When does the sea become ‘deep sea’? Or rather, when should the sea become deep sea?
Words by Alan Jamieson
Over 30 years ago, eminent deep sea biologists, John Gage and Paul Tyler, published the classic textbook: Deep Sea Biology; A Natural History of Organisms at the Deep Sea Floor. Early in that book they asked a solid opener: What is the deep sea? They explained that if you ask virtually any deep sea biologist, and you'll get a slightly different answer.
In most deep sea literature, the deep sea is considered to be the oceans deeper than 200 meters. Often this is given with a caveat of also being beyond the edge of the continental shelf that bounds the periphery of the ocean basins. It is also said that the upper limit can be based on the depth of the mixed surface layer; temperature; light penetration; or, the depth below which plant life is largely absent (which the latter two suggest as the limits of photosynthesis). A biological transition from shallow water fauna, to the shelf, to the deep sea fauna has also been defined for some time, or the combined descriptor of ‘penetration of sunlight’ and covariate depth by the associated fauna. But the general consensus is that the deep sea is the ocean beyond the shelf break at depths deeper than 200 meters, underlying the photic zone.
Establishing a solid definition requires a consensus for coherent communication, and that language use is a matter of social consensus. What’s more, as we delegate stronger authority to the use of certain terms to relevant authorities, this is why us as scientists need to clarify what we actually think the deep sea is.
The defined ocean layers (Image courtesy of NOAA)
‘Deep’ is subjective
The issue with the term ‘deep sea’ is that the word ‘deep’ is very subjective, and it can mean different things in different contexts. For example, the ‘deep end’ of a swimming pool can be 3 meters, deep sea sports fishing is generally fishing in waters greater than 30 metres. The deep sea fishing industry is generally in the top 1000 metres. Deep sea mining is 4000 to 6000m. The Challenger Deep is 11 kilometers underwater. And it gets really muddy when you start looking at deep space. ‘Deep’ by definition, simply means extending far down from the top of a surface. And far simply means: a great distance (and ‘great’ means ‘unusual’ or ‘comparatively large in size’).
“‘Deep’ by definition, simply means extending far down from the top of a surface. And far simply means: a great distance, and ‘great’ means ‘unusual’ or ‘comparatively large in size’.”
The term is used entirely in the context of the activity in which it's being used. While acknowledging times where something which once thought ‘too deep’, in hindsight, it’s no longer sufficiently challenging to feel unreachable. So historically, the definition of deep sea has been argued based on what was known (at the time) to be considered ‘deep’.
Where did the term ‘deep sea’ come from?
In the book ‘Fathoming the Oceans’, there's a story there that regales correspondence from 1869, where John Gwyn Jeffreys wrote to the journal Nature, claiming that 10 fathoms (which is 18 metres) was too shallow for the term ‘deep sea’. Which had apparently been the definition since the 1840s. He claims that 50 fathoms (which is 91metres) was a better delimiter for the ‘deep sea’ based on the depths that were readily being sampled at the time. Eight years later, he had further refined it to ‘Abyssal’ for anything from 1,000 fathoms (which is 183 to 1829 metres). This is arguably the first mention of what is now the Bathyal Zone. And therefore by defining the upper limits of the deep sea (approximately 200 to 2000 meters - albeit rounded, following a conversion from fathoms), refers to the Neritic Zone as depths less than 200 fathoms, and ‘Bathyal’ as depths between 200 and 2000 fathoms.
An illustration of Forbes’ Azoic Theory (1843) doi.org/10.1890/0012-9623-91.2.176, (Image courtesy of Georgia Wells)
The famous Edward Forbes described his regions of depth. The first was a Littoral zone for the space between the tide marks. The second was a circum-littoral or Laminarian Zone, coined after the tangles of seaweeds. And the third was the median Coralline Zone where seaweeds were mostly absent. The fourth region was infra-median, the ‘Abyssal Zone’ a region of deep sea stony corals, which can be scarcely said to be ‘developed within the British area’. Therefore, Forbes depths definitions were simply based on local biodiversity surveys around Europe. Regardless of how exactly that was established, it appears to be clear that it was based upon a 100 year old European or North Atlantic centric idea of waters that extend beyond the continental shelf in those areas.
Defining rules to the natural world as simply what is ‘deep’ and what is ‘not deep’, will always run into exceptions and variability that may lead to questioning the value of the rule at all. For example, 200 meters as the limit to plant growth is quickly challenged, since seaweeds have been found as deep as 265 meters, (albeit most are less than 200m). Solar light penetrates much deeper into the ocean than 200 meters, by another 800m in some places. 2.7 million square kilometers of continental shelf around Antarctica tends to be closer to 450 meters (over twice the norm of 200m). And in the absence of continental shelves in places such as islands and atolls, the 200 meter contour is often ignored altogether when defining transitions. Therefore, consistency in nomenclature when defining ocean zones is where the issues lie.
