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The Gulf of Mexico (GoM) is one of the most dynamic marginal seas in the world owing to the intrusion of the Loop Current and the shedding of anticyclonic eddies (LCE) that travel westward across the Gulf. However, the impacts of these mesoscale dynamics on the supply and removal of bioessential trace metals in surface waters remain unclear. We study the impact of mesoscale eddies on the distribution of dissolved nickel (Ni), a biologically active element scarcely studied in the region. The vertical distribution of Ni was determined in the deep-water region of the GoM during summer of 2017, when two anticyclonic LCE (Quantum and Poseidon) were present. Nutrient-like profiles of Ni in the GoM resemble those from the Atlantic Ocean, but they showed high spatial variability within the first 1000 m, which was associated with the impact of mesoscale eddies. Similarly to subtropical gyres, macronutrients were almost depleted in surface waters, while Ni never fell below 1.51 nmol kg-1, suggesting low Ni lability or alternatively, slow biological uptake compared to that of macronutrients. In particular, lowest levels of Ni and macronutrients (PO4 and NO3) were recorded in surface waters of the anticyclonic eddies and the Loop Current area. Anticyclonic LCEs deepened these Ni-poor waters pushing the Ni-rich core of Tropical Atlantic Central Water up to 600 m, whereas its shallowest position (up to 200 m) was recorded under cyclonic conditions in Campeche Bay. This eddy-induced vertical displacement of water masses also affected the integrated Ni and macronutrient concentrations in the upper 350 m but without modifying their stoichiometries. We suggest that a significant decrease in surface inventories of Ni and macronutrient in areas impacted by LCEs is a consequence of the trapping of the water within eddies, the biological uptake of Ni and macronutrients combined with their limited replenishment from below, which likely affects autotrophic groups. In conclusion, the mesoscale dynamic permanently present in the GoM play an important role in modifying the vertical distribution of Ni and macronutrients as well as their availability in the upper water column of this marginal sea.

In the Midriff Islands region (MIR) of the Gulf of California (Mexico), tidal mixing plays a very important role in the distribution of chemical properties. Although the temporal variability of the water masses is linked to the ocean dynamics of this region, its importance has not been considered. This work estimates the effect of water mass dynamics on the spatial distribution and seasonal variation of dissolved inorganic carbon (DIC) in the region, with special emphasis on the Ballenas Channel, during winter 2002, spring 2003, and summer 2004. Gulf of California Water (GCW) was found throughout the area in the first 100 m. Below, Subtropical Subsurface Water (SSW) filled the Delfín, Salsipuedes, and Tiburón basins, while to the southeast it was found between 150 and 350 m. Pacific Intermediate Water did not cross the sills. SSW enriches the surface water of the MIR with DIC when it increases in volume and when mixed with GCW by tidal effects. This contribution decreases towards summertime, when SSW sinks and the volume of GCW increases. In winter, GCW had three water types associated with oceanographic mixing processes and in combination with the presence of SSW. There was a strong DIC vs temperature relationship, which can be applied to analyze the carbon dynamics in the MIR, both at the surface using satellite imagery and vertically to simulate physical processes.

Three oceanographic cruises were carried out in the region of the Ballenas Channel (BC) during winter 2002, spring 2003, and summer 2004, in order to study the impact of vertical mixing of the water column (ф) on the surface concentration of dissolved inorganic nutrients (NO3, PO4, and Si(OH)4). Mean nutrient concentrations integrated over the upper 100 m of BC showed a clear seasonal variation, with higher values in winter (18.2 ± 1.1 ± µM NO3, 2.36 ± 0.08 µM PO4, and 35.4 ± 3.2 µM Si(OH)4), associated with a well-mixed water column (ф = 0.7–16 J m–3), and low values in summer (7.1 ± 1.0 µM NO3, 1.22 ± 0.06 µM PO4, and 16.0 ± 2.1 µM Si(OH)4), related to vertically stratified water column conditions (ф = 150–223 J m–3). In contrast, below 200 m the seasonal variability of the three nutrient concentrations was reduced. The hydrographic analysis indicated that the wintertime surface nutrient enrichment was due to an intensification of the water column mixing, which vertically transported Subtropical Subsurface Water (SSW, cold and nutrient-enriched) to the surface of BC. In contrast, nutrient impoverishment in summertime was due to biological consumption combined with the stratification of the water column, which reduced the vertical flow of SSW and, consequently, the supply of nutrients to the surface of this system. The N:P and Si:N ratios and an excess of PO4 > 0.65 µM in surface waters indicated that NO3 was the potential limiting nutrient for primary production in this oceanographic province of the Gulf of California.

Abstract Seabirds are known to play an important role in the geochemical cycling of macronutrients; however, their role in cycling elements of environmental interest has not been investigated. Guano is an important source of marine-derived nutrients and trace metals in seabird nesting areas, but most of the available information on this topic is derived from local studies. In the present study, we used a bioenergetic model to estimate the amounts of cadmium (Cd), mercury (Hg) and lead (Pb) that are deposited via faecal material in seabird colonies worldwide. The findings showed that the seabirds excreted 39.3 Mg (Mg = metric ton or 1000 kg) of Cd, 35.7 Mg of Hg and 27.2 Mg of Pb annually. These amounts are of the same order of magnitude as those reported for other fluxes considered in the geochemical cycling of these elements (e.g. sea-salt spray, cement production, soil loss to oceans). Most of the deposition occurs in circumpolar zones in both hemispheres and, interestingly, high proportions of the metals in the excrements occur in geochemically labile forms, which can be easily leached into coastal waters and assimilated by marine organisms.

The global impact of seabird populations on nutrient cycles is poorly understood. Here, the authors use a bioenergetic model and a global seabird population inventory to estimate the amounts of nitrogen and phosphorus excreted by seabirds and estimate them to be 591 Gg N y−1 and 99 Gg P y−1 respectively.

Dissolved iron (Fed) concentrations are reported for the Gulf of California oxygen minimum zone (OMZ), which was located in front of San Esteban sill, between Subtropical Subsurface Water (SSW) and Pacific Intermediate Water. Mean Fed concentration in the OMZ was 3.84 ± 2.48 nM, with minimum and maximum values of 1.48 and 10.02 nM. These concentrations are higher and are enriched in relation to the Tropical Northeast Pacific Ocean OMZ (0.25–1.2 nM), whose waters enter the gulf through SSW. Sediment resuspension and Fed-enriched waters cause the observed Fe increase at the stations located near San Esteban sill. On the other hand, the relatively lower Fed concentrations found at the stations farthest from the sill are the result of denitrification in the suboxic zone.