12.11: Biological Provinces and Zones

The communities of organisms in different parts of the oceans are as distinct and different from one another as the species that live in tropical rainforests, deserts, and Arctic tundra. The oceans are separated into distinct biological zones delineated by the availability of light, nutrients, and food, and by the temperature and salinity characteristics of the water.

Differences between communities that live in or on the seafloor and those that inhabit the water column are so great that the two locations are considered to be separate environments. The seafloor is the benthic environment, and the water column is the pelagic environment. In contrast to pelagic organisms, benthic organisms do not have to swim or control their density and/or size to avoid sinking, which can be energetically advantageous, but they must compete with each other much more intensely for living space and food. Although the pelagic and benthic environments support different communities, many species live part of the life cycle in one environment, then change form (in some species as dramatically as that of caterpillars turning into butterflies) and live in the other stage of metamorphosis.

Benthic Environment

The benthic environment is separated into zones by depth. The deepest zone, the hadal zone (Fig. 12-14a), which occupies less than 1% of the ocean floor, is limited to the seafloor of deep-ocean trenches at depths greater than 6000 m. The seafloor between 2000 and 6000 m is the abyssal zone, and corresponds roughly to the abyssal plains. The hadal and abyssal zones are generally covered with soft, fine-grained muds. Much of these zones is below the carbonate compensation depth (CCD; Chap. 6), and the surface muds generally contain little or no calcium carbonate.

Figure 12-14. The oceans are separated into zones. The assemblage of species in each zone is different from that found in the other zones. (a) The benthic environment is separated into a number of zones, each of which spans a different depth range. (b) The pelagic environment (water column) is separated into a neritic province (water over the continental shelf) and an oceanic province. The oceanic province is divided into several zones that each span a different depth range.

Sediment characteristics are important to the species composition of the benthos. For example, some polychaete worms feed by passing sediment through the gut to digest microbes and organic matter in the sediment, just as earthworms do. Such polychaetes are more successful in soft, fine-grained muds than in coarse-grained sediment, particularly if the muds contain a significant amount of detritus. Where sediments are coarse-grained with little organic matter, many benthic animals live on the sediment surface and obtain food from particles or other animals in the water column. Benthic animals are called infauna if they live much or all their life in sediments, and epifauna if they live on or attached to the seafloor.

The seafloor environment between 2000 m and the approximate depth of the continental shelf break (about 200 m) is the bathyal zone (Fig. 12-14a). Because sediments in the bathyal zone are more variable than those in the abyssal and hadal zones, the benthos is more variable in composition in the bathyal zone.

The hadal and abyssal zones and almost all of the bathyal zone are below the photic zone, so no photosynthetic organisms are present in the benthos. Instead, benthos in these zones are dependent on food transported to them from the overlying photic zone, primarily as detritus. All benthic communities in these zones thus belong to the detrital food web, with some exceptions. One is the unusual benthic communities around hydrothermal vents. These organisms are supported by a chemosynthesis-based food web (Chap. 15). A second exception occurs in locations where methane and other hydrocarbons are abundant, and methane based chemosynthesis occurs, generally at seeps of oil and gas (most of which are natural seeps). A third, and minor, exception occurs in the shallowest parts of the bathyal zone in areas of the clearest waters, where light levels are very low but sufficient to support limited photosynthesis by species adapted to very low light levels. Such conditions are generally present only on some seamounts far from land.

The benthic environment between the continental shelf break (200 m depth) and the land is divided into three major zones. The sublittoral zone (subtidal zone) is the continental shelf floor that is permanently covered by water. It extends from the low-tide line to the depth of the continental shelf break (about 200 m). The intertidal zone (littoral zone) is the region between the low-tide line and the high-tide line and is covered with water during only a part of each tidal cycle. The supralittoral zone, often called the “splash zone,” is the region above the high-tide line that is covered by water only when large storm waves or tsunamis reach the coast or during extremely high tides or storm surges.

Organisms that live in the supralittoral and intertidal zones must endure much more extreme conditions than other marine organisms. Such conditions include exposure to the temperature extremes of the atmosphere, loss of body fluids by evaporation while exposed to the atmosphere, salinity variations due to rainfall, predation by both marine and terrestrial species, and mechanical shock and turbulence created by breaking waves. In intertidal zones with sand or mud sediments, most organisms bury themselves to avoid extreme conditions. This is why beaches and mudflats appear to be unpopulated. Most sediment-covered shores support an abundance of infauna living a few centimeters beneath the sediment surface. Organisms that live in rocky intertidal zones adapt to their changeable environment in other ways (Chap. 17).

