16.4: Pelagic Depth Zones

Pelagic depth zones consist of horizontally arranged layers within the water column from the surface to the seafloor. Though often reported with fixed-depth ranges, their widths may vary. As Gage and Tyler (1991) put it, “this depth zone terminology cannot be rigidly applied, and vertical zonation of fauna . . . seems determined much more by a complex of sometimes interacting ecological factors than by simple physical variables associated with the depth gradient.” Nevertheless, pelagic depth zones provide a simple and convenient means to categorize oceanic environments and the major groups of organisms dwelling within them.

The Sea Surface Microlayer Zone

Though not commonly presented as an ocean depth zone, the sea surface microlayer zone represents as distinct an environment as any (e.g., Wangersky 1976; Hardy 1982; Wurl et al. 2017). As Macintyre says, “Perhaps nowhere else do microscopic physiochemical and hydrodynamic processes exert so profound an influence over macroscopic geochemical and geophysical phenomena” (Macintyre 1974). It’s been described as “that microscopic portion of the surface ocean which is in contact with the atmosphere and which may have physical, chemical, or biological properties that are measurably different from those of adjacent subsurface waters” (Hunter 1997). By definition, the sea surface microlayer occupies the top millimeter (1,000 µm) of the water column (e.g., GESAMP 1995).

This “vital skin” (e.g., Engel et al. 2017) gains its properties from the highly enriched, gel-like matrix of biogenic macromolecules that cover its surface. Despite its modest dimensions, the sea surface microlayer exhibits two sublayers, defined by Hardy and Word (1986) and Hardy (1997) and adopted by the UN’s Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP; 1995): (1) the nanolayer, the thin organic coating at the very surface of the ocean, from 0 to 1 micrometer in depth; and (2) the microlayer, inhabited by a unique assemblage of microbes and various abiogenic and biogenic particles, found from 1 to 1,000 micrometers in depth. The nanolayer interacts primarily with Earth’s atmosphere, while the microlayer connects with the water column below.

In recent years the sea surface microlayer has received attention for its role in cloud formation (e.g., Hendrickson et al. 2021), air–sea exchange of heat, water, and gases (e.g., Ribas-Ribas et al. 2018), and ocean biogeochemistry (e.g., Wurl et al. 2017; Ebling and Landing 2017; Mustaffa et al. 2018; Enders et al. 2023). Suppression of turbulence by the gelatinous surface film reduces the rate at which energy and matter are transferred across the air–sea interface. Gladyshev (2005) calls it a “bottleneck” for the exchange of heat and matter between the ocean and the atmosphere. In addition, the unique microbial populations of the sea surface microlayer (e.g., Sieburth 1983; Zäncker et al. 2021) and its importance as a nursery for diverse marine larvae, including larval fish (e.g., Whitney et al. 2021), establish this environment as a unique and critical zone at the very surface of the ocean.

The Epipelagic Zone (Surface Zone)

The lighted waters of the world ocean—those surface waters illuminated by the Sun directly beneath the sea surface microlayer—constitute the epipelagic zone (epi = “on top of”). This domain, though only some 4.5 percent of the ocean by volume (e.g., Bucklin et al. 2010), hosts those species of large animals best known and loved by the general public—the charismatic megafauna. Here you will find squids, whale sharks, great white sharks, sea turtles, sea otters, sea lions, dolphins, blue whales, humpback whales, and even polar bears. The animals that inhabit the epipelagic zone are most familiar to us because they live near the surface or are held captive in zoos and aquariums where we can see them. Some we recognize as guests on our dinner plates—members of the epipelagic nekton constitute the most commercially important species in the ocean (Moyle and Cech 2004).

The epipelagic zone—also known as the euphotic zone—represents the region of the water column sufficient to support the growth of plants (e.g., Warming 1909; Pearsall 1917; Gilson 1937; Sverdrup et al. 1942; Chandler and Weeks 1945; Ryther 1956; Banse 2004; NOAA 2023). However, because not all wavelengths of sunlight can be used by plants, oceanographers define a quantity known as photosynthetically available radiation (PAR), sunlight between the wavelengths of 400 and 700 nanometers (nm). Based on PAR, the boundaries of the euphotic zone range from PAR at (or just below) the ocean surface (Z₀^PAR or Z_0-^PAR) to the depth where the underwater light intensity reaches about 1 percent of its surface value (Z_1%^PAR; e.g., Mignot et al. 2014; Wu et al. 2021).

