What is the intertidal zone?

rockyThe intertidal zone is the area of the marine shoreline that is exposed to air at low tide, and covered with seawater when the tide is high. Intertidal zonation refers to the tendency of plants and animals to form distinct communities between the high and low tide lines. All the plants and animals that inhabit this zone are marine, but they have adapted to survive in this often challenging environment. At low tide they must be able to survive prolonged exposure to the air, large fluctuations in temperature and salinity, extremes in wave action, as well as predation from land animals (e.g. raccoons, birds) during low tide, and marine predators like sea stars and fish when the tide is in. Starvation is also a risk when the tide is out since most intertidal animals feed only when they are submerged. Also, most intertidal animals need to be underwater to respire and this can be a limiting factor affecting their distribution on the shore. These challenges are more pronounced higher on the shore where organisms are exposed for longer periods of time. On the positive side, animals such as mussels and barnacles can escape predation from sea stars by inhabiting the upper intertidal zone, above the limit where sea stars can thrive.

What plants and animals live in the intertidal zone?

The marine waters of the Pacific Northwest support a rich and diverse intertidal marine community, due in part to upwelling currents that bring nutrient-rich waters up to the surface. Despite the environmental challenges of the intertidal zone, there is a wide diversity of marine life forms that inhabit this ecosystem. Some types of intertidal species and their adaptation strategies include:

  • Mobile species such as crabs, limpets and snails hide under rocks, in moist crevices or under seaweeds to escape the sun and keep from drying out.
  • Some attached animals like barnacles and mussels can hold a small amount of seawater within their shells, and close up tightly during low tide.
  • Algae such as Porphyra, Fucus and Enteromorpha have the remarkable ability to survive losing 60-90% of their moisture, to the point of becoming brittle; when the tide comes back in, they reabsorb water.
  • Many non-motile (sedentary) species are adapted to withstand high wave action. For example, kelps attach to the substrate with strong holdfasts, barnacles stick to rocks by secreting cement, and sea stars have thousands of tube feet that hug the shore with tiny suction cups. Mussels secrete strong pliable “byssus” threads that anchor them to the shore.

Plants and animals with similar tolerances to the stresses of intertidal life tend to form communities or bio-bands along the shoreline. These horizontal bands are often easy to spot even from a distance, particularly on rocky shorelines, due to differences in colour and form. For example, just below the trees there is typically a black-grey band of encrusting algae and lichens; slightly below that will be a greyish band of barnacles with a yellowish-brown band of rockweed (Fucus) and mussels below that.

Zones along marine shores are generally divided into the following (refer to diagram above). The backshore is the upper area of land between the high tide line and an imaginary boundary 50 meters inland. Terrestrial species like forests and land animals dominate this zone.

Closest to the high tide line, the backshore zone is fringed with the spray zone. This area is not covered by the high tide, but it is exposed to salt spray. It is typically inhabited by periwinkle snails (Littorina), black encrusting lichens (Verrucaria) and orange lichen (Caloplaca).

The intertidal zone is the broadest zone. It lies between the highest high tide and the lowest low tide, and is flooded once or twice daily by the tide. The form and shape of a shoreline, as well as the degree of wave exposure determine what species of animals and plants are likely to be found living there. Rocky shores tend to be steeper than sediment shores, and some fine sediment shores, such as Patricia Bay on Saanich Inlet, have very wide shallow-sloped intertidal areas.

r-sOn a rocky shore, the intertidal zone is marked at its upper limit by barnacles, and closest to the low tide by large kelps (e.g. Laminaria and bull kelp). Other species present include Fucus (rockweed), barnacles, limpets, Nucella (a snail), hermit crabs, shore crabs, mussels, anemones, chitons, sea stars and many seaweed species, to name a few. On shores with fine sediments (sand, mud, small pebbles), many animals are found buried in the sediment or attached to pebbles and cobbles. These include clams, mud shrimp, sand dollars and many types of worms.

estuaryThe subtidal fringe is exposed only by the lowest of low tides. It borders the subtidal zone, which is always submerged. The subtidal fringe has the richest diversity of species of all the intertidal zones, and includes those that can tolerate only a short exposure to air. Some of the many types of species in this zone include kelp, eelgrass, many species of red algae, sea slugs, sponges, bryozoans, snails, limpets, chitons, crabs, shrimp, sea anemones, sea stars, sea urchins, sea cucumbers, moon snails as well as fish such as sculpins, clingfish and gunnels.

Where can the intertidal zone be explored around Victoria?

There are many excellent places to explore the intertidal zone in our region. Cattle Point and Clover Point are both great places to explore the rocky shoreline in the urban area. Further west near Port Renfrew, Botany Beach is well known for the beautiful tide pools full of marine life that can be viewed at low tide. In between there are many public beaches and parks along the southwest shores of Vancouver Island where the intertidal zone can be explored (e.g. East Sooke Regional Park or Sandcut Beach. In the opposite direction, Cordova Bay, Island View Beach and Patricia (Pat) Bay all have wide sand beaches that are easy to access.

Basically anywhere that the shore is safely accessible at low tide will be a good place to see intertidal marine life. Be sure that you check local tide tables before you head out, go prepared for weather changes, and never turn your back on the sea as large waves can reach far up the shore even on a calm day.

Why is the intertidal zone important?

The intertidal zone is a critical interface between terrestrial and marine ecosystems. In this region, characterized by mixed semi-diurnal tides (two high and low tides in 24hr, of unequal size), the ebb and flow of the tide determines the rhythm of life cycles and behaviours of many animals, both marine and terrestrial (including people!).

  • For migrating birds, low tides periods are critical feeding times when birds have access to the rich food that mudflats, estuaries and similar intertidal habitats provide.
  • Resident birds such as herons, eagles, gulls, and many species of shorebird feed in the intertidal zone during low tides.
  • Animals such as bears, wolves, raccoons, mink, otter and deer forage for a variety of marine life during low tide/
  • For thousands of years, intertidal animals and plants have provided an important food source for many human cultures. In many parts of the BC coast, aquaculture of species such as the Pacific oyster depends on a clean and healthy intertidal zone.
  • The intertidal zone provides a physical buffer protecting land from erosion by wave action during storms. This protection is critical for buildings, roads, bridges and other infrastructure built along shorelines.
  • A rocky shoreline at low tide provides an irreplaceable outdoor classroom for students of all ages. In this region of the Pacific northwest, we are fortunate to have a rich and diverse intertidal zone, and conservation and environmental stewardship of this ecosystem is essential.

How can I help protect it?

Learn more about the plants and animals that inhabit the intertidal zone, by visiting local beaches and quietly observing. Use proper beach etiquette while visiting the shore:

  • “Take only photographs, leave only footprints”. Empty shells are recycled and reused by animals such as hermit crabs. Seaweed tossed on the shore by fall and winter storms should be left there to decompose, rather than being gathered for gardens. Many marine species such as amphipods are dependent on these piles of seaweed for their food, and are essential for recycling the nutrients in the seaweed back into the marine ecosystem.
  • Avoid trampling plants and animals while visiting the seashore. Studies have shown that human trampling is often responsible for much of the disappearance of marine life on shores along popular public beaches.
  • Avoid turning rocks over – you may disturb many animals and plants taking refuge beneath rocks during low tide
  • Learn more about local marine life by visiting marine aquaria (see links below).
  • Visit Great Canadian Shoreline Clean-up to find out how you can help clean up a local shoreline
  • Join a local stewardship group working to protect shorelines and marine life
  • Learn how to protect shorelines
  • Learn how to limit the impact of shoreline development

More information:

© Images courtesy of Stewardship Centre for BC

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