Snow made by living things
In sunlit waters, phytoplankton use light and dissolved nutrients to grow. Tiny animals graze on them and become food for other animals. As cells die and animals feed, some of that material ends up back in the water.
Dead cells, faecal pellets and mucus drift alongside fragments of algae and animals. When they collide, they can stick together.
Some of these clumps, called aggregates, grow large enough to see. Their ingredients vary. A flake can hold plankton remains, mineral grains, bacteria and even tiny living organisms.
A descent full of interruptions
Watch a flake for a moment and you may see it travel sideways or spin. Currents move flakes around and turbulence can tear them apart. Some aggregates disintegrate within a few tens of metres. Others pick up particles and sink faster. Animals and microbes feed on them as they fall. Zooplankton can eat or fragment flakes. Bacteria grow on the particles and break down organic matter, some of which passes into solution.
A flake that reaches deep water can have a very different mix of material from the one that started sinking.
When snow feeds the deep
Several hundred metres down, too little sunlight remains for photosynthesis. Farther down, the water is permanently dark. Animals still have to eat. Marine snow brings some of their food from above, carrying organic carbon and nutrients produced in sunlit water. Some animals intercept flakes as they sink. Others feed on what settles, including material that microbes have already broken down in the sediment.
The supply can arrive in pulses. In some temperate seas, more material sinks after the spring plankton bloom. A bloom near the surface can therefore affect animals living thousands of metres below it.
What the snow means for carbon
The falling particles also move carbon between the surface and the deep ocean. Phytoplankton use dissolved carbon dioxide to make organic matter. When some of that material sinks, it carries the carbon with it. This downward transfer is one part of the biological carbon pump.
As organisms use or break down the particles, some of the carbon returns to the water as dissolved carbon dioxide. How long it stays out of contact with the atmosphere depends on how deep it was released and where the currents take that water.
Only a small share of the organic matter made near the surface stays buried in sediment over the long term.
What about the Mediterranean?
Marine snow also forms in the Mediterranean. It can be especially visible when plankton production increases or mucus-rich aggregates form in the water column. Divers may see translucent threads, fluffy clumps or particles that seem to fill the water. Sediment stirred up from the seabed can look much the same. The Mediterranean also sees large masses of gelatinous mucilage. They can contain many organic aggregates and form through processes distinct from the ordinary fall of marine snow.
Currents can carry material along drop-offs, through open water and over seagrass beds, bringing it within reach of filter feeders. Sponges, sea squirts and other attached animals capture different kinds of suspended particles, depending on particle size and how the animals filter water.
Marine snow helps move organic matter between different parts of the marine environment.
Interactive quiz
Marine snow quiz
5 question(s) to test your knowledge.
Interactive quiz
Marine snow quiz
Question 1 out of 5Score: 0
Why does the composition of marine snow vary from flake to flake?
What does the decline in particle flux with depth tell us?
How can marine snow feed organisms in the deep?
What can affect how long carbon carried down by marine snow stays out of the atmosphere?
What might explain flakes seen drifting over a seagrass bed?
Final result
Quiz completed
correct answers
What falls does not disappear
The next time flakes drift through your torch beam, think about where each one might have started. Some began as plankton cells. Others came from a dead animal, a meal that had already been digested or mucus released into the water. Most of the material will be eaten or broken down before it reaches the seabed. A small share will settle there.
That small share can feed animals that live far below the sunlit water where it formed.
Scientific references
These sources document the main processes described in the article.
