Shetland Islands Intertidal Zone

Category: Marine and Oceanic Biomes.

Geographic Location: Europe.

Summary: The diverse and rugged sub-Arctic coastline of the Shetland Islands is exposed to extreme wind and waves, and is home to populations of hardy plants, birds, and marine life.

The Shetland Islands is an archipelago of about 100 islands around 106 miles (170 kilometers) north of mainland Scotland. Shetland covers an area of 567 square miles (1,468 square kilometers), with a coastline of 1,679 miles (2,702 kilometers). The largest island is known as Mainland, and is one of the 16 inhabited islands, with a total population of about 22,000 people. Shetland was formerly called Hjaltland, possibly derived from the Old Norse word hjölt, meaning “the handgrip of a sword,” reflecting the island chain’s resemblance to a sword.

Shetland is sub-Arctic, but the strong influence of the North Atlantic Drift provides an oceanic climate, with long, mild winters and short, cool summers. The mean maximal temperature range is 41–57 degrees F (5–14 degrees C), and the sea temperatures vary from 45–55 degrees F (7 to 13 degrees C). In general, the climate is windy and cloudy. Fog is common during summer. The prevailing wind is southwesterly; storms in Shetland can be exceptional. Gusts of more than 124 miles per hour (200 kilometers per hour) have been recorded, and the islands have five times the number of gale days as mainland Scotland. Huge seas batter the west coast, creating storm beaches up to 59 feet (18 meters) above sea level.

The archipelago has a complex geology, with many faults and fold axes. During the ice ages, glaciers entirely covered the islands, as shown by the numerous voes (glaciated valleys drowned by rising sea levels). One intertidal geological feature of interest is the largest active tombolo (mound or sandbar) in the United Kingdom, which connects St. Ninian’s Isle to Mainland. Sixteen of the islands are inhabited, but many more are accessible by boat. Shetland gets up to 20 percent of its electricity from wind and tidal power, relying on oil for the remainder.

Intertidal Habitats and Species

Shetland’s coastline experiences a broad range of environmental conditions. The west coast is extremely exposed to waves, with warmer water from the North Atlantic Drift, whereas the upper reaches of the voes (small bays or narrow creeks) are almost completely sheltered, and the east coast is washed in cool water from the North Sea. This variety of conditions, combined with the long and complex coastline, creates a broad range of intertidal habitats.

Much of the coastline is rugged and rocky, with many high cliffs and a heterogeneous range of features, such as boulders, ledges, crevices, gullies, and rock pools. These features support many types of seaweed and diverse fauna, including a great variety of limpet (order Patellogastropoda), which are aquatic snails with conical shells; winkle (Littorina littorea), a small edible sea snail species; and dogwhelk (Nucella lapillus), a predatory sea snail also called Atlantic dogwinkle; as well as widely varied species of mussel, barnacle, anemone, and crustaceans such as crabs.

Well above the tidal zone, and out of reach of grazing sheep and human interference, coastal cliffs here support sizable breeding populations of seabirds including fulmar, gannet, storm petrel, shag, kittiwake, guillemot, razorbill, and puffin. Some of these birds interact heavily with the intertidal zone, preying upon its fauna.

At the back of numerous bays in the Shetland are beaches of golden sand. Salt marshes dominated by fine grasses are found at the head of some voes, and there are several expanses of intertidal mud. These areas can be rich in marine invertebrates, providing important foraging areas for waders and seabirds.

A total 274 species of benthic algae are recorded in Shetland. One is a rare, free-living form of knotted wrack (Ascophyllum nodosummackii), which occurs in sheltered water and grows in spherical masses up to 24 inches (600 millimeters) in diameter, giving it the name crofter’s wig (a croft being a small farm). The complex structure of the seaweed provides habitat for many small invertebrates.

Larger marine life here includes various trout varieties, mackerel, ling, haddock, halibut, porbeagle shark, scorpionfish, butterfish, assorted crabs, and urchin. All the land animals in the Shetland Islands—including otters, mice, and rats among those that frequent the tidal zones—were introduced by humans.

Shetland is largely treeless, but since the ground has an abundance of peat deposits, the islands likely did have a warmer climate at some point in its history. Local vegetation and habitat that is often washed in spray from the high-tide intertidal wave action includes blanket bog dominated by heather, cotton grass, dwarf shrubs such as crowberry and bilberry, and sphagnum moss. There are various greens such as heather, grasses and sedges, mosses, liverworts, ferns, and lichens

Human Settlement and Activity

People have lived in Shetland for more than 6,000 years. Stone Age hunter-gatherers harvested species that they found locally, leaving the remains in archaeological middens (rubbish dumps). These middens reveal a range of intertidal species, including oysters, mussels, limpets, crabs, and cockles, as well as fish and seabird remains. Limpets are abundant on the rocky shores and are particularly common in middens. Limpets also are important to aquaculture, primarily for salmon and mussels to feed upon, while Pacific and European oysters are often grown intertidally on racks.

In the U.K., the intertidal zone, known generally here as foreshore, is deemed to be owned by the Crown, although shores in Shetland are notable exceptions, where Udal Law generally holds. Udal Law is a near-defunct Norse system in which individuals have rights of ownership of the foreshore. This law has been upheld intermittently by Scottish courts, and has important implications for construction of pipelines, cables, or coastal defenses—as well as conservation efforts.

Discovery of North Sea oil provided a major economic boost—and the dangers that come with it. A large terminal at Sullom Voe processes and stores oil from the North Sea and Russia. The oil tanker Braer ran aground on the south coast of Mainland in January 1993, releasing 93,696 tons (85,000 metric tons) of crude oil. The spill had short-term effects on fish, shellfish, marine mammals, and seabirds, but fortunately long-term effects on intertidal species were less than anticipated.

The impact of climate change seems to pose some positives for the Shetland Islands. As the sea waters warm, fewer storms could hit the islands, moving at least some of the harshest weather further south to southern England and France. However, warming temperatures could spell more coastal flooding for the archipelago. Sea level is expected to rise more for the islands than for mainland Scotland and already threatens archaeological and historic sites. The increase in sediment flushed into the tidal areas, as well as erosion of some of the more sensitive zones, would create difficulties for many of the intertidal faunal communities here.

Bibliography

Baxter, J. and S. Mathieson. Broad Scale Habitat Mapping of Intertidal and Subtidal Coastal Areas: Busta Voe and Olna Firth, Shetland. Wallsend, Scotland: Entec, 1996.

Hamilton, Alastair. "Climate Change: The Challenge for Shetland." Shetland, 10 Nov. 2021, www.shetland.org/blog/climate-change-the-challenge-for-shetland. Accessed 31 Aug. 2022.

Johnston, Laughton. A Naturalist’s Shetland. London: A & C Black Publishers Ltd., 2002.

Scottish Natural Heritage. “Natural Heritage Futures: Shetland.” http://www.snh.gov.uk/docs/A306317.pdf.