Submarine canyons are steep-sided submarine valleys cut into the continental slope. They are considered to be the main pathways for sediment transport between the shelf (ca. 200 m depth), and the abyss (reaching depths over 4000 m). Unfortunately, in addition to that sediment, they also tend to funnel our human garbage and pollutants into the deep sea…
Submarine canyons occur worldwide, along all ocean margins. A recent study by Peter Harris and Tanya Whiteway counted at least 5849 of them! This work was based on a global bathymetric dataset of the oceans (ETOPO1) that is reasonably detailed. However, in some places we have much more detailed seabed maps, and we see that the better the maps, the more canyons we find. So there is still a lot to discover in the near future!
Some of the canyons that have been mapped are directly connected to a river system on land (e.g. the Mississippi Canyon or the Congo Canyon). Others do cut right into the shelf, but are not necessarily connected to a river. They may be following the course of a deep-seated fault in the geology (e.g. Nazaré Canyon offshore Portugal), or may catch the currents and sediments at the end of a Bay (e.g. Cap de Creus Canyon in the Mediterranean Sea).
Finally, there are also canyons that do not cut the shelf, but are formed deeper on the continental slope, sometimes as the result of repeated submarine slides cutting upslope into the continental margin sediments (e.g. canyons in the Rockall Trough, NE Atlantic).
Canyons are formed by a combination of the erosive and abrasive forces of sediment flows, and of slope failures such as submarine landslides occurring on the canyon flanks. The first process tends to make canyons deeper, while the second one makes them wider. The origin of the sediment flows could be anything from flooding in the feeding river (bringing a lot of material into the ocean) to the activity of storms on the shelf, which stir up al lot of sediment. Many canyons used to be much more active during the ice-ages, when the sea level was lower than today, and the shoreline with its river mouths was much closer to the shelf edge.
Because of the steep topography and the regular occurrence of erosive currents and submarine landslides, submarine canyons contain habitat types that are often rare along the smoother and more homogeneous continental slope. For example, in some places we find rock outcrops which form a solid anchoring point for a variety of sessile fauna that catch their food through filtering of the passing waters.
Figure 1: Stylasterid coral on the flanks of Whittard Canyon (©National Oceanography Centre, Southampton, UK)
In addition, the steep shape of the canyon tends to affect local current patterns, often pushing deeper, nutrient-rich waters to the sea surface (so-called ‘upwelling’). Once there, the nutrients stimulate extra primary production or plankton growth, which in turn is the basis for a rich food chain. On the other hand, the occasional downslope sediment flows that transport material from the shelf to the deep sea also bring fresh organic matter from the shallower to the deeper waters. This supports a richer fauna also in the deeper parts of some canyons, compared to the average continental slope.
Overall, canyons are often found to be areas with increased regional biodiversity and increased biological growth. This is a result of all the different habitat types that can be found so closely together and of the specific processes that bring nutrients to the sea surface, and organic matter to the bottom of the canyons. It makes submarine canyons very important locations when considering environmental conservation in the deep sea!
Unfortunately, still far too little is known about submarine canyons, about the exact way those current and sediment transport processes work, and about the enormous variety in biological life forms, species and ecosystems that occur. Again owing to their complex shape, marine research in submarine canyons has always been quite difficult. The steep walls cannot easily be sampled or photographed with traditional, drop-down equipment. It is only since the technological development of deep-water robotic vehicles that we can now slowly begin to ‘look around’ in submarine canyons, and can start to understand how they work.
This sometimes requires researchers to think outside the box and turn well-established methods on their head! Which is exactly what Veerle Huvenne and her colleagues did a few years back in the Whittard Canyon. They used ISIS, a Remotely Operated Vehicle (or ROV) to survey the steep canyon flanks. Instead of deploying the multibeam echosounder (the typical piece of acoustic equipment to make seabed maps) in its traditional down-ward pointing orientation, they placed it in a forward-looking position, and used that to make a series of maps of one of the steep canyon walls. To their surprise, they found that the wall was (a) not just steep, but actually overhanging, which is something you’d never be able to see from the sea surface, and (b) covered with a rich hanging cold-water coral habitat, including corals, sponges, fish, bivalves,…
Figure 3: Mapping the steep or overhanging walls of submarine canyons leads to the discovery of rich hanging ecosystems which were entirely unknown until a few years ago! (after Huvenne et al. (2011, PLoS One)).
Further research into this method is currently ongoing within the CODEMAP project, while other examples of rich hanging deep-sea communities have recently been discovered in different canyons around the world. In terms of marine conservation, these vertical communities could be very important. By grace of their steep morphology, they somehow protect themselves from a lot of the potential human impacts in the deep ocean (at least from trawling fisheries, for example, which nowadays can take place to >1500 m water depth). This way they could act as nursing grounds for several species, and we hope one day they might also provide the larvae needed for the restoration and re-colonisation of other parts of the deep sea that already have been affected by human impacts. Still, steep bathymetry will not protect the rich biodiversity from all the perils caused by human exploitation of the deep ocean – the corals in Whittard Canyon will still feel the effects of ocean acidification, while entangled fishing lines and plastic bags, swept down the canyon by the occasional sediment flows, could be seen in between the coral framework. So it is very important we are careful with those vulnerable marine ecosystems, and that we try to protect them as much as possible.
Here’s an interview with Veerle Huvenne herself, speaking about her work in the Hermione project.
Harris, P., & Whiteway, T. (2011). Global distribution of large submarine canyons: Geomorphic differences between active and passive continental margins Marine Geology, 285 (1-4), 69-86 DOI: 10.1016/j.margeo.2011.05.008
Huvenne VA, Tyler PA, Masson DG, Fisher EH, Hauton C, Hühnerbach V, Le Bas TP, & Wolff GA (2011). A picture on the wall: innovative mapping reveals cold-water coral refuge in submarine canyon. PloS one, 6 (12) PMID: 22194903
Thanks and acknowledgements go out to Dr. Veerle Huvenne of NOC for taking the time to produce this blog post and informing us of this relatively undiscovered seabed habitat.