Marine snow near the seabed (Source: NOAA Okeanos Explorer Program, Gulf of Mexico 2012 Expedition)

The deep-sea is like a winter wonderland; it’s often very cold, and it’s always snowing. These snowflakes aren’t at all like the ones we are used to up here on land. Instead of being icy hexagonal flakes, marine snow is made up of organic material- a combination of animal faeces, sloughed off scales, dying plankton, and sediments. As it sinks, the snow clumps together, forming aggregations with a diameter sometimes up to several centimetres, sometimes taking several weeks to reach the sea floor. The idea of falling detritus may not sound particularly pleasant to you or me, but it’s the basis of life in the deep-sea.

Marine snow is an incredibly nutritious and remarkably consistent source of food in an otherwise barren habitat. Prey items can be few and far between hundreds of metres underwater, but marine snow is always drifting by. In an environment where there is no sunlight to support the phytoplankton which forms the base of the food web in the shallows, the movement of organic material to the deep in the form of marine snow is incredibly important in supporting life. It makes up the majority of the diet of zooplankton in the deep, and there are even larger animals that utilise it as a reliable food source.

Vampire squid (Vampyroteuthis infernalis) surrounded by particles of marine snow (Source: MBARI)

The vampire squid (Vampyroteuthis infernalis) looks like an intimidating predator; they have webbed tentacles like Dracula’s cloak and are blood red in colour, and with a name that translates to “vampire squid from hell”,  they sound pretty menacing too. Given all other known cephalopods are predatory, early researchers assumed than the vampire squid was a hunter too. In reality, they simple extend two long, thin, sticky filaments into the water column, catching marine snow, which they then transfer to their mouths. They aren’t the only animals that catching the falling food. Larvaceans, free swimming tunicates that look rather similar to tadpoles, feed by catching marine snow in the mucus net they call . The net can be up to 1 metre in diameter, an impressive size for an animal that rarely exceeds 10cm in length.

A giant larvacean (Bathochordaeus stygius) in a flurry of marine snow (Source: MBARI)

Marine snow is not only useful as a food source. Human activities release a huge amount of carbon dioxide into the atmosphere. About a third of atmospheric carbon dioxide is absorbed by the ocean, where it dissolves easily. In order for the ocean to continue taking in carbon dioxide, the carbon dioxide must move from the surface waters to the depths. This is where the “biological pump” comes into play. As detritus, which is rich in carbon, aggregates and sinks in the form of marine snow, it is “pumping” carbon down towards the seafloor.

Approximately 15% of carbon in the ocean ends up trapped in seafloor sediments, where it becomes a part of deposits and is permanently removed from the carbon cycle. While 15% may not sound like much, it is a massive feat. Phytoplankton, considered to be highly important in the sequestering of carbon through photosynthesis, sink slowly on their own. When they aggregate into flakes of marine snow, they become heavier and sink faster, so they’re more likely to make it to the seafloor.

While marine snow may not look like much, and certainly isn’t something I’d enjoy to have falling from the sky, it is incredibly important in the ocean system. It feeds animals of all shapes and sizes, but also provides a huge ecological service to us by helping to in the removal of carbon from the system. Who knew fish poop could be so helpful?

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