The open ocean Flashcards
What defines the oceanic (pelagic) zone?
It is the open ocean area above the deep sea, beyond the continental shelf break.
Organisms live and move in 3D with no place to hide, burrow, or anchor.
Why is variability a defining feature of the pelagic zone?
Because of patchiness in physical properties, biological production, and biomass
What is the epipelagic zone?
The surface to 200 m “sunlit” zone where photosynthesis occurs and most ocean life depends on its productivity
Also known as euphotic zone
What is the mesopelagic zone?
The “twilight” zone with only ~1% of surface light and epi-meso transition marked by a thermocline
Limited photosynthesis and subject to thermal stratification
What defines the bathypelagic zone?
The “midnight” zone where no light penetrates and temperature is a constant ~4°C
What is the abyssopelagic zone?
A deep-sea zone covering ~40% of Earth’s surface with near-constant conditions
Fuelled by marine snow, most life benthic
hat is the hadalpelagic zone?
The deepest ocean zone (6,000+ m), including Challenger Deep at ~11,022 m
Located in deep sea trenches
How does light affect life in the pelagic zone?
It enables photosynthesis in the upper layers but also causes UV damage; deeper zones have no light.
What challenges do organisms face with depth in the pelagic zone?
Increasing pressure, reduced oxygen levels (from bacterial decomposition), and temperature shifts
What are plankton?
Drifting organisms, including phytoplankton (plants) and zooplankton (animals), that are moved by currents
What are nekton?
Free-swimming marine organisms like fish, mammals, and squid
Actively move, independently of currents
Why are phytoplankton important?
They account for over 95% of oceanic photosynthesis
Crucial for global oxygen production and food webs
What’s the difference between holoplankton and meroplankton?
Holoplankton spend their entire life as plankton; meroplankton only part of their life
E.g many larvae meroplankton
What is diel vertical migration (DVM)?
A daily movement of zooplankton to avoid predators and UV light
Largest daily migration on earth by biomass
Why are epipelagic food webs complex?
Organisms often feed across multiple trophic levels, and roles change with life stage and location
How does primary productivity influence epipelagic organism abundance?
It directly determines the availability of energy and food
More productivity = more biomass
Why is flotation a challenge in the pelagic zone?
Organisms are denser than seawater, so they risk sinking
Name three strategies marine organisms use to stay afloat
- Continuous swimming
- Increasing water resistance
- Increasing buoyancy
How do plankton increase buoyancy?
By storing lipids (e.g., oil droplets) or regulating body gases
Some excrete heavy ions like sulfate in favour of lighter ions
What is the role of water resistance in flotation?
More surface area increases drag, helping organisms float more easily
How do nekton maintain buoyancy?
Via swim bladders (in fish), oily livers (in sharks), or blubber (in marine mammals)
