Spatial variability of dissolved nickel is enhanced by mesoscale dynamics in the Gulf of Mexico

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

Wintertime enrichment of inorganic nutrients in the Ballenas Channel, Gulf of California

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.

Composición y biomasa fitoplanctónica bajo condiciones oligotróficas en la cuenca de Guaymas (golfo de California)

Con el propósito de determinar la biomasa y la composición del fitoplancton (picofitoplancton, nanodiatomeas y microdiatomeas) bajo condiciones oligotróficas, se realizó un estudio en la capa superficial de la cuenca de Guaymas, golfo de California, durante el verano tardío de 2016. El estudio contempló la medición de variables hidrográficas, químicas y biológicas en la capa superficial del área de estudio. Nuestros resultados mostraron una columna de agua cálida, fuertemente estratificada, empobrecida en nutrientes y con biomasas fitoplanctónicas reducidas (<1 mg Cla·m–3). Los cocientes promedio de N:P (0.55 ± 1.34), N:Si (0.13 ± 0.18) y Fe:N (52.70 ± 29.70) indicaron condiciones de limitación por N. La contribución de los grupos fitoplanctónicos a la biomasa fluctuó ampliamente por nivel de profundidad, y se observó  el predominio del picofitoplancton (85.0 ± 2.7%) en superficie (5 m) y de las nano-microdiatomeas (91.5 ± 5.9%) en el nivel más profundo (35-40 m), adyacente a la termoclina. Las diatomeas dominaron la biomasa integrada entre la superficie y 50 m de profundidad, con una contribución promedio ~6 veces mayor que la del picofitoplancton. A pesar de que se registraron condiciones ideales para la proliferación de cianobacterias diazótrofas, la presencia de Trichodesmium spp. y Richelia intracellularis fue muy irregular y en abundancias relativamente bajas (2,220 ± 1,575 célula·L–1). Nuestros resultados son especialmente relevantes, ya que sugieren que, bajo condiciones de limitación por N, el paradigma de la alta biomasa y dominancia de células grandes en el golfo de California puede verse desafiado durante la temporada de verano