Submarine Canyons: CBNMS

Bodega Canyon is the most prominent submarine canyon within Cordell Bank National Marine Sanctuary. This seafloor feature, which is 20 km long and over 1600 m deep, cuts across the continental slope in the northwest portion of the sanctuary and extends to the deep sea.

Several small submarine canyons also cut into the continental slope of Cordell Bank National Marine Sanctuary. Present scientific knowledge about these canyons is limited as they are largely unexplored.

The majority of canyon habitat is expected to be soft-bottom, while a much smaller portion is predicted to be hard-bottom. Much of the sediment carried by alongshore currents ends up in the axes of active submarine canyons. For example, it is estimated that approximately 500,000 cubic meters (17,657,333 cubic feet) of sand as well as large volumes of finer-grained material descend into Monterey Canyon each year. Submarine landslides from canyon walls also deposit sediments on the canyon floor.

Submarine canyon sediment transport events are thought to be episodic. Potential triggering events include storms, earthquakes, moderate sea and surf conditions, tidal fluctuation, and flooding rivers. The frequency of these events is not well known. Current monitoring efforts, including repeat bathymetric mapping and installation of instruments in the canyons, enable scientists to determine locations where deposition and erosion take place and to quantify the frequency and intensity of sediment transport events. The organic material associated with sediments provides nutrients to deep-sea organisms.

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 of the continental shelf to the deep sea and contain a wide range of habitats, they contain an incredible diversity of organisms.

Mobile fishes and invertebrates, such as prickly sharks and krill, have been found to aggregate in canyon heads and along canyon walls. Rocky outcrops along canyon walls are colonized by invertebrates – including feather stars, corals and tunicates – 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 (e.g., sea pens, sea cucumbers, brittle stars, sea stars) and fishes (e.g., flatfishes, ratfishes, whiptails, grenadiers, sablefish, hake, thornyheads).

Submarine canyons are vulnerable to a variety of human activities because they extend across a range of depths. 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 the Monterey Submarine 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, such as abandoned fishing gear, plastics and other man-made items, in canyons.