Soft Substrate Communities: The intertidal and subtidal zones

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Soft Substrate CommunitiesThe intertidal and
subtidal zones
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Intertidal Habitats
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• Exposed - sand beaches
• Protected - sand and mud flats
• Sand beaches
• Appear devoid of macroscopic life
• Virtually all organisms bury themselves
• Exposed to waves, face open ocean
• Pronounced slope
• Sand and mud flats
• Large numbers of visible macroscopic life
• Facing bay or lagoon
• Little or no slope

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Sandy shores

• Defined by three factors
• Particle size, wave action, and slope
• Interrelated
• Particle size
• Water retention
• Suitability for burrowing
• Substrate movement

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Slope

• Interaction between particle size, wave action,
  and swash/backswash
• Swash - water running up a beach
• Carries particles
• Accretion
• Backswash
• Removes particles

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Substrate movement

• Particles are not stable
• Continually moved and sorted
• Fines settle out in low wave action
• Coarses settle immediately
• Results in zonation based on grain size
• Different beach types

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• Dissipative beach
• Strong wave action
• Energy dissipated in broad flat surf zone
• Gentle swash
• Gentle slope
• Reflective beach
• Strong wave action
• Energy is not dissipated
• Strong swash
• Steep slope

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Seasonal changes

• Changes in wave intensity change in grain size
• Common seasonal shift in beach profile
• Fine sand in summer
• Coarse beach in winter
• Substrate may be moved a meter or more
• Few large organisms occupy the surface

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• Smooth uniform profile
• Lack topographical diversity
• Uniform action of physical factors
• Temperature
• Wave action
• Dessication

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• Sand is an excellent buffer
• Temperature changes
• Salinity
• Exposure to sunlight
• Oxygen
• Not limiting on surface
• May become limiting in substrate
• Interchange of surface water with interstitial
  water
• Exchange
• Fine - slow
• Coarse - fast
• Tube builders and burrowers may deepen oxygen

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Sand flats

• Consist of finer grained sand and sediment
• Waves and water currents affect grain size
• Very low slope
• Oxygen generally not limited
• Unless you go deep

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Muddy flats

• Characteristic of estuaries, salt marshes
• Restricted to completely protected areas (waves)
• Slope is flat
• More stable
• Conducive to permanent burrows
• Long retention time of water in sediment
• Low exchange rate with water above
• Results in anaerobic conditions below surface

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RPDRedox potential discontinuity layer

• Rapid change from aerobic to anaerobic layer
• Characterized by greyish color, below is black
• Below decomposition by anaerobic bacteria
• Biologically significant
• Reduced compounds diffuse upward
• Oxidized by bacteria in aerobic sediment
• Incorporated into bacterial biomass
• Form basis of food chains

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Subtidal habitats

• Turbulence eliminates thermal stratification
• Waves may affect stability of of substrate
• May suspend and move particles
• Determines types of particles present
• Removes fine particles
• Salinity is variable
• Temperature shows seasonal change
• Light penetration is reduced
• just a few meters

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Topography

• Vast monotonous expanses
• Ripple marks, worm tubes, fecal mounds
• Substrate grain size and composition only major
  differences
• Fewer habitats for animals to occupy
• infaunal species lt epifaunal species

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• Sublittoral - subtidal zone - area not exposed in
  tidal cycle but shallow (contintental shelf)
• Composed of soft sediments (mud, sand, some hard
  substrates)
• Communities dominated by infaunal organisms

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Physical Characteristics
Sand Beach Sand flats Muddy flats Subtidal
Wave energy High Low Low Variable
Grain size Large Med Fine Variable
Slope High Low Low Low
Stability Low Med High Variable
O2 availability High High Low Variable
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Organisms
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Size of infaunal organisms

• Macrofauna gt0.5 mm
• Meiofauna 0.5-0.062 mm
• Microfauna lt 0.062 (mostly protozoans and
  bacteria)

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Community organization

• Patchiness
• Time and space
• Horizontal and vertical
• Cyclical
• result of physical factors and interactions
  between organisms

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Community Organization

• Grain size sets limits for organisms
• Dominated by suspension feeders (filterers) and
  detritivores
• Generally separated
• Detritivores in fine sand
• Filterers in clean coarse sand
• Seasonal change

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Community structure

• Changes occur through physical or biological
  factors

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Parallel bottom communities

• Thorson 1955
• Similar communities in similar habitats found
  globally
• Similar sediments contain similar organisms
• Similar ecologically and taxonomically
• Pattern implies associations are not random
• Represent interacting systems with similar rules

