Submarine canyons are steep-sided gorges on the seafloor of the continental slope. These distinctive underwater features share several physical characteristics with onshore river valleys:
- They are erosional features that carve into the seafloor and expose older, underlying strata in canyon walls.
- They can have sinuous channel axes and may also have a number of branching channels.
- The positions of some channels coincide with geologic faults (such as Carmel Canyon, which lies just off the Monterey Peninsula and coincides with the San Gregorio fault).
Submarine canyons are the most prominent geomorphic features within the Monterey Bay National Marine Sanctuary. Monterey Canyon, in the center of Monterey Bay, is the largest submarine canyon along the coast of North America. Numerous smaller canyons also cut into the continental shelf and slope of the Monterey Bay sanctuary. No significant canyons exist in the Greater Farallones or Cordell Bank sanctuaries.
Much of the sediment carried by longshore currents ends up in the axes of active submarine canyons. Submarine landslides from canyon walls also deposit sediments on the canyon floor. The organic material associated with sediments provides nutrients to deep-sea organisms.
Sediment transport events are thought to be episodic. Potential triggering events include storms, earthquakes, moderate sea and surf conditions, tidal fluctuation, and flooding rivers.
Most organisms observed in canyons are not unique to canyon systems but are also found at similar depths outsides canyons. However, because submarine canyons extend from shallow waters to the deep sea, they contain an incredible diversity of organisms. For example:
- Mobile fishes and invertebrates aggregate in canyon heads and along canyon walls.
- Rocky outcrops along canyon walls are colonized by invertebrates and provide shelter for a variety of rockfishes.
- Clams and worms burrow into canyon walls.
- The soft sediments on the canyon floor support a diverse community of invertebrates and fishes.
Conservation and Management Issues
A Humboldt squid (Dosidicus gigas) dashing in front of the ROV camera. This photo was taken at a depth of -481.8 meters in Sur Canyon as a part of a deep-sea coral expedition conducted by NMFS aboard the R/V Shimida in December, 2010.
Because they extend across a range of depths, submarine canyons are vulnerable to a variety of human activities. For example, a comparison of contaminant loads in surface and deep-sea fishes in Monterey Bay found elevated concentrations of persistent organic pollutants, such as PCBs and DDT, in fishes collected from Monterey Canyon.
The risk of pollutant bioaccumulation is higher in submarine canyons than in surrounding waters because the flow of sediments and pollutants tends to be concentrated in canyons. These processes may also lead to an accumulation of marine debris.
One important management issue relating to this habitat revolves around whether to allow the installation of communication cables – and if so, where to place them. If routed across submarine canyons, the cables may break as sediments rush down these geologic features. In addition, loops from slack cables are potential entanglement hazards for mobile species, such as marine mammals.
The proximity of canyons to the shore in the Monterey Bay sanctuary provides scientists with a unique opportunity to study canyon habitats. Recent monitoring efforts have focused on sediment movement events, mapping of benthic habitats, and determining the abundance and distribution of fishes and invertebrates.
Within the three northern California sanctuaries, current submarine canyon monitoring efforts take place in the Monterey Bay sanctuary; neither the Greater Farallones nor the Cordell Bank sanctuary has monitoring efforts focused on this habitat.
Examples of related monitoring projects: