Zooplankton communities along a Southern Ocean monitoring transect at 110° E from three CPR surveys (Dec 2014, Jan 2015, Mar 2015)

Many studies using surface plankton data from the Continuous Plankton Recorder (CPR) have suggested that micro- and meso-sized zooplankton species/taxa can serve as indicators of environmental change. But while regular CPR sampling would allow for the detection of changes in zooplankton communities, few intra-annual CPR surveys have been undertaken along single transects. We report the temporal variability of zooplankton community structure using CPR data collected along the Japanese monitoring transect (110 degrees E) in December 2014, January 2015, and March 2015. We found zooplankton abundances in both the Polar Frontal Zone (PFZ) and Antarctic Zone (AZ) to increase from December to January, and mean AZ abundance to peak abruptly in January (381.9 ind. m(-3)) before decreasing to about 25% of that value in March. We attribute changes in total zooplankton abundance to fluctuations in the numbers of the two small dominant copepods, Oithona similis and Ctenocalanus citer. High AZ phytoplankton concentrations in December likely increased surface food availability, influencing population growth, and/or the timing of copepod seasonal ascension. We recommend future CPR surveys include sampling with finer mesh for more accurate determination of abundances of small-sized zooplankton, to better understand the ramifications of climate change on Southern Ocean trophic structure.

Automated summary

Studies using CPR data have shown that micro- and meso-sized zooplankton can indicate environmental changes. Zooplankton abundances in the Polar Frontal Zone and Antarctic Zone varied from December to March, mainly due to fluctuations in the numbers of two small dominant copepods.