Zooplankton for live experiments were collected by Tucker trawls done at similar locations and depths as the MOCNESS tows (table S1). The Tucker trawl used standard MOCNESS control software and sensors and had a large insulated cod end (26) so that many species remained alive when brought to the surface in water at their ambient temperature (13). Shipboard respiration measurements of key species (copepods, euphausiids, krill, shrimps, lophogastrids, crabs, squids, and octopods) determined their physiological tolerances [critical oxygen partial pressure (Pcrit)] and metabolic rates at 5°, 8°, 10°, or 20°C that could be related to species-specific distributions and habitat preferences. Following 6- to 12-hour acclimation at experimental temperature and air-saturated water, animals were placed in darkened sealed chambers filled with 0.2-μm filtered seawater treated with antibiotics (25 mg/liter each of streptomycin and actinomycin) to minimize bacterial respiration. Chamber size ranged from 80 μl to 750 ml, resulting in a ratio of chamber volume to animal mass of ~10 to 100. Seawater PO2 (oxygen partial pressure) was measured optically with a Loligo Systems Witrox 4 meter, PyroScience FireStingO2 meter, or Loligo Systems Sensor Dish Reader. Upon placement in the chamber, animals were allowed to breathe down the ambient oxygen until their oxygen consumption rate could no longer be sustained. Individual trial durations ranged from 6 to 48 hours. Temperature was maintained with Lauda E100 and Thermo Fisher Scientific NESLAB RTE-7 water baths. Oxygen meters were calibrated with air-saturated seawater and concentrated NaSO3 solution. Chambers were stirred with magnetic stirrers (Cole-Parmer Immersible Stirrer EW-04636-50) or via a shaker table. After the experiments were completed, animals were frozen at −80°C before being weighed and measured.

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