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The euphausiid exception in the oxygen minimum zone (OMZ)

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Samples obtained with a MOCNESS net were analysed for the vertical distribution of euphausiids, in the entrance of the Sea of Cortés, during November 2009. Two transects were followed: B influenced by the Southern California Current and a southern one, D, with more tropical conditions and shallower oxygen minimum. A stratified sampling was performed, with layers of 100 m from 500 to 200 m and every 50 m from 200 to 0 m. A fixed point, with continuous samplings, were performed, for 48 hours in transect D. Most zooplankton groups were scarce or absent, in hypoxic waters down from ~100 to 500 m depth, with the exception of euphausiids, which were present at all layers at all times of the day, showing their adaptability to low oxygenated waters. The low-oxygen adapted and neritic species Euphausia lamelligera was dominant, followed by Stylocheiron affine, a non-migrant or short-distance migratory species. These two species make 89% of the entire sample’s abundance. With the exception of Euphausia tenera, all species of the genus Euphausia showed vertical migration within the oxygen minimum layer. The rest did not show long vertical migrations, but stayed within the first 150 m layer, which is, however, well within the oxygen minimum layer, showing that these are capable of migrating in the low oxygen layer, probably for short time. Furcilia stages were present in shallower waters only in the first 150 m. These results demonstrate that euphausiids and especially the genus Euphausia are the best adapted zooplankton group to low oxygen conditions.

This talk is part of the British Antarctic Survey series.

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