Deep Zone

Background

The deep zone of the ocean extends from the depths of 1000m to the seafloor, encompassing the Bathypelagic, Abyssopelagic, and Hadalpelagic zones. In these regions, the pressure reaches high levels and temperatures approach freezing. But the most striking features are the lack of light and wide dispersion of organisms. These factors limit the growth of the brilliant foliage or complex structures of the organisms above, but the creatures of the deep are incredibly diverse, with complex chemical processes that help them survive in these depths. In addition, these three zones account for 90% of the ocean by volume, however, much is left undiscovered about these area of the ocean and the systems within. With new technologies such as autonomous robotics, scientists have been able to gather more information and reach new depths.

Habitats & Adaptations

Habitats in the Deep Zone of the ocean encompass a range of geological features, including deep-sea trenches, seamounts, hydrothermal vent systems, and abyssal plains. These diverse habitats offer specialized niches for various organisms, enabling them to exploit specific resources and evolve adaptations that allow them to thrive in the extreme conditions of their environment. The deep ocean, with its unique characteristics, presents a fascinating arena for studying the interplay between geology, biology, and adaptation.    

Learning Objectives:

• Explain the formation of hydrothermal vents, including the role of geothermal heating and the interaction of seawater with magma. 
• Explain the concept of symbiotic relationships and how organisms like tubeworms and mussels form mutually beneficial partnerships with chemosynthetic bacteria.
• Analyze how energy flows through the hydrothermal vent ecosystem via chemosynthesis and bioluminescence, and describe the roles of primary producers and consumers.

Next Generation Science Standards

Hydrothermal Vents

Hydrothermal vents are geothermally heated water discharges often found near volcanically active areas. Their chimney structures are created by sulfide-laden particles that emit from the cracks in the seafloor to cool and form vibrant communities in the open expanse. The critical change that transitions a simple vent into a place capable supporting of life is the composition of hydrothermally active fluid that is used by chemosynthetic organisms. 

These structures were discovered in 1977 when exploring an oceanic spreading ridge (mid-ocean ridge) near the Galapagos islands. This relatively new discovery is not completely understood and studied due to the difficult nature of obtaining samples and conducting experiments at that depth. But these communities are thriving with life, and scientists are finding new ways to explore the area to understand the ocean’s systems. 

Hydrothermal vents are formed when seawater percolates through the fissures in the ocean crust and reach magma. The seawater then rapidly heats up and rushes out the vents carrying minerals and metals to form sulfides outside. Due to the high pressure, the liquid does not boil but rather forms a less-dense stream of liquid flowing upwards. 

The two types of hydrothermal vents are black smokers and white smokers, differentiated by their chemical composition. Black smokers are formed by deposits of iron sulfide and emit a black smoke-like fluid containing iron oxide.

This fluid contains metals, unlike the white smokers which are made of barium, calcium, and silicon, forming a white-colored chimney and fluid. 

This extreme and complex chemical composition in the environment surrounding the vents supports a variety of deep-sea life, also known as extremophiles, such as hydrothermal vent worms which can grow to over six feet tall. These giant tubeworms are polychaetas (bristle worm) belonging to the phylum Annelida (segmented worms). Like other polychaetes, their offspring of free-swimming larvae are transported by currents until they reach a hydrothermal vent, where they form symbiotic relationships with chemosynthetic bacteria. However, scientists are still unsure of how tubeworms are able to travel and create colonies in the barren expanse. The most striking feature of these worms is the red plumes sprouting from the ends. These organs contain hemoglobin which binds with the hydrogen sulfide from vents to transfer to the chemosynthetic bacteria. Then, the chemosynthetic bacteria convert the compound into energy for itself and the worm.  

This symbiotic relationship is present in other organisms as well such as Bathymodiolus puteoserpentis, a type of mussel living at these locations. These mollusks contain symbiotic bacteria in their gills which are able to convert hydrogen into energy. These bacteria are similar to the ones in vent worms, or also deep-sea shrimp, containing an enzyme that is able to process this critical compound. 

These areas are still under research and are mostly done with ROVs, or remotely operated vehicles due to the extreme conditions. The immense pressure and low temperatures restrict humans from exploring but the robots allow us a glimpse of the world down below. In addition, scientists are seeking ways to protect these areas, which are critical to the processes of the ocean. 

Whalefalls 

Whalefalls are the fallen corpses of whales on the seafloor. This concentrated source of food attracts organisms from across the seafloor, who feast on this sudden abundance of food. In a few months, the soft tissue of the whale is consumed and broken up by scavengers such as giant isopods and sleeper sharks, leaving the bones bare and ready to be used by the other species. The following organisms such as certain types of tube worms and mollusks will colonize the hard surface of the bone and use the sulfides derivates from the decaying of this substrate. In a few years, or up to 10, these organisms will live on this surface and use their compounds to sustain themselves.

After all the material has been sued it, their lifecycles have been complete with their hundreds of planktonic offspring in the deep currents. This vicious cycle of chance will continue with the next generation, leading to the same cycle on another whale fall.

