What would be a scientific, clear name for the soil (or sand, or silt, or rocks) underneath the water?

In a stream or lake or pond, etc (freshwater environments). I'm trying to write a paper on the density of organisms in the water in these habitats vs. in the first 6 inches or so of ground below the water, and I don't know what to call it.

What would be a scientific, clear name for the soil (or sand, or silt, or rocks) underneath the water?
Sediment core samples?
Reply:Benthic zone? (and there is no soil under water, it is called sediment). Benthic organisms live on the bottom; pelagic organisms live in the water column. Clams and worms are benthic organisms, most fish are pelagic.