Strong Currents and Seabed Habitat Offshore of the Finnmark Coast

The strong currents off the coast of Finnmark are renowned by local seafarers, and our latest observations North of Nordkynnhalvøya (Figure 1) are shedding light on what affect these currents are having on the seabed.

By Valérie Bellec, Ned King, Tor Erik Finne, Preben Vindenes and Børge Holte

Figure 1. Study area showing the strandflat area off the the coast of Finnmark at Nordkynnhalvøya. The video lines already acquired in this MAREANO cruise are shown as red lines, together with corresponding figure numbers. Depths are about 200 m.

Our latest video and sonar data are mainly along the flank and beyond the bedrock-dominated strandflat, where storm waves have their affect (Figure 1). Typically, loose sediment originally left by the glaciers can be stirred up and transported in a variety of so-called bedforms. Where we used the Campod on the biggest forms (sandwaves), we indeed see active movement of sand and shell hash, and on the sandwaves are smaller ripples (Figure 2).

In other areas, only a dusting of sand and shell fragments cover glacial gravel on flat-lying bedrock, and the currents have moulded long bands of sand stringers (Figure 3).

Boulders and smaller cobbles are common, either over glacial deposits or directly on bedrock. These not only provide essential habitat for fish (including shelter and hiding places) but they influence the currents, locally accelerating the currents to cause scouring moats on the stoss or up-current side and leaving lee-side “comet marks”, cone-shaped sand deposits (Figure 3), or, depending on the current strength, long trails of gravel where the added turbulence won’t let the sand settle.

The glacial deposits are generally clay rich, but thousands of years of exposure to the currents have concentrated the large particles at the seabed, by removing and transporting the fines away. These deposits still retain the network of iceberg scours or ploughmarks stemming from just after the glacier left, and this creates another habitat of small-scale relief and a seabed texture patchiness (sand, gravel, cobbles, boulders) of which benthic organisms can take advantage.

Even beyond the strandflat, sometimes the bedrock sticks up through the glacial deposits, and our video dives there (Figure 4), five km seaward of the strandflat, show a homogeneous cover of gravel and cobbles but locally a similar scattering of small sand patches or stringers across the gravel. Of course, different life-forms will preferentially inhabit all of these seabed textures and seascapes.

The epifauna living on top of the sediments in these high-energetic bottom areas was not very diverse. The most common groups, although not represented by many individuals, were small-sized sponges, hermit crabs, sea stars, some bivalves and a few polychaetes. Filter-feeders such as sponges and bivalves take advantage of these high-energetic areas where food particles are being brought to the animals by the currents.

Carnivores were also observed, such as hermit crabs, sea stars and a few individuals of the polychaete Nothria conchylega which hunts across the sediment surface. The polychaetes comprise a group of animals that are very diverse. A great number of these species live below the sediment surface. This makes the polychaetes difficult to observe through a video camera such as the MAREANO's "Campod", that is hauled by the research ship "G.O. Sars" at a speed of 0.7 knots. Actually, Nothria conchylega was described in 1835 by Michael Sars, the father of Georg Ossian Sars who has given name to the present research ship.

Figures:

Figure 2. Pictures grabbed from HD video taken in a narrow channel between a high structure and the strandflat. The channel seafloor is covered by sandy sandwaves and ripples.

2.1 Sandwave crest looking almost down-current over the lee-side face. The sharp crest indicates active transport of these large sand bodies. Only the crest shows here but these form a large field with a regular pattern of dunes spaced at about 100 m. It is actually the much smaller superimposed ripples that do the work of carrying the sand and shell grains.

2.2 Ripples on sandwaves depict a swirling pattern in these so-called three-dimensional forms. The laser provides a 10 cm box for scale.

2.3 Sea Urchin (irregular).

Figure 3. Pictures from a video line acquired at about 5 km from the strandflat, on a large high structure.

3.1 Homogeneous cover of gravel show influence of the currents. The finer sand and clay particles are carried away leaving only the coarse material.

3.2 Outcropping bedrock covered in some places by coarse shelly sand, gravel, cobbles and boulders.

3.3 Sandy area showing comet marks due to turbulence behind the cobbles.

3.4 Shelly sand as in the photograph above.The red points are lasers allowing measuremet of the bottom features (10 cm separation). Visible are three or four worms that take formerly living carbonate shell fragments and glue them into a tube-shaped "house" (Nothria conchylega), each 3-4 cm long (close-up below).

3.5 This sand is not only fine rock fragments; this close-up shows a Nothria conchylega on sand with a high content of carbonate fragments that once were part of living organisms.

Figure 4. These pictures from the video recording are from a homogeneous gravel cover on a bedrock-cored hill protruding from the glacial sediments just beyond the submerged strandflat outside Porsanger. A variety of fauna make their home here.

 
4.1 Homogeneous cover of gravel in the current-influenced area beyond and deeper than the coastal submerged strandflat.

4.2 This shrimp-like mysida is happy to have the currents bring floating food particles which are picked out with the help of tiny feeders. Such shrimp can be abundant on the seabed but their small size keeps them relatively unknown to most.
 

4.3 As do plenty of other bottom dwellers living in strong currents, this sponge filters tiny food particles. The filtering efficiency in sponges may reach amazing 99%. Look closely for the "troll lobster" (Munida) claws just to the right of the white sponge. 
 

4.4 A hermit crab (Pagurida)  living in this gravelly sediment has its burdens. Not only does he have to carry his heavy home "stolen" from a dead snail to protect the vulnerable tail, but three attached anemones have become his permanent passengers.
 

4.5 Many benthic organisms have developed to take advantage of food in current-rich areas. This little pectinid mollusc in a sandy gravel area gets nourishment from tiny particles of detritus drifting past, essentially "served on a platter".

4.6 This starfish perched on a pebble within a shelly sand is a true predator, eating smaller forms by first encircling them with his underside and then stuffing the prey into its digestive system.