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Woodin 1983

• Classification of organisms into limited
  assemblages
• Functional groups

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Types of organisms

• Sediment stabilizers
• Organisms that secrete mucous or roots to bind
  sediment
• Amphipods, phoronid worms, anemones, polychaetes
• Sediment destabilizers (bioturbators)
• motile or sedentary organisms who cause sediments
  to move
• Cucumbers, mobile clams, whelks

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Community organization

• Four dominant taxonomic groups of macrofauna
• Polychaetes
• Tube building worms, Burrowing worms
• Crustaceans
• Ostracods, Amphipods, isopods, decapods, mysids,
  tanaids
• Echinoderms
• Brittle stars, urchins, sand dollars, sea
  cucumbers, sea stars
• Mollusks
• Bivalves, scaphopods, gastropods

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• Infaunal animals
• Deposit feeders
• Suspension feeders
• Predators
• Worms
• Crustaceans
• Mollusks
• Echinoderms
• Bottom fishes

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Adaptations
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Adaptations

• Deep Burrowing
• Get away from sediment affected by waves
• Heavy shells - anchors
• Long siphons
• Severe storm may wash the up on beach
• Harder to get back into water and burrow quickly
• Mercenaria, Pismo clam

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Adaptations

• Fast burrowers
• More common
• Burrow as soon as wave removes organism
• Annelid worms, small clams, crustaceans
• Short bodies, limbs
• Donax, Siliqua and Ensis (razor clams)
• Emerita (mole crabs)

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Adaptations

• Swash migration
• Find food
• Avoid predators

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Adaptations

• Smooth shells - reduce resistance of sand
• Ridges - grip sediment, aid in penetration
• Reduced spines (echinoderms, sand dollars)
• Weight belts - accumulation of iron compounds -
  sand dollars

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Adaptations (muddy shores)

• Burrow
• Permanent tubes
• Anaerobic adaptations
• Development of oxygen carriers (hemoglobin)
• Glycogen stores for anaerobic metabolism
• Bring surface water down

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Reproduction

• Iteroparous gt semelparous
• Coordinate spawning with tides
• Lunar rhythms
• Stranding
• Predation
• Latitudinal gradient
• Planktogrophic - tropics
• Lecitrophic - temperate zone

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Types of organisms - sand beach

• Lack of macroscopic plants
• Primary producers - benthic diatoms, surf-living
  phytoplankton
• Vertical migration in sediments, water column
• Polychaete worms, mollusks, crustaceans

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Feeding ecology - sand beach

• Very little primary production
• Organisms depend on phytoplankton in water,
  organic debris
• Filter feeders, detritus feers, scavengers
• Few resident carnivores
• Opportunistic carnivores, scavengers

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Types of organisms - sand flats

• Perrenial microscopic plants, seagrasses
• Ephemeral algae, seasonally abundant
• Large and diverse array of microflora
• Benthic diatoms, dinoflagellates, cyanobacteria
• Polychaete worms, mollusks, crustaceans

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Feeding Ecology - sand flats

• Productivity from microfloral films, seagrasses,
  macroalgae
• Not grazed extensively
• 90-95 broken down into detritus
• Scavengers, filterfeeders, and deposit feeders

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Types of organisms - mud flats

• Substantial plant life
• Diatoms, macroalgae, seagrasses
• Bacteria
• Highly abundant
• Sulfur bacteria (oxidize sulfur compounds for
  energy) (Chemolithoautotrophic bacteria)
• Two separate layers of productivity
• Macrofauna similar to sandy areas

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Feeding Ecology - mud flats

• More food available than in sand
• More large organisms
• Deposit and suspension feeders are dominant
• Deposit feeders (worms and bivalves)
• Burrow through substrate (earthworms)
• Surface feeding

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• Suspension feeders
• Mostly like others in sandy areas
• Must deal with fine suspended particles
• Partially feed on both particles and plankton
• Predators
• Fish, birds, moon snails, crabs, worms
• Few herbivores
• Trophic structure based
• detritus bacteria base
• Autotrophic base

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Types of organisms - subtidal

• Nutrients are rarely limiting
• Productivity is relatively high
• Large populations of zooplankon and benthic
  organisms
• Macroscopic plants contribute to primary
  production
• Runoff from land plays major role
• Few large grazing animals

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Organismal Characteristics
Sand Beach Sand flats Muddy flats Subtidal
Plants Low Med High Variable
Primary productivity Low Med High High
Epifaunal predators Low High High High
Detritus Low Med High Variable
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Community Organization of Soft Substrates
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What governs subtidal communities?

