Soft Substrate Communities: The intertidal and subtidal zones

Intertidal Habitats

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

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

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

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

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

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

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

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

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

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

RPD Redox 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

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

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 < epifaunal species

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

Sand Beach Sand flatsMuddy flats Subtidal Wave energyHighLow Variable Grain sizeLargeMedFineVariable SlopeHighLow StabilityLowMedHighVariable O2 availability High LowVariable Physical Characteristics

Organisms

Size of infaunal organisms Macrofauna: >0.5 mm Meiofauna: mm Microfauna: < (mostly protozoans and bacteria)

Community organization Patchiness –Time and space –Horizontal and vertical –Cyclical –result of physical factors and interactions between organisms

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

Community structure Changes occur through physical or biological factors

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”

Woodin 1983 Classification of organisms into limited # assemblages Functional groups

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

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

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

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

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)

Adaptations Swash migration –Find food –Avoid predators

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

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

Reproduction Iteroparous > semelparous Coordinate spawning with tides –Lunar rhythms –Stranding –Predation Latitudinal gradient –Planktogrophic - tropics –Lecitrophic - temperate zone

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

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

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

Feeding Ecology - sand flats Productivity from microfloral films, seagrasses, macroalgae Not grazed extensively 90-95% broken down into detritus Scavengers, filterfeeders, and deposit feeders

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

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

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

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

Sand Beach Sand flats Muddy flats Subtidal PlantsLowMedHighVariable Primary productivity LowMedHigh Epifaunal predators LowHigh DetritusLowMedHighVariable Organismal Characteristics

Community Organization of Soft Substrates

What governs subtidal communities? Predation Disturbance Recruitment Recolonization Competition

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

Community organization - sand Zonation present, but fuzzy –Habit of animals to migrate up and down beach –Lack of studies

Community organization - mud Intertidal area extensive Supralittoral –burrowing crabs Midlittoral –clams and polychaetes Infralittoral –No sharp boundary –Like midlittoral

Distribution Gregarious Crustaceans  exposed and tropical shores Bivalves  protected and temperate shores # macrofaunal sp.  decreasing wave exposure Biomass  exposed beaches

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

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

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

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

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

Wiltse 1980 Moon snail (Polinices duplicatus) –Active predator of bivalves Soft shell clam (Mya arenaria)

Wiltse 1980 Removed moon snail Increase in Mya Increase in infauna

Virnstein 1977 Green crab (Caenus maenas) Blue crab (Callinectes sapidus) Crabs which dig in sediment for food

Virnstein 1977 Exclusion resulted in an increase in infaunal densities

Woodin 1978 Horseshoe crab (Limulus polyphemus) Digs distinctive pits in search of food

Whelks (Busycon spp.) Devastating to sand flat clam populations Consume all sizes of clams No size refuge

Refugia? Woodin 1978 Tube-forming worm Diopatra cuprea Forms upright tubes Effectively deters both Limulus and Calinectes Infaunal abundances greater around tubes

Peterson and Peterson (1979) Effects of deposit feeders in NC Hemichordate worm (Balanoglossus aurantiacus) Funnel feeder, digs u-shaped burrow Ingests sediment

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

Adult-larval interactions (Woodin 1976) Predatory interactions

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

Brenchley and Carlton (1983) Littorina littorea (European), Ilyanassa obsoleta (native) Spatial segregation Littorina destroys egg capsules of Ilyanassa

Grant (1981) Two amphipod species –Acanthohaustorius - lives in oxidized layer –Pseudohaustorius - lives in anoxic layer Both prefer oxidized layer Acanthohaustorius superior competitor

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