A BLIZZARD is not something you’d expect to find in the deep ocean. But when sailors first travelled underwater in submarines, they thought they had been caught in a snowstorm.
Superficially, marine snow shows some surprising similarities with its atmospheric counterpart. It falls under gravity, from the ocean surface to the seafloor, and it is highly seasonal, with late spring and summer being peak times. But it is a very different substance from the stuff that falls from clouds: for one thing, it glows in the dark.
So what is it? Marine snowflakes are made up of an assortment of flotsam and jetsam glued together by copious amounts of mucus. Living and dead microscopic plants such as diatoms and cyanobacteria, faecal pellets, bacteria and tiny animals known as zooplankton are all bound together by polysaccharide tendrils to form flakes no bigger than half a centimetre wide. Bioluminescent bacteria give the flakes their eerie glow.
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How does it form? In the oceans of the northern hemisphere, it all starts about now. Tiny oceanic floating plants – phytoplankton – get trapped in the warm surface layers that have become enriched with essential nutrients stirred up from the deep by winter storms. It’s phytoplankton heaven. Basking in the spring sunlight they explode into photosynthetic action, sucking up carbon dioxide from the air and turning it into building blocks for their cells.
But all good things must come to an end. As little as a week after the bloom starts, the supply of inorganic nutrients becomes exhausted and the phytoplankton are forced to stop growing. Yet they are still busy making carbon compounds which they can’t use, and to get rid of this excess they exude it as mucus. Cells stick together as their gloopy tendrils become entwined, and other microscopic debris gets caught up in the tangle.
And then it starts to snow. Just like snowflakes in clouds, the flakes reach a critical mass and begin to sink. The bloom turns into marine snow, drifting towards the ocean bottom at a rate of about 50 metres a day.
What happens when it hits the seafloor? It piles up in a fluffy layer, like freshly fallen winter snow. For creatures living in the twilight zone of the deep oceans – everything from bacteria to fish – it is like manna from heaven. The snow is their major source of food.
Marine snow also plays a crucial role in the Earth’s climate. It acts as a conveyor belt, taking carbon from CO2 in the air and transporting it to the seafloor, where it stays locked up for decades – or more if the flakes become buried in sediments.
As you read this, the spring sunshine will be warming the northern oceans, and beneath the waves it will soon be snowing.