Suspension FeedingSuspension Feeding

Mostly mechanics and foraging Mostly mechanics and foraging theorytheory

TermsTerms

Definition — feeding on particles by removing Definition — feeding on particles by removing them from suspensionthem from suspension

Active — create own feeding currentsActive — create own feeding currents Passive — use ambient fluid motion and (or) Passive — use ambient fluid motion and (or)

gravitygravity Beware of classifications/dichotomiesBeware of classifications/dichotomies Nobody make a living by swallowing Nobody make a living by swallowing

seawater. Good particles must be seawater. Good particles must be concentrated.concentrated.

Suspension, like Deposit, Suspension, like Deposit, Feeding Says How, not WhatFeeding Says How, not What

PhytoplanktonPhytoplankton DetritusDetritus BacteriaBacteria ProtistsProtists AnimalsAnimals More than one of More than one of

the abovethe above

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

Passive & Active SFPassive & Active SF

In the plankton,In the plankton,thecosome pteropodsthecosome pteropods

In the plankton, copepods,In the plankton, copepods,salps, some fishes, etc.salps, some fishes, etc.

ManyManybenthicbenthicphylaphyla

ManyManybenthicbenthicphylaphyla

More terms and conceptsMore terms and concepts

BEWAREBEWARE of “volume filtered” or “volume cleared.” of “volume filtered” or “volume cleared.” It comes from the practice of measuring It comes from the practice of measuring CC((tt). If you ). If you

know know CC ( (xx) and ) and C C ((x x + + yy), then from the geometric ), then from the geometric mean concentration over the interval mean concentration over the interval yy and the time and the time yy and the volume of the experimental container you and the volume of the experimental container you can calculate what volume has been cleared of cells can calculate what volume has been cleared of cells in that time. in that time.

Just because you can do the calculation does not Just because you can do the calculation does not mean that the animal actually filters that volume.mean that the animal actually filters that volume.

Where to forageWhere to forage

Planktonic suspension feeders, where Planktonic suspension feeders, where C C is high, but observation is that they is high, but observation is that they forage where production is high.forage where production is high.

Benthic suspension feeders where Benthic suspension feeders where CCxxu u is highis high

Benthic suspension feeders can’t chase Benthic suspension feeders can’t chase patchespatches

What Particles to TakeWhat Particles to Take

For feeders on living organisms, take For feeders on living organisms, take particles larger than the mean sizeparticles larger than the mean size

For detritivores, take smaller particles and For detritivores, take smaller particles and ones lower in specific gravityones lower in specific gravity

If sorting is moderately expensive, show If sorting is moderately expensive, show partial preferencepartial preference

Sorting may be an issue for benthic Sorting may be an issue for benthic suspension feeders that experience high suspension feeders that experience high concentrations of poor foodsconcentrations of poor foods

How Fast to FeedHow Fast to Feed

II

Concentration [N L ]-3

M

Concentration [N L ]-3

A post-capture step, probablyrate of digestion, is rate limiting

Filtering rate declines,reducing costs andincreasing net gains

Fmax

Recall the digestionRecall the digestionlectures and reprintslectures and reprints

Filter only fast enoughFilter only fast enoughto keep the gut fullto keep the gut full

CautionsCautions

Aerosol filtration theory (Rubenstein and Aerosol filtration theory (Rubenstein and Koehl 1977) has a different goal and Koehl 1977) has a different goal and flow geometryflow geometry

The flow is often unbounded in filtration The flow is often unbounded in filtration of hydrosols by suspension feedersof hydrosols by suspension feeders

Beware of early aquatic applications Beware of early aquatic applications that focus on efficiency of encounter that focus on efficiency of encounter and fail to use excess particle densityand fail to use excess particle density

Flow and Collector GeometryFlow and Collector Geometry

Direct InterceptionDirect Interception

The only mechanism that does not cross streamlinesThe only mechanism that does not cross streamlines

Inertial ImpactionInertial Impaction

Fl = 2Culs lc . ls < rc

Fl = 2Curc lc . ls ≥ rc

Gravitational DepositionGravitational Deposition

F = 2CwG s(r + r ) lcc s

ws

GravitationalDeposition

Diffusional DepositionDiffusional Deposition

(r + r )F = 2 pD c sCDlcD s

u DiffusionalDeposition

D

IssuesIssues

The mechanisms can be interactive rather than The mechanisms can be interactive rather than additive (but often one will be so dominant that it additive (but often one will be so dominant that it does not matter).does not matter).

The mechanisms are linear in particle concentration.The mechanisms are linear in particle concentration. The velocity you need is a face velocity, not a bulk The velocity you need is a face velocity, not a bulk

velocity.velocity. The concentration you need is local to the collector.The concentration you need is local to the collector. Per particle, bacteria are hard to encounter by any Per particle, bacteria are hard to encounter by any

mechanism.mechanism.

Fenchel Closes the Microbial Fenchel Closes the Microbial Loop (Bombannes 1982)Loop (Bombannes 1982)

u

F = 2 p C u r rc s

Direct Interception

rc

Chrysomonad

u

F = A C u

A = area of sieve (proportional to r )c

2

rc

u = mean velocitythrough sieve

Choanoflagellate

Sieving

But leaves out fluid motionBut leaves out fluid motionDiffusional Deposition

rc

F = 4 p C D (r + r )s c

Heliozoan

Shimeta and Jumars (1993) Shimeta and Jumars (1993) added shear, and showed — added shear, and showed — as is true for most suspensionas is true for most suspensionfeeders and sit-and-waitfeeders and sit-and-waitpredators — that an intermediate predators — that an intermediate shear rate maximizes rate ofshear rate maximizes rate ofingestion (effective encounter).ingestion (effective encounter).

Exceptions to the ~ 5-10 µm Exceptions to the ~ 5-10 µm rule for bacterivoryrule for bacterivory

Tunicates that use very fine meshes and Tunicates that use very fine meshes and so have a small pressure drop (and other so have a small pressure drop (and other thin mucus strand makers)thin mucus strand makers)

VirusesViruses Shear from decaying turbulence pushes it Shear from decaying turbulence pushes it

upup Ability to use ambient flow (benthos)Ability to use ambient flow (benthos) FW daphnids (charge effects?)FW daphnids (charge effects?)

For benthos, encounter and ingestion For benthos, encounter and ingestion rates hard to matchrates hard to match

Local Local uu and and CC poorly known poorly known Re Re often > 1often > 1 Vertical gradients are strong, and food can be Vertical gradients are strong, and food can be

depleted near the bed by dense assemblages depleted near the bed by dense assemblages of suspension feeders.of suspension feeders.

Unsteady, active motions can be important for Unsteady, active motions can be important for encounterencounter

Calculated rates do “match” for one brittlestar Calculated rates do “match” for one brittlestar species and some protistsspecies and some protists

Bivalves are a messBivalves are a mess

There are no intelligible, mechanistic There are no intelligible, mechanistic models of encounter.models of encounter.

Particle detection and unsteady motion Particle detection and unsteady motion is involved but not modeled.is involved but not modeled.

Controversy over mechanisms and Controversy over mechanisms and rates has raged for > 20 yrrates has raged for > 20 yr

Medium-scale flow issuesMedium-scale flow issues

Rejection (exhalent) jets — fast & highRejection (exhalent) jets — fast & high Induced flow (Venturi effect)Induced flow (Venturi effect) Lee feedingLee feeding Vortex trappingVortex trapping Induced resuspensionInduced resuspension Location relative to obstacles & bedformsLocation relative to obstacles & bedforms Depleted boundary layersDepleted boundary layers