Benthos Community Structure

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• Benthos Community Structure
• Community Composition
• Descriptions of community composition and
  structure have been approached in several ways
• Indicator Species
• Broad sampling over large geographical areas used
  to define species composition and large-scale
  distribution in terms of dominant species,
  usually macrofauna
• Regular occurrence of a few dominant, conspicuous
  macrofaunal species over a wide area used to name
  a community in terms of one or two most abundant
  species
• Species used to characterize communities
  indicator species
• Often more convenient to refer to communities in
  terms of indicator species than entire species
  composition

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• Benthos Community Structure
• Community Composition
• Indicator Species
• Each community associated with a particular set
  of physical and, in all likelihood, chemical
  conditions
• Close relationship between environmental
  conditions and community species composition
  presents an intriguing possibility Is the same
  community associated with the same environmental
  conditions everywhere in the world?

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Fig. 5.3
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• Benthos Community Structure
• Community Composition
• Indicator Species
• Similar environmental conditions worldwide do not
  produce identical communities, but striking
  similarities in composition occur
• Concept of parallel bottom communities introduced
  in 1957 by Gunnar Thorsen (Univ. of Washington)
• Based on empirical observations of community
  similarities
• Notice that all of these communities would be
  characterized as Macoma communities, but
  different species of Macoma at each site
• Concept based on idea that similar selective
  forces and responses (e.g. predation, settlement)
  should select for similar groups of more or less
  closely related species
• Theory considered valid for many years, but
  observation of latitudinal gradients in benthic
  species diversity (higher diversity at low
  latitudes) created difficulties in applying
  principle globally

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• Benthos Community Structure
• Community Composition
• Indicator Species (caveats)
• Some benthic communities may be characterized in
  terms of a few dominant species, but abundances
  of less dominant species arent necessarily
  well-correlated with abundances of numerous
  species
• Little reason to assume that dominant species are
  functionally related to other, less abundant
  species
• Important premise community composition
  determined by specific physical biological
  interactions
• Alternative organismal groupings may be caused
  more by physical factors than biological factors
  that maintain associations
• Ex Species dominance often related to sediment
  texture macrofaunal species dominate sandy
  sediments meiofauna (e.g. nematodes) dominate
  muds

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• Benthos Community Structure
• Community Composition
• Community Associations
• A different approach involves description and
  enumeration of all major benthic species within a
  localized area
• Within a given, relatively restricted locale its
  likely that physical environment is relatively
  constant and (hopefully) quantifiable
• Benthic communities often characterized according
  to species assemblage in a given location
• Possible to separate soft-bottom benthic fauna
  into size classes
• Does this sort of classification really have any
  biological meaning, or is it an arbitrary scheme
  based on human assumptions about the relationship
  between size and ecological role?

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• Benthos Community Structure
• Community Composition
• Community Associations
• Schwinghamer (1981) pointed out that three major
  size groups into which infauna typically are
  classified correspond to three habitat types for
  benthic organisms in particular substrates

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• Benthos Community Structure
• Community Composition
• Community Associations
• Schwinghamer (1981) plotted biomass distribution
  against organism size on a logarithmic scale to
  produce a characteristic biomass spectrum

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• Benthos Community Structure
• Community Composition
• Community Associations
• Biomass spectra can be constructed for a variety
  of habitats
• Benthic communities typically have characteristic
  spectra, and similar communities from different
  locations have similar spectra
• Analogous in many ways to sorting sediments to
  obtain a characteristic size spectrum
• Absolute amount of biomass in one area typically
  depends on level of organic matter input
• Size distribution of organisms within a location
  may depend on physical factors, such as sediment
  grain size, flow regime, etc.
• Theory has same problem as indicator species
  theory latitudinal gradients in benthic species
  diversity (higher diversity at low latitudes)
  create difficulties in applying principle
  globally
• Lower latitude communities more diverse, less
  likely to be dominated by few species, vs. high
  latitude communities

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• Benthos Community Structure
• Community Composition
• Guilds
• Approach entails arranging fauna into
  functionally similar species groups called guilds
• Grouping of species into guilds may be based on
• Feeding characteristics (e.g. mode, location,
  diet)
• Mobility (mobile vs. stationary)
• Ability to process and modify sediments (e.g.
  through feeding, burrowing, excavation)
• Advantage of approach lies in treatment of
  organisms as functional elements of a community
  and not just simple taxonomic units
• Ex System doesnt treat suspension feeders and
  deposit feeders as equivalent elements

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• Benthos Community Structure
• Community Composition
• Guilds
• Difficulty with approach and defining guilds
  functional classification scheme requires
  substantial knowledge of species ecology and
  organismal behavior as well as taxonomic
  identification
• Approach structures study of communities in
  functional terms that may be more useful and
  meaningful than simple taxonomic classification
• Concept of guilds requires understanding of
  biological interactions within benthic
  communities and impacts of interactions on
  communities

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• Benthos Community Structure
• Biological Interactions
• Stabilization vs. Destabilization
• Composition of a benthic community may be
  affected by relative stability of environment
• Instability translates directly into disturbance,
  and species vary greatly in tolerance of
  disturbance
• Some species enhance disturbance of benthos,
  while others reduce disturbance
• Some species disturb sediments minimally, while
  others generate substantial disturbance
• Benthic species may be separated into sediment
  stabilizers and sediment destabilizers

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Fig. 5.4
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• Benthos Community Structure
• Biological Interactions
• Stabilization vs. Destabilization
• Destabilizers carry out bioturbation, involving
  movement of sediments through mixing,
  resuspension or erosion
• Nature of bioturbation strongly correlated to
  species and type of activity
• Ex Mobile deposit-feeding clams such as Nucula
  and Macoma are destabilizers
• Ex Holothuroids and echinoids that plow through
  sea floor may destabilize sediments, especially
  near sediment-water interface
• Ex Burrowing ophiuroids and asteroids also may
  contribute to destabilization of sediments

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http//www.niwa.co.nz/pubs/wa/11-4/burrowing
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• Benthos Community Structure
• Biological Interactions
• Stabilization vs. Destabilization
• Not all destabilizers carry out bioturbation all
  the time
• Ex Some ophiuroids and holothuroids that plow
  through sediments part of the time, leaving a
  distinctive trace, sometimes move across sediment
  surface without disturbing sediments or leaving a
  discernible trace

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• Benthos Community Structure
• Biological Interactions
• Stabilization vs. Destabilization
• Stabilizers typically build structures that bind
  sediment particles together or increase their
  resistance to movement
• Ex Zostera and other sea grasses with roots that
  bind sediments and upper portions that alter flow
  and enhance particle deposition
• Ex Tube-building amphipods, phoronids,
  polychaetes and anemones
• Effects of stabilizers typically are local,
  creating patches of stability and enhancing
  community diversity

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• Benthos Community Structure
• Biological Interactions
• Stabilization vs. Destabilization
• Large-scale stabilization requires large numbers
  of stabilizers
• Ex In Bering and Chukchi Seas, ampeliscid
  amphipods produce tubes in such large numbers
  that square kilometers of sea floor are affected
• Tubes produced by amphipods stabilize sediments
• Selective deposit feeding by amphipods enhances
  accumulation of fine particles and increases
  topographic diversity
• Amphipod tubes are removed by feeding of gray
  whales and walruses (seasonal process)
• Feeding scars are colonized rapidly by
  opportunistic species and have lower species
  diversity
• Tube building amphipods eventually recolonize
  scars

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• Benthos Community Structure
• Biological Interactions
• Stabilization vs. Destabilization
• Large-scale stabilization requires large numbers
  of stabilizers
• Ex Ampeliscid amphipods produce tubes that
  stabilize sediments and enhance diversity in
  Barnstable Harbor, Massachusetts
• Amphipod tubes in Barnstable Harbor are washed
  out by winter storms, and community dominated by
  snail (Ilyanassa) until amphipods rebuild tubes
  in spring
• When Ilyanassa dominates, bioturbation by snails
  destabilizes sediments, leading to disturbed,
  low-diversity, low-biomass community
• When Ampelisca dominates, fine sediments that
  accumulate are colonized by polychaetes,
  producing less disturbed, higher diversity,
  higher biomass community

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• Benthos Community Structure
• Biological Interactions
• Competition
• Competition within soft-bottom benthic
  communities primarily seems to be indirect
• Direct competition seems scarce, perhaps because
  of difficulty in overgrowing or engaging in
  aggressive interactions in an environment with
  little substrate to use for anchorage
• Many indirect interactions are related to feeding
  modes of various species

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• Benthos Community Structure
• Trophic Interactions
• Two dominant trophic modes in soft-bottom benthic
  communities are deposit feeding and suspension
  feeding
• Feeding activities by suspension and deposit
  feeders may affect community composition
  indirectly, usually by altering physical
  environment
• Ex Bioturbation and resuspension of sediments by
  deposit feeders create highly turbid conditions
  that clog filters of suspension feeders
• Ex Efficient filtration by suspension feeders
  removes most settling particles before they reach
  sea floor and become accessible to deposit
  feeders
• Ex Burrowing by deposit feeders creates a loose,
  unstable layer of very fine particles at sediment
  surface. This layer does not have enough
  mechanical strength to support suspension feeders
• Ex Bioturbation by deposit feeders disturbs
  sediment surface and buries larvae of suspension
  feeding species
• All of these mechanisms tend to favor survival of
  trophic group that controls physical regime
• This process (trophic group amensalism)
  essentially involves exclusion of one trophic
  group due to modification of environment by
  members of another trophic group

