Distribution of carnivorous gelatinous zooplankton in the upwelling zone off central Chile (austral spring 2001).

The distribution of gelatinous carnivorous zooplankton off central Chile (35–37°30′S) was assessed during upwelling conditions in spring 2001 using stratified zooplankton sampling. The hydromedusa Liriope tetraphylla (54.2 ind. m⁻²), siphonophore Sphaeronectes gracilis (16.5 ind. m⁻²) and ctenophore Pleurobrachia bachei (52.1 ind. m⁻²) were the most abundant species. A strong horizontal density gradient and the presence of low dissolved oxygen waters (<1 mL⁻¹) nearshore, characteristics of upwelling conditions in eastern boundary regions, occurred throughout the cruise. Two groups of gelatinous carnivorous zooplankton were identified associated with the density front structure: one located coastward from the front (Sphaeronectes gracilis, Euphysa aurata and Beroe cucumis) and another located seaward from the front (Liriope tetraphylla, Pleurobrachia bachei, Abylopsis tetragona and Thalassocalyce inconstans). The most abundant species occurred more frequently in well-oxygenated waters; however, high abundances (10–100 ind. 100 m⁻³) were also observed in suboxic waters (<1 mL O₂ L⁻¹). A significant relationship was detected between the depth of the centroids of the most abundant species and the depths of the suboxic waters, suggesting that the depth of the suboxic seawaters determines their mean vertical distribution. The ability of gelatinous carnivorous zooplankton to withstand a wide range of oxygen concentrations seems to be an important adaptation in coastal upwelling areas because oxygen conditions vary widely, over both small time and spatial scales.