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Notes 12.1
Midwaters
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Directions
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Introduction
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Midwaters
- Mesopelagic
Environment
- Overview
of Mesopelagic Food Webs
- Mesopelagic
Production
- Mesopelagic
Decomposition
- Mesopelagic
Zooplankton
- Mesopelagic
Nekton
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Mesopelagic
Environment
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Mesopelagic Zone
- Also called the midwaters
or the twilight zone
- 200 to 1,000 meters
- Disphotic
- Temperatures decline
- Salinity stabilizes
- Dissolved oxygen concentration
declines to minimum
- Pressure rises (to
100 atm)
- Nitrogen and phosphorus
concentrations rise to maximum
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Overview
of Mesopelagic Food Webs
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Mesopelagic Food Webs
- Shift to detritus
based food webs
- No photosynthesis
- Bacterial decomposers
abundant
- About 20 percent of
food produced in the epipelagic zone reaches the midwaters as marine
snow
- Zooplankton feed on
detritus and migrate upwards for epipelagic phytoplankton
- Biolumenescent krill
and shrimp are important
- Nekton of midwater
fishes and squid
- Bristlemouths and
lanternfish are important
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Mesopelagic
Production
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No Production
- Not enough light for
photoautotrophs
- 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.
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Mesopelagic
Decomposition
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Bacterial Decomposition
- Marine snow drifting
down from the epipelagic is decomposed by bacteria in the mesopelagic
- The detrital snow
is an important food component for mesopelagic zooplankton and fishes
- The decomposition
uses up oxygen which can only be replenished by water sinking from
above
Oxygen Minimum Layer
- The oxygen minimum
layer is a fairly well defined layer of water in the middle of the
mesopelagic that has been depleted of oxygen
- Respiration by animals,
protozoa, and bacteria use up oxygen
- 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
- Below the oxygen minimum
layer there is very little food left so respiration declines
- Oxygen is supplied
from below because of deep thermohaline currents
- 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
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Mesopelagic
Zooplankton
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Holoplankton
- Make up a greater
percentage of the midwater zooplankton in numbers and species
- Copepods, krill, and
arrow worms are abundant as in the epipelagic
- A variety of shrimps
are relatively more common
- Ostracods a tiny bivalved
crustacean can be very abundant
- Many species have
bioluminescent organs called photophores
Diurnal Vertical Migrations
- Many midwater zooplankton
species rise at night and sink by day
Deep Scattering Layer
(DSL)
- Made up of dense concentrations
of midwater zooplankton, fish and squid
- Cause of false bottom
soundings during the early use of sonar for depth sounding during
WWII
- There are migratory
and non-migratory DSLs
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Mesopelagic
Nekton
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Types of Mesopelagic
Nekton
- Cephalopods are abundant:
squids, octopi and vampire squid
- Small fishes are abundant:
Bristlemouths & Lanternfish account for 90% of fishes
Adaptations of Mesopelagic
Nekton
- Darkly pigmented:
silvery to black countershading
- Biolumenescent photophores
usually located along ventrum for counterillumination, silhouette
reduction, confusing displays and communication
Sensory Adaptations
- Large eyes adapted
to dim light
- Tubular shaped eyes
for acute, upward tunnel vision and less acute periperal vision
- Elongate bodies and
sensitive lateral lines
Adaptations to Limited
Food Supply
- Small size
- Broad diets
- Needle-like teeth
and hinged, extendable jaws
Mesopelagic Cephalopods
- Patterns of photophores
- Darkly pigmented
- Webbing between tentacles
Vampire Squid
- Not a squid or an
octopus
- Has ten arms but two
are retractable feelers
- Up to 8 inches in
length
Bristlemouths
- Long bristle-like
teeth
- Rows of photophores
on ventrum
- Most abundant fish
on earth
Lanternfish
- Photophores on head
and body
- Blunt heads, large
mouths & large eyes
- About 4 inches long
- Migrate vertically
Other Mesopelagic
Fishes
- Hatchetfish-large
upturned eyes and mouths, hatchet shape & ventral photophores
- Dragonfish-long chin
barbel with photophore at tip, elongate body
- Lancetfish-sail-like
dorsal fin, elongate body
- Barreleyes-tubular,
upturned eyes atop barrel-shaped cylinders
Migration of Mesopelagic
Fishes
- Migrating fish go
to the surface at night while non-migrators stay at depth
Migrators
- Have swim bladders
- Well developed bones
and muscles
- Wide temperature and
pressure tolerances
- ex. bristlemouths
Non-migrators
- Lack swim bladders
- Have weak bones and
flabby muscles
- Lack streamlining
- ex. dragonfish
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