by Matthew Norton
The deep sea might look like a dark and desolate world but look closer and you will find that it is full of wonderfully weird creatures who have thrive in this environment despite the unique challenges they face. One particularly big problem is that there is not enough sunlight to support any plants, so the traditional food chain (plant → plant eater → animal eater) doesn’t work down there. Some species instead turn to alternative energy sources, such as the chemicals that leak out from hydrothermal vents and methane seeps, while others scavenge from the corpses of large animals. There is however a third option, which is to pick out the many smaller pieces of food that rain down from the surface as ‘marine snow’.
Each particle of marine snow is made from numerous dead or dying organisms (detritus is the technical term) stuck together along with other bits and pieces, such as sand, dust and animal faeces poo. These particles can provide temporary oases for living microorganisms such as bacteria and protists, who live in much greater densities on marine snow than in the surrounding water. It’s almost a shame that at any moment a slightly larger animal could come along and eat it, and it will likely happen because these particles are easy meals that can’t run, hide, or fight back (unless there are toxic or disease-causing microorganisms in the mix). Should they reach the seafloor uneaten, these particles may still get dug out of the sand, or mud and gobbled down by various bottom dwelling animals. Very little goes to waste in the marine environment.
In theory, marine snow can be made of anything that was once alive in the sea, but there are some groups that make especially large contributions to making these particles. For example, phytoplankton leave behind large numbers of dead cells, especially in the aftermath of blooms (explosions in phytoplankton growth), along with the remains and faeces of animals that came to eat them. Larvaceans (tadpole-like animals) also leave behind ‘ready-made’ balls of marine snow when they dispose of their jelly-like ‘houses’. These houses, and all its inbuilt filters and funnels, are the key to their ability to filter out tiny pieces of food from the water, but they get clogged up very easily and the only solution is to get rid of them and start again. Still, this does leave behind plenty of food for other creatures in the water and a fair amount of slime to stick it all together.
With so much marine snow raining down from the ocean surface it seems almost inevitable that some of it will reach the deepest parts of the ocean, but that doesn’t mean that those sunlit waters get nothing in return. Some of the oxygen and nutrients that build up in the deep sea (fewer animals down there to use them) is eventually carried back up to the surface by ocean currents in a process called upwelling. In some coastal regions, this process can provide the ideal conditions for the growth of the very phytoplankton that plays such a big role in creating marine snow in the first place. The ocean surface and deep sea are two very different worlds, but there is still a connection between them.
From a human perspective
The idea that food rains down from the ocean surface has persisted almost as long as we have known that there was life down in the deep sea. As far as we know, the phenomenon was first suggested during the 1872-1876 voyage of the HMS Challenger which, like several voyages on other ships, recovered specimens from the deep sea by using a dredge attached to a very long steel cable. Apparently, at some point during the voyage someone suggested that those creatures survived, despite being isolated from the sun, by feeding on a ‘rain of detritus’ from the surface.
However, no one actually saw this until William Beebe, an American naturalist and explorer (among other things), conducted a number of deep sea dives in the 1930s. He achieved this feat in a vehicle called the ‘bathysphere’, a diving bell suspended from a steel cable attached to a ‘mother’ ship (pretty basic, but you’ve got to start somewhere). From the porthole windows of the bathysphere, Beebe could see and identify various animals as they swam past and crucially, he could see those strange particles of which he called marine snow. It’s hard to imagine what that experience must have been like, to see the underwater world at such depths and relatively unaffected by humanity. This is a privilege that is becoming increasingly rare in the modern world.
William Beebe was unlikely to see any plastic rubbish during his dives in the bathysphere, but now it is one of the biggest threats to marine animals in every corner of the ocean. Many animals get caught up in bags, discarded fishing lines and other plastic rubbish, and suffering deep cuts and broken bones from trying to break free and if they don’t succeed, they will either drown or starve. Some animals will also eat plastic (mistaking it for prey), which then gets stuck inside their stomach and leaves less room for real food. Often, the victim doesn’t realise they are starving because they still get that feeling of being full from all the plastic in their stomach as they waste away.
Even the deep sea is not safe with records of plastic rubbish along the seafloor and right down to the Mariana Trench as well as plastic being recovered from the guts of various animals including deep sea fish, lobsters, anemones and sea cucumbers. We can only hope that some of this rubbish gets buried so far down in the seafloor that it stays out of the reach of any living creature, human or otherwise. Who knows, in the far future these plastics may be discovered among the fossilised remains of animals who are alive today. Hopefully, such reminders of how reckless we used to be with the natural world will be rare.
Plastic pollution is a serious issue that requires urgent action and the deep sea is no exception. Frankly, this has been known for a long time, but the scientific evidence can easily go under the radar unless it is made accessible and communicated in a way that actually inspires people to take notice and do something about it. For example, there have been scientific papers about plastic pollution for decades and the threat of microplastics has been covered in the literature since at least 2004. The existence of rubbish in the sea was even casually depicted in a classic Tom and Jerry film from 1947 called “Saltwater Tabby” where Tom (the cat) is at the beach and accidently dives into a pile of rubbish that was hidden by the sea until the last second. Despite all this, the issue only got the worldwide attention it really needed when David Attenborough raised awareness of it in “Blue Planet 2”, which just goes to show that communication and reaching out to people is key to protecting the natural world.
NOAA. 2018. What is marine snow? https://oceanservice.noaa.gov/facts/marinesnow.html
Wikipedia. 2020a. https://en.wikipedia.org/wiki/Marine_snow
Miller et al. 2013. A low trophic position of Japanese eel larvae indicates feeding on marine snow
Shanks and Walters. 1995. Feeding by a heterotrophic dinoflagellate (Noctiluca scintillans) in marine snow
Wotton, R.S., 2004. The essential role of exopolymers (EPS) in aquatic systems
Silver. 2015. Marine Snow: A Brief Historical Sketch
Wikipedia. 2020b. https://en.wikipedia.org/wiki/Challenger_expedition
Wikipedia. 2020c. https://en.wikipedia.org/wiki/William_Beebe
Taylor et al. 2016. Plastic microfibre ingestion by deep-sea organisms
Thompson et al. 2004. Lost at sea: where is all the plastic?
Russell R. Hopcroft, Institute of Marine Science, University of Alaska Fairbanks. 2005. [CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/)]. https://commons.wikimedia.org/wiki/File:Euphausia_gills.jpg
opencage. (unknown date). [CC BY-SA (https://creativecommons.org/licenses/by-sa/2.5)]. https://commons.wikimedia.org/wiki/File:Anguilla_japonica.jpg
Mike Cole. 2009. [CC BY (https://creativecommons.org/licenses/by/2.0)]. https://commons.wikimedia.org/wiki/File:Bathysphere.jpg
U+1F360. 2018. [CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)]. https://commons.wikimedia.org/wiki/File:Plastic_Bag_Jelly_Fish.jpg
Clandon haverford. 2018. [CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)]. https://commons.wikimedia.org/wiki/File:Microplastic_sample.jpg
All other images are in the public domain and do not require attribution