For millions of years, dinosaurs dominated the land. Some of the largest dinosaurs were as large as Argentinosaurus, but none are known to have been as large as the blue whale. The blue whale (Balaenoptera musculus) is the largest animal ever to have existed. It grows to a total length of 100-110 feet and more than 150 tons (sometimes up to 200 tons) in weight. Twice the length of a basketball court, it weighs as much as several dozen elephants.
When you look at its diet, its size seems even more incredible. The blue whale is the largest among all animals on Earth, but it has no interest in hunting other large animals. It feeds primarily on small shrimp-like invertebrates called krill. A blue whale can eat up to several tons of krill a day during feeding times. It filters in tons of seawater and then extracts the krill with baleen plates in its mouth. That is, one of the largest animals to ever exist lives on the smallest of animals, just a few centimetres long.
There is another part of the blue whale story that is much less obvious. It isn't just about feeding. Whales are also part of the marine food web that transports nutrients. That's where the “whale pump” comes in. Whales will dive to deeper water to feed and come back to the surface to breathe and expel waste. They excrete nutrients (N, P, and Fe) that are vital in the growth of the phytoplankton close to the ocean surface.
Parts of the Southern Ocean are of special interest due to the scarcity of iron. Researchers have discovered that the faeces of baleen whales contain approximately 10 million times more iron than Antarctic seawater. A 2010 study investigated 27 samples of faeces from baleen whales of four species and found that the concentration of iron is very high. The discovery enabled researchers to gain more information about how whales can bring iron to the surface, where it can be utilised by tiny organisms.
It's easy to see the relationship of whales and phytoplankton to krill. Phytoplankton are the smallest of the photosynthetic living organisms that are the foundation of many marine food webs. Krill eat them, and whales eat the krill. After whales have consumed food and excreted nutrients into the water, some of these nutrients are once again available to the phytoplankton. The cycle links some of the smallest organisms in the ocean with one of its largest animals.
Phytoplankton are more than just food. Marine plankton are responsible for much of the oxygen production of the ocean, which generates about half of the oxygen produced on Earth as a whole, according to NOAA. The micro-organisms consume carbon dioxide by photosynthesis as well. Phytoplankton, as the base of marine food webs, are also a crucial component of the global carbon cycle, according to NOAA.
This is why scientists became interested in what happened when whale populations declined. Whale populations were drastically depleted in the 20th century due to commercial whaling. According to NOAA, whaling has depleted blue whale populations, and their numbers are just a small fraction of what they once were. NOAA material states that over 360,000 whales were killed in the Southern Hemisphere in the 20th century.
The decline may have affected more than the number of whales in the ocean. Less whale consumption of krill leads to less recycling of nutrients from whale waste. A study from the Southern Ocean indicates that baleen whales may have had a much greater role back in the day before large-scale hunting in recycling iron. In one study, it was estimated that the pre-exploitation whale population could have returned thousands of tonnes of iron through its faeces.
The connection with climate change, however, needs some caution. Whales do contribute to nutrient cycling, and phytoplankton remove lots of CO₂. NOAA has compared the amount of carbon captured by marine phytoplankton globally with the carbon captured by about four Amazon rainforests. This does not represent the total amount of carbon taken up by whales. Research is ongoing into the extent of carbon removal that can be attributed to the whales.
The story of the blue whale is, therefore, unusual in terms of size and influence. Some of the smallest animals in the sea provide the food for the largest animal, and the waste that the largest animal produces circulates through the same system, moving nutrients. Conserving blue whales is not just a blue whale conservation issue but could also be a way to help restore natural processes in the oceans, as well. They are not only significant due to their size. This can also be observed in other, smaller organisms and the broader ecosystem that they are part of.
References
- Blue Whale
- The Big-Hearted Blue Whale
- How much oxygen comes from the ocean?
- Whales and Carbon Sequestration
- Southern Ocean iron fertilisation by baleen whales and Antarctic krill