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Based on satellite, hydrographic, and zooplankton data, the mesoscale activity, and its effect on the three-dimensional distribution of fish larvae in the northeastern Pacific water mass convergence region off Mexico during October 2022 were analyzed. Three eddies were observed during the cruise, but two were measured. An anticyclonic eddy (not crossed by the cruise) was observed by satellite data in the mouth of the Gulf of California. It formed off Mazatlán in Transitional Water and moved southeast of Cabo San Lucas. It was the smallest (10,207.4 km) and least energetic (140.7 cm 2 s − 2 ) of the eddies. A parallel sampling transect to this eddy showed predominance of fish larvae in preflexion stage in the surface mixed layer with dominance of Benthosema panamense and Syacium ovale species (epi-mesopelagic and soft-bottom demersal species). A second anticyclonic eddy had formed about a month earlier in the Tropical Surface Water when this was measured. Its trajectory showed that it came from the mainland coast off Cabo Corrientes. It was the most energetic (216.6 cm 2 s − 2 ) and presented the maximum anomalies of temperature (7.4 °C) and Dissolved Oxygen (>50 μ mol kg − 1 ), due to the downwelling that it produced (up to 95 m depth). It was also associated with the highest concentration of fish larvae (231.3 larvae 100 m − 3 ) from ∼200 m depth to surface, observing dominance of larvae in flexion stage of B. panamense and Bregamaceros bathymaster (coastal neritic species). This eddy could also retain and transport species of soft-bottom demersal habits (Dormitator latifrons). One cyclonic eddy, located off Cabo San Lucas had California Current Water. It was the largest (26,032 km 2) and oldest forming about nine months earlier off mainland coast in front of Pabellones Cove. Due to the upwelling of deeper water (up to 530 m depth), the minimum anomaly values of temperature (−5.8 °C), salinity (<−0.5 g kg − 1 ) and Dissolved Oxygen (<−50 μ mol kg − 1 ) were present. Low abundance of species such as Vinciguerria lucetia and Diogenichthys laternatus (both epi-mesopelagic) in the postflexion stage were presented there. Results showed that mesoscale eddies affected the three-dimensional fish larvae distribution in different ways. This could broaden the vision of the mesoscale evolution of physical-biological interactions in the oceans. • Different eddy trajectories were observed in NE Pacific off Mexico. • Youngest and most energetic anticyclonic eddy had high fish larvae abundance. • Oldest and least energetic cyclonic eddy had low fish larvae abundance. • Mesoscale eddies influence fish larvae distribution and development differently. [ABSTRACT FROM AUTHOR]

Vertical distribution of the cephalopod paralarvae was investigated in relation to a system of two cyclonic and three anticyclonic eddies in the southern Gulf of California and a front in the adjacent Pacific Ocean. Results showed that the preferential habitat for the Sthenoteuthis oualaniensis – Dosidicus gigas “SD-complex” in both regions was the oxygenated surface mixed layer and the thermocline. The highest abundances occurred in of one of the anticyclonic eddies and a frontal zone, which are convergent structures. Enoploteuthid and Pyroteuthid paralarvae both displayed their highest abundances in the thermocline. Pyroteuthids dominated in the cyclonic eddy whereas Enoploteuthidae were less evident in the eddy system. Pyroteuthids were observed on the western (California Current) side of the frontal zone, and Enoploteuthids on its eastern (Gulf of California) side. The octopods and the complex of Ommastrephes-Eucleoteuthis-Hyaloteuthis paralarvae were present below the thermocline. Both groups had a scarce presence in the eddy system and high abundance near the frontal zone. The octopods abounded on the eastern side in association with the low dissolved oxygen concentrations (< 44 µmol kg −1 ) of Subtropical-Subsurface Water; the complex on the western front side was immersed in California Current Water. It may be concluded that the spawning and early stages of development of these cephalopod groups are associated with particular mesoscale structures of the water masses. For example, the “SD complex” inhabits the surface water masses, preferentially in convergence zones generated by mesoscale activity. [ABSTRACT FROM AUTHOR]

