Soft Substrate Communities
soft sediment = substrate of
sedimentary particles;
uncemented, unconsolidated or
loosely consolidated
epifauna – on the surface
infauna – in the sediment
Physical Environment
1. Grain size - particle size
high energy = large grain size; sand
low energy = small grain size; mud
median grain size – sandy silt, silty sand
sorting – range of particle sizes, biological
sorting
Substrate mobility
– influenced by animals – burrowing, binding in
tubes
– cohesiveness – microbes, mucus
Interstitial space – space between grains,
“pores”
– affects water drainage
– diffusion of chemicals
2. Organic Matter
- % organic matter
- substrate for microbial decomposition,
detritus feeders
3. Oxidation-reduction state
redox potential discontinuity layer (RPD)
- measured by electrode (Eh)
Above RPD – oxygen present
Below RPD – oxygen absent
Chemosynthetic
bacteria – use H2S
Sulfate-reducing
bacteria – produce
H2S (fermentation)
• Organisms affect the depth of the RPD
layer
in irrigated tubes, extend RPD into
sediments
• Organisms must adapt to anaerobic
conditions
– Bring oxygenated water down
– Tolerate H2S
4. Light – when present, plants present
- benthic diatoms
- macroalgae
- seagrasses
Size of infaunal organisms
• Macrofauna:
• Meiofauna:
• Microfauna:
>0.5 mm
0.5-0.062 mm
< 0.062 mm
Clams
Snails
Suspension feeders
Trophic Structure
• Suspension Feeders (filter feeders)
- primary food = plankton
- generalists, size selection by filter
• Deposit Feeders
- animal that feeds by consuming
particles in or on the substrate
- “detritivore”
Types of deposit feeders
• Surface deposit feeders
• Burrowing or deep
Microbial Stripping Theory – deposit
feeders don’t digest detritus, just
digest microorganisms on the
detritus and sediment particles
Fenchel:
- low assimilation efficiency detritus
(1-10%)
- high assimilation efficiency microbes
(40-80%)
Logical argument for microbial
stripping
• Composition of the detritus
- sources, age
- temporal variation
• Digestion detritus vs microbes
• Constancy and quality of microbes
- microbial colonization
- protein
Renewal rates - microbes
• Animal must not ingest again until
microbes recolonize
• pelletization
Predators
Physical or biological?
• Soft bottom benthic communities
structured by ???
• Expt evidence
Physical –
• Oliver 1979
– Subtidal zoned on gradient of wave energy
– < 14m – regular disturbance, small molbile
crustaceans
– > stable, tube polychaetes
Both biological and physical
• Mills 1969 – sandy area, low biomass,
density of IF
• Illyanassa – mech disturbance by snails
• Low Illyanassa density – colonization by
Ampeleisca tube building amphipod –
exclude Illyanassa
• Selective deposit feeding produces fine
sediments, tubes create topo diversity,
promotes colonization by polychaetes
• Winter storms, destabilize Ampelisca mats
• Illyanass recruits in spring,
Biological factors
• Competition
• direct displacement – rare ( no hard
surface to push against) , no colonials
• Food/space – evidence form regular
spacing of individuals
• interference- Levinton 1977
• Direct: Active Bivalve Yoldia limulata
disrupts burrows of less mobile Solemaya
velum; Illyanassa and Ampleisca
Indirect : more common
• Burrowing DF – muddy sand w/high [OM]
• Where DF present in high no. SF absent
• DF burrow, fecal material, create loose
surface layers, unstable, easily
resuspended
• Clogs SF feeding
• Buries SF larvae, DF larvae OK
• Exclusion of one trophic group by another
Rhoads and Young 1970
• Reworked sediments by 3 sp DF clams,
Yoldia, Nucula, Macoma – excludes DF
• Soft sediment animals affect the sediment
they live in
• Functional groups – animals that use/affect
the environment in the same way
– Trophic groups
– Sediment stabilizers/destabilizers
Types of organisms
• Sediment stabilizers
– Organisms that secrete mucous or
otherwise bind sediment; roots
– Amphipods, phoronid worms, anemones,
polychaetes
• Sediment destabilizers (bioturbators)
– motile or sedentary organisms who cause
sediments to move
– Sea cucumbers, mobile clams, whelks
• Deposit feeders produce fluid fecal rich
surface
• Easily resuspended by low velocity
currents
• Instability might interfere with suspension
feeders:
– Experiments with Mercenaria in trays above
bottom
– Deposit feeder larvae not affected
Trophic Group Amensalism
• Interaction between two trophic groups in
which one group is inhibited while the
other is not
– Inhibitors = deposit feeders; exclude
suspension feeders
– Physical instability of the sediment – clogs
filters, buries newly settled suspension feeder
larvae/juveniles; can’t maintain life position;
disturbed or eaten by deposit feeders
Rhodes and Young 1971
• Molpadia oolotica – large, high density,
sedentary, silt-clay mud, heads down DF
• Ingest sediment at depth. Deposit loose
fecal matter, form mounds
• Reworking produces loose high water
content easily susp. sediment
• Areas between – highly unstable
• Cones – stable, fecal pellets, bound
material
• attracts SF polychaete Euchone, other SF
tube builders
• SF tube builders stabilize sediment, extend
downwards into substrate
• Stabilize cones, prevent resuspnsion
attract more tube SF
• High tube density prevent settlement of
large DF/ burrowers – can’t penetrate
• Indirect restriction – competitive
interference
• Also filter out and prey on larvae of DF
Coexistence is possible
• SF prefer more sandy areas – firmer,
easier to build sfc tubes
• Sand: DF not favored, low [OM], difficult
to burrow
• Areas where both can live – sharp
boundaries but no physical differences
• Patches – removal of residents (rays,
storms)
• Little asexual reproduction - colonize by
larval recruitment or adult immigration
Woodin 1976
Suggests there are 3 major functional
groups:
• Mobile (burrowing) deposit feeders
• Suspension feeders
• Tube builders
None have overlapping distributions – why?
