Size matters: Krill and the transport of carbon to the deep sea | Polarjournal
This Antarctic krill has had a good meal of phytoplankton, indicated by the green colour of its stomach. The larger the krill, the more it can eat and excrete. And the bigger the faecal pellets, the higher the carbon export to the deep sea. (Photo: Uwe Kils/Wikipedia/CC BY-SA 3.0)

The amount of carbon transported from the water surface into the deep sea on the Western Antarctic Peninsula is primarily determined by the body size of Antarctic krill, not by its abundance. The rapid warming of the region could negatively affect this carbon export

Over a record-breaking period of 21 years (1992 – 2013), a US research team studied the carbon transport from the light-flooded zone to the deep sea on the Western Antarctic Peninsula. Antarctic krill play a key role in this process by eating large quantities of phytoplankton and excreting the indigestible, carbon-rich remains in the form of faecal pellets that sink into the deep.

In contrast to what was previously suspected, the results of the study, which was published in the journal Nature, show that the export of carbon to the deep sea is clearly related to the size and age of the krill, but not to its abundance. For example, the vertical carbon transport known as the ‘biological pump’ was particularly high every five years when the proportion of large adult krill (larger than 4 cm and aged four years and older) was unusually high. Conversely, carbon export is significantly lower in years when young, small krill dominate the swarms, even if these are formed by many more individuals.

The researchers therefore assume that the annual fluctuations in carbon export are closely linked to the life cycle of Antarctic krill, which can live for five to six years.

The research team obtained the krill faecal pellets from a sediment trap that has been capturing sinking particles at a depth of 170 m since 1992. It is moored near the US Palmer Station in the marginal ice zone west of the Antarctic Peninsula. (Map: Julia Hager, GoogleEarth)

The explanation for the fluctuations is quite simple: the larger the krill, the larger the excreted faecal pellets and thus their carbon content. In addition, the larger faecal pellets usually remain intact and sink faster into the depths at several hundred metres per day than those of the small krill, which are quite fragile.

The faecal pellets of krill and other zooplankton, together with other solid organic material such as microorganisms, dead cells and more, are referred to as particulate organic carbon (POC). The removal of these carbon-rich particles from the upper water layers and their long-term storage in deep-sea sediments are of great importance for the regulation of the global climate. The region west of the Antarctic Peninsula is one of those with the highest summertime POC export worldwide. At the same time, it is one of the areas that is warming fastest.

This faecal pellet, with the shells of the diatom cells still visible, originates from much smaller zooplankton. Antarctic krill faecal pellets, on the other hand, can be several millimetres long. (Photo: Julia Hager)

As Antarctic krill are highly dependent on sea ice, it is suspected that the swarms could become smaller due to warming and decreasing sea ice, as is already observed in the Atlantic sector of the Southern Ocean. As a result, the five-year cycle of krill growth could be interrupted, thus altering carbon export. Furthermore, the decomposition of organic material into its inorganic components by microorganisms in the water column seems to increase at higher temperatures. As a result, carbon storage in the deep ocean decreases.

Julia Hager, PolarJournal

Link to the study: Trinh, R., Ducklow, H.W., Steinberg, D.K. et al. Krill body size drives particulate organic carbon export in West Antarctica. Nature 618, 526-530 (2023). https://doi.org/10.1038/s41586-023-06041-4

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