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Abstract: The upper layer circulation in the Bay of Campeche is analyzed with three years of data recorded by surface drifters, current meter moorings, and satellite altimetry. The measurements show that the mean cyclonic circulation observed by previous authors extends below 1000m, and that its size and location are delimited by the particular topography of the region: a deep basin to the west, and a shallower and gentle sloping submarine fan to the east. An Empirical Orthogonal Function analysis and large correlations of the surface flow with the deeper currents suggest that the topographic constraint is the result of potential vorticity conservation for an equivalent barotropic flow. The variability of the surface currents in the western basin is mostly due to changes in the size, form, position and intensity of the cyclonic gyre due to its interaction with northern Gulf of Mexico eddies, particularly Loop Current Eddies traveling the southern route towards the western boundary. By contrast, the eastern basin is characterized by a weak northward drift, with the occasional generation of anticyclones in the southeastern boundary, the genesis of which remains to be understood. This suggests that the variability in the eastern basin is mostly driven by locally generated disturbances, rather than by an influx of northern Gulf of Mexico eddies. Strong northward flows in the central and eastern basins result from the flow convergence between locally generated anticyclones and the cyclonic gyre. [Copyright &y& Elsevier]

Abstract: The effects of the 1997–1998 and 2002–2004 El Niño on the upper waters in the continental shelf and slope regions off northwestern Baja California are explored with data from eight cruises taken in late spring from 1998 to 2004 and the summers of 1997 and 1998. Geostrophic velocities were calculated referenced to a specific volume anomaly surface separating the generally southward flowing California Current waters from the waters advected to the north by the California Undercurrent. The resulting fields show equatorward flow near the surface except in the summer of 1997, when a poleward jet was found in the upper 40dbar. This shallow jet advected anomalously warm and salty waters characteristic of the 1997–1998 El Niño, with its core found within 20–30km from the coast. By spring of 1998, the waters brought into the region by the jet had mixed across the pycnoline with the salty California Undercurrent waters below, resulting in high salinity levels on the density surfaces corresponding to the otherwise fresh California Current waters (25–26). By contrast, the 2002–2004 El Niño stands out for the very fresh and cold waters found on the same density surfaces in late spring of 2003 and 2004, marking a pronounced presence of subarctic waters. The fresh conditions found on the latter years represent a nearshore expression of the anomalous intrusion of subarctic waters observed 50–150km from the coast of Southern California and Punta Eugenia, reported from July 2002 until April 2003. Our results suggest that the presence of this intrusion has continued to influence the region at least until May 2004. [Copyright &y& Elsevier]

Abstract We evaluated the factors that control the variability in dissolved cadmium (Cd d), phosphate (PO 4 3−) and the Cd d /PO 4 3− ratio in the upper 1000 m of the deep region of the Gulf of Mexico (GoM), a marginal sea where trace element data are limited. These variables were compared to data from studies of adjacent areas (in the Western Central Atlantic), related to physical processes (mesoscale structures and upwelling events) occurring inside the GoM, and to dinoflagellate and diatom abundances in surface waters of this region. Vertical distributions of Cd d concentrations and the Cd d /PO 4 3− ratio displayed the typical nutrient profile that has been extensively reported for different oceans. Cd d and PO 4 3− concentrations in the GoM were higher than those in the Western Central Atlantic. The slope of the Cd d vs. PO 4 3− relationship for samples within the Loop Current (LC) in the GoM was similar (242 pmol μmol−1, R 2 = 0.96) to that calculated for the Western Central Atlantic waters (same depth range) that reach the GoM through the LC (235 pmol μmol−1, R 2 = 0.98). The linear regression for all the GoM samples, though having a similar slope (249 pmol μmol−1), presented a larger deviation (R 2 = 0.91). We propose that this deviation is due to spatial inhomogeneity that can be explained by 1) preferential remineralization of PO 4 3− vs. Cd d at subsurface waters in some regions, 2) Cd d inputs from the north, transported by mesoscale processes, and 3) interactions with the continental slope at the minimum O 2 layer. Local changes in the relationship between Cd d and PO 4 3− were traced with Cd*, which represents the deviation of measured Cd to that expected by the deep-water Cd d vs. PO 4 3− relationship. Cd* presented mainly negative values at surface and subsurface waters (~150 m) indicating preferential uptake of Cd d vs. PO 4 3− by the phytoplankton at the surface and preferential remineralization of PO 4 3− vs. Cd d from the organic matter at the subsurface waters. At deeper waters (~400–1000 m) Cd d was preferentially remineralized. In order to understand the inhomogeneity of the horizontal distribution of Cd d , PO 4 3− and the Cd d /PO 4 3− ratio in surface waters, three regions were defined considering their biological and physical characteristics. These were: I) the Oligotrophic region (in the northwest GoM) where the variability in the Cd d /PO 4 3− ratio was largely explained by salinity, II) the Upwelling region (off the coast of the Yucatan Peninsula) where the variability in the Cd d /PO 4 3− ratio was principally due to the diatom abundance and, III) Semi-permanent Cyclonic Gyre region (Bay of Campeche) where the variability in Cd d and the Cd d /PO 4 3− ratio were primarily influenced by the upwelling generated by the cyclonic gyre, and the variability in PO 4 3− was likely due to the consumption of this nutrient by phytoplankton. In summary, physical processes, river inputs, and phytoplankton abundance appear to play an important role in explaining the spatial variation in the Cd d /PO 4 3− ratio. Highlights • Dissolved Cd (Cd d) and PO 4 3− are enriched in the Gulf of Mexico (GoM). • Cd d and PO 4 3− concentration distributions showed high spatial variation in the GoM. • The GoM and Western Central Atlantic waters exhibit similar Cd d vs. PO 4 3− slopes. • Biological and mesoscale processes control the Cd d /PO 4 3− ratio in the GoM. [ABSTRACT FROM AUTHOR]

