#dysphotic

The world’s oceans are an infinitely complex assemblage of structures and processes. In order to try to make sense of them, some simple divisions are needed. Oceans occupy depressions, or basins, in Earth’s surface. The margins of these basins are an extension of the adjacent landmasses. As a result, there is a continental shelf at the margins of ocean basins and this is covered by relatively shallow seas. Most ocean life is formed in this zone.

The outer most edge of the continental shelf slopes down steeply to the much deeper ocean floor and the oceanic crustal plate. In this deep part of the oceanic basin, tectonic-plate processes have shaped the topography of the sediment-covered sea floor. This ranges from long, rugged ridges to enormous expanses of flat abyssal plains with even deeper trenches and arcs of volcanic islands at geologically active margins (check out the amazing Nat Geo documentary link at the bottom). Huge current systems, driven by solar heating, transport heat and chemicals around Earth’s oceans, and affect the weather over the oceans and on land (check out the link ‘perpetual Ocean”).

Oceans are a three-dimensional environment and the specific terminology used to describe the different parts of the oceans reflects this. The most basic distinction is between seafloor features and processes that are described as benthic, and those in the water itself, or water column, which are called pelagic features or processes. Moving horizontally out from the shore, the only boundary is at the top of the continental slope. The shallower inshore portion is known as the neritic zone, while the zone beyond this point is known as the oceanic realm. Within the neritic zone, the littoral zone occupies the shore between the tides. This is bordered by the supralittoral above high water and sublittoral below low tide. The zone from high water to the shelf break is the subneritic.

Within the water column, the pelagic zone has several layers. In the surface epipelagic (sunlight) zone there is sufficient light for photosynthesis. In the mesopelagic (twilight) zone surface light starts to disappear. In the dysphotic zone, animals can see in available light, but plants cannot photosynthesize. No surface light reaches the dark bathyal (midnight) and abyssal zones, or the hadal zone in the deepest trenches. Check out the links for more specific information on zones and awesome documentaries.

~ JM

Photo Credit: www.seasky.org

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