Benthic systems: Unvegetated Sediments

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Benthic systems Unvegetated Sediments

• Associations, Diversity, Disturbance

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Deep Sea Benthic diversity

• In on sediments
• Dominated by macrofauna
• Defined by size (gt 300 µm)
• Include polychaetes, molluscs, crustaceans,
  echinoderms
• Estimated to include between 500,000 and
  10,000,000 species
• Program to inventory under way (CeDAMar or
  Census of Diversity of Marine Life)

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Ecological importance of benthic macrofauna

• Nutrient cycling at ecosystem level
• Food resource for commercially important species
• Pollutant metabolism
• Dispersion burial
• Energy cycling
• Influence sediment structure turnover

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Why so many species of macrofauna?

• Why would we expect low diversity?
• Apparently low variety of habitats so apparently
  low number of different niches
• Low rate of input for new energy/nutrients
• Competitive exclusion principle predicts low
  diversity

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What ecological mechanisms would explain high
diversity?

• H1 Niches are defined by more dimensions than
  sediment type
• Location within sediments (e.g., vary in O2)
• Other organisms create biotic variation
• H2 Competition is not a major factor
• Predator influence
• Disturbance influence
• H3 Local diversity may be low but regional
  diversity can be high
• This is multiplied by a very large area of habitat

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Sediment variation Bioturbation
Variable sediment surface from biological
activity 1100 m
Box Core from 1900 m
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Spatial variability in distribution of polychaetes
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Continental Shelves overview

• 7.4 of ocean surface
• Average 78 km wide, 135 m deep
• Most above water during ice ages (18k bp)

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Topography of a typical continental shelf
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Characteristics of the habitat

• High productivity along continents
• Higher turbulence than in deep ocean
• More variable salinity and temperature
• Light penetration usually is reduced (often to 10
  to 20 m)

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Components

• Infauna classified by size
• Bacteria protozoa
• Macro- and meiofauna
• Major taxonomic groups
• Polychaetes, crustacea
• Echinoderms, molluscs
• In similar habitats, community composition
  suggests convergence in form

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Major questions

• What are the major components of communities in
  unvegetated sediments?
• What is the relationship between taxonomic
  classification and ecological function?
  (Functional equivalents)
• How can disturbance promote diversity?
• What are the consequences of bottom-dredging
  fishing techniques?

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Functional groups examples of sediment
stabilizers and bioturbators
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Parallel infaunal communities in North Temperate
oceans Ecological convergence
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Apparent paradox high diversity, low habitat
heterogeneity

• Competitive exclusion is observed
• Deposit feeders exclude suspension feeders
  (really amensalism)
• Competition for space is documented

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Why not have more competitive exclusion?

• More niches than we thought?
• Vertical sorting hypothesis

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More niches?, continued

• Biotic facilitation
• Mutualism
• Commensalism (e.g., inquilines in tubes)

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Can disturbance facilitate diversity?

• Sources of disturbance
• Biotic disturbance
• Burrowing, sediment processing
• predation (exclosure experiments)
• Water turbulence, wave action
• Human disturbance
• Beam trawls designed to scrape sediments a few
  centimeters below surface
• Otter trawls, etc. along surface
• Consider differences in scale

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Effects of trawling on bottom structure
Fished Unfished
Kaiser Spencer. 1996. Journal of Ecology 65
348-358
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Effects of local and mid-scale disturbance

• Very patchy distributions
• Leading to high species turnover in space
• Lower density than required for competitive
  exclusion
• Superior competitor is reduced before inferior
  competitors disappear
• Recolonization patterns add to diversity because
  larval settlement depends on timing

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Changes in community diversity with trawling
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Why does disturbance from trawling reduce
diversity?

• Direct effects on habitat
• Scale of disturbance
• Disrupt biotic interactions
• Reduce potential for recolonization

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Benthic processes deep ocean systems
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Benthic processes coastal, subtidal systems
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Benthic processes vegetated, subtidal systems