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The eastern Pacific intermediate oxygen minimum layer (OML) is particularly well‐developed and shoals close to the Mexican coast. We obtained hydrographic profiles including oxygen concentration [O], measured dissolved inorganic carbon (DIC), continuous surface pCO2 in water and air and took biological data south of the Gulf of California in March 2005. The core of the OML, with close to zero [O] was centered around 475 m with a thickness of the core varying between 80 m and 700 m and the upper limit of the core ranging from 500 m to 80 m. At the surface [O] was close to air‐saturation and showed no relation with the depth of the OML contrary to DIC and pCO2 concentrations. Below 50‐m depth the changes in DIC and Apparent Oxygen Utilization (AOU) yielding a molar ratio of δDIC = 0.79 x δAOU. When the OML shoaled, surface temperature, chlorophyll concentration [CHL], and depth‐integrated zooplankton increased, resulting in positive correlations among plankton biomass, pCO2, and DIC in surface waters. When [CHL] is recalculated as particulate organic carbon (POC), a linear relationship with the sum of DIC and POC is observed as expected for concomitant transport of DIC and inorganic nutrients supporting POC formation. Neglecting ventilation of carbon into the atmosphere and the production of dissolved organic carbon, the results suggested that ~16% of the carbon transported up from the OML was present in surface waters in the form of POC and the rest in inorganic form.

El Bajo Espíritu Santo (EBES) es una montaña submarina localizada a 9 millas náuticas del extremo norte del archipiélago del Espíritu Santo, afuera de Bahía de La Paz, el cual es considerado un punto caliente de biodiversidad y biomasa. Algunos procesos que enriquecen el bajo son advección y transporte vertical, ocasionado principalmente por mareas y corrientes que chocan con la montaña. Asimismo, existe retención de organismos, particularmente zooplancton, causando enriquecimiento biológico. Así, para identificar la variabilidad de los procesos estacionales e interanuales que la afectan se realizó una revisión bibliográfica sobre hidrografía, dinámica y biología del EBES, que se complementa con análisis de series de tiempo utilizando información satelital sobre temperatura superficial del mar, clorofila superficial, productividad primaria neta y profundidad de la zona eufótica. Los resultados indican que dentro de los principales procesos que enriquecen al bajo se encuentran la mezcla debida al corte vertical de las corrientes, el transporte vertical ocasionado por corrientes de mareas que chocan con la montaña, la advección mediante chorros de corriente originados en el interior de la bahía y/o remolinos del Golfo de California, y la retención de organismos del zooplancton en la vecindad de la montaña por el obstáculo que esta impone. Se muestran dos periodos de productividad:  primavera y verano, i.e., un régimen bimodal con períodos dominantes de 12 y 6 meses respectivamente. Adicionalmente, existe variabilidad inter-anual dominada por periodos de 5 y 7.5 años asociada a ENSO. La combinación de factores confiere a EBES capacidad de enriquecimiento de nutrientes y plancton a lo largo del año, lo que atrae consumidores secundarios y depredadores tope, convirtiéndolo en un punto de gran actividad biológica.Bajo Espitritu Santo; biological abundance hotspot off Bahia de La PazABSTRACT. Bajo Espíritu Santo (EBES) is a submarine mountain located 9 nautical miles from the northern end of the Espíritu Santo archipelago, outside Bahia de La Paz. It is considered as a hotspot of biodiversity and biomass that has motivated studies to understand the processes that make it an area of high biological activity. Processes that enrich the EBES are advection and vertical transport, mainly caused by tides and currents that collide with the mountain. Also, there is retention of organisms, particularly zooplankton, which favors a biological enrichment. A review on hydrography, dynamics and biology is presented, complemented with time series analysis of satellite information on sea surface temperature, surface chlorophyll, net primary productivity, and euphotic zone depth, to identify the variability of seasonal and interannual processes affecting the mountain. The main processes that enrich EBES are mixture due to the vertical cut of the currents; the vertical transport caused by tidal currents that collide with the mountain; advection by jet currents originating in the interior of the bay and/or eddies from the Gulf of California; the retention of zooplankton organisms in the vicinity of the mountain. Two periods of productivity, one in spring and the other at late summer indicate a bimodal regime with dominant periods of 12 and 6 months respectively. Additionally, there is an inter-annual variability dominated by periods of 5 and 7.5 years associated with ENSO. Combination of these factors give EBES its ability for nutrient and plankton enrichment throughout the year, which attracts secondary consumers and top predators, turning it into a point of great biological activity.

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Surface and Gulf of California water masses. Historical data includes observations of 10 cruises between 1991