Detection of foray behaviour in the zooplankton of the Antarctic Polar Front region

In addition to diel vertical migration, individual zooplankton may also make a number of shorter-term migrations, or forays, into the surface layers from deeper depths. Direct observation of these forays is limited, particularly in the open ocean, which hinders our understanding of carbon flux via the biological carbon pump (BCP). We designed a novel net device capable of trapping zooplankton during such forays. The motion-compensated upward and downward looking (MUDL) net device consisted of 2 conical nets, one looking upwards and the other looking downwards, designed for stationary deployment at a set depth, into which migrating individuals must swim to be captured. The device was deployed at different time points within the diel cycle and at contrasting environments across the Antarctic Polar Front region in the southwestern Atlantic sector of the Southern Ocean. A range of zooplankton species were captured, with differences in abundance and species composition between times of day, net directions and sites. Of particular note was the large contribution of the copepod Oithona spp. to catches of both the upward and downward looking nets. Our study demonstrates the utility of our MUDL net for future ecosystem studies in the open ocean, particularly in relation to quantifying vertical carbon flux via the BCP.