While the epipelagic zone and euphotic zone are often considered synonymous, most textbooks (and websites) define the epipelagic zone as the upper 200 meters (660 ft) of the ocean. This definition doesn’t work for the euphotic zone. Flucutations in the concentrations of phytoplankton, suspended particles (living and nonliving), and dissolved organic matter affect the depth of light penetration into the ocean. As well, recent observations suggest a need to refine the definition of the euphotic zone (e.g., Banse 2004), possibly using the 0.1 percent light level (Marra et al. 2014; Buesseler et al. 2020). That’s beyond our discussion here, but these studies acknowledge a need to consider the “spatially and seasonally varying depth of light penetration” (Buesseler et al. 2020). Even Hedgpeth (1957) acknowledged the need “to indicate the depths in a flexible manner.” Thus, if we follow the science, we expect the depth range of the epipelagic zone to vary with location (e.g., coastal versus oceanic, high versus low latitude), time of year (e.g., the spring bloom), and even over longer timescales. And indeed, that’s just what we see.

If we use observations of euphotic zone depths as an estimate for the depth of the epipelagic zone, we find a fiftyfold range, from less than 5 meters (17 ft) in river-influenced coastal waters (e.g., Arnone et al. 2018) to near 250 meters (820 ft) in the clearest subtropical oceanic waters (e.g., Xing et al. 2020). Buesseler et al. (2020) reported euphotic zone depths from 30 to 60 meters (98—197 ft) in polar and coastal locations and 140 to 175 meters (459—574 ft) in subtropical gyres. Annual time-series observations of the submarine light field using Biochemical Argo floats (Chapter 4) clearly demonstrate seasonal variations in the depth of the euphotic zone (measured as isolumes), perhaps by as much as 50 meters (164 ft) in some regions of the ocean (e.g., Mignot et al. 2014).

All of this suggests that we treat the epipelagic zone as a dynamic region of the upper ocean that varies over spatial and temporal scales. An epipelagic zone model varying over time and space serves as a dynamic physical driver for changes in the abundance, distribution, and behavior of drifting and mobile organisms. It brings the epipelagic zone to life—physically, chemically, and biologically—in a way that static, fixed-depth definitions and diagrams do not convey.

Light also plays a role in the behavior and ecology of organisms, especially nekton. Unlike terrestrial animals who can seek refuge in bushes and trees, epipelagic organisms must devise their own means to avoid being seen or to scare away those who would eat them. Gelatinous animals and some larval fishes rely on transparent bodies to reduce their visibility in the water column. Fish on the menu of top predators form large schools meant to discourage predators from attacking (like raising your arms and making yourself big if you encounter a bear). In the wild, wild west of the epipelagic, animals must evolve different strategies—anatomical, physiological, or behavioral—to avoid being eaten.

Obviously, other physical, chemical, and geological factors operate in the epipelagic as well. As discussed in Chapter 13, seasonal changes in the depth of the mixed layer create physical structure in the upper ocean. Stratification of the water column divides the epipelagic zone into sublayers: the surface mixed layer, the thermocline layer, and the deeper water. Differences in temperature, salinity, and dissolved oxygen may develop. Ocean currents, upwelling, downwelling, and seafloor features that interact with currents (e.g., Cascão et al. 2019) can create mesoscale and submesoscale environments (e.g., McGillicuddy 2016; Penna and Gaube 2020). Such variations may favor some organisms and disfavor others. Swimming animals may seek out or avoid these environments. For example, some species of tuna prefer the warm (>68°F), well-oxygenated waters above the thermocline, while others appear quite comfortable in colder and deeper waters, at least for a time (e.g., Bernal et al. 2017). The epipelagic features a tapestry of complex and perhaps highly specialized environments that we are just beginning to appreciate and understand. All of these factors make the epipelagic zone the most dynamic and heterogeneous zone in the ocean.

The Mesopelagic Zone (Twilight Zone)

As we descend below depths of 200 meters (660 ft), we enter what oceanographers refer to as the deep sea (e.g., Rogers 2015). Collectively, most of the depth zones belong here.

