Chapter 22: Ocean Depth Zones

We live in remarkable times. Humans inhabit space aboard an orbiting space station. Robots dot the surface of Mars. Space probes travel to the far reaches of our solar system and beyond. Just about every part of the surface of our planet can be accessed in person or virtually in real time. Yet despite these feats of technology and human endurance, one region remains tantalizingly out of reach, at least in any sense of “Yeah, we go there.”

The deep sea—the expanse of ocean deeper than 200 meters (660 ft)—continues to challenge our efforts to explore and understand it. Sure, more than two dozen people now have been ferried aboard a submersible to the deepest location in the world ocean. And robots and landers plumb these depths with increasing frequency. But our knowledge of these regions is woefully incomplete (e.g., Cui and Wu 2018). Nearly every new expedition uncovers something never before seen. 

Of course, discovering the unknown is part of the excitement of deep sea research. Where organisms spend all or part of their lives proves critical for their conservation and management. As the documentary The Living Sea (MacGillivray 1995) puts it, “We can’t protect what we don’t understand.” To ensure our sustainable use of ocean resources, we need to understand how organisms use different ocean environments and how our activities may affect them.

Our journey will take us from the surface to the seafloor and back again. We distinguish here between those organisms that inhabit the water column—known as the pelagos or pelagic organisms—and those that live on or near the seafloor—the benthos or benthic organisms. This distinction has led to two separate classification systems—one for the pelagic organisms and another for benthic organisms. For the most part, both classifications are based on depth. Take a deep breath. It’s a long trip to the bottom.

22.1 Defining an Organism’s Habitat

Like teenagers, mobile organisms­—the nekton—tend to congregate in locations best suited to their lifestyle and temperament. They have their preferred place to chill, their favorite fast food spot, their ideal spot to spend time with their special someone. Indeed, the whole advantage of being mobile is the ability to go where you want. Isn’t this why teens can’t wait to learn how to drive? The places where an organism spends time to feed, rest, hide, find mates, and reproduce define its habitat (e.g., Hall et al. 1997; Fraschetti et al. 2008; Bamford and Calver 2014; Boero et al. 2019). 

There’s a tendency in general textbooks to equate habitat with “the environment where an animal lives” (Castro and Huber 2019). But this definition contributes to an impression that habitat and environment are synonymous (e.g., Fraschetti et al. 2008; Costello 2009; Bamford and Calver 2014). In fact, an organism’s various activities may occur in different environments, the geological, chemical, physical, and biological conditions at a given time and place. In many cases, organisms inhabit a range of environments during their lives, and their habitat includes all these multiple environments. Thus, I define habitat as the range of environments that support the survival and reproduction of a species on a permanent or temporary basis (e.g., Lack 1933; Morris 2005; Bamford and Calver 2014). This definition especially applies to marine organisms.

Consider benthic species that produce planktic larvae. The larvae live in the water column for an extended period of time. A deep-sea benthic larva that travels to the surface will cross a range of environments twice, once on the way up and once on the way down. Salmon offer another example. They’re born in freshwater, migrate to saltwater to feed until adulthood, and then return to freshwater to spawn. Their habitat includes the rivers, estuaries, and oceanic regions where they spend at least part of their life cycle.

We also have to be careful to avoid equating habitats with specific geographic locations. Free-swimming organisms follow a specific set of geological, chemical, physical, and biological conditions wherever they occur. Changes in ocean climate—natural and human-caused—lead to changes in ocean conditions across multiple timescales. As a result of such changes, organisms may shift their location. For example, during periods of El Niño, when warmer-than-average water temperatures occur along the California coast, we often find tropical species—red crabs, mahi mahi, swordfish, and marlin—inhabiting our temperate shores (e.g., Lluch-Belda et al. 2005; Cimino et al. 2021; Broughton et al. 2022). Ocean heat waves can cause similar displacements of organisms and disruptions of regional food webs (e.g., Cavole et al. 2016 ; Arimitsu et al. 2021).

