13.1 Pelagic zones and their inhabitants

The ocean's pelagic realm is divided into distinct zones, each with unique characteristics and inhabitants. From the sunlit epipelagic to the dark bathypelagic, marine life has adapted to thrive in these diverse environments.

Vertical migration connects these zones, with organisms moving between depths daily. This process plays a crucial role in nutrient cycling and energy transfer throughout the water column, highlighting the interconnectedness of pelagic ecosystems.

Pelagic Zones

Zones of the pelagic realm

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• Epipelagic zone
  • Uppermost layer of the ocean extending from the surface to about 200 meters deep
  • Receives the most sunlight allowing for high primary productivity by phytoplankton (diatoms, dinoflagellates)
  • Supports a diverse array of marine life including zooplankton (copepods, krill), fish (tuna, sardines), and marine mammals (dolphins, whales)
• Mesopelagic zone
  • Extends from 200 to 1,000 meters deep with diminishing light levels and decreasing water temperature
  • Home to organisms adapted to low-light conditions such as lanternfish, hatchetfish, and squid
  • Serves as a daytime refuge for vertically migrating species (zooplankton, small fish) to avoid predation
• Bathypelagic zone
  • Extends from 1,000 to 4,000 meters deep characterized by complete darkness, high pressure, and cold, stable temperatures around 4°C
  • Inhabited by organisms adapted to extreme conditions such as anglerfish, viperfish, and gulper eels
  • Plays a role in nutrient cycling and carbon sequestration through the biological pump as dead organisms and fecal pellets sink to the seafloor

Adaptations in pelagic organisms

• Epipelagic zone organisms
  • Adaptations for high light levels and visual predation such as countershading (dark on top, light on bottom) and transparency (jellyfish, salps)
  • Examples: sardines, anchovies, and jellyfish
• Mesopelagic zone organisms
  • Adaptations for low-light conditions such as bioluminescence (lanternfish) and large, sensitive eyes
  • Reduced swim bladders to manage buoyancy in high-pressure environments
  • Examples: lanternfish, hatchetfish, and vampire squid
• Bathypelagic zone organisms
  • Adaptations for complete darkness and high pressure such as reduced eyes or lack of eyes (anglerfish)
  • Energy-conserving metabolic rates and opportunistic feeding strategies (gulper eels)
  • Examples: anglerfish, viperfish, and gulper eels

Vertical Migration and Pelagic Ecosystems

Vertical migration in ecosystems

• Daily movement of organisms between different depths in the water column
• Many species, particularly zooplankton (copepods, krill) and small fish (lanternfish), migrate to the epipelagic zone at night to feed and return to the mesopelagic zone during the day to avoid predation
• Connects different pelagic zones, facilitating the transfer of energy and nutrients
• Migrating organisms play a crucial role in the biological pump by transporting carbon and other nutrients from the surface to deeper waters through their fecal pellets and dead bodies

Pelagic organisms in marine systems

• Phytoplankton (diatoms, dinoflagellates) in the epipelagic zone form the base of most marine food webs, converting sunlight into organic matter through photosynthesis
• Zooplankton (copepods, krill) graze on phytoplankton and are consumed by larger organisms like fish (sardines, anchovies) and whales (blue whales, humpback whales)
• Pelagic fish (tuna, swordfish) are important predators in open-ocean food webs and support commercial fisheries
• Contribute to biogeochemical cycles through processes like the biological pump, which transports carbon dioxide from the atmosphere to the deep ocean
• Fecal pellets and dead bodies of pelagic organisms sink to the seafloor, providing a food source for benthic communities (crabs, sea cucumbers) and playing a role in nutrient cycling