Zooplankton was collected using a multiple opening-closing net and environmental sensing system (MOCNESS); cod ends were fitted with a 200 micron mesh. Onboard, zooplankton were wet-sieved in filter seawater using different size mesh sieves and frozen at -20 (Degree-Celsius).

The zooplankton was collected at Station ALOHA during the KM1418 and KM1506 research cruises. In the summer cruise, each tow collected material from 9 depth intervals spanning the ocean surface to 1500 m (0-50 m, 50-100 m, 100-150 m, 150-200 m, 200-300 m, 300-500 m, 500-700 m, 700-1000 m, 1000-1500 m). In the spring, four separate net tows were conducted to collected additional samples from a single depth interval (500-700 m). This was done to provide enough sample material.

For total Hg (THg) determination about 10 mg of zooplankton was digested in reverse aqua regis overnight. For THg isotope determination zooplankton samples were combusted in a two-stage combustion furnace and Hg(0) g was trapped in a 1% KMnO4 solution. The 1% KMnO4 solution was analyzed for Hg stable isotope composition using a multiple collector inductively coupled plasma mass spectrometer.

All the methods are detailed in Motta et al., (2019).