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Animal excrements and parts of dead organisms constantly sink from the surface of the oceans towards the deep sea. This particle flow plays an important role in the global carbon cycle and consequently for the climate. Little is known so far about its distribution in the water column. An international research team led by the GEOMAR has now published a detailed image of this distribution mechanism in the equatorial ocean in Nature Geoscience.

The great ocean currents, with their immense energy transport, have a decisive influence on the atmosphere, and thus the climate. Tiny planktonic organisms take up carbon near the surface, process it, build up their bodies with it or excrete it. The carbon incorporated in the excretory products or dead organisms then sinks to the seabed. The constant flow of organic particles towards the deep sea is also called "marine snowfall."

This snowfall is most intense where strong biological primary production can be observed near the surface. This, for example, is the case along the equator in the Pacific and Atlantic Oceans. However, it is not known how the particles are distributed at depth and which processes influence this distribution. Now, an international team of scientists led by the GEOMAR Helmholtz Centre for Ocean Research Kiel has published the first study with high-resolution data on particle density in the equatorial Atlantic and Pacific Ocean down to a depth of 5000 meters. "The analysis of the data has shown that we have to revise several previously accepted ideas on the flow of particles into the deep sea," says Dr. Rainer Kiko, biologist at GEOMAR and lead author of the study.

The team, which includes colleagues from France and the U.S., has analyzed data collected during several expeditions of the German research vessels METEOR and MARIA S. MERIAN, the U.S. research vessel Ronald H. Brown and the French research vessels L'Atalante and Tara. The data were obtained with sensors including the so-called Underwater Vision Profiler (UVP). The UVP is a special underwater camera that can be lowered down to 6000 meters. During the decent, it takes 10 images per second, which allows the researchers to count particles and identify small plankton organisms.

"Up to now, it was usually assumed that the largest particle density is close to the surface and that it decreases continuously with depth," explains Dr. Kiko. "Our data show, however, that the particle density increases again at 300 to 600 meters of depth." The researchers explain this observation with the daily migratory behavior of many plankton organisms, which retreat into corresponding depths during the day. "This depth seems to be the loo for many species. That's why we find a lot of particles there," says Dr. Kiko.

These microscopic particles sink deeper and are still detectable at 5000 meters depth. "This is also surprising, because it has been assumed that only few larger, rapidly sinking particles can be found deeper than 1000 meters," explains Dr. Kiko.

The team also explained another phenomenon. "In the equatorial region, the flow of particles into the deep sea is much greater than in regions that are only 100 kilometers further north or south," says Dr. Kiko. Prof.

Dr. Peter Brandt, an oceanographer at GEOMAR, says, "There are strong, eastward flowing deep currents north and south of the equator, both in the Pacific and the Atlantic. They form natural barriers that prevent further north-south propagation of the particles."

All in all, the scientists were able to show the importance of biological and physical processes for the biological carbon pump. "Of course, we need further observations on the distribution of different planktonic groups in the ocean in order to further refine the image," says lead-author Dr. Kiko. At non-scientists can help in the task of sorting the enormous number of plankton images the UVP delivers. "On the PlanktonID website, interested people can help us to identify zooplankton, but they will also find additional information on the current study, such as the functioning of the UVP," says Dr. Kiko.