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• Benthos Community Structure
• Trophic Interactions
• Trophic group amensalism not applicable
  universally
• Ex Deposit and suspension feeders often co-occur
  and neither is restricted to a specific sediment
  type
• Ex Some species switch between trophic modes,
  depending on hydrodynamic conditions
• In some cases, biological interactions are
  overwhelmed by physical factors
• Ex Low current flows dont bring adequate
  supplies of particulate material to passive
  suspension feeders
• Ex High levels of current flow resuspend
  sediments and deposited organic material, making
  them accessible to suspension feeders but not to
  deposit feeders
• Relative dominance of species exhibiting a
  particular trophic mode likely to result from
  interactions among biological and physical
  factors, notably boundary layer hydrodynamics

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• Benthos Community Structure
• Trophic Interactions
• Deposit Feeding
• Animals ingest sedimentary material and derive
  nutrition from some fraction of that material,
  usually a combination of particulate organic
  material, algae and bacteria
• Relative contribution of each of these fractions
  depends on organism and its feeding technique as
  well as characteristics of sediments
• Bacteria can decompose organic material that is
  relatively refractory to eukaryotes
• Many organisms (e.g. amphipods, gastropods,
  holothuroids) have relatively little ability to
  digest detrital plant material but are relatively
  efficient at digesting bacteria
• Bacteria that colonize organic material may be
  very important in benthic food webs
• Idea that POM is relatively indigestible and that
  microbes are main source of nutrition for deposit
  feeders is the microbial stripping hypothesis

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• Benthos Community Structure
• Trophic Interactions
• Deposit Feeding
• Hypothesis suggests that POM must be decomposed
  and converted by microbes into digestible
  material
• Most POM is relatively refractory and therefore
  poorly digestible, while most animals dont have
  sufficient cellulase activity in their guts to
  digest complex carbohydrates, notably cellulose,
  in POM
• As organic material is degraded, its chemical
  composition is altered
• Microbial colonization leads to enrichment in
  nitrogen content of POM

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• Benthos Community Structure
• Trophic Interactions
• Deposit Feeding
• Microbial stripping hypothesis somewhat flawed
• Digestion and assimilation of POM may be much
  less efficient than digestion and assimilation of
  microbes, but POM generally is much more abundant
  than microbes
• Poor rate of utilization may be balanced by
  discrepancy in abundance between poor food source
  and rich food source
• Some forms of POM are more digestible than others
• Initial conclusions about digestibility of POM
  were based on studies with sea grass
• Algal detritus and phytodetritus much more
  digestible appear far more abundant than sea
  grass detritus
• Ex Benthic suspension feeders in kelp forests
  fueled primarily by decomposing kelp detritus
  from canopy
• Limit on how much deposit feeder biomass can be
  supported exclusively by organic material from
  microbial populations
• In general, heterotrophic bacterial biomass in
  sediments and in association with POM may be too
  sparse to support aerobic metabolic requirements
  of deposit-feeding population within a given area
• Bacteria are capable of producing certain trace
  fractions (e.g. micronutrients like specific
  fatty acids, amino acids and vitamins) that are
  relatively rare, otherwise
• Microbes may be important components of deposit
  feeder diets, not as a primary source of carbon
  and nitrogen but as sources of important minor
  dietary components

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• Benthos Community Structure
• Trophic Interactions
• Deposit Feeding
• Deposit feeders may use a variety of techniques
  related to their phylogenetic origins as well as
  their dietary preferences

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• Benthos Community Structure
• Trophic Interactions
• Deposit Feeding
• Deposit feeders may be classified as either
  selective or non-selective
• Some species eat bulk sediments, limited only by
  upper size limit of particles that can be
  ingested
• Animals digest labile organic material and
  defecate remainder
• Strategy used by many holothuroids and
  polychaetes, as well as bivalves that use siphons
  like suction hoses
• Other species ingest particles selectively and
  may spend considerable handling time selecting
  and/or modifying particles prior to ingesting
  them
• Ex Some amphipods tear apart larger pieces of
  particulate material and ingest only relatively
  small particles
• Ex Fiddler crabs handle sediments extensively,
  selecting fine organic particles and rejecting
  inorganic sand grains prior to ingestion
• Ex Some suction-feeding polychaetes vacuum
  particles non-selectively then sort them on palps
  before ingesting them

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• Benthos Community Structure
• Trophic Interactions
• Deposit Feeding
• Some deposit feeders are head-up feeders that
  have their mouths near or on the sediment surface
  when feeding
• Other species are head-down feeders that consume
  particles at depth but defecate on sediment
  surface
• These species often produce characteristic
  feeding traces, such as mounds and fecal casts
• Sediments with high abundances of burrowing fauna
  may have dramatically different characteristics
  from sediments that are relatively animal free