Vertical distribution of zooplankton groups, with an emphasis on fish larvae, in the oxygen minimum zone off southern México (December 2020)

The distribution of zooplankton groups, with an emphasis on fish larvae, in the Oxygen Minimum Zone off southern Mexico (December 2020) was analyzed. A hydrographic section of five sampling stations was made in the confluence of Transitional Water and Tropical Surface Waters. In each station, horizontal zooplankton trawls on three different dissolved oxygen conditions (~100, < 44 and < 4.4 μmol kg⁻¹) were carried out by a MOCNESS net (333 μm). The 100 μmol kg⁻¹ oxypleth (oxic condition) was ~60 m depth along the section, but the 4.4 μmol kg⁻¹ oxypleth (suboxic) rose southward from Transitional Water (~ 150 m) to Tropical Surface Water (~ 90 m), approaching the well oxygenated layer. The distribution of the zooplankton biomass, and the most abundant zooplankton groups (e.g. copepods, chaetognaths, ostracods, euphausiids) and fish larvae showed statistically significant differences (P < 0.01) between the oxic (100 μmol kg⁻¹) and the deeper suboxic conditions. The larvae of typically dominant fish species such as the bathypelagic Vinciguerria lucetia, Diogenichthys laternatus, Diaphus pacificus and Cubiceps pauciradiatus, were present only in the oxygenated depths in the Transitional Water, and were almost absent from all depths in the Tropical Surface Water, where the oxycline shoaled. These differences in larval fish abundance were found despite little change in chlorophyll a concentration (relative units “r.u.”) along the sections, indicating that the oxycline is a limiting factor for the fish larvae. The fish larvae results contrast with previous observations from the mouth of the Gulf of California, where some species have distributions independent of water column dissolved oxygen conditions, probably as a consequence of coastal processes. Overall, our results show that even within the OMZ, variations in oxycline depth have biological implications, particularly on meroplanktonic organisms.