Benthic Fauna

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Benthic Fauna
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Benthic Fauna

• Extremely diverse group of organisms
• Many different groups, difficult to generalize
  about patterns

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Distribution and Abundance

• Limited by few general characteristics
• Food availability
• Type of substrate
• Few phys/chem factors, especially dissolved oxygen

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Littoral vs. Profundal

• Benthic animals living in littoral region more
  varied than those in profundal region
• Reflection of
• 1) abundance of microhabitats
• 2) less stressful living conditions

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Littoral Benthos

• Protozoans, sponges, coelenterates, rotifers,
  nematodes, bryozoans, decapods, ostracods,
  cladocerans, copepods, bivalves, snails, insects,
  leeches

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Littoral Benthos

• Microbenthos - very tiny
• Vastly outnumber macrobenthos, and may contribute
  up to 50 of benthic production

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Littoral Benthos

• Seldom food limited
• Proximity to phytoplankton, macrophytes

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Sublittoral Benthos

• Boundary between littoral profundal
• Species diversity drops off sharply
• Mussels, ostracods, copepods, cladocerans from
  littoral - few typical dwellers

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Profundal Benthos

• Very poor diversity
• Oxygen limited
• Other stressors
• Colder
• Lower pH
• Higher CO2, CH4, organics, P, NH3
• Few can survive under these conditions

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Profundal Benthos

• Profundal benthos in eutrophic lakes resemble
  those of grossly polluted systems
• Low diversity, monotony - great number of
  individuals, but only 1 or 2 species represented

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Typical Profundal Assemblage

• Chironomus midge larvae
• Hemoglobin picks up limited oxygen
• May also be able to use anaerobic respiration
  excrete products of this process

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Typical Profundal Assemblage

• Oligochaete worms Tubifex, Limnodrilus
• Bury heads in organic sediments, wave tails with
  gills
• Can develop huge populations (10,000/m2)
• Preyed on heavily by predaceous Chironomus

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Typical Profundal Assemblage

• Fingernail clams in genus Pisidium
• Become dormant during anaerobic periods
• May also inhabit temporary ponds

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Typical Profundal Assemblage

• Phantom midge larvae - Chaoborus
• Temporary occupants - spend day on sediments,
  migrate into water column at night to prey on
  zooplankton
• Not very tolerant of anoxia

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Typical Profundal Assemblage

• Also several microscopic forms that tolerate low
  oxygen
• Some protozoan ciliates and flagellates, some
  nematodes

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General Standing Crop

• Most lakes have profundal benthos that averages
  5 g wet weight/m2
• 1/2 g dry weight/m2

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Less Productive Lakes

• Profundal benthos more diverse in less productive
  (oligotrophic) lakes
• Major reason oxygenated sediment-water interface

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Less Productive Lakes

• More species of midge larvae, oligochaetes,
  immature insects like mayflies
• Hexagenia - burrowing mayfly

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Less Productive Lakes

• Also more crustaceans like the amphipod
  Pontoporeia
• Makes vertical migrations up to metalimnion at
  night (doesnt eat zooplankton)

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General Benthos Pattern

• Diverse group in heterogenous, oxygenated
  littoral zone
• Less diversity in more homogeneous profundal zone
  (less in more productive lakes)

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Maxima of Abundance
Two maxima - one in littoral - one in profundal
biomass
depth
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Maxima of Abundance
As systems become more productive, zone of max.
production shifts from littoral to
profundal, then declines in profundal
biomass
depth
Midges replaced by oligochaetes
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Seasonal Abundance Patterns

• Lowest in summer (especially in insect-dominated
  communities)
• Emergence of adults, high predation
• Maximum densities and growth typically in autumn
  and winter in temperate zone

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Predation by Fish

• Predation can drastically reduce invertebrate
  standing crop
• May be gt50 of populations in some littoral areas
• Predation losses in profundal areas generally
  much lower

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Predation by Fish

• Despite intense predation pressure, benthos
  dynamics and production mostly controlled by food
  supply