The ocean depth zone immediately beneath the epipelagic—the mesopelagic zone (meso = “middle”)—gains its nickname, the twilight zone, from its very dim and diffuse light, the kind of light you see in the sky just before sunrise or after sunset. Nevertheless, there are strong arguments to be made that this zone resembles the television show of the same name (the original or the modern series). A number of bizarre and enigmatic creatures roam these depths, almost supernatural in their appearance and behavior. The vampire squid, Vampyroteuthis infernalis—a relict cephalopod species, neither squid nor octopus—resembles a vampire with its purplish-black color and cloak-like webbing that it rolls up over its head when disturbed. But you won’t find any blood on its lips. The vampire squid eats detritus. Need I say more?

Reported at a fixed-depth range from 200 to 1,000 meters (660—3,281 ft), the mesopelagic zone occupies an estimated 17 percent of the world ocean volume (e.g., Bucklin et al. 2010). Nevertheless, like the epipelagic zone, the boundaries of the mesopelagic zone vary with oceanographic conditions. According to Robinson et al. (2010), we can place “the top of the mesopelagic as the base of the euphotic zone, where light is too low for photosynthesis, and the bottom of the mesopelagic as the depth where downwelling irradiance is insufficient for vision to be effective in capturing prey.” Organisms, rather than physical or chemical factors, define the lower boundary of the mesopelagic zone.

During daylight hours, the mesopelagic is bathed in a diffuse glow of blue light (~490 nm). American marine biologist William Beebe (1877–1962) described this glow following his epic half-mile descent in a bathysphere in 1930:

Thousands upon thousands of human beings had reached the depth at which we were now suspended, and had passed on to lower levels. But all of these were dead. . . . We were the first living men to look out at the strange illumination. . . . It was of an indefinable translucent blue quite unlike anything I have ever seen in the upper world. . . . I think we both experienced a wholly new kind of mental reception of color impression. (1934, 109)

In the clearest waters, light may penetrate as deep as 1,100 meters (3,609 ft). In turbid coastal waters, light may not penetrate deeper than 100 meters (328 ft; Kaartvedt et al. 2019). A considerable body of evidence—much of it based on acoustic measurements—shows that mesopelagic animals move up and down in the water column in relation to varying intensities of light (e.g., Staby and Aksnes 2011; Røstad et al. 2016; Aksnes et al. 2017; Bosswell et al. 2020; Langbehn et al. 2021). They may even respond in a matter of hours to changes in weather—a passing storm—that reduce the intensity of the submarine light field (e.g., Kaartvedt et al. 2017). Remarkably, moonlight and even starlight may be sufficient to induce movements of mesopelagic organisms. During a full moon, a mesopelagic zone may be present to depths of 600 meters (1,969 ft) in the clearest waters. Starlight on a clear, moonless night may be visible to depths of 280 meters (919 ft; Kaartvedt et al. 2019). If you have ever visited Joshua Tree National Park during a full Moon—or any similar location far from city lights—you’ve likely experienced the “brightness” of moonlight, sufficient to scramble safely across rocks or follow a path. It’s an otherworldly experience.

Studies of the movements of nekton and other animals in response to light further suggest that organisms prefer a range of light intensities, a light comfort zone (e.g., Røstad et al. 2016), as first proposed for vertically migrating jellies (Dupont et al. 2009). This flexibility in preferred light intensities provides a balance between finding food and avoiding predation (e.g., Aksnes et al. 2017), what Langbehn et al. (2019) call “a game of hide and seek.”

The mesopelagic also contains oxygen minimum zones (OMZs), regions of reduced or absent dissolved oxygen caused by microbial respiration of organic matter (e.g., Robinson 2019). As phytoplankton and fecal material sink from the euphotic zone, microbes busily break it down, consuming oxygen and liberating carbon dioxide as they do so (Chapter 10). OMZs result from the decomposition of this rich supply of organic matter (e.g., Wyrtki 1962) and other factors (e.g., Oschlies et al. 2018). Below the OMZs, the diminshed supply of organic matter reduces rates of respiration and permits higher dissolved oxygen concentrations. Colder and well-oxygenated deep ocean currents also contribute to higher dissolved oxygen concentrations below the OMZ. Organisms that can tolerate hypoxic and anoxic conditions can find shelter within an OMZ, especially if their predators are less tolerant of reduced oxygen conditions. Thus, the OMZ may act as a barrier to some species and segregate their distributions to the upper or lower mesopelagic (e.g., Maas et al. 2014; Wishner et al. 2018; Deutsch et al. 2020).