Here, we report different lipophilic toxins (LTs) detected by LC–MS/MS in Mediterranean mussels ( Mytilus galloprovincialis ) collected through 2012 in Todos Santos Bay, northwest Baja California, Mexico. The concentration of okadaic acid (OA), dinophysistoxin 2 (DTX2), and pectenotoxin 2 (PTX2) reached 500 μg kg −1 during July and increased to 1647 μg kg −1 in October. These toxins were associated with the presence of Dinophysis fortii and Dinophysis acuminata and a strong stratification of the water column. Other LTs present were yessotoxins, with a maximum concentration of 1080 μg kg −1 in June. Cyclic imines (13-desmethyl spirolide and gymnodimine) and azaspiracid 1 were also detected in the mussels but at low concentrations. Diarrhetic toxins concentrations evaluated by LC–MS/MS were compared with the results of two mouse bioassay protocols. Positive results were obtained with both MBA protocols in several samples that presented toxicities below 160 μg OA-eq kg −1 , as estimated by LC–MS/MS results whereas other samples returned negative MBA results in samples with concentrations above this level. Therefore, analytical methods need to be applied to confirm the presence of regulated LTs. This is the first report of LTs in mussels cultivated in Mexico. The occurrence of these toxins represents an emerging problem in the region. [ABSTRACT FROM AUTHOR]

This study uses ROMS (Regional Ocean Modeling System) with realistic forcing to analyse cross-shelf transport associated with the presence of mesoscale anticyclones as they interact with the western shelf of the Gulf of Mexico. Daily outputs from this validated 15-year numerical simulation were used to calculate the depth and time averaged cross-shelf volume transport through the 200 m isobath, used as a proxy for the shelf break. Composites for cases in which mesoscale anticyclones (MAs) are close or far away from the shelf break were performed to show that cross-shelf exchange has remarkable differences when they interact with the shelf. These differences consist of: a) enhanced off-shelf transport on its northern rim (at ~ 25.5 o N), b) a flow with multiple inversions associated with the main body of the MA where both on and off-shelf transports are considerably increased, and c) enhanced on-shelf transport at its southern edge, linked to the interaction of the MA with the persistent mesoscale cyclonic circulation in the Bay of Campeche. The multiple transport inversions associated with the main body of the MA is clearly linked with spatial patterns of baroclinic and barotropic instabilities that develop at the rings edge. The time-evolution of a representative event in which an MA interacts with the shelf is analysed to verify the influence on cross-shelf transport observed in the composites. Results show that enhanced cross-shelf transport is driven by the proximity, shape, and angle of orientation of the anticyclone with respect to the shelf. The Eulerian analysis of the event coincides with the composite results described above with enhanced cross-shelf exchange when the MA is rounder and closer to the shelf. A Lagrangian experiment is carried out to quantify the exchange and fate of particles during this "typical eddy event", seeding drifters within the shelf, inside the MA and in the open waters outside the eddy. Results show a substantial increase in exchange rate and distance traveled by particles (they travel 200 km farther) when the MA is closer to the shelf. • Cross-shelf exchange enhanced by eddies in the western Gulf of Mexico. • Eddy-shelf break distance and eddies' shape modulate cross-shelf flows • Model results in agreement with Lagrangian observations.. • Flow instability linked to cross-shelf transport variability. [ABSTRACT FROM AUTHOR]

In the Gulf of Mexico, the distribution of larval fish assemblages have been linked to continental shelf vs oceanic habitats and mesoscale structures such as cyclonic and anticyclonic eddies. Myctophids have a worldwide distribution and are one of the mesopelagic fish families with higher biomass, thus playing a key role in the trophic structure of oceanic communities. However, studies focused on their larval ecology and distribution within the gulf are limited. This study evaluated whether the formation of myctophid larval assemblages can be linked to local adult population spawning patterns and/or oceanographic conditions and surface transport. Ichthyoplankton samples were collected in August–September 2015 using standard bongo net tows. Stations extended throughout Mexico's Exclusive Economic Zone (19°N–25°N) deep water region, including the Yucatan Channel. Myctophid comprised 24.6% of the standardized total larval abundance and were present at all stations. Two dominant assemblages (out of six identified with a Bray–Curtis dissimilarity analysis) were found in stations of the central gulf and the Bay of Campeche. The high proportion of preflexion larvae coupled with differences in hydrographic parameters and surface circulation between the central gulf and the Bay of Campeche, indicated that local adult fish populations and their spawning patterns likely contribute more to the formation of these assemblages than oceanographic conditions and surface transport. Stations from the northern Yucatan Peninsula located within the region of influence of the Loop Current shared a common assemblage, which may reflect biogeographic differences in the myctophid species composition between the northern Caribbean and western gulf. These results highlight the importance of adult distribution and local spawning, providing insight into biogeographic patterns during the early life stages of mesopelagic fishes in the Gulf of Mexico. • Myctophids were abundant throughout the deep water region of the southern GoM. • Ubiquitous presence of preflexion larvae indicates widespread spawning. • A predominant assemblage was found in the western of the GoM. • Assemblages are determined by adult spawning rather than surface transport. • Oceanographic conditions and larval assemblages differed among regions. [ABSTRACT FROM AUTHOR]