However, this host to deep-sea biodiversity is at risk. With the introduction of large-scale whaling in the 1800s, the number of natural deaths in whales are decreasing, with many being taken out of their ecosystem before they can complete their cycle. Due to this, many species are at risk, and processes that scientists are still currently researching are under threat. It is estimated that as much as 95% of whale fall habitats have been reduced due to whaling in the 19th and 20th centuries. However, with new regulations around noise pollution, ship collisions, and pollution, whales have the ability to make a comeback, bringing back the keystone species of an important ecosystem. 

Environmental Importance

Deep-sea currents, known as thermohaline circulation, move slowly along the ocean floor due to density differences caused by temperature and salinity variations. These currents follow the seafloor's topography and are vital for ocean circulation. Bottom trawling is a destructive fishing method that uses weighted nets to capture seafloor creatures. It damages habitats, causes bycatch of unintended species, and poses environmental challenges. Efforts have been made to mitigate the impact, such as turtle exclusion devices and policy measures like banning bottom trawling in certain depths. However, sustainable alternatives are needed to protect deep-sea biodiversity while addressing the demand for seafood.

Learning Objectives:

• Explain the concept of deep-sea currents and thermohaline circulation 
• Analyze the environmental impact of bottom trawling fishing on deep-sea ecosystems

Next Generation Science Standards

Deep Sea Currents

Currents flowing along the surface of the ocean are easily tracked with physical drifters released into the water or more advanced methods of acoustic profiling, surface temperature measurements, or color differences from phytoplankton. From these currents, merchants have mapped out routes to efficiently transport goods and travel between continents, but the deep ocean currents have not been studied to such an extent. 

Deep ocean currents or thermohaline circulation, span the entirety of the ocean basins, transporting cold water at slower speeds of two to ten centimeters per second compared to 250 centimeters per second of the Gulf Stream. In the deep ocean, these speeds are driven by the differences in density caused by water density differences, creating slower-moving currents that spread out in the bottom from melting hotspots. When the seawater above freezes, the denser salt precipitates sink to the bottom while the ice floats to the surface. This brine forms a continuous stream towards the bottom following the halocline until it reaches its appropriate depth. With this pattern, the water flows along the bottom following the topography of the seafloor, winding between underwater mountain ranges. The most famous example of these deep water masses is the North Atlantic Deep Water, which is a mixture of multiple other water masses in the ocean. Scientists use this stream as a standard of measurement for thermohaline circulation, as well as a bellwether for the conditions in deepwater formation in surrounding seas. 

Fish-Bottom Trawling

Bottom trawling is a method of fishing using a weighted net to capture the creatures dwelling around the seafloor. This net, kept open at the mouth with heavy trawl doors, contains a sweep on the bottom lip of the mouth, sweeping everything in its path into the net. With some of these nets reaching over 650 feet, the trawl footprint is huge, destroying all bottom habitats, and reducing the area to an inhospitable landscape for organisms. Closer to the surface, this is devastating for corals and seagrass beds, and in the deep, it destroys organic structures created by organisms such as mussels and seaweed. 

But a larger problem lies in the bycatch produced with these trawls. Without a method of sorting which organisms are to be caught, all organisms are caught, and when brought to the surface, they often die before being sorted and released back into the water. In the past, turtles were commonly caught in by shrimp trawlers as they dragged their wide nets through the water scooping up shrimp. However, sea turtles live in this environment as well, leading to them being caught in the net, and dragged to their death, due to not being able to return to the surface for air. Other organisms include sharks and rays, that also live near the seafloor. However, progress has been made in these fields, with the creation of turtle exclusion devices, the bycatch is able to escape, and thus reduce deaths for these organisms. In the US and Mexico, these devices have been required by law for shrimp trawlers, cementing a future of progress toward protecting these environments. 

Other policy measures have been made to protect these bottom communities in the Mediterranean Sea. In 2005, the General Fisheries Commission for the Mediterranean banned bottom trawling in waters in depths over 1000 meters. This monumental legislation protects the deep sea bed communities not yet fully understood by scientists and allows them to be protected, unlike the already overexploited shallow fisheries.  Especially important is that over 25% of Mediterranean aquatic plants are endemic to the waters, meaning their geographic distribution is limited to this small haven. In addition, this enactment will protect vulnerable seafloor communities such as seamounts, submerged mountains, submarine canyons, and cold-water corals. These easily disturbed communities may take years to return to their original state after a disturbance due to the low populations of keystone species and a wide dispersion of organisms in the deep. Economically, this change protects the future of the shrimp sector, protecting the nurseries of deep-water shrimp that rely on these features to grow into commercially viable catch. 

However, reducing the amount of bottom trawling would simply lead to an increase in other equally environmentally harmful methods of fishing. Alternatives such as gillnets, purse seine nets, or pots and traps have their own issues such as entanglement and increased bycatch, leading to the only solution of reducing fishing in these areas. The only currently viable method of retaining this biodiversity is phasing out the use of bottom trawling, and the development of truly sustainable fisheries. But with a growing population, the advancement in aquaculture and fish farms is slow to catch up. Currently, methods such as bottom trawling cannot be completely eliminated, and the biggest steps would be making strides toward creating alternative food sources or sustainable methods of harvesting these organisms. 

Activities

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