Does ‘the area beyond the continental shelf’ work as an accurate baseline for the start of the deep sea?
So let's think about the Continental shelf. The Continental shelf only accounts for 8.9% of the 32 million square kilometers of ocean. Plus, the significance of the 200 meter contour is greater in the western regions where early marine science emerged, than in other regions across the globe. Using some stats from a paper by Harris et al. (2014) on the global area of the under-sea geomorphology, it was found that as a percentage surface area, the percentages in the aforementioned western areas, (the North Atlantic, Mediterranean and Black Sea) were 16 and 23%, respectively. These are, in contrast to the Indian Ocean - 5.7%, Atlantic Ocean - 5%, North Pacific Ocean - 7.5%, South Pacific Ocean - 2.9%. There are, however, large percentages in the polar regions with 51% and 13%. So basically, there's far more continental shelf around the North Atlantic, Mediterranean, and Black Sea than there are in other oceans.
The Arctic arguably played an important role in the early western exploration by bounding the North Atlantic Ocean, between North America and Europe. The combined continental shelf area of the Arctic, North Atlantic, Mediterranean, Black Sea is 14 million square kilometers. Therefore 45.7% of global shelf areas are situated in these areas that account for just 16.8% of the oceans, that happened to be regions that have a strong legacy in marine science at the time of defining basic terms of oceanography.
Antarctica, including the Antarctic continental shelf (Image courtesy of NOAA)
Gunther and Deckert (1956), described the deep sea as driven by geomorphology, where down to 200 meters, the bottom has a relatively gentle gradient; and that below 200 meters the bottom generally falls more steeply. But what they are describing here is the North Atlantic, and not necessarily a global scenario that is easily recognised around the Pacific rim or global south. In the inhabited global south which is the South Atlantic, South Pacific and Indian Oceans (which is 55% of the ocean combined), the continental shelf accounts for just 4% of that combined area or 26% of the global shelf area. Alternatively, the combined shelf area of the North and South Pacific represents just 5.1% of the area. Furthermore, the continental shelf around Antarctica is much deeper than other continental shelves, at an average of 450 meters, which can extend over a thousand meters. And it's also unique in that some of the shelf area features a glacially excavated inner basin that can be over 1,500 meters deep. On average, the Antarctic shelf is almost twice as deep as other continental shelves elsewhere.
So the 200 meter shallow-to-deep boundary is likely an artefact from the days of early scientific exploration originating from around the North Atlantic, and does not offer any practical, bathymetric or geomorphological boundaries in most of the rest of the world. Although, there are large areas of shelves in other areas, for example Australia or Argentina, and so on. So perhaps, light would be a major consideration.
Should light be the determining factor?
Snell’s Window (a phenomenon by which an underwater viewer sees everything above the surface through a cone of light of width of about 97 degrees)
Solar light is often posited as a driver of marine diversity and biomass due to photosynthesis-derived phytoplankton increasing available energy in the upper ocean, indirectly supporting a complex food web that supports the deeper waters below. At 100 meters below the surface of a clear ocean, the quantal spectrum of daylight narrows significantly and becomes much dimmer, declining by about 2.6 log units, but can vary considerably in different clarities of ocean water. Below a hundred meters, coinciding with the decrease in plant and suspended organic manner, light intensity declines by about 1.5 orders of magnitude for every 100 meters of water depth. By 600-700 meters, it reaches starlight levels during the day. Beyond a thousand meters, daylight no longer penetrates sufficiently to be significant to deep sea animals. Seawater has a higher refractive index than air, which results in the entire 180 degree dome of the sky being compressed underwater to a 97 degree cone, known as Snell's Window, which is always brighter than surrounding space light. Although it is dominated by the position of the sun and the surface layers, its dominance declines with depth, disappearing altogether below the so called asymptotic depth (approximately 400 meters) in the clearest ocean water, albeit shallower in more turbid water. Therefore, solar light seems to be quite significant at twice the depth of what we're calling the ‘deep sea’.
is there a distinction in shallow vs deep fauna?
So let's think about where shallow species go deep. Many large marine taxa that are synonymous with the surface layers are known to make excursions to the deep sea beyond 200 metres. For example: the deepest arctic whale is 1,592m, deepest sperm whales: over 1,000 meters, deepest beaked whale is nearly 3,000 meters. Dolphins are known to go 300 meters, great white sharks: 1000m and so on. Even reptiles and birds are known to cross the 200 meter contour. And I was quite surprised to learn that sea snakes go to 250 m.
