Nepheloid layer

Nepheloid layer or nepheloid zone is a layer of water in the deep ocean basin, above the ocean floor, that contains significant amounts of suspended sediment.^[1] It is from 200 to 1000 m thick. The name comes from Greek: 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 of particles and turbulence of the current.

In a similar way, a surface nepheloid layer may be created, due to particle flotation, while intermediate nepheloid layers may be formed at the slopes of the ocean bed due to the dynamics of internal waves.

The existence of the nepheloid layer complicates bathymetry: one has to take into account the reflections of lidar or ultrasonic pulses from the upper interface of this layer, as well as their absorption within the layer.

Gulf of Mexico

A prominent nepheloid layer exists in the Gulf of Mexico extending from the delta of the Brazos River to South Padre Island. The layer of turbid 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. Due to the blockage of light by this nepheloid layer, algae and coral are sparse, resulting in an animal dominated community. This community is largely composed of infauna and consists of a detrital-based food chain. Many species of polychaete worms, amphipods, and brittle stars inhabit the benthic surface and can also be accompanied by some secondary consumers such as flounders, shrimp, crabs, and starfishes.

References

1. ^ Glossary of Geology, 5th Edition (American Geological Institute)