# Deep sea

This article describes the habitat of the deep sea. It is one of the sub-categories within the section dealing with biodiversity of marine habitats and ecosystems. It gives an overview about the characteristics, the bottom topography, the adaptations to the environment of the biota and the threats.

## Introduction

The deep sea floor represents the largest habitat on earth. It ranges from the edge of the continental shelf at 200m to the bottom of the ocean. At the edge of the continental shelf is the shelf break, where the gradient of the floor increases down the continental slope. Below the '''continental slope''' lies the continental rise, which has a more gently slope. At around 4,000 meters depth, the ocean floor is reached and extends over the ocean basins at depths of 5,000 meters on average. This is called the abyssal plain. The zone between the continental shelf and the abyssal plain is the bathyal zone. In some places, the sea floor drops again into elongated trenches with depths of 10 to 11 kilometers. This region is the hadal region. The ocean floor is interrupted by mountain chain known as the mid-oceanic ridge system. Other features on the ocean floor are seamounts and hydrothermal vents. [1]

Ocean zones [2]

## Characteristics

The temperature of the waters of the deep sea varies from 4°C to -1°C. Exceptions are the Mediterranean Sea with an average temperature of 13°C in the upper 4,000 meters and the Red Sea where the bottom water can reach temperatures up to 21.5°C at depths of 2,000 meters. The lowest temperatures are found in the Antarctic Ocean and are about -1.9°C. The salinity of the seawater is relatively constant. Below 2,000 meters, the salinity is close to 35 ‰ (34.8‰ ± 0.3 ‰) and at the very deepest layers it is 34.65‰. The oxygen concentrations are near saturation except in the oxygen minimum layer between 0.5 and 0.6 kilometers depth. In enclosed basins such as the Black Sea, the water is anoxic below 250 meters. When the deep water is further from its origin the oxygen concentration declines by metabolic processes. This is e.g. the case in the deep North Pacific. The most predictable physical variable is hydrostatic pressure. Per 10 meter the depth increases, the pressure increases by 1 atmosphere ($10^5$ Pa). [3] In the deep ocean, light is almost absent. It is a dark environment and the organisms are adapted to it. The circulation in the deep sea is explained in the article ocean circulation.

## Deep sea bottom

The bottom of the deep sea has several features that make the bottom a more diverse ecosystem. The main features are mid-oceanic ridges, hydrothermal vents, mud volcanoes, seamounts and cold seeps. But features such as canyons can also occur. Special forms of a diverse environment are the carcasses of large animals.

### Mid-oceanic ridges

A mid-oceanic ridge is an underwater mountain range and is formed by plate tectonics. It is characteristic of an oceanic spreading center. The uplifted sea floor results from convection currents which rise in the mantle as magma and emerge as lava, creating the new crust upon cooling. It is the boundary between two tectonic plates. The mid-oceanic ridges are connected and form a single global mid-oceanic ridge system that is part of every ocean. The rang is approximately 65,000 kilometers long and the total length of the system is 80,000 kilometers. It can be occupied by a rift valley that is formed by geological forces that pulled apart and broke the solid rock. This process is called faulting. They are active features characterized by frequent, shallow earthquakes, many faults and widespread volcanism. It is also segmented by transform faults, where fractured rocks slide past one another. This causes a zigzag pattern. Hydrothermal vents and cold seeps are frequently found on these mid-oceanic ridges. [4]

Mid-oceanic ridge [5]

### Hydrothermal vents

Due to the use of submersibles and more intensive acoustic mapping, unique and often diverse communities are found on features such as hydrothermal vents. These are islands in a sea of mud. The hydrothermal vents are associated with parts of the ocean floor that exhibit high levels of tectonic activity such as mid-oceanic ridges. In these regions, hot magma chambers occur near the seabed and heat up water that has permeated into the ocean floor. The water that has sinks down into cracks of the crust, are free of oxygen and potassium. After this step, calcium, sulfate and magnesium are also removed. When the water sinks deeper into the crust, calcium, sodium and potassium are added from the surrounding crust. At the deepest point, the fluids have reached their highest temperatures (350-400°C). Copper, zinc, iron and sulfur dissolve into the fluids. These hot fluids rise up and are released in the cold, oxygen-rich seawater at the sea bottom. The metals and sulfur combine to form black metal-sulfide minerals. [6] This type of hydrothermal vents is called a black smoker. White smokers also exist and are a little bit cooler.

Hydrothermal vents - Black smokers [7]

A huge biomass of associated organisms can be found. Because of the extreme conditions, the organisms need an alternative method of food supply. This production is autochthonous and related to the supply of reduced compounds from the vents. The primary production is generated by bacteria through chemosynthesis. The bacteria are chemoautotrophic and tend to be members of the most ancient Archaea. They can tolerate extremely high temperatures (called hyperthermophiles (80-115°C) and superthermophiles (>115°C)). Chemosynthetic bacteria can produce their own food from inorganic compounds without sunlight. They use energy derived from chemical reactions that involve substances such as ammonia ($NH_3$), sulfides ($S^{2-}$), nitrates ($NO_3^-$) and sulfates ($SO_4^{2-}$). The bacteria of the vents use sulfide ions and oxidize it to sulfur and sulfates. The energy produced by this process is used to produce food. [8]

Reaction:

$CO_2 + H_2S + O_2 + H_2O \rightarrow CH_2O + H_2SO_4$

### Mud volcanoes and seamounts

A mud volcano or mud dome is used to refer to formations created by geo-excreting fluids and gasses. The most abundant gas that is released is methane. Seamounts are mountains rising from the ocean floor that do not reach the water’s surface. They are typically formed from extinct volcanoes, which rise abruptly. It is estimated that there are 30,000 seamounts in the ocean, but only a few have been studied. They are hotspots of marine life. They can have their own localized tides, eddies and upwelling zones.

Distribution of seamounts [9]

### Cold seeps

Cold seeps are found along active and passive continental margins related to geological processes such as tectonically induced high-fluid pressures, petroleum or natural gas escape, catastrophic erosion and slides. The source of energy is methane-rich fluids of thermogenic and/or biogenic origin. But production of sulfide by sulfate reduction also plays an important role. Metanotrophic bacteria use methane to produce carbonates by methane reduction. The carbonates can react with calcium to form calciumcarbonate. Sulfate reducing bacteria use sulfate to form sulfides and this sulfide is oxidized by sulfur oxidizing bacteria and they release sulfate again into the seawater.

Overall reaction:

[itex] CH_4 + SO_4^{2-} \rightarrow HCO_3^- + HS^- + H_2O

Cold seep with tube worms and bivalves [10]

### Carcasses

Carcasses of large animals from the overlying water column sink to the bottom. They produce much localized food hot-spots. A similar food fall is the nutritious marine snow. They accumulate in tick patches separated by relatively barren areas.
1. Kaiser M. et al. 2005. Marine ecology: Processes, systems and impacts. Oxford University Press. p.584
2. Gage J.D. and Tyler P.A. 1991. Deep-sea biology – A natural history of organisms at the deep-sea floor. Cambridge University Press. p. 504
3. http://en.wikipedia.org/wiki/Mid-oceanic_ridge
4. http://en.wikipedia.org/wiki/Mid-oceanic_ridge
5. http://www.divediscover.whoi.edu/vents/vent-chemistry.html
6. http://www.mpi.org.au/campaigns/waste/deepsea
7. Kaiser M. et al. 2005. Marine ecology: Processes, systems and impacts. Oxford University Press. p.584
8. http://en.wikipedia.org/wiki/Seamount
9. NOAA