These are examples of species most often considered ‘surface-dwelling species’ that happen to make brief deep dives or excursions greater than 200 meters. Other uses of deep sea by ‘shallow species’ include the most dominant member of the zooplankton: the copepods. These creatures use the deep for diapause (a kind of dormancy that is temporarily initiated in a direct response to unfavourable and environmental conditions) to thrive in high latitude environments. To access areas where temperatures are generally colder and predators are less abundant or absent, diapause is initiated by a vertical migration to depths of 100-1000m. There was a study that reviewed six Pacific species, four Atlantic species and one Southern ocean species, and reported that all but one had a diapause range of more than 200 meters. Therefore the important and abundant surface dwelling groups, such as marine zooplankton, are using the deep sea for a significant phase of their life history.
Skipjack tuna (Katsuwonus pelamis)
So in addition to shallow water species that dive deep or temporarily diapause at depth, there are also species commonly associated with being shallow that have depth ranges that extend beyond the 200 meter contour. Of the top 10 most commercially-landed fish species identified by the FPO, only two of them are known exclusively from less than 200 meters. The rest transcend this magical 200 meter deep sea boundary. For example, the Alaska pollock, the skipjack tuna, Atlantic herring chub mackerel, the elephant tuna, the Chinese anchovy, the largehead hairtail and the Atlantic cod; these are all regularly found beyond the 200 meter limit.
So what about when deep sea species go shallow? The deep pelagic zone greater than 200 meters, contains almost 95% of the ocean's volume. Diel vertical migration is ubiquitous across the mesopelagic taxa. Mesopelagic fishes are among the most abundant marine organisms on Earth, and are usually found at depths between 100-1000m. Most mesopelagic species make extensive upward migration into the epipelagic zone during the night, and thereafter migrate down several 100 meters to their daytime depth. Myctophid larvae remain in the epipelagic zone, which is less than 200m, and then move to relatively deeper depths to adapt to their later adult life in the mesopelagic zone. After which, most species start diel vertical migration. This suggests that the deep mesopelagic is a fundamental element of the ecology of large epipelagic fishes, and the epipelagic is therefore integral to the ecology of nearly all mesopelagic fishes. This means that the mesopelagic fishes (amongst the most abundant marine organism on the planet) are neither shallow nor deep sea fishes, who rather utilise its top 1000 meters.
is there such a thing as shallow-water taxa?
So what about depth ranges of common marine taxa? In addition to shallow-water species going deep, and deep-water species going shallow, many marine taxa that are quite familiar in our daily lives, extend far deeper than is generally appreciated. For example, marine groups such as the cephalopods are all found well below the 200 meter mark. Even cuttlefish at 600m, nautilus - 700m, vampire squid - 5000m, squid - 6000m, octopus - 7000m. Other mollusks, such as snails and clams are known from over 10,700 meters, and sea slugs, with 4400 metres. Many orders of cnidarians are known from hadal depths. Hydrozoan jellyfish, now known from over 10,000 meters, shark rays and chimeras are found beyond 3000, 4000, and 3000m, respectively. Jawless fishes such as the hagfish can be found at 3000m. Bony fish can be found over 8000m. Echinoderms are represented to hadal depths with sea stars to nearly 10,000, brittle stars to 8500m, urchins to 7500m, sea lilies to nearly 10,000m, and so on. At the taxonomic class level, there are very few examples of those who do not use the deep sea at all.
“At the taxonomic class level, there are very few examples of those who do not use the deep sea at all.”
The industrialisation of the deep.
Someone else uses the deep sea is us. So what about the industrialisation of the deep sea? Another aspect to the subjectiveness of the term ‘deep’ is: how out of reach and difficult to access is it, really? We can think about how much access we have to depths greater than 200m, by using various industrial activities. For example, deep sea fisheries. Deep sea fishing was once a very challenging and dangerous activity, but not so much in modern times. There are 72 species or species groups being caught primarily with bottom trawls, mostly at depths greater than 400 meters. There was a paper that found that the most fished and vulnerable deep sea fishery species were the Greenland halibut, which go down to 1000m, orange roughy which go down to 1800m, red-nose grenadier - 1200m, slender armorhead - 600m, Patagonian toothfish - 1500m, blue ling - 500m, and the longnose velvet dog fish - 1500m.
