Nepheloid layer, sediment

The lower water column in most parts of the ocean, both shelf waters and the deep sea, shows a large increase in light scattering and attenuation conferred by the presence of increased amounts of fine-grained particulate material. This has been confirmed by size measurements and filtration of seawater with gravimetric analysis. This part of the water column is termed the bottom nepheloid layer (BNL) (Figure N3). Optical work shows that the BNL is up to 2000 m thick and generally has a basal uniform region, the bottom mixed nepheloid layer, corresponding quite closely to the bottom mixed layer defined by constant potential temperature. Above this light scattering falls off more or less exponentially to the clear-water minimum marking the top of the BNL.

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