Calanus finmarchicus builds a large oil sac during spring feeding, then descends to several hundred metres and enters diapause, a dormant overwintering state, before ascending to reproduce. That lipid store is what makes the animal worth eating: an individual copepod is tiny, but a dense aggregation is an energy-rich resource.
North Atlantic right whales feed by skimming with baleen through such aggregations, and they require patches far denser than average water to make a feeding dive profitable. Their historical concentration in the Bay of Fundy and Gulf of Maine tracked reliable Calanus patches. When warming in the Gulf of Maine altered copepod availability during the 2010s, right whales shifted substantially into the Gulf of St Lawrence, an area without corresponding vessel-strike and fishing-gear protections, and mortality rose.
The general point is the scale mismatch. A whale weighing tens of tonnes is coupled directly to the physiology of a millimetre-scale crustacean, so oceanographic change reaching the copepod reaches the whale within a season, with no intervening buffer in the food chain.