This distinction between habitat and environment proves critically important for the protection of species whose life cycles include multiple environments. Efforts to restore commercially important fish populations, for example, must recognize the importance of the different habitats that affect their birth, growth, survival, and successful reproduction (e.g., Lowerre-Barbieri et al. 2019). Such efforts have necessitated conversations between people not typically accustomed to talking to each other: commercial and recreational fishermen, loggers, tribal councils, water and hydroelectric managers, and city, state, and federal government officials (e.g., Heikkila and Gerlak 2005; Cote et al. 2021). A complete understanding of an organism’s habitat necessarily takes into account the range of environments required by that organism to survive and reproduce.

22.2 Marine Biogeography

The study of the relationship between the distribution of organisms and their environment belongs to a field of science known as biogeography. Marine biogeography—concerned with the distribution of marine organisms in the water column and on and below the seafloor—represents one branch of this field. Biogeography serves as a complementary field to ecology, the study of interactions between a co-occurring community of organisms and their environment. If there’s any distinction between the two, it’s that biogeographers tend to examine broader spatial and temporal scales (regional to global over geologic timescales) and make greater use of tools that generate large data sets. Ecologists focus on smaller spatial and temporal scales (local to regional, days to decades) and rely more on experimental methods (Jenkins and Ricklefs 2011). But in recent decades the distinctions between these two fields have blurred. Biogeography and ecology share a common interest in understanding the geological, chemical, physical, and biological factors that govern the distribution, ecology, and evolution of organisms (e.g., Jenkins and Ricklefs 2011; Heads 2015; Bianchi 2021).

22.3 Oceanic Environments

The broadest classification of the ocean follows Haeckel (1893) and draws a boundary between waters overlying the continental shelves—the coastal waters, or neritic waters—and those above the ocean basins—the oceanic waters. In 1957 Scripps marine biologist Joel Hedgpeth (1911–2006) proposed a system for pelagic and benthic realms (Hedgpeth 1957). He sought to establish a consistent terminology to describe “the environment and its life.” Notably he designated the upper lighted waters of the ocean as the photic zone and the dark waters as the aphotic zone. These zones were further subdivided into several other zones. With few modifications, Hedgpeth’s system remains in use today. These environments—both in the water column and along the seafloor—have come to be known as ocean depth zones (also ocean life zones). Though idealized, they offer a remarkable framework for understanding the evolution and adaptations of marine organisms. They’re a classic example of  the way in which geological, chemical, and physical processes shape ocean life.

22.4 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.

22.4.1 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. 

22.4.2 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.

22.4.3 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).

22.4.4 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.

22.4.5 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.

22.4.6 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). 

22.4.7 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).

22.5 Benthic Depth Zones

We finally arrive at the seafloor environment and the benthic depth zones. Should you be wondering if you will ever make it back to the surface, rest assured that our ascent to the surface will be much quicker than our descent into the abyss. Many of the properties of the benthic zones—pressure, temperature, and salinity—are quite similar to those of the pelagic ones. But some properties do vary, so we’ll need to stop at these depths and take a look around. A big theme in the benthic depth zones is the heterogeneity of the seafloor on spatial and temporal scales. Ripples, scarps, and dunes—features you might see on a sandy beach—appear on the seafloor as well. Disturbances such as turbidity currents or seasonal pulses of food generate temporal variability. This heterogeneity contributes to greater diversity in the benthos than might otherwise be expected based on physical properties alone (e.g., Sanders 1968). The seafloor also boasts rich mineral deposits (Chapter 7). A major driver of modern deep-sea exploration—especially on the seafloor—concerns a desire to assess the economic potential of deep-sea mining. Before such mining, however, it’s essential to document and understand the biodiversity of the deep sea and assess its potential for pharmaceutical and other human uses (in addition, of course, to its ecological importance for sustaining the ocean). We begin where we left off—in the oceanic trenches.

22.5.1 The Hadal Benthic Zone (Hadobenthic Zone)

The hadal benthic zone (or hadobenthic zone) comprises the seafloor deeper than 6,000 to 6,500 meters (19,685–21,325 ft; UNESCO 2009; Jamieson 2015). Though mostly contained within oceanic trenches, hadal environments can be found in deep basins, fracture zones, and transform faults, too. The largest in terms of area—the Izu-Bonin trench that extends north from the Mariana Trench to coastal Japan—covers some 100,000 square kilometers (38,610 mi²), an area roughly the size of Iceland (Jamieson 2015). As in the hadopelagic, seawater temperatures are cold except where hydrothermal vents are present. And, of course, extreme water pressure exists everywhere.

