<Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> The benthic zone is the ecological region at the lowest level of a body of water such as an ocean or a lake, including the sediment surface and some sub-surface layers . Organisms living in this zone are called benthos, e.g., the benthic invertebrate community, including crustaceans and polychaetes . The organisms generally live in close relationship with the substrate bottom and many are permanently attached to the bottom . The superficial layer of the soil lining the given body of water, the benthic boundary layer, is an integral part of the benthic zone, as it greatly influences the biological activity that takes place there . Examples of contact soil layers include sand bottoms, rocky outcrops, coral, and bay mud . </P> <P> The benthic region of the ocean begins at the shore line (intertidal or littoral zone) and extends downward along the surface of the continental shelf out to sea . The continental shelf is a gently sloping benthic region that extends away from the land mass . At the continental shelf edge, usually about 200 meters deep, the gradient greatly increases and is known as the continental slope . The continental slope drops down to the deep sea floor . The deep - sea floor is called the abyssal plain and is usually about 4,000 meters deep . The ocean floor is not all flat but has submarine ridges and deep ocean trenches known as the hadal zone . </P> <P> For comparison, the pelagic zone is the descriptive term for the ecological region above the benthos, including the water - column up to the surface . Depending on the water - body, the benthic zone may include areas that are only a few inches below water, such as a stream or shallow pond; at the other end of the spectrum, benthos of the deep ocean includes the bottom levels of the oceanic abyssal zone . </P>

What happens in a photic part of a body of water apex