Other oceanographic processes may also modify organisms’ distributions. Subsurface currents can affect the distribution of subsurface water masses. Off the California coast, strengthening of the California Undercurrent (Chapter 17) has brought warmer, saltier, and less-oxygenated water to the subsurface (and surface) in recent decades (e.g., Meinvielle and Johnson 2013; Bograd et al. 2015). This redistribution of water masses—possibly due to the Pacific Decadal Oscillation—has brought shifts in mesopelagic communities (e.g., Brodeur et al. 2003; Koslow et al. 2019). Climate change has also been suggested for shifting species’ distributions (e.g., Xiu et al. 2018; Bograd et al. 2023).

Subsurface eddies can pinch off portions of mesoscale water masses and take their organisms for a ride. These eddies may carry their nekton passengers thousands of miles (e.g., Garfield et al. 2001; Zhang et al. 2017; Penna and Gaube 2020; Wang et al. 2023). On their journey, these mobile “oases” may attract other species and create a kind of subsurface eddy community (e.g., Godø et al. 2012; Arostegui et al. 2022).

In low- and midlatitude waters, the mesopelagic zone hosts the permanent thermocline at depths from 200 to 1,000 meters (660–3,281 ft). The permanent thermocline represents Central Waters and exhibits a wide range of temperatures (5°–24°C; 41°–75°F) and salinities (34.3–36.4), which may further partition where predators and their prey hang out (e.g., Emery and Meincke 1986; Talley et al. 2011).

We should also be aware of pressure. As you know, each 10 meters (33 ft) of depth adds another atmosphere of pressure to the water column (Chapter 3). For organisms in the mesopelagic, water pressure may be 20 to 100 times that experienced at the surface. Despite this seeming obstacle, organisms inhabit nearly all depths in the ocean and exhibit a wide range of adaptations that enable them to do so (e.g., Menzies 1974; MacDonald 1997; Yancey 2020). The hadal snailfish—a fish related to tide pool sculpins—has been observed at 8,178 meters (more than five miles) deep in the Mariana Trench, the deepest fish found to date in the world ocean (Gerringer et al. 2021). Great white sharks in the North Atlantic make daily excursions from the surface to depths of more than 1,100 meters (3,608 ft; e.g., Skomal 2017; Gaube et al. 2018). Satellite-tagged Cuvier’s beaked whales have been logged at depths close to 3,000 meters (9,843 ft), representing a three-hundred-fold change in pressure (Schorr et al. 2014).

Finally, the mesopelagic zone represents a kind of new frontier for fisheries. As global demand for seafood and nutraceuticals rises, fishers are targeting deeper fish populations as an abundant and profitable source. Estimates of mesopelagic fish populations rival or exceed current catches (e.g., Proud et al. 2019), but so little is known about this region of the ocean and potential effects on epipelagic fisheries that scientists and resource managers urge caution (e.g., Hidalgo and Browman 2019). We have much to learn before we can responsibly and sustainably harvest this region of the ocean (e.g., St. John et al. 2016; Fjeld et al. 2023).

The Bathypelagic Zone (Midnight Zone)

Below 1,000 meters (3,281 ft), the bathypelagic zone (bathy = “deep”), also known as the midnight zone, represents the largest depth zone in the world ocean, comprising an estimated 59 percent of its volume (e.g., Bucklin et al. 2010). Despite this, we probably know less about the bathypelagic zone than any other region of the ocean. That’s because technology for exploring the water column at great depths has been limited. Even the abyssal seafloor is easier to sample. But interest in the deep sea for its role in climate change (specifically its ability or lack of ability to store carbon), its importance in ocean biodiversity (especially microbial diversity), and its susceptibility to seafloor mining (and how plumes of sediments may impact its species) have brought renewed attention to these waters. The need to study, understand, and even manage all organisms within the deep ocean has probably never been greater (e.g., Robison 2009; Roemmich et al. 2019; Danovaro et al. 2020; Amon et al. 2022; Bravo et al. 2023).

