Horseshoe and cold-water coral reefs
This well-known fishing bank is localized in the Norwegian Sea, southwest off Lofoten, and about 2-3 hours sailing time from the Træna islands in Nordland county. The morphology of the bank is largely uniform, except for a prominent feature named Hesteskoen (Horseshoe), which is located in the eastern part of Trænabanken (Figure 1).
Hesteskoen was formed during the last ice age by the glaciers transporting large sediment masses from the seabed, leaving behind a large hole of about 40 km2. Later the sediment masses detached from the glacier and formed a semi-circular large hill composed of various sediment types, from fine-grained clay to large cobbles and boulders (to learn more about these features, see Rise et al. 2016). Hesteskoen is an example of a landform called a “hill-hole pair”. It is one of many well-known glaciotectonic structures from the ice age on the Norwegian continental shelf.
The top of Hesteskoen hill reaches a depth of 180 m, about 100 m above the surrounding seafloor, while the deepest part of the hole is almost 350 m deep (Figures 1 and 2).
The highest part and the sides of Hesteskoen are composed of coarser material than the deeper flat seabed. Gravel, cobbles and boulders, often associated with finer sediments (clay, silt and sand), are quite common in this type of formation seen on video. However, the deepest part of Hesteskoen shows the same soft sediments as the surrounding area (mainly mud and some sand).
Cold-water coral reefs, formed by eye corals (Lophelia pertusa), thrives on hard bottom exposed to bottom ocean currents. This explains why the ongoing Mareano cruise found coral reefs on top of both sides of the elongated Hesteskoen formation where cobbles and boulders are common (Figure 2).
Not only corals, but also other seafloor living invertebrates thrive in these specific physical environments which are formed by geological processes. Species that need solid ground and physical support in order to stay upright in the water masses to filter out food particles from the water are relatively common in the hard-bottom locations of Hesteskoen. Borrowing species are living from eating muddy sediments rich in organic material accumulated on the seafloor, like in Hesteskoen’s hole soft sediments. These examples may illustrate how the benthic community composition varies from place to place, thereby indirectly also influences on demersal or pelagic organisms feeding on seafloor living invertebrates.
Reference: Rise, L., Bellec, V.K., Ottesen, D., Bøe, R., Thorsnes, T., 2016. Hill-hole pairs on the Norwegian continental shelf. Geological Society, London, Memoirs, 46, 203-204, 2016..