Autumn Vertical distribution of zooplankton in the Oxygen Minimum Zone of the Eastern Tropical North Pacific.

A general pattern of vertical zooplankton distribution is described, including 22 MOCNESS tows with a 48-h cycle of stratified zooplankton sampling; it showed no day-night differences in the vertical distribution of zooplankton, except for euphausiids. Most zooplankton groups sampled between 500 m and 0 m migrate only across the upper 100 m. Zooplankton carbon was three times higher in the upper 100 m than in the 100–500 m layer of the water column sampled. A difference of 1/24 between the shallower layer (0–50 m) and the 150–200 m layer was found. Euphausiids migrated into the Oxygen Minimum Zone (OMZ) during the day. Zooplankton migration was restrained to the upper 100-m layer by the shallow OMZ. Twenty-four zooplankton groups were studied, showing a ~ 50% reduction in the number and abundance of these groups inside the OMZ. These findings support the Habitat Compression Hypothesis (HCH). A Principal Component Analysis showed that vertical zooplankton distribution is limited by oxygen, concentrated mostly within the upper 100 m. The two most abundant euphausiid species were the vertical migrator Euphausia lamelligera (49.3%) and the non-migrator Stylocheiron affine (39.8%). Most zooplankton assemblages do not migrate across the oxycline.