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"pertaining to the sea," 1650s, from Latin pelagicus, from Greek pelagikos, from pelagos "sea, high sea, open sea, main," from PIE *pelag- "to spread out" (cf. Greek plagos "side," Latin plaga "hunting net, curtain, region"), possibly from root *plak- (1) "to spread out, be flat" (see placenta).
Relating to or living in or on oceanic waters. The pelagic zone of the ocean begins at the low tide mark and includes the entire oceanic water column. The pelagic ecosystem is largely dependent on the phytoplankton inhabiting the upper, sunlit regions, where most ocean organisms live. Biodiversity decreases sharply in the unlit zones where water pressure is high, temperatures are cold, and food sources scarce. Pelagic waters are divided, in descending order, into the epipelagic, mesopelagic, bathypelagic, abyssopelagic, and hadopelagic zones.