Sample collection
Environmental and biological data were collected from 125 stations covering the EEZ of Mexico in the deep water region of the GoM during three oceanographic campaigns led by Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE; Fig. 1). The XIXIMI-04 campaign was held from 27 August to 15 September 2015 (n = 47 stations), while the XIXIMI-05 campaign took place from 10–24 June 2016 (n = 33 stations), and the XIXIMI-06 campaign was conducted from 15 August to 8 September 2017 (n = 45 stations). The sampling stations were organized latitudinally from ~25 °N (Line A) to ~20 °N (Line J). Abiotic data and zooplankton samples were collected from 3–8 stations per line. The number of stations sampled during each oceanographic campaign, their geographic positions, and geo-physical parameters are reported in Supplementary Information Table S1.
At each station, zooplankton were collected with a 3m-long bongo net (0.60-m diameter mouth, 335-μm mesh) with a double oblique tow spanning the surface to 200 m. The content of one net was split in two parts. From the first portion, 4/5 of the sample was immediately preserved in 96% ethanol and stored at 4 °C until further analysis. Continuous measurements of temperature (°C), salinity (psu), dissolved oxygen (DO; ml/L), chlorophyll fluorescence (fru), and pressure (Db) were recorded with a SeaBird SBEplus9 CTD (Sea-Bird Electronics, Bellevue, USA) equipped with calibrated sensors. Raw environmental data were processed with SBE Data Processing software (Seasoft V2 Software Suite, 2013; Sea-Bird Electronics). Given that bongo sampling covered the upper portion of the water column, the average temperature, salinity, DO, fluorescence, and density in the upper 200 m were calculated and used in statistical analyses.