Copepods, tiny zooplankton, are the most abundant animals living in ocean waters. They play fundamental roles in Arctic food webs by consuming phytoplankton and protozoa and serving as food for fish, marine mammals, and birds. The Arctic is warming four times faster than the global rate, potentially putting many species at risk. The ability to predict the response of copepods to warming depends on robust knowledge of their upper thermal limits. This project will provide novel information on the critical thermal limit of key Arctic copepod species and whether or not this limit reflects their ability to succeed in their environment. The work will also provide insights into the success of a copepod species that is dramatically increasing its abundance in the southern Arctic, which may alter the transfer of energy in the ecosystem. The project’s findings, highlighting Arctic research, will reach broad audiences via social media and activities with K-12 students. The project will also train one Ph.D. student. Arctic species, which typically are cold temperature specialists, may be at risk of extinction because of the fast change the Arctic is experiencing. Thus, there is an urgent need to understand the physiological limits to survival in these species and whether or not these limits are a good measure of fitness. The project’s goals are: 1) Quantify critical thermal limits in three key copepod species of Disko Bay, Greenland, a widely studied Arctic ecosystem. 2) Evaluate