Vertical distribution of copepod eggs in the oxygen minimum zone off Mejillones Bay (23°S) in the Humboldt current system

In the Humboldt Current System (HCS) off northern Chile (23° S), 6 samplings were conducted in 2013 and 2014, each at 8 depths ranging from 5 to 80 m, to study the role of environmental variability on the abundance and vertical distribution of the eggs of dominant copepods, with special emphasis on hypoxia associated with the oxygen minimum zone (OMZ). The highest levels of abundance of Acartia tonsa and Paracalanus cf. indicus eggs were in surface waters above the OMZ. The distribution of eggs of both species correlated positively with chlorophyll a and stratification (Brunt-Väisälä frequency). Calanus chilensis eggs were found at greater depths, and their distribution did not correlate with any abiotic variables, whereas the distribution of C. chilensis females was associated with subsurface water upwelling due to a positive correlation with Ekman transport and salinity. The vertical distribution of copepod eggs in the HCS can be explained by abiotic variability, the sinking velocities of the eggs and the resuspension effect of upwelling vertical velocities, all of which may retain the eggs of smaller copepods in waters above the OMZ while C. chilensis eggs sink more rapidly, reaching the OMZ in less than 2 d. The survival of early-stage copepods is vital for the success of populations; therefore, any changes in conditions, like upwelling intensification, shoaling, and expansion of OMZs due to climate change, can affect the productivity of the pelagic realm in coastal upwelling ecosystems.