An interesting feature of trench hadal environments is their asymmetry. Oceanic trenches form where two plates collide. The denser underthrust plate slides beneath the less dense overthrust plate, and a trench forms. But because the motion is not smooth—the plates tend to lock up—the overthrust plate may arch upward, elevating the seafloor. Thus, the overthrust side of a trench tends to be steeper, while the underthrust side slopes more gradually. This asymmetry produces a rockier terrain and more frequent sediment slides on the overthrust side. The underthrust side more resembles an abyssal plain with thick sediments and soft features. These differences may give rise to quite different communities of organisms within the same trench (e.g., Jamieson et al. 2010).

Food supply, predictably, might be considered low given the distance of hadal environments from productive surface waters. But hadal zones are not the biological deserts they were once thought to be. Cold seeps have been found in hadal environments supporting a diverse community of chemosynthetic organisms and their associates (e.g., Rathburn et al. 2009; Suess 2014; 2020; Nanajkar et al. 2022). And the V shape of trenches acts as a funnel for sinking particles and debris, concentrating organic materials along the trench axis, the bottom of the V. These sites of enhanced deposition—called depocenters (short for depositional centers)—rival coastal areas in their concentrations of organic matter and microbial activity (e.g., Danovaro et al. 2003; Glud et al. 2021). Earthquakes may suspend continental shelf and slope sediments, which can then be carried by currents into nearby trenches (e.g., Itou et al. 2000; Oguri et al. 2013; Oguri et al. 2022). 

Unfortunately, any suspended material that sinks may accumulate in hadal zones. High concentrations of microplastics—weighted down by their microbial passengers—have been found in deep ocean and trench sediments (e.g., Woodall et al. 2014; Jamieson et al. 2019). Whether trenches represent “the ocean’s ultimate trashcan” (e.g., Peng et al. 2020) remains to be seen. As Jamieson et al. (2010) admit, “Trenches are poorly sampled and our knowledge of the ecological structure and functioning of this environment remains rudimentary.” We have much to learn.

22.5.2 The Abyssal Benthic Zone (Abyssobenthic Zone)

The abyssal benthic zone (or abyssobenthic zone) occurs at depths from 3,000 to 6,500 meters (9,843–21,325 ft). It covers nearly 74 percent of the entire seafloor—267 million square kilometers (103 million square miles; e.g., Menard and Smith 1966), the greatest percentage of any benthic zone. That’s slightly more than half of Earth’s surface! Abyssal hills and abyssal plains dominate this region. At least some portion of the base of many oceanic ridges also occurs at these depths (e.g., Harris 2014). Thus, if you had to generalize about the abyssal zone—and Earth’s surface, for that matter—you would not be wrong to say that most of it looks like Chino Hills, without the ranch homes and excellent Mexican restaurants. (Mi Ranchito is my favorite.) In fact, food here, as you might expect, arrives as a slow rain of organic matter interrupted by pulses of phytodetritus and carrion falls. It’s a bit colder than Chino Hills too. Abyssal zone temperatures average about 39°F (4°C), whereas average temperatures in Chino Hills vary from 68° to 77°F (20°–25°C) seasonally (World Weather and Climate Information 2023).

Until recently, the abyssal zone was characterized as flat and featureless expanses of sediments—mostly sands, silts, and clays (e.g., Smith et al. 2008). But as we continue to explore these depths using sound and deep-sea robots equipped with cameras, a different picture of the abyss has emerged. Movements of the seafloor along transform faults and their associated fracture zones expose chunks of oceanic crust in what has been called “abyssal rock patches” (Riehl et al. 2020). Imagery taken using camera-equipped epibenthic sleds and high-resolution bathymetry obtained using multibeam sonar reveal a patchwork of exposed “hard rock” intermixed with various forms of sediments, manganese nodules, and polymetallic crusts. The geologic heterogeneity of the abyssal seafloor offers a broad range of microenvironments for organisms, solid substrate for attachment, and caves and crevices for hiding (Riehl et al. 2020). These rock exposures interact with abyssal currents like rocks in a river, creating eddies of faster water that suspend and transport sediments or slower water that permits sediments to settle and accumulate (Chapter 6). Accumulations of sediments increase the likelihood of turbidity flows and sediment slumps—especially during earthquakes—adding environmental disturbance to the list of possible traits to which organisms may adapt. 