Adult-larval interactions
• Deposit feeders – change nature of
sediment (trophic group amensalism),
feed at surface
• Suspension feeders – consume larvae
while filtering
• Tube builders – dense assemblage creates
mat that larvae can’t penetrate; feed at
surface
Leads to:
• Strong dominance by year classes
• Inhibition model of succession – multiple
stable states
Ilyanassa (Nassarius) – mud snail, mobile DF
Ampelisca – amphipod, tube builder
Sand vs Mud
Role of Predation and Competition
in Soft Sediment Communities
• Sediments – 3D
• Refuge form non-digging predators
• Ability to divide resource and avoid
competition
Woodin – predator trophic types
• Surface –, juveniles vulnerable, affect size
classes., esp those with refuge in
size/depth
• Browsers – nippers, rob energy
• Burrowers – “weasel” predators
(nemerteans, Pisaster brevispinis)
• Digging – excavate holes, change
sediments, indiscriminate
• Infaunal – burrowing nemerteans,
polychaetes
Large predator/disturbers
Caging Results:
• Virnstein 1977– crabs/epifaunal or sfc
predators: change in numbers but not
composition
• Ambrose 1991 – infaunal – reduce
infaunal populations, eat other predators,
multiple layers of predators
Cage results overall – removal
of predators
• Increase in total density
• Increase in species richness
• No tendency for competitive exclusion
Why No Competitive Exclusion?
• Reduced opportunity for interference
competition
Vertical distribution
• Competition for food and space SF and DF
• Subtidal – food abundant – detritus
• SF – partition space by depth, feeding
structure (callianassa)
• DF – feed at different levels
• Peterson 1977
– –removal of some sp from a depth level –
increase in abundance of other sp at that
strata – competition
– Adding sp to a depth level caused emigration
by others – vertical spacing and maintain
density
Why No Competitive Exclusion?
• Reduced opportunity for interference
competition
• Extreme importance of adult-larval
interactions
Why No Competitive Exclusion?
• Reduced opportunity for interference
competition
• Extreme importance of adult-larval
interactions
• Developmental plasticity of marine
invertebrates
• Lack of clear competitive dominant
Caging Artifacts
• How could these change the results?
Wilson 1991
• No evidence for both predation and
competition affecting benthic community
structure
• No evidence for a competitive dominant in
any soft bottom system
• Can’t fully predict effects of predation or
competition
• Lack knowledge of growth, life spans,
trophic types, pop dynamics , esp DF
• No unifying theroy of community
organization for sof bottom envtsd
Multiple Stable States
• Long term stability – eg Molpadia
• Cyclic oscillation – Mills – Illynassa –
Ampelisca, biological and physical factors
• Multi- year long term – Baltic - alternating
states affected by variable recruitment
• Pontoporeia affinis – Macoma baltica
• High Pontoporeia keeps out Macoma, poor
year for Pontiporea allows Macoma, keeps
out Pontoporea
• Predators affects r selected species
• END
Recruitment Dynamics
• Loss of larvae in the water column
• Larvae as passive particles
• Larval site selection
• Adult-larval interactions
• Meiofauna-macrofauna interactions
Meiofauna – Macrofauna
Interactions (Watzin 1983)
• Larval/Juvenile macrofauna are the same
size meiofauna (temporary meiofauna)
• Among the meiofauna are potential
predators and competitors
Meiofauna – Macrofauna
Interactions (Watzin 1983)
• Manipulations in small boxes
– Increased densities of turbellarian predators
– Increased densities of “other meiofauna” –
potential competitors
• Exposed to recruitment for one week
– Macrofauna larvae avoid treatments, or lower
survival of juveniles after settlement
• Must survive the “meiofauna bottleneck” –
escape in size
Meiofauna as Food for Bottomfeeding Fishes
Other Roles of Meiofauna
• Meiofauna as food for deposit feeders
• Meiofauna stimulate bacterial productivity
– Speed break-down of detritus
– Ingest bacteria – turnover
– Mucus – release of DOM
Conclusions
• Trophic group amensalism
• Disturbance
• Predation
• Competition
• Recruitment