In section 1.2 we learned about the regions that make up the continental margins. So before we leave this topic, we will look at some of the other ways we can categorize the ocean environments.
The first major distinction is between the pelagic and benthic zones. The pelagic zone refers to the water column, where swimming and floating organisms live. The benthic zone refers to the bottom, and organisms living on and in the bottom are known as the benthos.
The pelagic zone is divided into two provinces: the neritic province corresponds to all of the water from the low tide line to the shelf break, while the oceanic province represents all of the other water in the open ocean regions.
The oceanic province is divided into depth zones (Figure 1.3.1):
- 0-200 m is the epipelagic zone (“epi” = “upon”, as in on top of the pelagic zone). This is the region where enough light penetrates the water to support photosynthesis (see section 7.3), so it is also called the euphotic or photic zone.
- 200-1000 m is the mesopelagic zone (“meso” = “middle”). There is some light here, but not enough for photosynthesis, so it is called the dysphotic zone, or the twilight zone.
- 1000-4000 m is the bathypelagic zone (“bathy” = “deep”). There is no light at these depths, so it is referred to as the aphotic zone. About 75% of the living space in the ocean lies at these depths or deeper.
- 4000-6000 m is the abyssopelagic or abyssalpelagic zone, which extends to the seafloor in most areas.
- 6000 m and below is the hadopelagic or hadalpelagic zone (named for Hades or “hell”). This refers to the water in deep ocean trenches.
Inhabitants of these regions are referred to according to their habitat, for example mesopelagic fish, or epipelagic squid.
The benthic environment is also divided into zones, most of which correspond to the pelagic divisions:
- The supralittoral zone lies above the high tide line. Also called the spray zone, it is only submerged during storms or unusually high waves.
- The littoral zone is the region between the high and low tides. Thus it is also referred to as the intertidal zone.
- Below the littoral zone is the sublittoral (shelf) zone, extending from the low tide mark to the shelf break, essentially covering the continental shelf.
- The bathyal zone extends along the bottom from the shelf break to 4000m, so it generally includes the continental slope and rise.
- The abyssal zone is found between 4000-6000 m, including most of the abyssal plains. The abyssal zone represents about 80% of the benthic environment.
- The hadal zone includes all benthic regions deeper than 6000 m, such as in the bottom of trenches.