Continental Shelf

• Continental shelves


  Contribute only 8% of the global sea-surface
• Continental shelves


  • Shallow nature
  • Most of the waters fall within the euphotic zone
  • Organic and mineral inputs via riverine discharges
  • Strong physical mixing of the water column and seabed
• Continental shelves are among the most productive and economically important regions of the world's oceans
• Regions with narrow continental shelves

  • Nutrients are additionally supplied through upwelling of water from the deep sea
• High productivity found on continental shelves fuels high levels of secondary production
• Primary production in shelf seas fuels 90% of the world's fisheries landings
• Future environmental changes that impact upon primary production

  Likely to have major consequences for shelf sea ecosystems and the goods and services that they provide
• Shelf seas


  • Abundant plankton and fish fauna
  • Key food resource for populations of top predators (seabirds and marine mammals)
  • Proximity to coastline
  • Shallow nature
• Shelf seas have become the focus of intensive human activities
• Shelf seas receive agricultural and industrial contaminants from terrestrial run-off
• Human activities in shelf seas

  • Shipping
  • Fishing
  • Exploration for hydrocarbons
  • Extraction of hydrocarbons
  • Mining of sediments and minerals
  • Underwater cable laying
  • Wind farm development
  • Tidal barrage development
  • Offshore aquaculture
  • Recreation
• The ecology of the continental shelf is under ever-increasing levels of human usage
• This has resulted in major changes in ecosystem structure in some localities
• The intensive use of the coastal shelf inevitably increases the risk of environmental damage from activities such as over-fishing, eutrophication, mineral extraction, dumping of waste, and oil spill accidents. The effects of these human activities may be exacerbated by the additional stress imposed by the current rapidly changing environmental conditions, particularly if they act synergistically.
• Continental shelf


  Extends from the extreme low-water mark on the shoreline down to a depth of approximately 200 m, termed neritic
• Continental shelf


  • Extends beyond the land from between nearly zero up to 1500 km offshore out to the shelf break
• Shelf break


  Area where biological and geological material from the continental shelf is supplied to the shelf slope, through a variety of processes such as the death of organisms or more dramatically via submarine mudslides
• Continental slope


  • Generally has a shallow gradient of approximately 1 degree except in regions where glacial activity has sculpted a more dramatic seabed
• Continental shelf


  • Shallow depth and position adjacent to the physical barrier of the land mass mean that it is strongly influenced by physical forcing processes, such as glaciation events, currents, waves, the formation of fronts and water turbidity
• Interaction of physical processes on continental shelf

  Influenced by shelf width and geographic disposition and is strongly related to consistent patterns in regional ecosystem structure
• Physical processes perform a key role in the continental shelf environment due to its proximity to land, inputs of fresh water, seabed topography, and shallow depth.
• The physical and biological characteristics of the continental shelf habitat are strongly influenced by the geological composition of the seabed, much of which has resulted from past glacial events that have had a profound influence on coastal margins and shallow near-shore seabed structure. The melting and formation of ice sheets causes the sea-level to rise and fall respectively, while the Earth's crust is lowered under the weight of ice, but rebounds (rises) in its absence. Thus areas hundreds of kilometers offshore may have been dry land at some point in recent geological history.
• The continental shelf is relatively young in geological terms and has undergone dramatic expansion and contraction as a result of glaciation events. Evidence for these events can be seen vividly on the seabed as drowned river plumes, boulder fields, and glacial scouring
• Wave effects


  • Important consequences for the ecology of the shallower areas of the shelf
  • Effects reach down to a depth of 80 m on open Atlantic coasts during gale-force conditions
• Depth to which waves influence benthic ecology


  Depends on the extent to which the coastline is sheltered from prevailing winds and the extent of fetch
• Where the physical force of waves causes sediment movement

  Wave action can be a major cause of mortality among benthic animals and has been shown to affect secondary production by limiting the body size of organisms that can survive in a highly energetic environment
• Water movement


  Generates currents and these affect both the shallow and deeper parts of the shelf
• Flow


  • Extremely important for the ecology of the continental shelf
  • Affects the passive and active transport of organisms, their gametes, and larvae
  • Affects the rate of supply of food from the pelagic system to the seabed
  • Places upper physical constraints on the type of organisms that live in particular habitats
• Close to the shoreline


  The seabed is affected most strongly by both waves and currents
• As we move further offshore

  The effects of waves reduce and the effects of currents begin to dominate the physical and biological processes on the seabed
• This gradient


  Is clearly reflected in the biomass of sessile filter and deposit feeding biota, which increase with distance away from shallow into deeper water where the physical stress associated with waves and currents decreases and the seabed is therefore less frequently subjected to physical disturbance
• Fetch is the uninterrupted distance over which winds exert friction at the sea surface.
• The ecological importance of wave action diminishes with reducing fetch and distance from the shore as water depth increases.
• Animals that are either attached to a substratum or that move relatively little or infrequently are termed sessile.
• Rising and falling water mass


  • Important implications for organisms that live on the shore
  • Generates flow or current as the water floods into, or ebbs from restricted areas of coastline
• Current


  • Increased when a water mass moves through, or around, land-bounded restrictions or across irregularities in the seabed topography
  • Typically strongest in straits and the narrow mouths of estuaries, reaching speeds of up to several metres per second
  • Strong currents around the apex of headlands and bedrock protrusions from the seabed due to restrictions to the flow of water
• Current velocity generated through the tidal rise and fait of water in a sea basin is exacerbated dose to the coast, where coastal morphology and restrictions increase the speed of flow
• High levels of shear stress

  • Cause scouring of the seabed and its biota
  • Inhibit effective feeding activity
• Biota that live in such environments

  • Have characteristics or behaviours that enable them to cope with the extremes of physical stress
  • Attached biota are highly flexible or encrusting
  • Mobile fauna will often seek shelter from currents within the sediment or in crevice habitats
• At a certain current velocity threshold

  Food particles transported from other areas will begin to sink to the seabed, where they become available as food to the benthos