The snow that falls to the bottom of the sea

Marine snow plays an important role in carbon storage and nutrient recycling in our oceans, but our impacts are starting to alter this natural phenomenon in a big way.

heavy snow
A flurry of marine snow falls to the seafloor

Every day large amounts of waste materials generated by natural processes fall through the water column in our oceans and settle on the seafloor. This is known as marine snow and, unlike the cold icy variety we experience, it is significantly more abundant, diverse and ecologically important. It provides food to the creatures of the deep and traps carbon absorbed by the oceans deep underwater. However this incredible natural process is starting to show signs of being impacted by our actions right as we start to need it the most. So what really makes up marine snow? And why is it so vital to marine life and us?

What is marine snow?

Marine snow is the general term given to particles of waste that fall through the water column towards the seafloor. There are two main types of marine snow, organic waste and inorganic waste. The majority of it is organic and consists of things like dead plankton, fish scales, decaying flesh and faeces, collectively known as detritus. The smaller inorganic component of marine snow consists of sand, soot and other dusts. The particles or ‘snowflakes’ of marine snow are usually around half a millimetre in size, but as they slowly reach the deep ocean they clump together and accumulate extra waste and can reach up to 10cm in size. This can take days and even weeks as the snowflakes slowly fall through the dense seawater, often appearing to barely move at all.

types of snow
Marine ‘snowflakes’ may not be as beautiful as real ones but they are much more varied and diverse

Settling on the seafloor

After days and weeks of falling into the deep the majority of marine snow will actually never make it to the bottom of the ocean (only around 3% reaches the seafloor) as it is instead consumed and recycled by marine creatures. However the snow that does not get re-used does end up settling on the seafloor like dead leaves falling to the forest floor. In coastal waters and on shallow continental shelves this layer of waste will quickly be utilized by crustaceans, molluscs and other bottom dwellers. However in the deep ocean open it joins a thick layer of debris commonly referred to as ‘ooze’. In some parts of the ocean this ooze can be hundreds of metres thick and is millions of years old.

Marine snow builds up on the seafloor and is utilised by bottom dwelling organisms

Seasonal patterns

Although marine snow occurs 24 hours a day throughout the entirety of our oceans it does not show consistent patterns of ‘snowfall’ throughout the year. Like real snow this is a result of changes in seasons, but rather than falling hardest in the winter, marine snow peaks in the spring. This is because in the oceans spring provides the ideal conditions for phytoplankton blooms. An increase in sunlight, temperature and upwelling of nutrients causes an explosion of the tiny photosynthetic algae at the surface, which in turn leads to a knock-on increase in the overall productivity of the oceans. The phytoplankton have a short lifespan and as a result there is also a big die-off during this period and so we see a spike in the concentration of marine snow falling to the seafloor.

vampire squid
A deep sea vampire squid swims through some heavy marine snow

Recycling nutrients

All this death and debris raining down on the deep ocean is actually extremely important because for many creatures living in the deepest darkest parts of the sea it is their main source of food. Detritus in particular is rich in the essential nutrients life needs to survive including nitrogen and carbon, as well as energy. However inorganic marine snow can also contain essential elements that certain species would not normally be able to access on their own. It’s not just deep dwelling species that rely on marine snow, filter feeders in the top few hundred metres of the ocean also consume a lot of marine snow before it falls very far at all.

A diagram of the nutrient recycling and carbon storage capacities of marine snow (via American Scientist)

The energy and nutrients from marine snow are re-used by animals as food, but it is by no means a one off event. Deep dwelling marine species also produce waste and die creating even more marine snow in the deep ocean. In places where ocean currents push water towards the surface, known as upwelling regions, those nutrients are pushed back up to the surface for things like plankton to feed on. Not only does this mean marine snow can also ‘fall upwards’, but it also creates a closed loop of nutrient recycling without which life in the oceans would not be able to survive in its current capacity.

Trapping carbon

If most of these nutrients like carbon are being taken in and re-used by marine life, or pushed back to the surface, you may be wondering how marine snow is able to trap any carbon at all. The answer is in the ‘ooze’, that deep layer of carbon-rich marine snow that reaches the seafloor. Over millions of years and the crushing pressures of the deep ocean that ooze eventually gets compacted into carbonate structures, such as the white cliffs of Dover, that trap the carbon inside. This extremely long process may not seem like it would have a very large effect, but research by the University of Sydney has shown that marine snow has played a critical role in cooling our planet over millennia.

white cliffs of dover
The white cliffs of Dover were created out of marine snow compacted over millions of years

Due to human caused climate change radically effecting our atmosphere and ocean systems marine snow is therefore more important than ever. The researchers predict that the rate of carbon being deposited in the ooze by marine snow is now 200 times greater than it was 80 million years ago. However there is big uncertainty about how climate change, in particular ocean acidification, may impact the ability of marine snow to trap atmospheric carbon. Lead researcher Dr Adriana Dutkiewicz told science daily that “even small changes in the sequestration of carbonate carbon into this enormous sink are quite important for understanding net changes in atmospheric carbon dioxide and climate”. The real worry among experts is that if marine snow can no longer trap carbon then it may make it even harder to undo damage done by climate change.

Poisoned with plastic

Unfortunately marine snow is now starting to include a more unnatural component thanks to humans, plastic. Like so many other things in our oceans plastic is getting tangled up with the marine snow in a way which can lead to some nasty consequences. In particular microplastics, the small fragments of plastic used in pharmaceutical products and created by the breakdown of other plastics in the ocean. Research from the University of Exeter has now shown that marine snow is one of the primary routes for these plastics into the deep ocean. This is bad on two fronts. Firstly organisms that feed on marine snow (of which there are a lot) are now going to be unknowingly consuming large amounts of plastic which will impact their health. Secondly like nutrients these plastic pieces are likely to just get continually recycled throughout the ocean spreading into the most remote parts and, unless we come up with an advanced technological solution, will remain in our oceans for thousands of years.

Unfortunately marine snowflakes like this one can contain lots of tiny microplastic pieces

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