Greater environmental heterogeneity promotes greater organismal diversity (e.g., Barry and Dayton 1991; Snelgrove and Smith 2002; Sigwart et al. 2023). The paradigm of the abyssal seafloor as a relatively homogenous and sparsely populated environment no longer holds true (e.g., Snelgrove 1999; Ramirez-Llorda et al. 2010; Ramirez-Llorda 2020). As Leray and Machida (2020) express it, “The deep seafloor is teeming with life, most of which remains poorly known to science.” A recent study on the seafloor of the Clarion Clipperton Zone between Mexico and Hawaii underscores their remark. Of the 5,578 species of animals found there, more than 90 percent may be new to science (Rabone et al. 2023).

These organisms hold potential for natural products and marine drugs. They also provide clues about the origins, evolution, and ecology of life in one of the most extreme environments on Earth. And we’re still learning about the role of the deep sea in biogeochemical cycles, carbon storage, and marine food webs (e.g., Glover and Smith 2003; Stratmann et al. 2021). New tools for observing the deep sea are starting to provide new insights into this vast wilderness. As we look to the abyssal seafloor for extraction of mineral and other resources (e.g., Hein et al. 2020; Hyman et al. 2022), oceanographers urge caution. “Humans are in danger of modifying one of the largest, most intriguing, ecosystems long before its natural state is fully understood” (e.g., Glover and Smith 2003).

22.5.3 The Bathyal Benthic Zone (Bathybenthic Zone)

The bathyal benthic zone (or bathybenthic zone) has been described as “where shallow meets the deep” (Levin and Dayton 2009). Indeed, extending from the shelf break, at about 200 meters (656 ft), to the bottom of the continental rise, at 3,500 meters (11,483 ft), the bathyal benthic represents a zone of transition from shallow- to deep-water environments. Though mostly associated with continental margins (Chapter 7), a significant percentage of bathyal depths can be found along the flanks and tops of seamounts and oceanic ridges (UNESCO 2009). Combined, bathyal depths cover 17.8 percent of the seafloor (Zezina 1997). The zone is often divided into an upper bathyal (200–800 meters; 656–2,625 ft) and a lower bathyal (800–3,500 meters; 2,625–11,483 ft), but given the enormous diversity of environments it encompasses, such depth-based boundaries may be more hopeful than useful.

The bathyal zone exhibits wide variability in pressure (20–300 atmospheres), temperature (<0°–10°C), and dissolved oxygen (<1–7 milliliters per liter ; e.g., UNESCO 2009). These factors vary with depth and location. Bathyal zones beneath swift-moving western boundary currents experience greater turbulent energy than their eastern boundary counterparts. Polar bathyal environments differ from tropical ones.

Like we found with the abyssal benthic zone, scientific characterization of the bathyal benthic zone has changed upon further scrutiny. Robotic and acoustic technologies have revealed an environment far more interesting than the “monotonous mud slopes” it was once thought to be (Levin and Sibuet 2012). The bathyal zone offers a rich collection of flat-topped guyots, rugged seamounts, steep submarine canyons, plunging vertical walls, massive rocky outcrops, cold seeps, and soft sediments shaped into ripples, mounds, or depressions (e.g., Gage and Tyler 1992; de la Torriente et al. 2018; Sutton and Milligan 2019; Leitner et al. 2021). These diverse features offer environments for habitation and ecological specialization (e.g., Sanders 1968; Menot et al. 2010; Levin and Sibuet 2012; Paulus 2021). 

