Oceanographic variables and nutrient concentrations in an oxygen-deficient upwelling system during La Niña (2018) and El Niño (2019)

Hydrographic data and seawater samples were collected in four stations along a cross-shore transect in front of the Port of Mazatlán (Mexico) during the oceanographic cruises Maz IV (April 2018) and Maz V (April 2019) on board the R/V “El Puma” (UNAM). Temperature, dissolved oxygen, salinity, and chlorophyll-a fluorescence profiles were measured with a SBE-19plus CTD probe equipped with a WET labs fluorometer sensor. Seawater samples for nutrient analyses were collected at different depths based on the oxygen and chlorophyll-a profiles: surface (5 m depth); mixed layer, deep chlorophyll maximum (at 10–45 m depth); oxycline, base of the oxycline (at 60–125 m depth); upper OMZ (only for Maz V); OMZ core (at 250 m depth); and bottom (between 23 m depth in S1 and 670 m depth in S4). Seawater samples were filtered through cellulose acetate syringe filters (0.22-μm pore, Merck Millipore) and nutrient analyses were determined using a Skalar San-Plus segmented-flow autoanalyzer.