The vast majority of Channel Islands National Marine Sanctuary consists of open waters – three-dimensional habitats not associated with the seafloor. Water column (or pelagic habitats) are comprised of discrete portions of ocean waters categorized by variation among multiple factors, such as light penetration, temperature, oxygen concentration, and density. Based on variation among these factors the water column is divided into numerous vertical and horizontal sub-habitats.
Major vertical zones within the water column begin at the ocean surface with the microlayer, a fine film of organic molecules. Next, the photic zone, from the surface to a depth of approximately 201 meters (660 feet), is the portion of the water column in which there is sufficient light for photosynthesis. Within the photic zone there is an important temperature and density gradient called the pycnocline that separates warm, mixed surface water from cool, dense water below. The surface water may reach depths between approximately 40 to 101 meters (130 to 330 feet) or more. Below the photic zone lies the mesopelagic zone, from approximately 201 to 1006 meters (660 to 3,300 feet), and the bathypelagic zone, from approximately 1006 meters to 3505 meters (3,300 to 11,500 feet). Due to the location of sanctuary boundaries, water column habitats within the majority of the sanctuary do not extend deeper than the mesopelagic zone, although the southern reaches of the sanctuary boundary near the mouth of Santa Cruz Canyon (a submarine canyon between and offshore from southeastern Santa Rosa Island and southwestern Santa Cruz Island) approach bathypelagic depths. In general, horizontal variation in water column habitats occurs from the coast to the open ocean, within currents, at differing latitudes, and among gyres (Thorne-Miller 1999).
Pelagic organisms are highly diverse and many have interesting and unique traits. Pelagic organisms living in the water column are classified as either plankton (passive drifters moving with the water) or nekton (actively swimming organisms). Some of these organisms are found exclusively in the microlayer, others occupy it only for a part of their life history (e.g., as eggs and larvae), and still others are found in different water column zones. The photic zone represents the range limit of phytoplankton, microscopic marine plants requiring light to synthesize their food. Many of the organisms living in the mesopelagic and bathypelagic zones produce light biochemically for such purposes as attracting prey or disorienting predators. In general, the mesopelagic zone has the greatest species diversity of pelagic fish (Thorne-Miller 1999).
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