Biological structures also contribute to the environmental heterogeneity: deep-sea glass sponges (so named for their siliceous skeletons), deep-sea corals, tube-building worms, and various kinds of bivalves—deep water oysters, clams, and mussels. The structures of these “ecosystem engineers” modify the physical and geological environment and contribute to establishment of a mosaic of microenvironments (e.g., Jones et al. 1994; Levin et al. 2001; Wright and Jones 2006; Cordes et al. 2009; Levin and Sibuet 2012; McClain et al. 2020). Whereas the unexpected high biodiversity of bathyal environments was once difficult to explain, it’s now becoming clear that environmental heterogeneity (e.g., substrate, sediment size, biological engineeering), food supply (e.g., food pulses, chemosynthetic food factories, nutrient depocenters, food falls), and environmental disturbances (e.g., turbidity flows, water mass shifts, oxygen minimum zones) help to structure these diverse ecosystems (e.g., Rex and Etter 2010; McClain et al 2020; Furness et al. 2021; Bryant and McClain 2022). The degree to which each or all of these factors maintain deep-sea biodiversity remains to be seen.

As humans increasingly look to the deep sea for energy, minerals, food, biotechnology, and other goods, the need to understand deep-sea ecosystems is greater than ever. As Levin and Sibuet (2012) put it:

This high regional biodiversity is fundamental to the production of valuable fisheries, energy, and mineral resources, and performs critical ecological services (nutrient cycling, carbon sequestration, nursery and habitat support). . . . Serious actions are required to preserve the functions and services provided by the deep-sea settings we are just now getting to know. 

22.5.4 The Sublittoral Zone (Coastal Benthic Zone)

Covering nearly 9 percent of the seafloor (e.g., Harris et al. 2014), the sublittoral zone, the region below the littoral zone (from the Latin litoralis, or “shore”), extends from the zero tide height (i.e., 0 meters) to the shelf break—about 200 meters (656 ft), in theory. Of all the terms in the oceanographic literature, “sublittoral” and “littoral” are my least favorites. In my opinion, the term “littoral zone” lacks context and meaning for most American readers. In addition, it sounds too much like “literal” and is frequently spelled as such. Where we encounter “littoral zone” we could (and often do) substitute “intertidal zone.” But, as noted by Hedgpeth (1957), many regions of the world lack discernible tides, making the term less satisfactory on a global basis. We would substitute “subtidal” for “sublittoral” if this term didn’t already restrict its meaning to nearshore waters immediately below the zero tide height.

Efforts to improve the terminology (e.g., Dauvin et al. 2008) urge adoption of the region below the littoral zone as the infralittoral zone, exposed only by the lowest tides and often occupied by seaweeds, and the circalittoral zone, whose lower boundary extends to the 1 percent light level (i.e., the euphotic zone depth). Of course, we already know the 1 percent light level depends greatly on water clarity, which can be highly variable. Such a definition would place the lower boundary of the circalittoral at quite shallow depths. The term elittoral zone has been proposed, in this case, to account for depths deeper than the circalittoral but shallower than the bathyal. Other alternatives (e.g., Pérès 1982) include phytal (suitable for photosynthesis) and aphytal (not suitable for photosynthesis), but these have been vigorously contested (e.g., Golikov 1985). If you’re expecting a satisfactory outcome here, you won’t find one. Different terms remain in use in different parts of the world, often because the environments are quite different. While it may be desirable to establish a common terminology, the sublittoral zone—for historical and practical reasons—may defy such a simple classification (e.g., Costello 2009). The etymology aside, we’ll stick with the term sublittoral and let 0–200 meters (656 ft) define its boundaries.

That said, we immediately encounter exceptions. Continental shelves, to which this zone mostly applies, vary widely in width (from less than a mile to hundreds of miles); their geology may be quite different (from rocky to muddy); they may be incised by deep submarine canyons (e.g., Monterey Bay Submarine Canyon); and they may be subject to extensive modification by terrestrial processes, especially in the vicinity of major rivers, such as the Amazon (e.g., Lavagnino et al. 2020). As one example of a regional approach to defining sublittoral, Valentine et al. (2005) extend the lower boundary to 400 meters (154 ft) to include the very wide continental shelves of the Gulf of Mexico and even deeper—up to 800 meters (309 ft)—in the vicinity of submarine canyons. 