Pelagic Environment 

The pelagic environment consists of the neritic province and the oceanic province. The neritic province is the water column between the sea surface and seafloor in water depths to about 200 m. Thus, it consists of the water overlying the continental shelf and shallow banks (Fig. 12-14b). The neritic province experiences greater variability in salinity, temperature, and suspended sediment concentrations than the oceanic province (Chap. 13). Further, the mixed layer reaches the seafloor in most of the neritic zone, at least for part of the year, which is important to nutrient cycles (Chap. 13).

The oceanic province comprises the entire water column of the open oceans beyond the continental shelf. It consists of several zones, each of which is a water layer distinct from the layer above or below because of differences in salinity, temperature, and light intensity. The epipelagic zone extends between the surface and 200 m, the approximate depth where light intensity becomes too low for photosynthesis. Note that this depth is not the same as the shallower (usually less than 100 m) compensation depth, where photosynthesis occurs just rapidly enough to match respiration (Fig. 12-5). The epipelagic zone is the only zone in the deep oceans in which food can be produced directly by phototrophy. In all deeper zones, the original source of food is primarily detritus falling from the epipelagic zone. Minor additional food sources include vertically migrating animals and chemosynthesis, especially at hydrothermal vents (Chap. 15).

Below the epipelagic zone, between 200 and 1000 m is the mesopelagic zone (Fig. 12-14b). Although light intensity is too low to support phototrophy, many organisms in this zone have photoreceptors or eyes adapted to detect faint ambient light or bioluminescence (Chap. 15). Some of these species migrate upward into the epipelagic zone at night to feed and return to the mesopelagic zone during the day. Below the mesopelagic zone are the bathypelagic zone (1000–2000 m), the abyssopelagic zone (2000–6000 m), and the hadopelagic zone (>6000 m). These are zones of perpetual darkness, high pressure, and low temperature, and their inhabitants feed on detritus or on each other (Chap. 15). The boundary between the bathypelagic and abyssopelagic zones is essentially the boundary between relatively young bottom water masses of the ocean basins and older overlying deep water masses (Chap. 8). The hadopelagic zone is restricted to the deep trenches, in which water movements are generally very slow and water residence time is long (CC8).

Latitudinal Zones

Most benthic and pelagic environments below the photic zone are relatively uniform at a given depth. Latitudinal variations in temperature and salinity are less in waters or sediment below the photic zone than in the epipelagic zone and in the pelagic and benthic environments of the continental shelf. Most marine organisms are adapted to live successfully within a narrow range of environmental conditions, particularly a narrow range of temperatures.

Variations in water temperature with latitude (Figs. 7-14, 7-24) act as effective barriers to latitudinal dispersal of many marine species. Hence, the marine biota of each biological zone varies with latitude (although the variation is small in the abyssal benthos and the abyssopelagic zone). For example, coral reefs are present in warm-water areas in all oceans, but not at cold, high latitudes. The fauna and flora of the Arctic and Antarctic regions are also very different. Polar bears and most penguin species live only at high latitudes, but polar bears live only in the Arctic region, and most penguin species live only in the Antarctic.

In some cases, continents act as geographic barriers to the distribution of marine species. The tropical Atlantic Ocean is effectively separated from the tropical Pacific and Indian Oceans by the continents. To move between the Atlantic Ocean and either the Pacific Ocean or the Indian Ocean, marine organisms would have to pass around the southern tip of Africa or South America or through the Arctic Ocean. Many tropical species could not tolerate the low water temperatures they would encounter in such journeys. Therefore, almost all tropical Atlantic species are different from those in the tropical Pacific and Indian Oceans. In contrast, the tropical Pacific and Indian Oceans (the Indo-Pacific) have many species in common because they are connected at tropical latitudes.

Tropical Atlantic and Indo-Pacific species, although different, are more closely related than Arctic and Antarctic species. The reason is that the continents have drifted to their present positions in the latter stages of the present spreading cycle (Chap. 4). Previously, all the oceans were connected at tropical latitudes, and tropical species could move freely around the globe during this relatively recent period in evolutionary history.