The term “pelagic” refers to the vast open water column of the ocean, from the surface down to the seafloor, excluding the bottom and coastal areas. This expansive aquatic environment represents Earth’s largest habitat.
What is the Pelagic Zone?
The pelagic zone defines the entire body of water in the ocean, extending from the surface to just above the seabed. It is distinct from the seafloor (benthic zone) and near-shore coastal regions (littoral zone). The term “pelagic” originates from the Ancient Greek word “pélagos,” meaning “open sea.” This zone covers over 70% of the Earth’s surface, representing an immense volume of water, estimated at 1.33 billion cubic kilometers. Many organisms spend their entire lives suspended here, never encountering land or the ocean floor.
Exploring the Pelagic Depths
The pelagic zone is vertically stratified into distinct layers, each with unique conditions influenced by light, temperature, and pressure.
Epipelagic Zone
The uppermost layer, the epipelagic zone, extends from the surface to 200 meters (660 feet) deep. Called the “sunlight zone,” it receives ample sunlight, allowing for photosynthesis and supporting most of the ocean’s primary production.
Mesopelagic Zone
Below this is the mesopelagic zone, or “twilight zone,” ranging from 200 to 1,000 meters (660 to 3,300 feet). Sunlight here is faint, insufficient for photosynthesis, and temperatures begin to drop. Many organisms in this zone exhibit bioluminescence.
Bathypelagic Zone
The bathypelagic zone, the “midnight zone,” extends from 1,000 to 4,000 meters (3,300 to 13,000 feet). This realm is in perpetual darkness, with consistently cold temperatures around 2-4°C and immense pressure.
Abyssopelagic Zone
The abyssopelagic zone, known as the “abyss,” spans from 4,000 to 6,000 meters (13,000 to 20,000 feet). It has near-freezing temperatures, complete darkness, and crushing pressures.
Hadalpelagic Zone
The deepest parts of the ocean, found within oceanic trenches, form the hadalpelagic zone, extending below 6,000 meters (20,000 feet) down to approximately 11,000 meters (36,000 feet). Pressure here is extreme, exceeding 1,000 times that at the surface, and temperatures remain just above freezing.
Life in the Open Ocean
Life in the pelagic zone is categorized into two main groups: plankton and nekton. Plankton are organisms that drift with ocean currents. This group includes microscopic phytoplankton, which form the base of marine food webs through photosynthesis, and zooplankton, small animals that feed on phytoplankton.
Nekton are free-swimming animals capable of moving independently through the water column. This group includes fish species like tuna and sharks, marine mammals such as whales and dolphins, and cephalopods like squid. Pelagic organisms have developed adaptations to survive in this vast environment. Many have streamlined bodies for efficient swimming and buoyancy control. Camouflage is common, with epipelagic species often exhibiting countershading (darker on top, lighter below) or transparency to blend with light. In deeper, darker zones, some animals are red or black, as these colors appear invisible in the absence of red light. Others use bioluminescence for communication, attracting prey, or evading predators.
The Pelagic Zone’s Global Importance
The pelagic zone plays a significant role in Earth’s ecological balance and global climate regulation. Phytoplankton within the sunlit epipelagic zone absorb vast amounts of carbon dioxide from the atmosphere through photosynthesis. This process removes greenhouse gases and produces at least 50% of the oxygen in our atmosphere.
The biological carbon pump, driven by pelagic organisms, transports carbon from surface waters to the deep ocean when organisms die and sink. This mechanism helps sequester carbon dioxide, influencing atmospheric concentrations. The pelagic zone forms the foundation of marine food webs, supporting a wide array of marine biodiversity, including commercially important fish species that sustain global fisheries.