The sublittoral also differs in its physical, chemical, and biological properties (e.g., Brown et al. 2011). Waves, tides, currents, solar radiation, temperatures, salinities, terrestrial inputs, and biological productivity are among the many possible players in the sublittoral environment. The degree to which these properties influence the distribution of benthic organisms in a given region depends on oceanographic and terrestrial processes. The highly dynamic coastal benthic zone—as I prefer to call it—awaits further study and characterization. Given its importance to human enterprises, the need for young scientists to study this zone couldn’t be greater.

22.5.5 The Littoral Zone (Intertidal Zone)

Humans have long been fascinated by life at the ocean’s edge. The shore offered the world’s first unlimited buffet with food that “littorally” could not be fresher. Evidence of shell middens—piles of discarded and presumably eaten shellfish—on the shores of Africa date back some 120,000 years (e.g., Niespolo et al. 2021). A wide variety of material resources and even energy were available in the remains of organisms, including whales (e.g., Erlandson et al. 2015; Kishigami 2021). 

Scientific observations of the distribution of marine organisms along the shore span nearly two centuries (e.g., Forbes 1840; Verrill 1874; Sumner 1910; Colman 1933; Doty 1946; Stephenson and Stephenson 1949; Ebling et al. 1960; Dayton 1971; Connell 1972; Lubchenco 1980; Gaines and Roughgarden 1985; Paine 1994; Robles and Desharnais 2002; Menge et al. 2003; Bird et al. 2013; Weitzman et al. 2021). Naturalist Edward Forbes (1815–1854), born on the Isle of Man (in the Irish Sea), defined the littoral zone as “the tract that lies between the high and low water marks” (Forbes 1840). Indeed, the littoral zone encompasses the region of the shoreline regularly submerged—what’s known as submersion—or exposed to air—that is, emersion. Because it occupies such a narrow strip of seafloor along the sea’s edge, it’s been callled the ocean’s “bathtub ring” (Menge and Branch 2001).

Organisms living here straddle oceanic and terrestrial environments, the “sea-land gradient” (Raffaelli and Hawkins 1999).  Both oceanic and terrestrial conditions establish the environments available for habitation by organisms. Raffaelli and Hawkins (1999) stress that tides represent only one of the many environmental factors that influence the distribution of organisms along the shore. Waves (high- versus low-energy), geologic substrate (sedimentary or rocky), and salinity (from brackish to highly saline) also play roles. Because many factors are at play, they prefer the term “littoral” (i.e., the shore) to “intertidal” (between the tides) but acknowledge that “intertidal is in such common usage that it would now be difficult to replace.” 

From the earliest days of research in the littoral zone, marine biologists sought to explain the visible and persistent patterns in the distribution of organisms. Even a casual visitor can discern the distinct regions of color and texture that often appear at different “heights” above the water’s edge. These regions represent the phenomenon of intertidal zonation, the grouping of organisms into horizontal bands along vertical gradients in elevation along a shoreline. Note that height refers to the vertical distance above or below the zero tide height (i.e., mean sea level). In some cases, the zones and their community of organisms are distinct. In others they appear as a gradient of species, with some more abundant at different heights (e.g., Helmuth 2015).

The observation of discrete bands of organisms within the littoral zone (and above and below it) has led to a set of terms to classify these subdivisions. As you might expect, the terms vary, but attempts have been made to standardize the terminology to make it applicable worldwide (e.g., Stephenson and Stephenson 1949; Lewis 1964). We’ll follow subdivisions proposed by Raffaelli and Hawkins (1999)—largely based on the work of Stephenson and Stephenson (1949, 1972) and Lewis (1964), but also Ricketts and Calvin (1962). These latter authors worked along the Pacific Coast, so their zones apply better for US shores. 

