Large whales are usually described as consumers at the top of the ocean food web. They also function as a transport system that moves nutrients against the direction gravity takes them.
The open ocean is short of nutrients at the surface
Photosynthesis requires light, so plankton growth is confined to the upper layer of water. Nutrients, meanwhile, sink continuously as particles and dead organisms fall.
The result is a persistent mismatch: the light is at the top and much of the iron and nitrogen ends up below the depth where it can be used.
Where deep water is driven upward by currents, productivity is high. Across much of the open ocean it is not, and surface nutrients are the limiting factor.
Feeding deep and defecating shallow moves material upward
Many whales feed at depth on fish and krill, then return to the surface to breathe, where they release buoyant, nutrient-rich waste.
That material stays in the sunlit layer long enough for plankton to take it up, effectively returning nutrients that would otherwise have continued sinking.
Iron matters particularly, because in some ocean regions plankton growth is limited by iron rather than by nitrogen, and whale waste is comparatively rich in it.
Migration transports nutrients between regions
Several baleen whale species feed intensively in cold, productive waters and then migrate to warm, nutrient-poor breeding areas where they eat little.
During that period they release urea and, when calves are born, placental material and other tissue into waters that receive few nutrient inputs.
The transfer runs from productive high-latitude seas to unproductive tropical ones, which is the opposite of how most ocean nutrient movement works.
Carcasses deliver a large pulse to the deep sea
A whale that dies and sinks brings a concentrated mass of organic material to the seafloor, where food arrives normally as a slow drift of fine particles.
These falls support a sequence of specialised communities over years, beginning with scavengers and progressing to organisms that exploit the bones themselves.
Some species associated with whale falls are known from little else, so the frequency of carcasses reaching the seabed shapes deep-sea biodiversity.
Reduced whale numbers reduced the flux
Industrial whaling removed a very large share of great whale biomass, and with it a proportionate share of this nutrient movement.
Recovering populations restore part of the function, though the effect is regional and depends on which species return to which feeding grounds.
The wider point is that population size determines ecological function. A species present in small numbers is not performing the role it once did.