1. Mesopelagic Environment
2. Overview of Mesopelagic Food Webs
3. Mesopelagic Production
4. Mesopelagic Decomposition
5. Mesopelagic Zooplankton
6. Mesopelagic Nekton

Mesopelagic Zone

1. Also called the midwaters or the twilight zone
2. 200 to 1,000 meters
3. Disphotic
4. Temperatures decline
5. Salinity stabilizes
6. Dissolved oxygen concentration declines to minimum
7. Pressure rises (to 100 atm)
8. Nitrogen and phosphorus concentrations rise to maximum

Mesopelagic Food Webs

1. Shift to detritus based food webs
2. No photosynthesis
3. Bacterial decomposers abundant
4. About 20 percent of food produced in the epipelagic zone reaches the midwaters as marine snow
5. Zooplankton feed on detritus and migrate upwards for epipelagic phytoplankton
6. Biolumenescent krill and shrimp are important
7. Nekton of midwater fishes and squid
8. Bristlemouths and lanternfish are important

No Production

1. Not enough light for photoautotrophs
2. The mesopelagic zone is below the compensation depth for phytoplankton
   • The compensation depth is the depth where the light intensity supports enough gross production (GP) to equal respiration (R) by the phytoplankton.
   • At the compensation depth net production (NP) is equal to zero.
   • Below the compensation depth the energy needs of phytoplankton are not met and the phytoplankton die.

Bacterial Decomposition

1. Marine snow drifting down from the epipelagic is decomposed by bacteria in the mesopelagic
2. The detrital snow is an important food component for mesopelagic zooplankton and fishes
3. The decomposition uses up oxygen which can only be replenished by water sinking from above

Oxygen Minimum Layer

1. The oxygen minimum layer is a fairly well defined layer of water in the middle of the mesopelagic that has been depleted of oxygen
2. Respiration by animals, protozoa, and bacteria use up oxygen
3. There is relatively little oxygen entering the layer because it is cut off from sources of fresh oxygen such as photosynthesis and the atmosphere

Below the Oxygen Minimum Layer

1. Below the oxygen minimum layer there is very little food left so respiration declines
2. Oxygen is supplied from below because of deep thermohaline currents
3. Dense, cold, salty, oxygen rich water sinks from the surface at the polar regions and spreads throughout the ocean basins restoring oxygen levels below the oxygen minimum layer

Holoplankton

1. Make up a greater percentage of the midwater zooplankton in numbers and species
2. Copepods, krill, and arrow worms are abundant as in the epipelagic
3. A variety of shrimps are relatively more common
4. Ostracods a tiny bivalved crustacean can be very abundant
5. Many species have bioluminescent organs called photophores

Diurnal Vertical Migrations

1. Many midwater zooplankton species rise at night and sink by day

Deep Scattering Layer (DSL)

1. Made up of dense concentrations of midwater zooplankton, fish and squid
2. Cause of false bottom soundings during the early use of sonar for depth sounding during WWII
3. There are migratory and non-migratory DSLs

Types of Mesopelagic Nekton

1. Cephalopods are abundant: squids, octopi and vampire squid
2. Small fishes are abundant: Bristlemouths & Lanternfish account for 90% of fishes

Adaptations of Mesopelagic Nekton

1. Darkly pigmented: silvery to black countershading
2. Biolumenescent photophores usually located along ventrum for counterillumination, silhouette reduction, confusing displays and communication

Sensory Adaptations

1. Large eyes adapted to dim light
2. Tubular shaped eyes for acute, upward tunnel vision and less acute periperal vision
3. Elongate bodies and sensitive lateral lines

Adaptations to Limited Food Supply

1. Small size
2. Broad diets
3. Needle-like teeth and hinged, extendable jaws

Mesopelagic Cephalopods

1. Patterns of photophores
2. Darkly pigmented
3. Webbing between tentacles

Vampire Squid

1. Not a squid or an octopus
2. Has ten arms but two are retractable feelers
3. Up to 8 inches in length

Bristlemouths

1. Long bristle-like teeth
2. Rows of photophores on ventrum
3. Most abundant fish on earth

Lanternfish

1. Photophores on head and body
2. Blunt heads, large mouths & large eyes
3. About 4 inches long
4. Migrate vertically

Other Mesopelagic Fishes

1. Hatchetfish-large upturned eyes and mouths, hatchet shape & ventral photophores
2. Dragonfish-long chin barbel with photophore at tip, elongate body
3. Lancetfish-sail-like dorsal fin, elongate body
4. Barreleyes-tubular, upturned eyes atop barrel-shaped cylinders

Migration of Mesopelagic Fishes

1. Migrating fish go to the surface at night while non-migrators stay at depth

Migrators

1. Have swim bladders
2. Well developed bones and muscles
3. Wide temperature and pressure tolerances
4. ex. bristlemouths

Non-migrators

1. Lack swim bladders
2. Have weak bones and flabby muscles
3. Lack streamlining
4. ex. dragonfish