Despite this urgency, many people, including some scientists, consider the bathypelagic to be a dark, homogeneous, relatively uninteresting place. Other than its oft-publicized, wicked-looking predatory fishes (e.g., Simon 2020), it excites little public attention. In 1964 Canadian-born fisheries biologist Clarence P. Idyll (1916–2007)—author of Abyss: The Deep Sea and the Creatures That Live in It—described the bathypelagic in his classic book as follows:

The deep sea is pitch black, without the least glimmer of the sun’s rays to give it cheer; it is cold, only a little above freezing; it is under enormous pressure, with power to crush to a shapeless mass any body not constructed to combat it; it is salty and laden with nutrient minerals, but these are useless since the energy of the light is missing; it is virtually still, with only the most languid currents moving. Yet this unlikely living space is inhabited by a huge variety of fishes and squids and other curious animals that are lucky enough not to realize that their home seems so unlivable. (1964, 49)

Life always finds a way to take what seems like an uninhabitable place and make it home. Archaea and other microbes appear to thrive in the bathypelagic. And so do dragonfish, anglerfish, and giant squid. It’s a bit like The Addams Family (e.g., Koslow 2007), but it is a vibrant, thriving, and important community of organisms, nonetheless.

Scientists report the boundaries as fixed depths, albeit variable ones, from 1,000 to 3,000 (3,281–9,843 ft), 4,000 (13,123 ft), or 5,000 meters (16,404 ft), depending on the reference (e.g., Priede 2017; Bucklin et al. 2010; and Nagata et al. 2010, respectively). The upper boundary of the bathypelagic, of course, varies with the depth of light penetration from 100 to 1,100 meters (328–3,609 ft), as we saw above. Bruun (1956) sets the lower boundary at the depth of the 4°C isotherm (39.2°F), but this is probably too shallow given that in the Pacific, Indian, and parts of the Atlantic Ocean, this isotherm generally can be found at 1,000 meters (3,281 ft)—the lower limit of the mesopelagic zone (Talley et al. 2011).

Nagata et al. (2010), reviewing microbial processes in the bathypelagic zone, extend the lower boundary of the bathypelagic zone to 5,000 meters (16,404 ft). And Priede (2017) dispenses with any lower boundary at all, proposing that we use the term bathypelagic for all depths below 1,000 meters (3,281 ft) except the hadopelagic, the water column contained within the oceanic trenches (deeper than 6,000 meters; 19,685 ft). However, as we improve our technology for making measurements of temperature, salinity, dissolved oxygen, and other properties at depths from 2,000 meters (6,562 ft) to the seafloor, we’ll get a clearer picture of this environment and likely find ecologically relevant boundaries (e.g., Jamieson et al. 2010; Kawaguchi et al. 2018; Liu et al. 2020). For now, we shall have to be content that any well-defined lower boundary has yet to be established.

Perhaps the one defining feature of the bathypelagic is that sunlight cannot penetrate these depths. To that extent, characterization of this zone as “the dark ocean” (e.g., Arístegui et al. 2009) or the “midnight zone” (e.g., Hardt and Safina 2010) seems appropriate except for one small fact: organisms produce light here, living light, otherwise known as bioluminescence. Found at all depths and visible throughout the water column at night, bioluminescence becomes more visible (and useful) as the submarine light field diminishes and disappears.

In the bathypelagic zone, bioluminescence prevails as the sole light source. Discrete points of light are emitted from the specialized light organs of animals, their photophores. Used to attract prey, ward off predators, blend in, or communicate with potential reproductive partners, bioluminescence has become something of an art form in the otherwise “pitch black” waters of the bathypelagic (e.g., Haddock et al. 2010; Martini and Haddock 2017). Though natural levels of bioluminescence may be quite low at these depths—as few as one or two flashes per hour (e.g., Buskey and Swift 1990)—in the presence of hydrodynamic disturbances, such as a predator, flashes may increase substantially (e.g., Vacquié-Garcia et al. 2012). Given the extraordinary visual systems of deep sea fishes, bioluminescence likely plays an important role in their survival and reproduction (de Busserolles et al. 2020). So while the presence of light in the zones above creates an environmental gradient that drives the ecology of organisms, the absence of light—along with bioluminescence—does the same in the bathypelagic (e.g., Cohen et al. 2020; Davis et al. 2020).

Some of the organisms that live in the bathypelagic zone seek prey in the zone above—the mesopelagic. But many simply wait for unsuspecting prey to cross their path. Given its distance from the epipelagic zone, the bathypelagic zone relies entirely on food produced elsewhere. For that reason, it is generally characterized as “food-poor,” or sparse in terms of food supply. However, this view may be changing. While oceanographers once thought that most particulate organic matter is recycled in the mesopelagic, recognition of the importance of slowly sinking particles and fast-sinking aggregates (e.g., phytodetritus and the carcasses of gelatinous organisms) has led to reassessment of the role of the “dark ocean” in ocean food webs and biogeochemical cycles. New appreciation for physical and biological mechanisms that deliver significant quantities of food-rich particles—what are called “particle injection pumps”—promise to “reshape” our view of deep-sea food webs and the role of the deep ocean in carbon and other biogeochemical cycles (e.g., Arístegui et al. 2009; Danovaro et al. 2014; Boyd et al. 2019; Buesseler et al. 2020; Baltar et al. 2021; Herndl et al. 2023; Lappan et al. 2023).

Indeed, if we follow the microbes, as it were, we discover an environment that is more heterogeneous than previously appreciated (e.g., Hewson et al. 2006; Nagata et al. 2010). Microbial “hotspots”—patches of elevated microbial activity in response to an uneven distribution of particulate or dissolved organic matter—may be more common than once thought (e.g., Azam et al. 1994; Bochdansky et al. 2017; Rahav et al. 2019). Patches of actively growing microbes offer a local food source for gelatinous, filter-feeding predators. These predators, in turn, appear on the menu of smaller predators that feed larger predators, and so on up the food web (e.g., Chi et al. 2020). Thus, environmental heterogeneity in the bathypelagic, especially as it affects the abundance and distribution of secondary producers (i.e., microbes), can have a significant effect on the transfer of energy and matter into pelagic food webs (e.g., Boyd et al. 2019; Iversen 2023).

Of all the zones in the ocean, the bathypelagic is perhaps the most isolated, at least ecologically. The supply of matter and energy here relies on internal oceanic sources, unlike upper ocean depth zones, which regularly interact with the atmosphere. In any case, it’s certainly not the “unlivable” zone envisioned by Idyll because life is clever and remarkably tenacious.

The Abyssopelagic Zone (Mystery Zone)

The abyssopelagic zone (abysso = “bottomless”) encompasses some 19 percent of the ocean volume at depths from 4,000 to 7,000 meters (13,123–22,966 ft; e.g., Bucklin et al. 2010). More commonly, the depth range is set from 3,000 to 6,000 meters (9,843 ft–19,685 ft; e.g., Herring 2002; Priede 2017). Not everyone recognizes the abyssopelagic as a distinct zone. Herring (2002) includes it with the bathypelagic (starting at 1,000 m, or 3,281 ft). Everything deeper he refers to as hadopelagic (see below). Sutton (2013) similarly includes the abyssopelagic with the bathypelagic and sets the lower boundary at 100 meters (328 ft) above the seafloor. A few studies cite peaks in fish abundance at depths greater than 2,500 meters (8,202 ft) as evidence of a distinct environment (e.g., Sutton 2013; Sutton and Milligan 2019). And studies of microbes hint at discrete communities at abyssopelagic depths (e.g., Walsh et al. 2016). In truth, a lack of studies at these depths makes it challenging to affirm or reject the existence of an ecologically distinct abyssopelagic zone (e.g., Costello and Breyer 2017). Including this zone as part of the bathypelagic seems the most parsimonious interpretation of existing data. Still, like Pluto’s status as a planet (e.g., Carter 2021), the abyssopelagic could be reinstated if convincing arguments can be made.

The Hadopelagic Zone (Trench Zone)

Should you ever be told to spend some time in the underworld (“Go to the devil!”), you may take less offense if you consider that the ocean region named after the Greek god of the underworld, Hades, can be quite peaceful. Removed from the hustle and bustle of life on the abyssal plains and out of the main freeway of the strongest abyssal currents, the hadopelagic zones—the regions of the water column bounded by oceanic trenches—represent the deepest and most remote parts of the ocean. Defined as depths greater than 6,000 meters (19,685 ft; e.g., Jamieson 2015—the authority we’ll accept here), greater than 6,500 meters (21,325 ft; e.g., UNESCO 2009), or greater than 7,000 meters (22,966 ft; e.g., Bucklin et al. 2010), the hadal zone represents a tiny fraction of the volume of the world ocean. Nevertheless, the hadal depth range, from its shallowest at 6,000 meters (19,685 ft) to its deepest at 10,925 meters (35,843 ft, the depth of the Challenger Deep), includes 45 percent of the total depth range of the ocean.