These fisheries are occurring well within the deep sea, and demonstrate an intrinsic, albeit negative, link to the deep sea irrespective of what we may consider to be ‘deep’. However the vast majority, in terms of catch weight landed, is generally in the top 1000m. Furthermore, the industrialisation of fishing fleets has resulted in bottom trawling on the continental slopes, reshaping the sea floor over large spatial scales. Trawling induces sediment displacement, and removal from fishing grounds reduces seafloor complexity, smoothing it over time. Papers have compared the variable effects on the deep sea floor by trawling, to those generated by agricultural ploughing on land. Estimations of the daily amount of organic carbon removed by commercial trawling in the Mediterranean between 200-800m, could be as high 60 to 100% of the input flux. This results in the degradation of sedimentary habitats and a significant decrease in organic carbon turnover, therefore, reducing meiofauna abundance and biodiversity. There are studies that concluded commercial, deep sea trawling represents some major threats to the deep sea ecosystem at the global scale, but albeit largely within the top 1000m.
other deep sea extractions
The deep sea is not the only a place to extract food. There is also an emerging industry in Asia that promotes the drinking of deep sea water for health benefits. There are papers that describe the health benefits of water to which a mineral extract obtained from the deep sea water is added, where it is defined as seawater from the depth of greater than 200 meters. It is believed that drinking deep-sea-derived mineral water could have numerous health benefits, including improving gut ecosystem and intestinal health, possibly alleviating obesity and diabetes, and a range of other benefits. Who knows?
The Deep Ocean Water Research Journal, published by the Deep Ocean Water Application Society.
In these studies the water is actually pumped from a depth of 1100m around 18km off South Korea. The usage of deep sea water is an emerging industry, with some commercial brands boasting their deep sea water-based drinks are concentrated from water pumped from 662m off Taiwan. This particular site has inlets 600, 660, and 710m deep. It's also emerging in Japan with great economic values worth billions of dollars. The United States also extract deep water from 600m, 670m and 900m deep off Hawaii, to applications for bottled drinking water, agricultural, pharmaceutical cosmetic products. There's even a Japanese journal dedicated to research into applications for deep sea water that published from 2000 to 2018.
In many regions of the world, increasing water demands and diminishing ground water supplies, are prompting a greater reliance on desalination. As the production of desalinated water increases, methods to make the industry more efficient are being sought. One issue of efficiency is that of the entrainment of marine organisms that could be substantially reduced or eliminated by placing intakes away from biologically productive areas, such as the deeper water farther offshore. Such intakes are supposed to be as deep to 600m.
Another global industry present in the deep sea, greater than 200m is the oil and gas industry. There are currently an estimated 12,000 offshore platforms and 180,000 kilometers of subsea pipelines. Furthermore, offshore marine renewable energy developments would likely increase the amount of subsea infrastructure over the next 20 years. Although most platforms are situated between 30 and 150m of water on the continental shelf, there is deep water greater than 200m productions in areas such as the Arctic, North Atlantic Ocean east of West Africa, Gulf of Mexico, South America, India, Southeast Asia and Australia. The majority of these are in water depths of less than 1,000 meters, although production in waters greater than 1,000 meters does occur. In these early stages, they’re limited to areas such as the Gulf of Mexico, sometimes West Africa. Most deep water fixed platforms in the Gulf of Mexico are located across the edge of the continent slope, at depths up to about 400 meters. Here, approximately half of the fixed platforms are in 120 - 150m depths, with the remainder in approximately 150 to 300m depths. Only six fixed platforms resided in depths greater than 300 meters, and some floating platforms are located throughout the region in depths of 500 to 3000m.
So in modern times, we're readily fishing the top 1000m. We're reshaping it with bottom trawls. We're drinking water from it. We're extracting hydrocarbons from it. Our most recognisable surface fauna, including air breathers, are utilising water greater than 200 meters. The largest marine biomass is largely confined to the top 1000 meters to migrate, while plankton use the top 1000m as overwintering refugia.
None of these excursions, migrations or anthropocentric activities were known, or were occurring at the time that the deep sea was considered to begin at 200 meters. So I think when you start putting all this together, and you think about the subjectivity of the word ‘deep’, ‘deep’ means: it’s kind of out of reach. It’s a place which is quite difficult to get to. And that was once true. Plus, the 200 meter mark being associated with the continental shelf is something that was primarily driven by the early marine scientists operating around the North Atlantic.
I propose that the deep sea should really start at 1000m, because at 1000 meters, it does become quite challenging. And if we're regularly fishing and eating and drinking and extracting stuff, and animals are utilising this whole body water, the deep sea doesn't seem that deep anymore. 1000m, to me, no longer feels like it's out of reach; it's not challenging anymore. So it’s about time we redefined what the deep sea was, and changed it to depths greater than 1000m.
Literature referenced (in order of mention):
John Gage and Paul Tyler (1991), Deep Sea Biology; A Natural History of Organisms at the Deep Sea Floor
Helen M. Rozwadowski (2008), Fathoming the Ocean
Edward Forbes (1843), Azoic Theory
Harris et al. (2014), Geomorphology of the oceans
Günther and Deckert (1956), Creatures of the deep sea
Deep Ocean Water Application Society, The Deep Ocean Water Research Journal
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This was originally broadcast on The Deep-Sea Podcast: episode 44 - Alan Takes Over.