Deep ocean

Vertical Life Zones and Biodiversity

In this session we will define the four vertical life zones found in all oceans, and examine how and why some areas of the ocean are home to much greater numbers of species than are others.

Vertical life zones

Oceanographers have defined two fundamental features about life in the deep ocean. First, although animals can be found at all depths in the seas, there is a clear decrease in the amount of life with increasing depth, with a particularly marked fall between about 100 and 1,000 metres. Second, although some species can be found over very wide depth ranges, most species are much more restricted in their vertical distribution, and live in fairly distinct communities, which can be found at the same depths over very wide areas of the oceans. These communities live in quite well-defined horizontal slices or zones, some very thin and others extremely thick. Although the boundaries between the zones are not always very obvious, and some animals regularly move from one zone to another, the broad zones can be recognised anywhere in the ocean.

Sea multiple corer being deployed from a research ship. This instrument has revolutionised deep-sea biology since it was developed in the 1970s. Unlike previous sediment samplers, it takes totally undisturbed samples of the bottom mud with the sediment-water interface intact. (Image: Tony Rice)

There are three main mid-water zones. The shallowest and thinnest is the epipelagic (which means upon the sea). It extends from the surface down to about 100-150m. It is lit by sunlight, which allows plant growth (all the deeper layers are too dark for this). The epipelagic zone impinges on the sea floor only in shallow inshore waters on the continental shelves. In the deeper parts of the ocean its lower margin simply merges with the second depth zone.

The second zone is the mesopelagic (meaning middle sea). It is often called the twilight zone because, while its upper regions where it meets the epipelagic zone are still quite light during the day, its lower boundary at about 1,000m is perpetually dark because none of the Sun's rays can penetrate this far. Many of the animals in this strange twilight world produce their own light, bioluminescence (living light), which they use as camouflage or to find, or attract, potential food or mates. The mesopelagic zone impinges on the sea floor on the outer parts of the continental shelves and the upper part of the continental slopes. In the deep sea proper the mesopelagic zone overlies the vast, dark waters of the last great depth zone.

The third zone receives no sunlight at all, so it is always absolutely dark apart from the occasional flashes of ghostly blue light produced by bioluminescent animals. It is rather arbitrarily divided by ocean scientists into the bathypelagic (deep sea), which extends down to about 3,000m, the abyssopelagic (bottomless sea) from 3,000 to 6,000m, and the hadal (unseen) for the ocean trenches. But these zones can all be grouped together into the abyss.

Underlying the water column is the fourth zone, the sea floor itself. This is the benthic zone, and the animal communities living there are called collectively the benthos, names that are derived from a Greek word meaning deep. The benthic zone is the thinnest and most clearly limited zone of all, because most benthic animals live either on the surface of the sea-bed sediments or within the top few centimetres of bottom mud. Even this zone is blurred because some sea-floor animals, particularly the shrimps and fish, move away from the bottom from time to time, sometimes swimming tens or even hundreds of metres above it.

As on land, with very rare exceptions (discussed below), the animal life in all of these oceanic zones is ultimately dependent for its existence on photosynthesis. This is the process by which plants use the energy in sunlight to convert simple inorganic chemicals into the complex organic substances which animals need, but cannot make for themselves. Photosynthesis can take place only in the sunlit waters near the ocean surface. Therefore, large seaweeds attached to the bottom are restricted to the very small part of the ocean with shallow waters. Instead, most plant production in the sea is by tiny single-celled algae, the phytoplankton, which float freely in the water column far from land. The phytoplankton is the collective name for about 4,000 species ranging in size from a few microns to a millimetre or so; despite their small size, collectively they produce about 20,000 million tonnes of new growth each year.

Biodiversity in the deep ocean

Biodiversity refers to the variety of living forms with which we share the planet Earth. It has a huge range of applications but a relatively simple feature--the number of species found in any particular area or environment - is often used to measure and compare biodiversity. Classical high-biodiversity environments include tropical rain forests on land, with their amazing variety of plants and animals, and coral reefs in the seas, with their equally impressive communities of invertebrates and fish. With the exception of coral reefs and one or two other marine habitats, the oceans have traditionally been considered to be much less biodiverse than the land.

Life on Earth almost certainly originated in the sea and was more or less restricted to the oceans for the first 3 billion years of evolution. As a result, there is a much greater variety of animal forms in the marine environment than on the land. The seas contain representatives of 28 phyla--the major groupings of animals, such as sponges, crustaceans, molluscs and so on--whereas terrestrial environments have only 11 phyla. The situation is very different, however, at the species level. There are only about 4000 marine-plant species compared with an estimated 250,000 species of terrestrial green plants. Similarly, of the 1.7 million-or-so known animal species, only about one-tenth are from the oceans, with most of these living on or close to the bottom in relatively shallow water.

No barriers

The mid and deep-water ocean realms are particularly species-poor compared with the land or fresh water. For example, copepods, the dominant animal group in the plankton throughout the oceans, are represented by fewer than 2,000 species, whereas among insects, the copepods' nearest terrestrial equivalents, there are several hundreds of thousands of species in just one order--the beetles. Similarly, despite the enormous size of the oceans compared with fresh-water systems, only just over half of the estimated 25,000 fish species on Earth are marine. Most of these live in shallow, warm waters, with the vast deep-ocean water column harbouring only 1,000 or so species.

The reason for this relatively low species-richness in the open sea seems to be that the oceans have fewer major physical or ecological barriers than terrestrial and fresh-water environments. As a result, many mid-water marine species are very widely distributed, with quite a few being found throughout the world's oceans. Thus there are far fewer opportunities for populations of animals to become separated, preventing the possibility of divergent evolution and the formation of new species. At the same time, at least in mid-water, there are fewer specialist roles to be occupied by individual species than there are on land, so there is less encouragement for the evolution of lots of different species. For these reasons, while there are doubtless many more mid-water animals to be discovered, it seems likely that the total number of species in this environment will not exceed 10,000-20,000.

Species-rich sea floors

By contrast, the sea-floor communities are much more diverse. Almost 200,000 sea-floor species have already been described, mostly from the continental shelves and slopes and just a few thousands from the tiny proportion of the true deep-sea floor so far sampled. Ocean scientists predict that many more, possibly millions, remain to be discovered and that the vast majority of these will be tiny creatures living in the muddy sediments at the bottom of the deep ocean.

This is an amazing change of heart since, until the 1960s, most deep-sea biologists believed that the deep-sea floor was a very monotonous environment harbouring a rather small number of different species. More recent observations indicate that, far from being monotonous, the bottom of the deep sea exhibits an amazing variety at all scales. Large animals move over the bottom, churning it up into mounds and depressions; water currents move the sediment surface around, scouring it from some areas and depositing it in others; carcasses of large animals from the overlying water column sink to the bottom, producing localised food hot-spots; other potential food accumulates in thick patches separated by relatively barren areas; and from time to time slumps and slides of sediment like underwater avalanches sweep everything away so that the whole colonisation process starts all over again. These processes are very similar to those going on in tropical rain forests and in coral reefs, which encourage the evolution of the species-rich communities typical of these environments.