benthic-zone-hhWhat are Rocky Subtidal Benthic Communities?

Rocky subtidal benthic communities are groups of plants and animals that inhabit rocky areas of the ocean floor in the subtidal zone. The subtidal zone ranges from the shoreline at the lowest low tide to the edge of the undersea continental shelf, at a depth of about 200 metres. In this case, we will be discussing much shallower depths that occur in and around the harbours of Victoria, usually less than 20 metres, and often 2-5 m. Benthic describes the ocean bottom, and the term communities refers to groups of plants and/or animals that live in close proximity, and are often interdependent.

The substrate (the basic mineral material) of benthic environments may be composed of either soft sediment or rock or a combination of the two. In soft sediment, burrowing animals predominate, and vegetation is usually absent or limited (an exception is in some sheltered areas where eelgrass beds grow).

Many species spend their adult lives attached to the substrate, and must compete for limited space. Rocky substrates occur in areas where strong tidal currents and/or significant wave action cause fine sediments such as sand grains to be removed, leaving behind larger sediments such as gravel, cobbles and boulders. Tidal currents and wave action also constantly mix oxygen, nutrients and organic matter throughout the water column, creating a favourable environment for many species of marine organisms.

Where are subtidal benthic communities found?

These communities are generally found where there is a rocky substrate and moderate to strong tidal currents. The largest rocky subtidal benthic associations in the Victoria area are found in the narrow sections of the Gorge Waterway. Smaller communities are found in Esquimalt Harbour near Inskip and Fisgard Islands.

What kinds of organisms live in rocky subtidal benthic communities?

Many types of algae grow on rocky substrates, including brown algae such as kelp and rockweed (Fucus), green algae and red algae. Kelp forests are a special case and provide a complex, three-dimensional ecosystem.

Many animals in rocky subtidal benthic communities are filter feeders. They obtain their nutrients from detritus (decaying plant and animal matter) or plankton that is suspended in the water column. A smaller percentage of animals are grazers that feed on algae (e.g. sea urchins), or predators (e.g. sea stars) that eat grazing and filter feeding animals.

Filter feeding communities may be composed of encrusting species that spread asexually, such as sponges, tunicates, bryozoans and cnidarians (e.g. anemones); or they may consist of individuals that settle on the rock, often in large groups, such as mussels, sea urchins and gastropods (e.g. snails and limpets). More mobile species such as crabs may also be present.

The subtidal benthic communities in high-current areas of the Gorge Waterway are composed primarily of the following organisms:
  • Aggregations of sponges such as the yellow bread-crumb sponge (Halichondria panacea) and purple sponge (Haliclona permollis)
  • Ascidians such as sea pork (Aplidium californicum), red ascidian (Aplidium solidum), and sea squirts (e.g. Cnemidocarpa finmarkiensis)
  • Bryozoans or “moss animals”
  • Anemones such as the plumose anemone (Metridium senile) and Tealia
  • Mussels (Mytilus and Modiolus species) and Olympia oysters (Ostrea lurida)
  • Filamentous red algae, and kelps (e.g. Laminaria species)
Many of these species are also present in the smaller rocky subtidal benthic communities in Esquimalt Harbour.

Why are rocky subtidal benthic communities important?

Competition and predation within the communities helps to maintain balance in the marine environment. For example, sea stars prey upon mussels and keep them from taking over large areas of the subtidal habitat (mussels resist desiccation better than sea stars, so they can colonize higher on the shoreline in the intertidal zone).

Other animals feed on species that live in benthic communities. For example, birds such as white-winged scoters, black scoters and the greater scaup eat mussels and snails, and river otters feed on crabs.

Filter feeding animals in these communities help to clarify the water by ingesting particles of organic matter and other nutrients. This benefits other species such as kelp, eelgrass, fish and mammals.

What threatens rocky subtidal benthic communities?

Pollution such as heavy metals, oils, pesticides and household chemicals may damage or kill organisms within these communities. These chemicals can enter the harbours by being washed off roads, disposed of down storm sewers or spilled into the ocean by boaters.

Construction activities in or near marine areas can damage fragile organisms. Building new structures such as seawalls, bridges or wharves may destroy subtidal benthic habitat.

How can I help protect rocky subtidal benthic communities?

For information on protecting rocky subtidal benthic communities, please visit our How Can I Help? section.

Additional Links & Information



  • Archipelago Marine Research Ltd. Subtidal Survey of Physical and Biological Features of Portage Inlet and the Gorge Waterway. October 2000
©Image courtesy of Minette Layne

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