Material and methods
Sample collection
We chose the CPR dataset, assembled by the Global Alliance of CPR Surveys (Batten et al. 2019), because it provides the largest, consistent, most spatially-extensive, species-resolved plankton dataset (Richardson et al. 2006). Data were sourced from four surveys: the North Atlantic CPR Survey, the Scientific Committee on Antarctic Research (SCAR) Southern Ocean CPR Survey, the North Pacific CPR Survey, and the Australian CPR Survey (Fig. 1).
Although CPR surveys have better coverage in polar and temperate regions than in subtropical and tropical regions, samples have been collected in waters as warm as 30°C. All surveys use similar methods to collect and count copepods (see Reid et al. (2003), Richardson et al. (2006) for more details). Specifically, all CPRs use the same mesh size (270 µm), the same mesh material (silk), the same size mouth opening (1.61 cm2), towed at the same depths (5-10 m), and have similar designs (Hosie et al. 2003; Reid et al. 2003). The CPR is mainly towed behind ships of opportunity on their normal trading routes, but also behind research vessels. Each CPR tow is usually up to 450 nautical miles. The internal silk roll that captures the plankton is cut up into samples representing either 5 or 10 nautical miles, and microscopic counts of copepods are converted to number per m3.