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Anepheloid layer ornepheloid zone is a layer of water in thedeep sea basin, above theseabed, that contains significant amounts of suspendedsediment.[1] It is from 200 to 1000 m thick. The name comes fromGreek: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.[2] Its thickness depends on bottom current velocity and is a result of balance betweengravitational settling 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 measuringbathymetry as well as affect the types of marine life in an area. There are several significant examples of nepheloid layers across the globe, including within theGulf of Mexico and thePorcupine Bank.
Asurface nepheloid layer (SNL) may be created, due to particle flotation, whileintermediate nepheloid layers (INL) may be formed at the slopes of the ocean bed due to the dynamics ofinternal waves. These intermediate nepheloid layers are derived frombottom nepheloid layers (BNL) after the layers become detached and spread alongisopycnal surfaces.[3]
Open ocean convection has a prominent effect on the distribution of nepheloid layers and their ability to form in certain areas of the ocean, such as the northernAtlantic Ocean and the northwesternMediterranean Sea.[4] Nepheloid layers are more likely to form based on patterns of deep ocean circulation that directly affect theabyssal plain.[5] 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 viaocean gyres also affect the concentration and relative sizes of the suspended sediments, and by extension the area's correspondingbiotic activity.
The existence of the nepheloid layer complicatesbathymetric measurements: one has to take into account the reflections oflidar orultrasonic pulses from the upper interface of this layer, as well as their absorption within the layer.[3] Interference from the thick layers of suspended sediments can ultimately produce inaccurate results concerning submarinetopography.
Depending on the characteristics of a particular nepheloid layer, they can have a significant impact on marine life in the area.[6] The layers of sediments can block natural light, making it difficult forphotosynthetic organisms to survive. In addition, the suspended particulates can harmfilter feeding organisms andplankton by blocking their gills or weighing them down.
A prominent nepheloid layer exists in theGulf of Mexico extending from the delta of theBrazos River toSouth Padre Island.[2] The layer ofturbid 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 highturbidity. Due to the blockage of light by this nepheloid layer,algae andcoral are sparse, resulting in an animal-dominated community. This community is largely composed ofinfauna and consists of adetrital-based food chain.[7] Many species ofpolychaete worms,amphipods, andbrittle stars inhabit thebenthic surface and can also be accompanied by some secondary consumers such asflounders,shrimp,crabs, andstarfishes.
A prominent nepheloid layer exists in thePorcupine Bank.[8] Geographically, the nepheloid layers are more detectable and prominent along the Porcupine Bank's western slope.[6] Both the bottom and intermediate nepheloid layers form due to a myriad of factors such asinternal tides, waves, and subsequent bottomerosion. 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, andsalinity.