The three-dimensional distribution of larval fish habitats in a mesoscale dipole eddy was analyzed in the Southern Gulf of California in July 2010. Detection of the characteristic mushroom-shape using chlorophyll and sea surface temperature satellite images was used to direct sampling of two transects across the structure with conductivity, temperature, depth casts and stratified oblique zooplankton hauls with opening-closing net. The latter sampled every 17 m layer down to the thermocline and every 50 m layer from the thermocline down to 200 m depth. Lowered Acoustic Doppler Current Profiler velocities revealed that the dipole structure persisted down to 800 m depth. The “cap” showed two opposite poles: one cyclonic with diameter ~50 km, and the other anticyclonic with diameter~80 km. It had a vigorous central jet with speeds>25 cm/s. The Θ-S A properties were consistent with formation of the structure outside the Gulf of California in Tropical Surface and Subtropical Subsurface water masses. In this context, three larval fish habitats were statistically defined ( P <0.001). One habitat, located in the surface mixed layer (~15 m depth), was characterized by the highest species richness, high abundance of the mesopelagic species Vinciguerria lucetia and the presence of epipelagic species like Auxis sp.; this habitat was correlated with the warm (24–25 °C) and oxygenated (4–5 mL/L) Tropical Surface Water. The other two larval fish habitats were defined below the thermocline (between ~15 and 50 m depth) and comprised mainly by mesopelagic species. One of them occurred in the cyclonic pole and central jet zone with the highest larval abundance, dominated by Diogenichthys laternatus; the other habitat was located in the dipole margins, or zones of return flow, with the lowest larval abundance, characterized by Psenes sio and Bathylagoides wesethi, species frequent outside of the Gulf of California. These two habitats were correlated with cooler (~13–22 °C) and hypoxic (0.03–1 mL/L) Subtropical Subsurface water masses. The results suggest that during the formation of the dipole, water masses dominated by different species were trapped, forming larval fish habitats. [ABSTRACT FROM AUTHOR]

The distribution and transport of paralarvae of the complex Sthenoteuthis oualaniensis - Dosidicus gigas (SD complex, Cephalopoda: Ommastrephidae) in the tropical-subtropical convergence off Mexico were analyzed under contrasting interannual environmental conditions. Two El Niño years were sampled (2010 and 2016), in which the SST were up to 3 °C above the average throughout the study area, while during La Niña year (2012) the SST anomalies were up to 2 °C below the average. During El Niño years, a relative shallow thermocline (<60 m depth) was detected around Cabo Corrientes, where the paralarvae were found with mean abundance of 2 org 1000 m−3. During La Niña condition, a shallower thermocline was detected in the entrance of the Gulf of California (~20 m depth) and the paralarvae abundance rose to 84 org 1000 m−3 with high frequency of occurrence. In both El Niño and La Niña conditions the paralarvae were mainly found in the strongly stratified waters (>80 cycles h−1). In addition, a simple dispersion Lagrangian model based on observed satellite geostrophic currents, suggested association of the SD complex paralarvae with mesoscale structures, in which the eggs and hatched paralarvae had an appropriate habitat for a nursery, retaining the paralarvae until their recruitment in a ~100 km radius. But during warm extreme events, with less mesoscale activity, the paralarvae could be transported as far as 500 km southeast of the spawning zone. Even though the interannual ENSO conditions modulate the magnitude of the paralarvae abundance, the shallow and well developed thermocline could help paralarvae to increase their survival chances, while mesoscale activity retains them near the productive coastal waters. • ENSO phases modulate Sthenoteuthis-Dosidicus complex paralarval abundance. • Mesoscale eddies retain paralarvae near coastal waters. • Lack of mesoscale eddy circulation leads to high paralarval dispersion rate. [ABSTRACT FROM AUTHOR]