• Predation
• Disturbance
• Recruitment
• Recolonization
• Competition

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Community organization - sand

• Grain size sets limits for organisms
• Dominated by suspension feeders (filterers) and
  detritivores
• Generally separated
• Detritivores in fine sand
• Filterers in clean coarse sand
• Seasonal change

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Community organization - sand

• Zonation present, but fuzzy
• Habit of animals to migrate up and down beach
• Lack of studies

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Community organization - mud

• Intertidal area extensive
• Supralittoral
• burrowing crabs
• Midlittoral
• clams and polychaetes
• Infralittoral
• No sharp boundary
• Like midlittoral

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Distribution

• Gregarious
• Crustaceans ? exposed and tropical shores
• Bivalves ? protected and temperate shores
• macrofaunal sp. ? decreasing wave exposure
• Biomass ? exposed beaches

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McLachlan 1983

• Abundance and diversity correlated with particle
  size and slope
• Faunas ?if beach is dissipative
• Wave action dissipated in surf zone
• Flat slopes
• Less movement
• High biomass of filter feeders

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Community regulation - sandy beaches

• Sandy beaches - not studied so extensively
• Competition for space not major contributor to
  patterns
• Three dimensional space
• Extreme patchiness
• Competition for food
• Abundant plankton
• Sparse populations

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• Most sand beach animals are opportunistic
• Few indigenous invertebrate predators
• Few exclusion experiments, diversity ?
• Filter feeders - ample food

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Community regulation - sand and mud flats

• Physical factors important
• Grain size
• Trophic group amensalism
• Exclusion of one trophic group by another
• Deposit feeders exclude suspension feeders
• Burial of newly settled suspension feeder larvae
  by deposit feeders

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• Seasonal weather changes
• Migration to deep water
• Sea ice - scouring
• Prime factors are
• Predation
• Competition
• Disturbance
• Predation, predation/disturbance significant

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Wiltse 1980

• Moon snail (Polinices duplicatus)
• Active predator of bivalves
• Soft shell clam (Mya arenaria)

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Wiltse 1980

• Removed moon snail
• Increase in Mya
• Increase in infauna

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Virnstein 1977

• Green crab (Caenus maenas)
• Blue crab (Callinectes sapidus)
• Crabs which dig in sediment for food

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Virnstein 1977

• Exclusion resulted in an increase in infaunal
  densities

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Woodin 1978

• Horseshoe crab (Limulus polyphemus)
• Digs distinctive pits in search of food

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Whelks (Busycon spp.)

• Devastating to sand flat clam populations
• Consume all sizes of clams
• No size refuge

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Refugia? Woodin 1978

• Tube-forming worm Diopatra cuprea
• Forms upright tubes
• Effectively deters both Limulus and Calinectes
• Infaunal abundances greater around tubes

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Peterson and Peterson (1979)

• Effects of deposit feeders in NC
• Hemichordate worm (Balanoglossus aurantiacus)
• Funnel feeder, digs u-shaped burrow
• Ingests sediment

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Peterson and Peterson (1979)

• Consumes small infaunal organisms
• Causes death of others it does not ingest
• Keeps of infaunal organisms low
• Compounded by sea cucumber (Leptosynapta tenuis)
• Also ingests sediment

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Adult-larval interactions (Woodin 1976)

• Predatory interactions

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Direct Competition

• Levinton et al. (1985)
• Hydobia totteni (Eurpoean), Ilyanassa obsoleta
  (native mud snail)
• H. totteni lives in high intertidal
• Avoids Ilyannasa
• H. totteni is an inferior competitor

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Brenchley and Carlton (1983)

• Littorina littorea (European), Ilyanassa obsoleta
  (native)
• Spatial segregation
• Littorina destroys egg capsules of Ilyanassa

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Grant (1981)

• Two amphipod species
• Acanthohaustorius - lives in oxidized layer
• Pseudohaustorius - lives in anoxic layer
• Both prefer oxidized layer
• Acanthohaustorius superior competitor

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Predators (Petersen 1991)

• Rocky intertidal
• Starfish, mollusks
• Slow moving
• Limited to inundated parts
• Soft intertidal
• Dominated by highly mobile predators
• Crabs, fishes, birds
• Ranges throughout intertidal