A nepheloid layer or nepheloid zone is a layer of water in the

deep ocean The deep sea is broadly defined as the ocean depth where light begins to fade, at an approximate depth of 200 metres (656 feet) or the point of transition from continental shelves to continental slopes. Conditions within the deep sea are a combin ...

basin, above the

ocean floor The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...

, that contains significant amounts of suspended

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...

. It is from 200 to 1000 m thick. The name comes from

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

: ''nephos'', "cloud". The particles in the layer may come from the upper ocean layers and from stripping the sediments from the ocean floor by currents. Its thickness depends on bottom current velocity and is a result of balance between

gravitational settling Radiative levitation is the name given to a phenomenon that causes the spectroscopically-derived abundance of heavy elements in the photospheres of hot stars to be very much higher than solar abundance or than the expected bulk abundance; for examp ...

of particles and turbulence of the current. The formation mechanisms of nepheloid layers may vary, but primarily depend on deep ocean convection. Nepheloid layers can impact the accuracy of instruments when measuring bathymetry as well as affect the types marine life in an area. There are several significant examples of nepheloid layers across the globe, including within the

Gulf of Mexico The Gulf of Mexico ( es, Golfo de México) is an oceanic basin, ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North American continent. It is bounded on the northeast, north and northwest by the Gulf Coast of ...

and the

Porcupine Bank Porcupine Bank is an area of the Irish shelf, on the fringes of the Atlantic Ocean approximately west of Ireland. The relatively raised area of seabed, 200 m below sea level at its highest, lies between the deep-water Porcupine Seabight and Rock ...

Formation mechanisms

A surface nepheloid layer (SNL) may be created, due to particle flotation, while intermediate nepheloid layers (INL) may be formed at the slopes of the ocean bed due to the dynamics of

internal wave Internal waves are gravity waves that oscillate within a fluid medium, rather than on its surface. To exist, the fluid must be stratified: the density must change (continuously or discontinuously) with depth/height due to changes, for example, in ...

s. These intermediate nepheloid layers are derived from bottom nepheloid layers (BNL) after the layers become detached and spread along

isopycnal Isopycnals are layers within the ocean that are stratified based on their densities and can be shown as a line connecting points of a specific density or potential density on a graph. Isopycnals are often displayed graphically to help visualize " ...

surfaces.

Open ocean convection Open ocean convection is a process in which the mesoscale ocean circulation and large, strong winds mix layers of water at different depths. Fresher water lying over the saltier or warmer over the colder leads to the stratification of water, or i ...

has a prominent effect on the distribution of nepheloid layers and their ability to form in certain areas of the ocean, such as the northern

Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...

and the northwestern

Mediterranean Sea The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the ea ...

. Nepheloid layers are more likely to form based on patterns of deep ocean circulation that directly affect the

abyssal plain An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between and . Lying generally between the foot of a continental rise and a mid-ocean ridge, abyssal plains cover more than 50% of the Earth's surface. T ...

. This is largely through the disruption of accumulated sediments in areas that deep ocean currents interact with. Convection currents that disturb areas of the ocean floor such as those that circulate via

ocean gyre In oceanography, a gyre () is any large system of circulating ocean currents, particularly those involved with large wind movements. Gyres are caused by the Coriolis effect; planetary vorticity, horizontal friction and vertical friction determine ...

s also affect the concentration and relative sizes of the suspended sediments, and by extension the area's corresponding biotic activity.

Impacts

Bathymetry

The existence of the nepheloid layer complicates

bathymetric Bathymetry (; ) is the study of underwater depth of ocean floors (''seabed topography''), lake floors, or river floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The first recorded evidence of water de ...

measurements: one has to take into account the reflections of

lidar Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. It can also be ...

ultrasonic Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies fr ...

pulses from the upper interface of this layer, as well as their absorption within the layer. Interference from the thick layers of suspended sediments can ultimately produce inaccurate results concerning submarine

topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sci ...

Marine life

Depending on the characteristics of a particular nepheloid layer, they can have a significant impact on marine life in the area. The layers of sediments can block natural light, making it difficult for

photosynthetic Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in c ...

organisms to survive. In addition, the suspended particulates can harm

filter feeding Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...

organisms and

plankton Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a Ocean current, current (or wind). The individual organisms constituting plankton are called plankt ...

by blocking their gills or weighing them down.

Examples

Gulf of Mexico

A prominent nepheloid layer exists in the

extending from the delta of the

Brazos River The Brazos River ( , ), called the ''Río de los Brazos de Dios'' (translated as "The River of the Arms of God") by early Spanish explorers, is the 11th-longest river in the United States at from its headwater source at the head of Blackwater Dr ...

South Padre Island South Padre Island is a barrier island in the U.S. state of Texas. The remote landform is located in Cameron County, Willacy County, and accessible by the Queen Isabella Causeway. South Padre Island was formed when the creation of the Port Mans ...

. The layer of

turbid Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality. Fluids can ...

water can begin as shallow as 20 meters and is caused mostly by clay run-off from multiple rivers. The silty bottom of the gulf also contributes to the high

turbidity Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality. Fluids can ...

. Due to the blockage of light by this nepheloid layer,

algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...

and

coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and sec ...

are sparse, resulting in an animal-dominated community. This community is largely composed of

infauna Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as '' biota''. Zoo ...

and consists of a

detrital Detritus (; adj. ''detrital'' ) is particles of rock derived from pre-existing rock through weathering and erosion.Essentials of Geology, 3rd Ed, Stephen Marshak, p G-7 A fragment of detritus is called a clast.Essentials of Geology, 3rd Ed, Stephen ...

-based food chain. Many species of

polychaete Polychaeta () is a paraphyletic class (biology), class of generally marine invertebrate, marine annelid worms, common name, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that ...

worms,

amphipods Amphipoda is an order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. Amphipods range in size from and are mostly detritivores or scavengers. There are more than 9,900 amphipod species so far descri ...

, and

brittle stars Brittle stars, serpent stars, or ophiuroids (; ; referring to the serpent-like arms of the brittle star) are echinoderms in the class Ophiuroidea, closely related to starfish. They crawl across the sea floor using their flexible arms for locomo ...

inhabit the benthic surface and can also be accompanied by some secondary consumers such as flounders,

shrimp Shrimp are crustaceans (a form of shellfish) with elongated bodies and a primarily swimming mode of locomotion – most commonly Caridea and Dendrobranchiata of the decapod order, although some crustaceans outside of this order are refer ...

crabs Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen) ( el, βραχύς , translit=brachys = short, / = tail), usually hidden entirely under the thorax. They live in all the ...

, and starfishes.

Porcupine Bank

A prominent nepheloid layer exists also in the

.Dickson RR, McCave IN (1986) ''Nepheloid layers on the continental slope west of Porcupine Bank''. Deep Sea Res 33:791–818 Geographically, the nepheloid layers are more detectable and prominent along the Porcupine Bank's western slope. Both the bottom and intermediate nepheloid layers form due to a myriad of factors such as

internal tides Internal tides are generated as the surface tides move stratified water up and down sloping topography, which produces a wave in the ocean interior. So internal tides are internal waves at a tidal frequency. The other major source of internal w ...

, waves, and subsequent bottom

erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is distin ...

. The intermediate nepheloid layer can also manifest by breaking off from the bottom layer, and the water column above the area in which the bottom nepheloid layer forms is marked by significant differences in temperature, density, and

salinity Salinity () is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal ...

References

{{reflist Oceanography Geology