The open ocean is relatively unproductive because of a lack of nutrients, yet because it is so vast, it has more overall primary production than any other marine habitat (Fig. 4.1). Only about 10 percent of marine species live in the open ocean. But among them are the largest and fastest of all marine animals, as well as the animals that dive the deepest and migrate the longest. In the depths lurk animal that, to our eyes, appear hugely alien.
Figure 4.1: From top: The pelagic food web, showing the central involvement of marine microor-ganisms in how the ocean imports nutrients from and then exports them back to the atmosphere and ocean floor.
Much of the aphotic zone’s energy is supplied by the open ocean in the form of detritus. In deep water, “marine snow” is a continuous shower of mostly organic detritus falling from the upper layers of the water column.1Its origin lies in activi-ties within the productive photic zone. Marine snow includes dead or dying plankton, protists (diatoms), fecal matter, sand, soot and other inorganic dust. The “snowflakes” grow over time and may reach several centimetres in diameter, travelling for weeks before reaching the ocean floor. However, most
or-2The word diel comes from the Latin dies, day, and means a 24-hour period.
ganic components of marine snow are consumed by microbes, zooplankton and other filter-feeding animals within the first 1,000 metres of their journey, that is, within the epipelagic zone. In this way marine snow may be considered the foun-dation of deep-sea mesopelagic and benthic ecosystems: As sunlight cannot reach them, deep-sea organisms rely heavily on marine snow as an energy source.
4.1.1 Surface Waters
The surface waters are sunlit. The waters down to about 200 metres are said to be in the epipelagic zone (Fig. 4.1). Enough sunlight enters the epipelagic zone to allow photosynthesis by phytoplankton. The epipelagic zone is usually low in nu-trients. This is partially because the organic debris produced in the zone, such as excrement and dead animals, sinks to the depths and is lost to the upper zone. Photosynthesis can happen only if both sunlight and nutrients are present.
In some places, such as the edge of continental shelves, nutrients can upwell from the ocean depth, or land runoff can be distributed by storms and ocean currents. In these areas, given that both sunlight and nutrients are now present, phytoplankton can rapidly establish itself, multiplying so fast that the water turns green from the chlorophyll, resulting in an algal bloom. These nutrient-rich surface waters are among the most biologically productive in the world, supporting billions of tonnes of biomass.
Phytoplankton are eaten by zooplankton - the most abun-dant zooplankton species are copepods and krill: tiny crus-taceans that are the most numerous animals on Earth. Other types of zooplankton include jelly fish and the larvae of fish, marine worms, starfish, and other marine organisms. In turn, the zooplankton are eaten by filter-feeding animals, including some sea birds, small forage fish such as herrings and sar-dines, whale sharks, manta rays, and the largest animal in the world, the blue whale. Yet again, moving up the food chain, the small forage fish are in turn eaten by larger predators, such as tuna, marlin, sharks, large squid, sea birds, dolphins, and toothed whales.
In response to the predatory pressures in the surface water food chain, many planktonic and nectonic organisms engage in diel2vertical migration (DVM), also known as diurnal vertical migration. The migration occurs when organisms move up to the uppermost layer of the sea at night and return to the bottom of the daylight zone of the oceans during the day (Fig. 4.2). In terms of biomass, it is the greatest migration in the world! It is not restricted to any one taxon as examples are known from crustaceans (copepods), molluscs (squid), and ray-finned fishes. The phenomenon may arise for a number of reasons, though it is most typically to access food and avoid
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Figure 4.2: Daily migration of marine life to and from the twilight zone to the ocean surface. During day, much zooplankton and nekton stays deep in the water, returning to the surface layer at night.
(Still of a NASA animation aten.wikipedia.org/
Diel_vertical_migration.)
Figure 4.3: Underwater sonar 2D-slice through the water column: The green layer is the deep scatter-ing layer of diel vertically migratscatter-ing mesopelagic zooplankton and fish.
3Viruses do not, of course, contribute directly to recycling of organic matter. However, they kill microbes, thereby cycling material even within the microbial loop.
predators. While this mass migration is generally nocturnal, with the animals ascending from the depths at nightfall and descending at sunrise, the timing can be altered in response to the different cues and stimuli that trigger it. Some unusual events impact vertical migration: DVM is absent during the midnight sun in Arctic regions and vertical migration can occur suddenly during a solar eclipse.
4.1.2 The Mesopelagic
Somewhat a simplification, the mesopelagic is the zone where the microbial loop comes into its own. Although some light pene-trates the mesopelagic zone, it is insufficient for photosynthe-sis (Fig. 4.1). The biological community of the mesopelagic zone has adapted to a low-light, low-food. This is a very effi-cient ecosystem with many organisms recycling the organic matter sinking from the epipelagic zone resulting in very little organic carbon making it to deeper ocean waters. The general types of life forms found are daytime-visiting herbivores, de-tritivores feeding on dead organisms and fecal pellets, and carnivores feeding on those detritivores. Many organisms in the mesopelagic zone move up into the epipelagic zone at night, and retreat to the mesopelagic zone during the day (Fig. 4.2). There is so much biomass in this migration that sonar operators in World War II would regularly misinterpret the signal returned by this thick layer of plankton as a false sea floor (Fig. 4.3).
