Oceans play a major role in everything – from the air we breathe to daily weather and climate patterns – and represent a major ‘carbon sink’. They are home to a vast array of marine wildlife, but we are still trying to explore and understand the basic processes that occur in their depths.
Phytoplankton at the bottom of the Antarctic food chain
Phytoplankton are tiny oceanic plants that take in carbon dioxide and produce oxygen. Up to 50% of the oxygen we breathe comes from the ocean. Phytoplankton form the base of the marine food chain and without it marine life cannot exist. In large areas of the Southern Ocean, the growth of phytoplankton is limited by iron, as there isn’t enough iron in the surface of the seawater to fertilise it.
This is where marine bacteria come in. They recycle iron and produce complex compounds called ‘organic ligands’, which keep iron on the surface of the ocean for phytoplankton to absorb more readily. Bacteria also play an important role in storing more carbon in the ocean. In the Southern Ocean, the growth of marine bacteria is limited by iron and dissolved organic carbon.
Phytoplankton form the base of the marine food chain and without it marine life cannot exist
I have been very intrigued by this for years. As everything is connected, I hope to figure out how the pieces of the puzzle connect. Working on the established base that phytoplankton and bacteria are linked, I set out to investigate what else is connected to this loop, and how.
The role of Krill
I started off by looking at Antarctic krill. They aren’t just a nutritional supplement available at your local pharmacy – the entire Southern Ocean ecosystem depends on them. This is why they are a ‘keystone species’ and their fishing is regulated by the Convention for the Conservation of Antarctic Marine Living Resources.
Antarctic krill feed on phytoplankton, so a transfer of nutrients takes place. Krill retain the nutrients they need to grow, and release the rest as iron-rich faecal pellets. These faecal pellets quickly sink to deeper levels of the ocean, with some remaining near the surface where they are a source of nutrients for bacteria and phytoplankton.
Moving up one trophic level, we get to the baleen whales. Baleen whales are filter feeders, with baleen plates instead of teeth. In the Southern Ocean and Antarctic waters, baleen whales such as the blue, fin and humpback whales, feed on Antarctic krill. A blue whale gulps up to 50 tonnes of water and krill in one mouthful, but swallows only the krill. An adult blue whale can eat up to two tonnes of krill a day.
Baleen whales, like all mammals, need iron to grow. A newborn blue whale calf weighs 2.5 tonnes and can, in the latter stages of suckling, put on 100kg a day.
Marine animals are therefore important players in the maintenance of a healthy planet. During the 20th century nearly 1.3 million blue, fin and humpback whales were killed through industrial whaling. Although there was an abundance of whales in the Southern Ocean, commercial whaling decreased their population by over 90%. With the cease of commercial whaling, the population is slowly recovering. The tragic irony is we often see the effects of marine animals after they have been lost from the ecosystem.
Unfortunately, the cryptic nature of these interactions, and the remoteness and difficulty of getting to the Southern Ocean and Antarctic waters, makes it difficult to examine the feedback of Antarctic krill and whales on the marine ecosystem
Baleen whales in the Southern Ocean
- Blue whale (Balaenoptera musculus)
- Fin whale (Balaenoptera physalus)
- Humpback whale (Megaptera novaeangliae)
- Antarctic minke whale (Balaenoptera bonaerensis)
Written by Dr Lavenia Ratnarajah – Follow Lavenia on Twitter
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