The hadopelagic represents several independent zones, mostly unconnected to each other. Using 6,000 meters (19,685 ft) as a cutoff depth, Jamieson (2015) identified 46 distinct hadal environments. These occur mostly in trenches, but waters deeper than 6,000 meters (19,685 ft) can also be found in deep basins, fracture zones, and transform faults (Jamieson and Stewart 2021). The deepest, of course, is the Mariana Trench, but trenches vary considerably in their depth, volume, morphology, and degree of isolation (e.g., Stewart and Jamieson 2018). Even the Mariana Trench can be subdivided into five distinct regions. At its northern end, a “bridge” of seafloor shallower than 6,000 meters (19,685 ft) divides it from the Volcano Trench, whose depths exceed 8,500 meters (27,887 ft; Jamieson and Stewart 2021). This complex seafloor morphology has led oceanographers to view hadal zones as isolated “islands” where species evolve independently with little exchange between adjacent regions and trenches (e.g., Stewart and Jamieson 2018). The evolution of different species of finches on the various Galapagos Islands—observed by Charles Darwin and acknowledged in modern times as evidence for speciation by evolution—serves as an example of what may occur in trenches.

Seawater temperatures in the hadopelagic resemble those of abyssal waters in the bathypelagic, with a range of 1° to 4°C (33.8°–39.2°F). But extreme pressure presents the biggest challenge to life here. Using the pressure equation introduced in Chapter 3, we calculate a pressure 600 to 1,100 times greater than sea level. Snailfish are the deepest species (e.g., Gerringer et al. 2021), but no fish have ever been observed at depths deeper than 8,400 meters (27,559 ft; e.g., Yancey et al. 2014). Decapod crustaceans (e.g., shrimp) can only make it to 7,700 meters (25,262 ft). Benthic lander video of an octopus swimming at 6,957 meters (22,825 ft) is the deepest observation of a cephalopod (Jamieson and Vecchione 2020). On the other hand, deep sea amphipods—relatives of the beach hoppers you often see around kelp and other beached seaweeds—have been collected with a hadal lander at the very bottom of the Challenger Deep in the Mariana Trench at 10,929 meters (within the margin of error of the deepest official depth; e.g., Lan et al. 2016). Foraminifera—mostly the unshelled variety—also thrive at these depths (e.g., Zeppilli et al. 2018).

The Benthopelagic Zone (Near-Bottom Zone)

The benthopelagic zone represents the portion of the water column within 100 meters (328 ft) of the seafloor. Thus, its lower boundary varies with the bathymetry of the seafloor from 0 to 10,925 meters (35,843 ft). Though typically considered a part of the benthic environment, the benthopelagic, as its name implies, proves an important part of the water column as well. It represents a kind of transition zone for exchanges between the water column and the seafloor (e.g., Marshall and Merrett 1977 ; Wishner 1980; Sutton and Milligan 2019). Diverse and abundant organisms—planktic and nektic—can be found in the benthopelagic, supported by pulses of phytodetritus that arrive on the seafloor or larger bounties, like the carcasses of fishes or whales (e.g., Mauchline and Gordon 1991).

Organisms at these depths may travel higher in the water column and prey on pelagic species (e.g., Mauchline and Gordon 1991) or be preyed upon by pelagic species hunting in the deep (e.g., Drazen et al. 2008). Importantly, the connections between benthic and pelagic food webs have implications for the fate of carbon in the deep sea. Organic matter consumed by benthic species may do an about-face when consumed by a pelagic predator that returns to the upper water column.

Though the upper boundary of this zone is defined as 100 meters (328 ft) above the bottom, this boundary likely fluctuates depending on geological and physical conditions. Seafloor features, such as seamounts and oceanic ridges, interact with abyssal currents and generate turbulence in the overlying water column. This effect is most pronounced where seafloor bathymetry varies abruptly—characterized as “rough topography”—versus the relatively smooth topography of abyssal plains (e.g., Orellana-Rovirosa and Richards 2017). With sufficient current speeds, the bottom-enhanced turbulence may reach all the way to the surface (e.g., Polzin et al. 1997). On average, this effect is most pronounced within 200 to 300 meters (656–984 ft) of the bottom (e.g., Waterhouse et al. 2014). Recent theoretical work and general circulation models confirm that bottom-enhanced mixing may be sufficient to drive abyssal circulation (e.g., Callies et al. 2018; Drake et al. 2020). Thus, we find a possible pathway for particles and properties to return from the seafloor to the surface where they originated. In any case, a better understanding of the benthopelagic and all deep-sea zones will be needed to assess their connectivity and their role in food webs, biogeochemical cycles, and ocean conservation (e.g., Sutton 2013).