Communities of benthic animals in the North Sea

The benthic life in the North Sea is extremely diverse because animals can live on as well as in the bottom. And the bottom itself can have differences. There are soft bottoms with sand or silt and hard bottoms with gravel, stone or rock. Every benthic animal has a preference for a specific type of bottom. Digging worms cannot live in or on rocky bottoms, nor can animals which attach themselves to hard surfaces survive on soft bottoms.

An estimated 600 to 220 animals live in every square meter of the bottom of the southern North Sea. Together, they weigh 30 to 90 grammes (not including the shell) and produce 70 to 150 g of new life per year. In areas with lots of silt, more animals will be found than on sandy bottoms because silty bottoms contain many more remnants of dead plants and animals, whereby more food is available.

Benthic fauna caught close to the shore with a research beam trawl with small mesh size: sole, dab, pipefish, starfish, brittle stars and American razor shells

Benthic fauna, caught far from the coast with a research beam trawl with small mesh size: dab, poor cod, sand stars and a lot of brittle stars

Communities of benthic fauna in the southern and northern North Sea

In the northern North Sea, the communities of benthic animals are clearly different than in the southern North Sea. Sponges, bryozoans, sea anemones and 'sea pens" (around 50 centimeters high, colonies of polyps resembling bird feathers) cover the sandy surface and the stones. All of these groups of animal species filter their food particles out of the water. The variety (in animal species) is much greater here than in the southern section, while the density and the season dependant differences in water temperature and numbers of animals is much less.

Mostly echinoderms and crustaceans live in the southern North Sea, eat organic matter or scavenge. The fish species also differ. It is probable that the intensive beam-trawl fisheries have had a definitely long-lasting measurable influence on the communities of benthic animals in the southern North Sea, which used to be rich in young fauna. Presently, mostly small, short-lived yet very productive species are the most dominant.

The bottom of the Dutch section of the North Sea can be roughly divided into four regions of different nature. Since 1986, scientists from the NIOZ have been taking regular samples from the sea bottom in order to establish the bottom fauna. Slowly but surelly one is getting sufficient data to determine the developments in the species composition.

There is a matter of fine sand with an average level of silt in the coastal zone. There is a matter of a dynamic environment from the large supply of nutrients and the working of the surf. There are not many different species of animals but the animals which live in or on the bottom can be present in enormous numbers. One example is the cut trough shell (spisula) where large banks are created which can also quickly disappear. There are also characteristic species for the coastal zone: the American razor shells, worms and shrimp. Different shellfish (Fabulina fabula) and predator snails (Alder's necklace shell) have clearly decline in significance in the period '86-'96.

Sandy bottoms of the Southern Bight, with the occasional gravel bank (such as Klaverbank) lie westward of the coastal zone and directly to the north of the Wadden Islands. Many offshore platforms stand in this region, which is why it is sometimes referred to as the offshore area. Just about all of the silt has disappeared from the bottom due to continual erosion. Coarse sand remains in its place. The bottom fauna is not very numerous here: the densities are low. The fauna, however, is species-richer than in the coastal waters. The two species which are considered characteristic for this region (Alder's necklace shell and the ragworm nephtys cirrosa) declined significantly between 1985 and 1993, but is showing an increasing trend as of 2000.

Further to the north and northwest lie the Oystergrounds. Water masses from the English Channel and the Northern Atlantic Ocean join together and silt is deposited. The sea bottom is therefore extremely muddy and rich in food, which is reflected in the bottom fauna: densities and richness in species are high. In one square meter of sea floor lives an average of 250 to 500 Amphiura filiformis, around 25 to 75 specimen of callianassa, 50 to 200 magelona papillicornis (worms) and 200 to 400 Mysella bidentata. Another characteristic bottom inhabitant of the Oystergrounds is the long-lived quahog. There are no clear trends observable in the composition of the benthic fauna, except that the decline in the amounts of the rag-worm nephtys hombergii. In 1997, the number of callianassa suddenly increased.

In the most northern point of the Dutch section of the North Sea lies the southern edge of the Doggersbank. The bottom here consists of fine sand with an average silt level and is subject to erosion: the silt level has been declining since 1991.Even here is a matter of a rich benthic fauna and a large variety in animal species. Remarkably enough, most of the characteristic animal species for this region increased significantly till 1993, but sall howed declining trends up to 1996. In 1997, the diversity stabilised at a low level. It concerns the gammarid bathyporeia elegans (an average of 550 in 1993 per square meter and around 200 in 1997), the sand mason (in 1993, an average of 100/m2 , in 1997 practically gone), the Mysella bidentata (an average of 100 animals per m2 in 1993, much less in 1997) and the rag-worm nephtys cirrosa (an average of 100 animals per m2 in 1993, around 50 in 1997). The only species which continually increased in number between 1986-1997 was the Alder's necklace shell, a predator snail.

Source: de Vleet, Ecomare