While the names differ, all authors agree on at least three subdivisions of the littoral zone: (1) the splash zone (Ricketts and Calvin 1962), also known as the supralittoral fringe (e.g., Stephenson and Stephenson 1972; Bertness 1999) and littoral fringe (Raffaelli and Hawkins 1999), characterized by organisms that require occasional wetting; (2) the intertidal zone (Ricketts and Calvin 1962), also referred to as the eulittoral zone (Raffaelli and Hawkins 1999; eu = “true” or “real”), the region of alternating submersion and emersion; and (3) the subtidal zone (Ricketts and Calvin 1962), infralittoral (Stephenson and Stephenson 1972), or sublittoral (Raffaelli and Hawkins 1999), the region rarely exposed to air. See what I mean about names? 

Though generally in agreement with the above scheme, Ricketts and Calvin (1962) offer a slightly different approach for subdividing the intertidal zone. Their epic and aptly named book Between Pacific Tides, first published in 1939 and now in its fifth edition (1985), was heavily criticized by marine biologists prior to publication. Fortunately, it gained popularity and went on to sell more than 100,000 copies, a huge success for its publisher (Tamm 2004). Writing for “beachcombers” as well as “professors,” Ricketts and Calvin divide the intertidal (their preferred term for the littoral) into four zones based on tide heights, which they numbered one through four. Their numbering system provides a convenient shorthand for notetaking in the field—not an easy task when you’re ever watchful for crashing waves and slippery surfaces. 

The uppermost Zone 1, the upper intertidal, centers around the high high tide (Chapter 20)—roughly at a tide height of 1.5–2 meters (5–7 ft) and higher, inclusive of the splash zone. Zones 2 and 3 represent the middle intertidal, occupying heights up to 1.5 meters (5 ft)  above the zero tide height. This zone is further subdivided into an upper middle and lower middle intertidal, respectively, with the dividing line set at the height of the mean high low tide, roughly 0.5 to 1.5 meters (2.5–5 ft). The lower intertidal, Zone 4, extends from the zero tide height to the lowest minus tide. Raffaelli and Hawkins (1999) refer to Zone 4 as the sublittoral fringe. Stephenson and Stephenson (1972) call this the infralittoral fringe (infra = “below”). Ricketts and Calvin’s subtidal zone—the nearshore region below the intertidal—has been defined as the region in which seaweeds and seagrasses flourish (e.g., Dauvin et al. 2008). Others simply call this region the sublittoral (e.g., Dauvin et al. 2008). If you’re a student in an oceanography or marine biology laboratory or field course on the North American West Coast, you’re likely to encounter upper, upper middle, lower middle, and lower as the four subdivisions of the intertidal. 

Broadly speaking, environmental factors explain the upper limit of most organisms’ distributions on the shore (e.g., Dayton 1971). As noted above, wave energy, substrate heterogeneity, temperature, salinity, and other factors interact along with tides to modify where they can live. For example, wave-exposed regions develop a different assemblage of organisms than sheltered locations. The type of substrate—sand, gravel, cobble, boulder, or solid rock—determines sediment stability under different wave conditions. It’s hard to stay attached if your home is rolling in the waves. Mixed substrates—including fractured or eroded stretches of rock—offer the potential for tide pools and nooks and crannies in which organisms can hang out to avoid adverse conditions. Temperature variations—especially during low tides—can bake or freeze organisms exposed to the air. Whether an organism survives depends on its physiology and the duration of exposure to such adverse conditions. 

Salinity comes into play when tidal fluctuations bring saltier or fresher water to the intertidal. Precipitation can lower salinities, even to zero during prolonged storm events. On the other hand, evaporation in tide pools—especially those less frequently submerged—can cause salinity to increase.

Physical factors may vary over temporal scales as well—from days to decades. Thus, the assemblage of species at a given location may represent a montage of different events separated in time. For example, a random log that crashes onto a shore and clears a space will look quite different from the surrounding rock—especially in its initial stages—as new larvae and mobile organisms grab the space and make a home.

