The seasonal variations and the interactions of the water masses in the tropical Pacific off central Mexico (TPCM) and four surrounding areas were examined based on an extensive new hydrographic database. The regional water masses were redefined in terms of absolute salinity (S A ) and conservative temperature (Q) according to the Thermodynamic Equation of Seawater 2010 (TEOS-10). Hydrographic data and the evaporation minus (precipitation 1 runoff) balance were used to investigate the origin and seasonality of two salinity minima in the area. The shallow (50-100 m) salinity minimum originates with the California Current System and becomes saltier as it extends southeastward and mixes with tropical subsurface waters while the surface salinity minimum extends farther north in the TPCM in summer and fall because of the northward advection of tropical surface waters. The interactions between water masses allow a characterization of the seasonal pattern of circulation of the Mexican Coastal Current (MCC), the tropical branch of the California Current, and the flows through the entrance of the Gulf of California. The seasonality of the MCC inferred from the distribution of the water masses largely coincides with the geostrophic circulation forced by an annual Rossby wave.

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Vertical distribution and abundance of three numerically dominant krill species (Nyctiphanes simplex, Nematoscelis difficilis, and Euphausia eximia) were surveyed in the Gulf of California to understand the role of oxidative stress in their daily vertical migration (DVM) and zoogeographic patterns. Superoxide radical production, lipid peroxidation, and antioxidant enzyme activities were analyzed from krill collected with stratified nets from the surface down to 200 m during January, July, and October 2007. The upper boundary of the oxygen minimum zone (OMZ) was significantly shallower during October than during January. N. simplex was always distributed above the hypoxic layers, mostly in coastal upwelling areas. Ne. difficilis and E. eximia were relatively abundant during January, but detected mostly during their ascending migration. N. simplex was the most sensitive species to high temperatures and low oxygen concentrations, showing evidence of oxidative stress during summer (100 times more lipid peroxidation and 30 times more antioxidant enzyme activities than in winter). Ne. difficilis had higher glutathione peroxidase activity than N. simplex, which could facilitate its larger DVM. Low abundance of Ne. difficilis at 100 m during summer suggests that high temperature was also an environmental limiting factor. Oxidative stress indicators could explain the absence of N. simplex and Ne. difficilis in the eastern tropical Pacific and the ability of E. eximia to live in the OMZ and the eastern tropical Pacific. The latter had higher superoxide radical production and smaller lipid peroxidation during October. This suggests that E. eximia antioxidant enzyme activities are enough to avoid oxidative damage when exposed to hypoxic conditions during DVM.