Sandy and Muddy Shores

  • Created by: rosieevie
  • Created on: 19-03-17 22:25

The Sedimentary Environment

Rocky shores - 2D environment, sessile and mobile epifauna

Soft shore - 3D, mobile epifauna and infauna (within or on top of sediment)

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Porosity and Permeability

Porosity - volume of pore space in between particles

Small particles reduce porosity

Permeability - rate of percolation of water through sediment

Low porosity = low permeability

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Water/Mud Content

Water content relates to particle size, beach profile, water table height

Dilatant sands - pressure applied causes dry and hard packed sand, difficult to burrow (low diversity)

Thixotropic sands (quick sands) - high clay content so when wetter easily penetrated when agitates, easy to burrow (high diversity)

Muds - don't drain (saturated), soft and easy to burrow (higher diversity)

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Oxygen and Sediment Chemistry

Heterotrophic bacter decomposes organism material at surface -abundant oxygen

Oxygen consumption depreives deeper layers = lower surfaces anaerobic

Less oxygen deep go = anaerobic bacteria dominate = sulphur smell

Depth of oxygenated layer depends on grain size - permeability

Exposed shores = larger grains = deeper oxygen layer

  • Burrowing animals generate respiratory current within burrows
  • Others extend siphones into oxygenated areas to feed while being sheltered from predators e.g. soft shelled clams

Redox discontinuity layer - transition layer between oxygen rich and oxygen poor layers

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Organic Content

Coarse sands - high turbulence environment = low organic content

Fine sands = high organic content

Amount and quality of organic loading effects oxygen concentration and biogeochemical processes

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Plants and Algae

Less macroalgae - hard substratum needed

Blooms of green algae on mudflats and brown algae if on pebbles

Benthic diatoms present as biofilms on sand/mud

Spartina plants - make up saltmarshes

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  • Deposit feeders e.g. amphipods, lungworms, muscles
  • Predators e.g. shore crabs, anenome, ragworm
  • Detritovores e.g. heart urchin
  • Filter feeder e.g. soft-shelled clams, cockles
  • Grazers e.g. hydrobia snail
  • Macoma balthica bivalve changes from deposit feeder on sheltered shores to filter feeder on exposed shores
  • Infauna very mobile and make deep burrows

Ratios of different fauna types depend on sediment type - microfauna in fine sand while macro/meiofauna dominate in fine sand

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  • Nematodes
  • Polychaetes - larval
  • Ostracods
  • Ciliates
  • Gastrotrichs
  • Harpacticoids

1 million species

Over a million individuals found per square metre

80% of individuals on Earth

Types of fauna on sandy shores inversely related in terms of biomass and abundance

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Fossil Record

Exceptional fossil record

Found on all continents, very high elevations

Dated back to 3.5 billion years -first living organisms (Stromatolites)

Stromatolites - formed by cementtion of sediment grains by biofilms

Diversity of muddy shore organisms changed over history

Mass extinction events e.g. Permian-Triassic, Cretaceous-Paleocene

Bivalves common throughout Phanerozoic

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Macrofauna Diversity and Biomass vs Particle Size

Zonation schemes related to hydrodynamics (water table/wave action).

Lack of distinct vertical zonation

Sediments buffer physical stresses - temp fluctuation, desiccation so organisms mobile and burrow into intertidal region

Progressive addition of species from exposed to sheltered byt loss of those unable to tolerate more reducing environment

Species richness, abundance and biomass increase with decreasing exposure and increasing sediment stability

Vertical distribution within sediment more pronounced than up a shore - organisms prefer more shallower oxygenated sands but because interspecific competition reside in deeper sands.

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Feeding Modes and Burrowing

Deposit feeders - ingest sediments and derive nutrition from extracting detritus/organic materia

Surface deposite feeders - surface, benthic microalgae and bacteria

Head-down deposite feeders - depths, defecate at surface

Filter feeding bivalves - characteristic of sediments

  • Burrowing animals either use hydro-mechanical or simple digging mechanisms
    • Thixotrophic sediments required - less resistant w/ concentrated force
    • Invetebrates not only burrowers - eels, rays, walruses
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Fossilised burrows (trace fossils) - created by diagenesis where sediments change into each other

Few examples of infaunal life in Cambrian - more common in Ordovician - extenside/deep burrows

After late Permian extinction, benthic animals increasingly infaunal, cosequence of increased durophagy (eat hard-shelled food) evolution during MMR.

Infaunalization large scale in Mesozoic

  • Lungworms originate in the Triassic period
  • Bivalves diversify post late Permian

Number of taxa moved from shallow waters to deep ocean and 'safe' places - direct response to late Permian extinction and subsequent diversification of durophagous predators during MMR?

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Sediment Stabilisation

Biostabilisers e.g. diatoms

  • Increase cohesiveness
  • Sediment surface smoother
  • Form protective layer

Benthic microalgae - form biofilms and secrete a mucus (EPS) in order to migrate. EPS increases cohesiveness and reduces bed roughness. Effect greatest in spring/summer on upper shores - increased photosynthesis

High sand mason (tube worm) diversity = stabilisation by 'skimming' flow and protecting bed from turbulence. Few tube worms = complete destabilisation by promoting bed scour through wake turbulence - water trapped between worms

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Sediment Destabilisation

Bioturbators e.g. whales, crabs, walruses

  • Sediment surface rougher
  • Regrade sediment particle structure
  • Reduce sediment strength
  • Oxygenate sediment
  • Mofidy geochemistry profiles
  • Exclude sessile filter feeders - cannot cope

Hydrobia, lung worms and sand masons - change bed roughness

Bamboo worm - head-down conveyor-belt deposit feeders - vertically rework sediments so smaller excreted at top = lower sediment size distribution at top

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Sandy Beach Community Dynamics

Lower species diversity and simpler food web:

  • Subjected to heavy wave action or sheltered bays
  • Costal geomorphologists classify beaches using slope, particle size and wave action into morphodynamic ranges from: Reflective to Dissipative
  • Low primary production and dependent on imported surf-zone phytoplankton with swash
  • Few deposit feeders 
  • Most infauna filter feeders
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Mudflat Community Dynamics

Higher species diversity

  • Upper reaches of estuaries in sheltered areas
  • Up to 50% surface area of estuaries
  • Shallow aerobic surface with large anaerobic, black smelly layer
  • Primary production dominated by benthic diatoms, macroalgae uncommon
  • Microbial communities dominant
  • Benthic invertebrates dominated by grazers and deposit feeders
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Human Impacts

  • Climate change
  • Ocean acidification
  • Habitat alteration - beach replenishment schemes
  • Harvesting
  • Pollution
  • Invasive species
  • Freshwater inflow
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Freshwater Inflow Case Study - Colorado River

  • 14 main dams built - no freshwater reaches Gulf of California
  • Prior, mud flat habitats and high tidal range
    • 2 clam types
      •  M. coloradoensis - brachkish water
      • Chione cortezi clams - marine
    • Clam diversity reduced
    • M clam massively decreased
    • Also affected predators of clams e.g. crabs, snails
  • Fish and porpoises dependent on inflow of water
    • T. macdonaldi - extensive overfishing and dam building reduced size and number
    • Blocked fish moving upstream to lay eggs 
    • Stopped juvenile fish moving to brackish water - required for early growth
    • Post dam T fish - slow growth rates, smaller sizes, reduced pop growth, critically endangered
    • Key reasons - past fishing, illegal fishing, lack of freshwater inflow
    • Swim bladders worth lots on black market
    • Porpoises feed in fish - decreased
    • Solutions - pulse flows and preventing illegal fishing
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