About one quarter of the Earth's land is covered by grass - it's a familiar sight in areas as diverse as chilly Britain to the baking savannah of Africa. Indeed, this massive biome can be found on every continent except Antarctica. The grasslands of the sea are not so well known, however, but are ecologically important. Though on a much smaller scale than land-based grasslands, they are key communities. Since humans have tended to concentrate towns and cities around coasts, seagrass habitats are under constant threat.
When is a grass not a grass?
In spite of the name, seagrasses are not true grasses. They belong to another group of flowering plants, but the resemblance of their long, flattened, blade-like leaves to ordinary grasses gives them their name. 58 species are known in 12 genera. They usually have a well developed root system and an underground rhizome to store food. They grow in marine or estuarine waters with most living in soft, silty or sandy sediments. Seagrasses are believed to have evolved from terrestrial plants which returned to the sea by gradual steps from shallow brackish water to eventual total submersion in salt water. Their adaptation to life submerged in sea water has led these plants to develop a unique set of features.
One seed leaf or two?
True land grasses and sea grasses are both monocotyledons: they germinate with just one seed leaf, unlike the dicotyledons, such as beans and peas, which have two. Most flowering plants are dicotyledons. True grasses can be found almost anywhere that terrestrial plants can grow and have adapted to a huge range of soil types and climates. In spite of this, they all look remarkably similar.
Pollen
All hay-fever sufferers know that grasses are capable of producing huge amounts of pollen which can travel great distances. This is one of the main reasons for their success. By spreading genetic material between different populations, the grasses have ensured their adaptability.
Seagrass ecology
Seagrass provides habitats for a broad range of marine life. They have a relatively high rate of primary production which drives detritus-based food chains. Sea grasses are also consumed by birds, dugongs, manatees and turtles.
Useful seagrass
Studies of seagrass meadows show they have several important ecological purposes. They keep water clear by trapping fine sediments in their leaves - this is especially important where there are coral reefs that need clear water in order to survive. Grasses also stabilise the sea bottom with their roots and rhizomes and provide a habitat for many fish, crustaceans and shellfish, which in turn provide food for many other marine mammals.
The grasses provide direct nourishment for some animals. Perhaps most relevant for humans, they provide a nursery for recreational and commercial fisheries. In recent years however, humans have had a major impact on coastal ecosystems.
The green planet?
Temperate grasslands once covered much of interior North America, and were more common in Eurasia and South America. Like seagrass meadows, grasslands are highly productive and the soil is deep and fertile. However, these grasslands have been converted into agricultural land on a vast scale.
In North America, the prairies were once inhabited by huge herds of bison and pronghorns, which were hunted by wolves and bears. The grasslands thus supported a large and diverse food web. Now, the herds are virtually gone and the prairies have been converted into the richest agricultural region on the planet. The wild grasslands have been tamed for human use and the ecology has been completely altered in the process.
Seagrasses in industry and coastal protection
In Western Australia, 27 species of seagrasses cover 20,000 square kilometres of seabed and support a multi-million dollar rock-lobster fishing industry. The larvae shelter in inshore meadows prior to moving offshore. Waters off the Western Australian coast are considered relatively nutrient poor, but seagrasses act as an energy store, releasing nutrients as they decompose. Over time seagrasses are capable of building up banks, up to 10 metres high, and gradually alter water flow to provide considerable protection for parts of the coast.
Grass houses
Dried seagrass was commonly used as housing insulation well into the 20th century. The dried seagrass contains many air spaces, meaning it has excellent sound-proofing and thermal insulation properties. The high level of silicon in seagrass tissues means it is non-flammable.
Seagrasses were also used for thatching roofs, especially in rural coastal areas in Europe. It was often used as a substitute for straw and was in some ways superior because it was slower to rot, non-flammable, and apparently flea proof. Its use dates from at least as early as the 17th century, so the latter property would have been a definite bonus!
In Europe and the USA, seagrass was once used to stuff pillows, mattresses and upholstery. These versatile plants have also been used for the following: to feed poultry, as a garden mulch, to make mats, to pack crabs and even to fill leaks in the hulls of 17th century ships.
Dykes to dunes
Seagrass was extensively used to bind clay and soil in the dykes of the Netherlands. Today it has found a similar use in the stabilisation of sand dunes. In Australia in particular, layers of sea grass mulch have been shown to form an erosion resistant mat. This helps seedlings to become established which in turn stabilises the dune.
Food for thought
There are many minor uses of grasses, but the main one is culinary. Humans are dependant on both seagrass and grasslands to provide them with food. With grasslands, the food source is both direct and indirect - grain is harvested and consumed on a vast scale, and animals which live on the grasslands are also consumed. With seagrasses the food source is largely indirect. Humans eat the products of fisheries which depend on seagrass beds as nurseries. Protein is a vital part of the human diet, especially in developing countries, so both systems are vital in an increasingly crowded world.