Biological factors—especially competition for food and space and predation—generally set the lower limits of species’ distributions (e.g., Dayton 1971). A classic set of field experiments carried out in the rocky intertidal of Scotland by UC Santa Barbara ecologist Joseph Connell (1923–2020) demonstrated the ability of the larger and faster-growing common rock barnacle (Semibalanus balanoides) to displace and outcompete the smaller and slower-growing stellate barnacle (Chthamalus stellatus). The larger barnacle simply “smothered, undercut, or crushed” the smaller one and prevented it from occupying its preferred habitat. When the larger barnacle was excluded (by removing newly settled larvae with a needle), the smaller barnacle did quite well (Connell 1961). These experiments set the stage for decades of experimental manipulations in the intertidal to study interactions among species. Of course, generalizations only take us so far. Biological factors may set the upper boundary for some organisms and physical factors the lower boundary. In ecology, there are always exceptions to “rules” (e.g., Bird et al. 2013; Underwood 2000; Hawkins et al. 2020).

Beaches and estuaries with sandy, silty, and muddy shores experience many of the same environmental challenges as rocky shores (e.g., Knox 2001; Kennish 2016). Waves, tides, and currents act on sediments and move them in relation to their energy and particle size (Chapter 6). In some environments, their zonation remains hidden beneath their sediments. In others, the presence of marine plants—seagrasses and cord grasses—structure the environment (e.g., Degraer et al. 1999; Knox 2001; Semeniuk and Brocx 2016). We don’t have the space here to discuss soft-bottom environments, but know that they are every bit as fascinating. We’ll reserve our discussion of soft bottoms to the chapters ahead on the organisms that live there.

22.6 Ocean Depth Zoning Out

Aquatic scientists have long debated the terms and definitions used to classify regions of the seafloor (e.g., Forbes and Hanley 1853; Southern 1915; Ekman 1953; Hedgpeth 1957; Pérès 1982; Golikov 1985; Dauvin et al. 2008; Fraschetti et al. 2008; Costello 2009; Althaus et al. 2015; Costello et al. 2017). As pointed out by Trombley and Cottenie (2019), “The usage of ambiguous terminology can cause confusion or discord among members of the same field and fuel unproductive debates.” On the other hand, they acknowledge that “ambiguity can speed up the acquisition of knowledge by allowing definitions to change as knowledge on the subject increases.” 

Now, I admit it may have been simpler (and occupied far fewer pages) to simply present the “textbook” scheme for classifying marine environments. But that approach ignores two very important principles of science (as we discussed way back in Chapter 2). First, science is an active, ongoing process that yields new data and new ideas subject to revision, review, and rejection; and second, science communication, especially to the public, demands a clear and transparent accounting of new data and ideas. Thus, while it may seem tedious and even a bit annoying to get the play-by-play as the science unfolds—like listening to a neighbor describe the installation of power outlets in the remodel of their home—in truth, nearly all science requires revision and freshening up. I feel strongly committed to promoting a view of science as a process and helping the public understand what scientists do and how they do it. It’s important to know where those terms and facts come from—and their limits—so that you are better skilled at using science in your own life. 

The regular “upheaval” of ideas in science was not lost on the California-born American writer John Steinbeck (1902–1968). A close friend of marine biologist Ed Ricketts (1897–1948), Steinbeck frequently incorporated marine science in his work, especially Cannery Row (1945) and The Log from the Sea of Cortez (1951). He also wrote a foreword for the Third Edition of Between Pacific Tides (Ricketts and Calvin 1962), mentioned above. Commenting on modern science, Steinbeck wrote: 

And in many fields young, inquisitive are seeing new worlds. And from their seeing will emerge not only new patterns but new ethics, disciplines, and manners. The upheaval of the present world may stimulate restive minds to new speculations and evaluations. . . . The world is being broken down to be built up again.

Steinbeck ends with a quote from the book: 

There are good things to see in the tidepools and there are exciting and interesting thoughts to be generated from the seeing. Every new eye applied to the peep hole which looks out at the world may fish in some new beauty and see some new pattern, and the world of the human mind must be enriched by such fishing.

New tools for observing the ocean, especially robots and drones, offer opportunities to sample across larger areas on scales not possible with traditional methods. With changes in the ocean happening at an unprecedented rate, our understanding of this environment may be more important than ever. We still have much to learn. Be enriched, ocean fisher. And carry your ocean message to others. We live because the sea lives!

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