Temporal changes in abundances of large calanoid copepods in the Scotia Sea: comparing the 1930s with contemporary times

To investigate whether impacts of reported climate change in the Antarctic marine environment have affected mesozooplankton populations, we compared the summertime abundances of four species of large calanoid copepods from samples taken during the Discovery Investigations (1926-1938) and contemporary times (1996-2013). Discovery samples were obtained using an N70V closing net fished vertically through three depth horizons encompassing the top 250 m of the water column, whereas contemporary samples were obtained using a Bongo net fished vertically through 200-0 m. Data from a previous study comparing catch efficiencies of the two nets were used to generate calibration factors which were applied to the N70V abundances. Following further corrections for net depth differences and seasonal biases in sampling frequency, three of the four species, Calanoides acutus, Rhincalanus gigas and Calanus simillimus, were found to be between similar to 20-55% more abundant in contemporary times than they were 70 years ago. Calanus propinquus was marginally more abundant in the Discovery era. These results were robust to sensitivity analyses for the net calibration factor, seasonal bias and net depth corrections. Although near-surface ocean temperatures within the Scotia Sea have increased by up to 1.5 A degrees C during the last 70 years, we conclude that the most likely causes of increased copepod abundances are linked to changes in the food-web. In particular, we discuss the reported decrease in krill abundance in the South Atlantic that has potentially increased the amount of food available to copepods while at the same time decreasing predator pressure.