Suspension Feeding

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Suspension Feeding

• Mostly mechanics and foraging theory

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Terms

• Definition feeding on particles by removing
  them from suspension
• Active create own feeding currents
• Passive use ambient fluid motion and (or)
  gravity
• Beware of classifications/dichotomies
• Nobody make a living by swallowing seawater.
  Good particles must be concentrated.

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Suspension, like Deposit, Feeding Says How, not
What

• Phytoplankton
• Detritus
• Bacteria
• Protists
• Animals
• More than one of the above

• One or more from column A, plus something
  acquired in another feeding mode (e.g., deposit
  feeding or osmotrophy)

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Passive Active SF
Many benthic phyla
Many benthic phyla
In the plankton, thecosome pteropods
In the plankton, copepods, salps, some fishes,
etc.
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More terms and concepts

• BEWARE of volume filtered or volume cleared.
  
• It comes from the practice of measuring C(t). If
  you know C (x) and C (x y), then from the
  geometric mean concentration over the interval y
  and the time y and the volume of the experimental
  container you can calculate what volume has been
  cleared of cells in that time.
• Just because you can do the calculation does not
  mean that the animal actually filters that volume.

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Where to forage

• Planktonic suspension feeders, where C is high,
  but observation is that they forage where
  production is high.
• Benthic suspension feeders where Cxu is high
• Benthic suspension feeders cant chase patches

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What Particles to Take

• For feeders on living organisms, take particles
  larger than the mean size
• For detritivores, take smaller particles and ones
  lower in specific gravity
• If sorting is moderately expensive, show partial
  preference
• Sorting may be an issue for benthic suspension
  feeders that experience high concentrations of
  poor foods

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How Fast to Feed
Recall the digestion lectures and reprints
Filter only fast enough to keep the gut full
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Cautions

• Aerosol filtration theory (Rubenstein and Koehl
  1977) has a different goal and flow geometry
• The flow is often unbounded in filtration of
  hydrosols by suspension feeders
• Beware of early aquatic applications that focus
  on efficiency of encounter and fail to use excess
  particle density

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Flow and Collector Geometry
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Direct Interception
The only mechanism that does not cross streamlines
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Inertial Impaction
Fl 2Culs lc . ls lt rc
Fl 2Curc lc . ls rc
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Gravitational Deposition
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Diffusional Deposition
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Issues

• The mechanisms can be interactive rather than
  additive (but often one will be so dominant that
  it does not matter).
• The mechanisms are linear in particle
  concentration.
• The velocity you need is a face velocity, not a
  bulk velocity.
• The concentration you need is local to the
  collector.
• Per particle, bacteria are hard to encounter by
  any mechanism.

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Fenchel Closes the Microbial Loop (Bombannes 1982)
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But leaves out fluid motion
Shimeta and Jumars (1993) added shear, and
showed as is true for most suspension feeders
and sit-and-wait predators that an intermediate
shear rate maximizes rate of ingestion
(effective encounter).
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Exceptions to the 5-10 µm rule for bacterivory

• Tunicates that use very fine meshes and so have a
  small pressure drop (and other thin mucus strand
  makers)
• Viruses
• Shear from decaying turbulence pushes it up
• Ability to use ambient flow (benthos)
• FW daphnids (charge effects?)

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For benthos, encounter and ingestion rates hard
to match

• Local u and C poorly known
• Re often gt 1
• Vertical gradients are strong, and food can be
  depleted near the bed by dense assemblages of
  suspension feeders.
• Unsteady, active motions can be important for
  encounter
• Calculated rates do match for one brittlestar
  species and some protists

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Bivalves are a mess

• There are no intelligible, mechanistic models of
  encounter.
• Particle detection and unsteady motion is
  involved but not modeled.
• Controversy over mechanisms and rates has raged
  for gt 20 yr

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Medium-scale flow issues

• Rejection (exhalent) jets fast high
• Induced flow (Venturi effect)
• Lee feeding
• Vortex trapping
• Induced resuspension
• Location relative to obstacles bedforms
• Depleted boundary layers