The classification of the ocean in water masses with similar physical and/or biogeochemical characteristics provides an ideal framework for an efficient monitoring of the change in ocean properties. Particularly, the combination of the seawater temperature and salinity set the stratification of the water column and impacts ocean circulation, with important implications for the surface-to-interior propagation of climate signals. The objective of this study is to find spatially coherent thermohaline structures in different regions of the global ocean, as well as their link with regional dynamics. To this end, we apply clustering techniques to identify water masses delimited by coherent thermohaline structures at different spatial scales over the global ocean. The clustering technique known as K-mean was used with a wide range of k values for conservative temperature and absolute salinity profiles of the entire ocean. Our analysis revealed the impact of the main dynamical oceanic structures (such as ocean fronts, currents, or regions of sluggish circulation) on the vertical thermohaline component. We present three cases of study, the California Current System, the Southern Ocean, and the Eastern Tropical Latitudes, where we could identify regions with common thermohaline characteristics (despite being from different ocean basins), as well as to identify seasonal changes and anomalous profiles. This method makes it possible to reduce the dimensionality of the water column, and allows for the establishment of regional limits driven by their vertical thermohaline structure instead of more rigid, and not always appropriate geographical borders. This has potential important applications for the monitoring and prediction of ocean variability in the context of a rapidly changing climate. [Display omitted] • A clustering method to identify coherent thermohaline structures is presented. • The thermohaline clusters follow known dynamic features at different spatial scales. • The thermohaline clusters identify seasonal changes and anomalous profiles. • This analysis allows the reduction of dimensionality of the global ocean. • A cluster database ready to use in future studies is provided. [ABSTRACT FROM AUTHOR]

We describe the seasonal changes in the horizontal and vertical distribution and abundance of euphausiid species associated with seven physical and 61 biological variables in the Gulf of California (24–31°N). Euphausiid community structure was explored in the epipelagic habitat (<200 m) in January, July, and October 2007 and in epipelagic to bathypelagic habitats (<1400 m depth) in May 2015. Twelve euphausiid species comprising two distinct regional assemblages were identified. Nyctiphanes simplex and Nematoscelis difficilis were the most abundant species (>90%) in all cruises carried out in the 26–31°N region and Euphausia distinguenda in the 24–26°N region (mostly in October >90%). We confirmed that Euphausia gibboides and Nematobrachion flexipes inhabit the mesopelagic habitat, adapted to <1 ml O 2 l –1 environmental condition. Although Euphausia lamelligera and N. simplex populations were concentrated in well-oxygenated water (>3 ml O 2 l –1 ) near the surface (<50 m), they were also detected in low densities in the hypoxic mesopelagic habitat (250–800 m), but only at oxygen levels less than 0.09 ml O 2 l –1 . Stylocheiron affine and Stylocheiron carinatum were numerous well below the seasonal thermocline (100–350 m). Nematoscelis difficilis and N. simplex extended into the hypoxic mesopelagic habitat, but at low densities. Multivariate analyses showed six seasonal and regional krill assemblages, which are characterized by different species (Similarity percentage analysis). Canonical Correspondence Analysis show that, of 68 variables, three abiotic factors (temperature, dissolved oxygen, and OMZ depth) and eight biotic factors (accessory pigments; zeaxanthin and 19'-hexanoyloxyfucoxanthin, and the abundance of six copepod species; Candacia pectinata , Canthocalanus pauper , Centropages furcatus , Rhincalanus nasutus , Scolecithrix danae , and Temora discaudata ), were the most influential variables associated with the vertical distribution and abundance of euphausiids. Krill‒copepod (predator–prey) relationships may be important determinants of daily and seasonal vertical distribution patterns. We conclude that the euphausiid vertical habitat reaches down to 900 m depth (with historical records of six species collected between 1000 and 2280 m) but with lower diversity and abundance in the mesopelagic and bathypelagic habitats than in the epipelagic habitat. [ABSTRACT FROM AUTHOR]