Losing the fat in the chilly north Atlantic

Colder oceans are full of animals that need to fatten up to survive, but a key source of lipids — a tiny, oil-stuffed critter called a copepod — is feeling the heat, affecting a vital food web Cape Cod Bay is a shallow expanse of murky water off the coast of Massachusetts, sheltered from the wider ocean by a curl of land that looks like a flexed arm. Some years back, Karen Stamieszkin looked out at its gray surface and saw it aswirl with pink plumes that endangered North Atlantic right whales “were sucking up like spaghetti,” she says. Those pink plumes — small marine creatures like protozoans and crustaceans, fish larvae and jellies, collectively known as zooplankton — had risen through the water to gobble up the annual spring bloom of billions of tiny phytoplankton busy soaking up sunlight and turning it to sugars.

Whenever the plumes show up, says Stamieszkin, a zooplankton ecologist at the Bigelow Laboratory for Ocean Sciences in Maine, “that’s what whales are after.” An endangered North Atlantic right whale skim-feeding on zooplankton. KEN UNDER NOAA PERMIT 25740-02 The whales she saw that day were after _Calanus finmarchicus_, a rice-sized crustacean called a copepod that resembles a ruddy, translucent flea and is a dominant species of zooplankton in the bay. Copepods prop up marine life in the world’s cold, northerly oceans, and _C._ _finmarchicus_ is the most important _Calanus_ species in the North Atlantic.

Like all _Calanus_, it accumulates and synthesizes energy-dense fats, or lipids, from the phytoplankton it eats. Those lipids are essential to the baleen whales and herring that gobble _Calanus_ straight; they also indirectly support the cod, bluefin tuna, humpback whales, seals, sea birds and human beings whose diets are rich in those herring. Fat-stuffed _Calanus_, in other words, is “the critical connection” between the spring phytoplankton bloom and life higher up the food chain, says Andrew Pershing, an ecologist formerly at the Gulf of Maine Research Institute.

It is the anchor for the lipid food web, as it’s known to ocean scientists, which stokes the engine of marine life in chilly seas. That web needs an ample _Calanus_ supply — mostly _C. finmarchicus_ in the North Atlantic, other _Calanus_ species in other waters with seasonal ice. But today, climate change, along with other human factors like overfishing in some places, is altering that web.

This could reorganize ecosystems in dramatic ways. The North Atlantic is warming precipitously, particularly in shallower shelf regions of the northeastern coast of North America and the Barents Sea off Norway. The former has warmed by almost four times the global average, and it experienced three significant heat waves between 2012 and 2018.

Warmth-related disruptions are most dramatic in the Gulf of Maine, above Cape Cod Bay, which as of 2022 was warming faster than 97 percent of the rest of the world’s oceans. Decades ago, warming and other ocean changes drove out one _Calanus_ species there: a copepod 10 times the mass of _C._ _finmarchicus_ called _Calanus hyperboreus_. Now it’s _C. finmarchicus_ that is vulnerable.