Microbes in the mesopelagic Very little is known about the microbial community of the mesopelagic zone because it is a difficult part of the ocean to study. Recent work using DNA from seawater samples emphasized the importance of viruses and microbes role in recycling organic matter from the surface ocean, known as the microbial loop.3These many microbes can get their energy from different metabolic pathways. Some are autotrophs, heterotrophs, and even some chemoautotrophs
Figure 4.4: Helmet jellyfish Periphylla periphylla is a luminescent, red-colored jellyfish of the deep sea.
Size 30 cm. From: Arctic Ocean Diversity.
Figure 4.5: Pyrosoma, a colonial tunicate; each individual zooid in the colony flashes a blue-green light. Size 20 cm.
Figure 4.6: Gnathophausia millemoesii, one of the deep-sea mysidacea not unlike G. ingens. Size 3 cm.
gr: a groove dividing the last abdominal somite.
(oxidising ammonium as their energy source).
Microbial biomass and diversity typically decline exponen-tially with depth in the mesopelagic zone, tracking the general decline of food from above. The community composition varies with depths in the mesopelagic as different organisms are evolved for varying light conditions. Microbial biomass in the mesopelagic is greater at higher latitudes and decreases towards the tropics, which is likely linked to the differing productivity levels in the surface waters.
Zooplankton in the mesopelagic The mesopelagic zone hosts a diverse zooplankton community. Common zooplank-ton include copepods, krill, jellyfish, siphonophores, lar-vaceans, cephalopods, and pteropods. Gelatinous organisms are thought to play an important role in the ecology of the mesopelagic and are common predators. Though previously thought to be passive predators just drifting through the wa-ter column, jellyfish could be more active predators (Fig. 4.4).
Mesopelagic zooplankton have unique adaptations for the low light. Bioluminescence is a very common strategy in many zooplankton (Fig. 4.5). This light production is thought to function as a form of communication between conspecifics, prey attraction, predator deterrence, and/or reproduction strategy. Another common adaption are enhanced light organs (read: eyes), which is common in krill and shrimp, so they can take advantage of the limited light. Some octopus and krill even have tubular eyes that look upwards in the water column.
Most life processes, like growth rates and reproductive rates, are slower in the mesopelagic. Metabolic activity has been shown to decrease with increasing depth and decreasing temperature in colder-water environments. For example, the mesopelagic shrimp-like mysid, Gnathophausia ingens (Fig. 4.6), lives for 6 to 8 years, while similar benthic shrimp only live for 2 years.
Fish in the mesopelagic The mesopelagic is home to a sig-nificant portion of the world’s total fish biomass; one study estimated mesopelagic fish could be 95% of the total fish biomass. Another estimate puts mesopelagic fish biomass at 1 billion tons. This ocean realm could contain the largest fishery in the world and there is active development for this zone to become a commercial fishery.
The Gonostomatidae, or bristlemouth, are common mesopelagic fish (Fig. 4.7). The bristlemouth could be the Earth’s most abundant vertebrate, with numbers in the hun-dreds of trillions to quadrillions. Another dominant family of fish in the mesopelagic zone are lanternfish (Myctophidae, Fig. 4.8 top), which include 245 species distributed among 33different genera. They have prominent photophores along
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Figure 4.7: Veiled anglemouth, Cyclothone microdon, a mesopelagic (to abyssopelatic) bristlemouth of the family Gonostomatidae. Size 10 cm.
Figure 4.8: From top: Lanternfish Myctophum punctatum (10 cm); Atlantic silver hatchetfish Argyropelecus aculeatus (7 cm); Slender Lightfish Vinciguerria attenuata (4 cm); an anglerfish Bufocer-atias wedli (10 cm). Note extreme eye size, easily larger than the brain itself.
their ventral side.
Food is often limited and patchy in the mesopelagic, lead-ing to dietary adaptations. Common adaptations fish may have include sensitive eyes and huge jaws for enhanced and opportunistic feeding. Fish are also generally small to reduce the energy requirement for growth and muscle formation.
Other feeding adaptations include jaws that can unhinge, elas-tic throats, and massive, long teeth. Some predators develop bioluminescent lures, such as the tasselled anglerfish, which can attract prey, while others respond to pressure or chemical cues instead of relying on vision.
4.1.3 Deep Sea
In the deep ocean, the waters extend far below the epipelagic zone (Fig. 4.1), and support very different types of pelagic life forms adapted to living in these deeper zones. Some deep-sea pelagic groups, such as the lanternfish, ridgehead, marine hatchetfish, and lightfish families (Fig. 4.8) are sometimes termed “pseudoceanic”, because rather than having an even distribution in open water, they occur in significantly higher abundances around structural oases, notably seamounts and over continental slopes. The phenomenon is explained by the likewise abundance of prey species which are also attracted to the structures.
The fish in the different pelagic and deep water benthic zones are physically structured, and behave in ways that differ markedly from each other. Groups of coexisting species within each zone all seem to operate in similar ways, such as the small mesopelagic vertically migrating plankton-feeders, the bathypelagic anglerfishes, and the deep water benthic grandiers.