The deep-sea habitat

What is the deep sea? This is actually difficult to define. For the purposes here, I will consider the waters below the continental shelves as the deep sea. That is all the water greater than ~200 meters depth.

The deep-sea is the largest ecosystem on the planet. The oceans cover about 71% of the earth's surface. Of that area ~90% is open ocean below which lies "deep" water. The depth of the oceans ranges from shallow bays, estuaries, and tidepools to a depth of 11,000 meters in the Marianas Trench. The average depth of the ocean is ~4000 meters (2.5 miles)!! When you take into consideration the vertical extent of the deep-sea it represents 90% of the volume of the world's ocean.

While this environment is vast and undoubtedly an important component to the world biosphere it is the least explored habitat on our planet. More people have been to the moon than to the bottom of the Mariana Trench! The deep sea has been relatively inaccessible because it is a difficult place to work in. However, many new technologies such as remotely operated vehicles (ROVs), robotic autonomous underwater vehicles (AUVs), and a variety of benthic cameras and sampling devices are currently blazing trails into the deep frontier.

The deep sea is a challenging environment. It is particularly cold. At the average ocean depth of 4000 meters the temperature hovers at ~2 °C (see figure below). The deep sea is under great pressures. The deeper you go the greater the pressure becomes. With every 10 meters of water another atmosphere of pressure is added. Consequently, at 4000 meters the pressure is a crushing 400 atmospheres (5880 pounds per square inch)! This environment is also very dark. As depth increases, the quantity of light decreases (see figure below) and the quality of the light changes. Water absorbs reds and yellows much more rapidly than blues so even when light is available for vision some colors (primarily red) actually appear black.

Productivity is very low in the deep-sea. Without light the deep sea has no primary productivity via the photosynthesis of plants and algae, so there is very little food compared to the shallow seas. This results in very low abundance and biomass of the organisms living there (see figure below). Many animals in the mesopelagic zone (see below) migrate nightly up into the productive surface waters to feed. Otherwise the deep-sea food web is fueled by a rain of dead plants and animals from surface waters. In the ocean when microscopic plankton or even a whale dies it usually sinks. This "food" sinks through the midwaters where it can be consumed by bacteria or animals to fuel the food web. The sinking material is transient in the midwaters and if not consumed it is lost for good. But, on the seafloor the dead organisms stop their journey and can accumulate. Therefore, the bottom habitat is not as food poor as the midwaters and there is a larger biomass there.

I must mention that primary productivity is very high at hydrothermal vents and methane seeps where bacteria can use sulfide and methane as energy sources but these places are infrequent in the deep sea. If you are interested in vents and seeps please see the deep-sea biology texts in the bibliography. I hope to include some information about hydrothermal vents and their fishes in the future.

Several zones divide up the deep sea. It is important to describe these zones here because I will be referring to them in describing the habitats of various fishes. The figure below provides a diagrammatic view of the ocean and the zones. First the deep-sea is divided into midwater habitats (pelagic - in blue type) and bottom habitats (benthic - in red type).

The epipelagic zone is the one we are most familiar with. It is the brightly lit surface waters inhabited by phytoplankton, sardines, salmon and tuna. Below this region and often bounded by the sharp drop in temperature called the permanent thermocline, is the mesopelagic zone where there is still some light but it is not enough to fuel photosynthesis. Most creatures here have very well developed eyes to see in the dim light and this zone holds the shallowest deep-sea community. In the bathypelagic zone there is no light, very low organismal biomass, and very low temperatures. The boundary between the bathy- and abyssopelagic is very obscure. The abyssopelagic is considered the deep water column directly above the continental rise and abyssal plain up to about 3000 meters water depth. Some scientists consider this region as part of the bathypelagic zone. Finally, there is the hadalpelagic which is the midwaters that occur in the oceans very deep trenchs. We know extremely little about this habitat and what lives there.

The benthic habitats are divided based on the geology of the continental boundaries. Please consult an earth science or geology text for a complete explanation of the geologic features in the figure. Among benthic habitats the one that we know from fishing, tidepooling, and scuba diving is the sublittoral zone. This is the shallow bottom environment present from the shoreline and across the continental shelf. It is sunlit except in the most turbid waters and hosts a variety of corals, algae and surfgrasses. The bathyal environment is the bottom of the continental slope. It is generally steep and often cut by even steeper submarine canyons such as the one present in Monterey Bay. Where the topography begins to flatten out on the continental rise and finally turns into the abyssal plain the benthic habitat is called the abyssal zone. In the extreme deep of the trenches the benthic habitat is termed the hadal zone.

The boundaries of some of these zones and even the distinctions between pelagic and benthic can be blurry in the deep-sea. Nevertheless, each zone or habitat generally has a set of environmental characteristics that defines it and a typical community of organisms.