Vertical pelagic habitat of euphausiid species assemblages in the Gulf of California

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We describe the seasonal changes in the horizontal and vertical distribution and abundance of euphausiid species associated with seven physical and 61 biological variables in the Gulf of California (24–31°N). Euphausiid community structure was explored in the epipelagic habitat (<200m) in January, July, and October 2007 and in epipelagic to bathypelagic habitats (<1400m depth) in May 2015. Twelve euphausiid species comprising two distinct regional assemblages were identified. Nyctiphanes simplex and Nematoscelis difficilis were the most abundant species (>90%) in all cruises carried out in the 26–31°N region and Euphausia distinguenda in the 24–26°N region (mostly in October >90%). We confirmed that Euphausia gibboides and Nematobrachion flexipes inhabit the mesopelagic habitat, adapted to <1ml O2 l–1 environmental condition. Although Euphausia lamelligera and N. simplex populations were concentrated in well-oxygenated water (>3ml O2 l–1) near the surface (<50m), they were also detected in low densities in the hypoxic mesopelagic habitat (250–800m), but only at oxygen levels less than 0.09ml O2 l–1. Stylocheiron affine and Stylocheiron carinatum were numerous well below the seasonal thermocline (100–350m). Nematoscelis difficilis and N. simplex extended into the hypoxic mesopelagic habitat, but at low densities. Multivariate analyses showed six seasonal and regional krill assemblages, which are characterized by different species (Similarity percentage analysis). Canonical Correspondence Analysis show that, of 68 variables, three abiotic factors (temperature, dissolved oxygen, and OMZ depth) and eight biotic factors (accessory pigments; zeaxanthin and 19'-hexanoyloxyfucoxanthin, and the abundance of six copepod species; Candacia pectinata, Canthocalanus pauper, Centropages furcatus, Rhincalanus nasutus, Scolecithrix danae, and Temora discaudata), were the most influential variables associated with the vertical distribution and abundance of euphausiids. Krill‒copepod (predator–prey) relationships may be important determinants of daily and seasonal vertical distribution patterns. We conclude that the euphausiid vertical habitat reaches down to 900m depth (with historical records of six species collected between 1000 and 2280m) but with lower diversity and abundance in the mesopelagic and bathypelagic habitats than in the epipelagic habitat.