and can contribute to finding some undetected members of ichthyoplankton communities as well as geographical differences in their species richness.

The 2015–2016 El Niño was one of the most intense events in the Mexican Tropical Pacific and the Gulf of California, characterized by increased temperatures and shifts in water-mass proportions. To quantify these variations and their impact on carbon biogeochemistry, an Optimum Multiparameter (OMP) analysis was implemented using temperature, salinity, and dissolved oxygen (O2) data from three oceanographic cruises (June 2015, March 2016, and October 2017) and Argo float profiles. Results showed that between June 2015 and March 2016, Tropical Surface Water (TSW) dominated the upper 70 m (>50%), deepening both the oxycline and the dissolved inorganic carbon gradient (hereafter carboncline, defined as the depth of maximum DIC vertical gradient). This TSW intrusion reconfigured the vertical structure of the water column, directly affecting the biogeochemistry of O2 and dissolved inorganic carbon (DIC). Estimates of biogeochemical residuals (DIC and O2) revealed distinct processes: in June 2015, the distribution of residuals suggests a signal consistent with aerobic remineralization, located in the subsurface layer (70-140 m), driven by the mixing of Gulf of California Water (GCW) and Subtropical Subsurface Water (StSsW). In contrast, in March 2016, a cyclonic eddy acted as a localized productivity oasis, pumping nutrient-rich subsurface waters into the euphotic zone and driving a statistically significant photosynthetic signal (∆DIC < 0, ∆O₂ > 0) that challenged the expected regional oligotrophy of El Niño conditions. During the El Niño-Southern Oscillation (ENSO) neutral conditions in October 2017, physical water-mass mixing overrode any clearly distinguishable biogeochemical signal. These findings demonstrate that the use of biogeochemical residuals is a robust mechanism for assessing O2 and DIC biogeochemistry within the oxygen minimum zone (OMZ) of the Mexican Tropical Pacific under interannual fo

Entrance to the Gulf of California is a hydrographically complex region where the interaction among regional circulation, contrasting water masses, and intense mesoscale activity promotes the formation of thermal fronts. In this study, we analyzed the surface and subsurface distribution of thermal fronts in the region and evaluated the role of mesoscale eddies in their formation. Daily satellite-derived sea surface temperature and three-dimensional GLORYS fields (temperature and velocity) were analyzed for 2003-2025. Thermal fronts were identified from temperature gradients, their frequency was quantified as the probability of occurrence, and the temporal variability of the gradient magnitude was characterized through wavelet analysis. Eddy kinetic energy was used to assess coupling with mesoscale variability, with HYCOM outputs used to examine eddy-driven frontogenesis. Results show that thermal gradients are dominated by the annual cycle, with intermittent intraseasonal variability, while thermal fronts exhibit a well-defined three-dimensional structure, with maximum intensity and occurrence consistently found around 50 m depth. Surface monthly climatologies of frontal probability displayed a marked seasonal cycle, with maxima concentrated near coastal regions during spring, while remaining relatively low over much of the oceanic domain due to the transient nature of frontal structures. Selected snapshots revealed that mesoscale eddies enhance frontogenesis through advection, trapping, and flow deformation, with the strongest gradients developing along eddy peripheries and during dipolar interactions, whereas in later stages, enhanced mixing can weaken thermal contrasts. These results highlight the role of mesoscale variability in the water-mass distribution and the eddy evolution as controlling the formation and vertical structure of thermal fronts.

In the oxygen minimum zone of the eastern tropical North Pacific off the Gulf of Tehuantepec, mesoscale eddies and offshore jets often develop in response to Tehuano wind events. These dynamic features play crucial roles in modulating the vertical distribution of nutrients, dissolved oxygen, biogeochemical cycling, and regional primary productivity. This study analyzed data from two BGC‐Argo floats deployed from 2022 to 2023 that followed distinct northward and southward trajectories while collecting data on a wide range of oceanographic conditions. The floats sampled anticyclonic and cyclonic eddies, a shear zone, and Tehuano wind events. The results indicate that the base of the oxycline coincided with the upper boundary of Subtropical Subsurface Water. Anticyclonic eddies deepened the oxycline, creating oxygen‐rich and nitrate‐poor conditions within the upper 100–200 m of the water column. In contrast, cyclonic eddies, the shear zone, and Tehuano wind events shoaled the base of the oxycline to 25–50 m, enhancing hypoxia, increasing nitrate availability in surface waters, and elevating chlorophyll‐a concentrations. Notably, some eddies sampled by the southern float exhibited elevated nitrate concentrations between 200 and 400 m, suggesting a seasonal intrusion of oxygenated equatorial waters that may have inhibited nitrogen loss. Overall, these findings highlight the intricate links between physical forcing, biogeochemical structures, and microbial activity in this region.

The 2015–2016 El Niño was one of the most intense events in the Mexican Tropical Pacific and the Gulf of California, characterized by increased temperatures and shifts in water-mass proportions. To quantify these variations and their impact on carbon biogeochemistry, an Optimum Multiparameter (OMP) analysis was implemented using temperature, salinity, and dissolved oxygen (O2) data from three oceanographic cruises (June 2015, March 2016, and October 2017) and Argo float profiles. Results showed that between June 2015 and March 2016, Tropical Surface Water (TSW) dominated the upper 70 m (>50%), deepening both the oxycline and the dissolved inorganic carbon gradient (hereafter carboncline, defined as the depth of maximum DIC vertical gradient). This TSW intrusion reconfigured the vertical structure of the water column, exerting direct effects on the biogeochemistry of O2 and dissolved inorganic carbon (DIC). Estimates of biogeochemical residuals (DIC and O2) revealed distinct processes: in June 2015, the distribution of residuals suggests a signal consistent with aerobic remineralization, located in the subsurface layer (70-140 m), driven by the mixing of Gulf of California Water (GCW) and Subtropical Subsurface Water (StSsW). In contrast, in March 2016, a cyclonic eddy acted as a localized productivity oasis, pumping nutrient-rich subsurface waters into the euphotic zone and driving a statistically significant photosynthetic signal (∆DIC < 0, ∆O₂ > 0) that challenged the expected regional oligotrophy of El Niño conditions. During the El Niño-Southern Oscillation (ENSO) neutral conditions in October 2017, physical water-mass mixing overrode any clearly distinguishable biogeochemical signal. These findings demonstrate that the use of biogeochemical residuals is a robust mechanism for assessing O2 and DIC biogeochemistry within the oxygen minimum zone (OMZ) of the Mexican Tropical Pacific under interannual fo