16.5: 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.

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.

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).

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.

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.

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.