LEARN MORE ABOUT THE DEEP SEA
The major structuring variable in the pelagic ocean (ocean waters away from the coast and above the bottom) is depth and its covariance with temperature and the penetration of sunlight. This results in a layering of the ecosystems of the pelagic ocean. The deep-pelagic ocean is generally considered to mean deeper than the penetration of sunlight sufficient to support photosynthesis. This depth varies geographically but is generally considered to be below about 200 m depth, which coincides with the maximum depth of seasonal variability in temperature, the seasonal thermocline. Below the upper 200 m (the “epipelagic zone”) is a layer where sunlight penetrates during the day (but with insufficient intensity to support primary production), called the “mesopelagic zone,” (also called the "twilight zone"). At 200-1000 m depth, this zone is about four times as thick as the epipelagic and is coincident with the vertical temperature gradient known as the permanent thermocline. In some geographic areas, microbial degradation of organic matter sinking from the surface zone results in low oxygen concentrations in the mesopelagic, called oxygen-minimum zones. Below the depth to which sunlight can penetrate (about 1000 m at noon on a sunny day in clear water) is the largest layer of the deep-pelagic realm and by far the largest ecosystem on our planet, the bathypelagic zone. The bathypelagic zone comprises almost 75% of the volume of the ocean and is generally remote from the influence of the bottom and its ecological communities.
The deep- pelagic environment is even more alien to human experience than the bottom of the ocean. As terrestrial animals, our interactions with other organisms are essentially two dimensional. Familiar terrestrial life is tightly bound to the surface of the land. Even flying animals are constrained by gravity to return to land. This two-dimensionality makes it relatively easy for us to envision similarly two- dimensional benthic (bottom-living) ecosystems in the ocean. In contrast, the pelagic is a three-dimensional environment, most of which has little or no interaction with the interfaces at the ocean's bottom and surface. In fact, many organisms may never see a solid object in their lifetime except for other organisms. The deep-pelagic environment encompasses over a billion cubic kilometers with animals and microbes living out their lives throughout that volume. Unlike in air, the density of water allows organisms to attain neutral buoyancy that frees them from the restraint of life on the bottom.
Living in an environment that, for the organisms, effectively has no bottom requires staying within the appropriate depth range. This means actively swimming against gravity (an energetically costly strategy in a food-poor environment), increasing drag to prevent sinking, or achieving neutral buoyancy. The latter can be accomplished by special flotation structures to offset the weight of muscles and skeleton or by reducing the overall density of the tissue to near that of seawater. Reduction of density to a gelatinous consistency is very common among deep pelagic animals and is found in many phyla. Furthermore, as more direct observations have been made with various submersibles, it has become apparent that the deep-pelagic fauna is dominated by gelatinous zooplankton, such as cnidarians (jellyfishes and siphonophores), ctenophores (comb-jellies), and salps. Although little is known about their feeding rates, their sheer abundance suggests ecological importance in the ecosystem. Evidence also is now developing that many other members of deep-pelagic food webs are dependent either directly or indirectly on these jellies.
The deep-pelagic enivironmentis vast and very diffuse, with generally low abundances of inhabitants, although submersible observations indicate that some species may concentrate into narrow depth bands. A vast, diffuse environment implies low encounter rates for both food and potential mates. Availability of food is even lower than on the deep seafloor because sinking food accumulates at the interface of the bottom but passes through the water column. The popular concept of deep-sea animals as fishes with large mouths and long, sharp teeth results from these fishes' need to catch and swallow whatever prey they chance upon or can lure into range.
Whereas bioluminescence is observed in a variety of coastal marine communities, and is not unusual on land (e.g., fireflies), it is almost universal among deep-pelagic organisms. Some animals produce the light independently whereas others are symbiotic with luminescent bacteria. Biological production of light comes in many forms and has many functions. Examples include an amazing variety of lures, searchlights, species- and sex-specific mate signaling/recognition light organs (photophores), and various forms of predator distraction and avoidance. A common form of bioluminescence that has evolved repeatedly among deep-pelagic animals, especially the vertical migrators of the mesopelagic, is counter-illumination. Ventral photophores produce light that matches the faint blue remnant of sunlight coming from above and disrupts the silhouette of the animal when seen from below by a predator.
The life cycles of deep-pelagic animals often involve shifts in vertical distribution among developmental stages. Additionally, many deep-benthic species spend part of their life cycles, typically the early stages but for some the reproductive stage, at some level in the pelagic realm. Such ontogenetic vertical migrations expand the dependence of species on the physical and biological dynamics of the various layers, often including the surface layer. Even more spectacular are the daily (or diel) vertical migrations of very many species typical of the mesopelagic and upper bathypelagic. Although there are various detailed patterns, this shift is generally upward at night to feed in the higher biomass closer to the surface and back down during the day, perhaps to avoid visual predators during daylight or perhaps for energetic efficiency in the colder, deeper waters. Diel vertical migration in the deep sea comprises the largest migration on earth. The argument has recently been made that so many animals are swimming up and down regularly that they add substantially to the physical mixing of the ocean water. All of this vertical migration also actively contributes to the "biological pump" that substantially accelerates the movement of carbon compounds and nutrients from the epipelagic into the deep ocean. When the temporal component is superimposed on the massive volume of the deep ocean, the deep- pelagic environment can be considered to be effectively four-dimensional.
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