WE often read about the demise of forests and the effect it has upon increasing the carbon dioxide content of our air, but we seldom come across two greater worldwide absorbers of carbon – seagrass and sphagnum moss.
Seagrass can capture carbon dioxide up to 35 times faster than tropical rainforests, yet it covers less than 1 per cent of the ocean’s floors and is responsible for 15 per cent of the seas total carbon capture. Rainforests store in their wood about 30,000 tonnes of carbon per square km whereas, for the same area, seagrass beds can store up to 83,000 tonnes in the soils beneath them.
In the last 20 years about 30,000 square km of seagrass has been lost, amounting to nearly 20 per cent of the world’s underwater meadows. Not only are seagrass beds the most productive ecosystems in our world and huge carbon ‘sinks’ but they provide habitats and fodder to a huge range of marine life much like coral reefs.
They are found in estuarine and marine environments on all continental shelves apart from Antarctica. There are 72 known species of seagrass, divided into five families, growing from six to 12 feet in height in waters not deeper than 200 metres for they need sunlight for photosynthesis.
Seagrass beds are composed of a single plant species in temperate zones but in the tropics the beds show a great variety of species. Unlike land grasses, these species lack stomata. But their specialised leaf structure includes an epidermis as the main photosynthesising tissue. With underground stems or rhizomes, they can firmly anchor on the seabed.
Some species form seed banks of small seeds with hard cases which remain dormant for several months. These species are short lived. Other species take the form of buoyant fruits with inner seeds ready to burst into life. Undoubtedly, seagrasses are a complex form of plant life with many marine biologists involved in research into their evolution and structure. It is thought that they emerged 140 million years ago.
Threats to seagrass meadows
Currently our seagrass meadows are lost at a rate 1.5 per cent each year and 25 per cent of the known species are classified as Threatened or Near Threatened on the IUCN Red List of species. Already we have seen the destruction of these meadows through overfishing by trawling methods, scrapping the sea floors clear of all living creatures and plants.
Trawling also causes die back in seagrasses as it disturbs sand particles which are scattered. With high light requirements, the sea waters lack clarity and block out sunlight and so the plants die off. Seagrass is also very susceptible to high nutrient loads contained in sewage and thus experience the growth of algal blooms through eutrophication. These blooms block sunlight.
Already, we have seen anthropogenic induced threats causing more frequent storms, rising sea levels, and increased heat all having an effect on seagrass meadows. Extreme heat in the intertidal zone causes the desiccation of these grasses and thus die back when exposed at low tide.
More frequent and intense storms rip out seagrass from its anchorage and lead to huge deposits of rotting grass at high tide mark on otherwise pristine beaches. This causes extra expense in beach tourist resorts in keeping the sands free of rotting debris. Migrating Brent geese who feed essentially on eelgrass have relocated from traditional habitats on British beaches affected by storm damage and increasing desiccation of these plants.
Conservation of seagrass beds
Many governments are now creating Marine Reservation Areas to protect seagrass meadows for finally the value of these plants have been realised for several economic reasons. Seagrass species produce an extensive root system thereby trapping and stabilising sediment and thus reduce coastal erosion. These plants enhance water quality by stabilising heavy metals, absorbing pollutants, and some nutrients.
The very length of these grasses slows the movement of seawater through the absorption of wave energy thus acting as protectors of the shoreline as well as oxygenating seawater. The meadows harbour, for shelter and foraging, a vast range of marine species from juvenile coral reef fish and shrimps to sharks and dugongs (sea cows), which happily graze the meadows. They provide local fisheries with a valuable source of income and cash to the scuba and snorkel diving tourist industry.
China’s government has declared some areas as Marine Reserves as has Malaysia and only very recently in the county of my birthplace, Cornwall, UK, a huge 900-acre seagrass meadow in St Austell Bay has been declared as a national legacy. Some 122 different species and animals have been identified in this relatively small area. While most of us are wary of swimming over seagrass beds, they are a valuable part of our world’s ecosystems to be neglected.
This moss is but one of nearly 400 species of moss worldwide and is also known as bog moss which absorbs carbon dioxide and converts it into food through photosynthesis. Mainly located in the Northern Hemisphere, it is also found in southern Argentina and Chile, New Zealand and Tasmania as well as in subtropical mountainous areas of Brazil.
I believe that I found a variety of sphagnum on the highest altitudes of Mount Kinabalu, Sabah. It is a freshwater colonising plant filling in any natural hollow and is thus found mostly in highland environments with plenty of rainfall. In such waterlogged conditions these plants are massive carbon storers in anaerobic conditions.
Sphagnum moss consists of a single stem with tightly clusters of branches which hold two cell types – small, green, living cells with chlorophyll. They produce food and barrel shaped cells with a pore at each end, helping the plant to allow water absorption and retain it during sunshine. These plants grow into carpets, smothering the landscape. As is typical of many moss types, it spreads through seed dispersal by the wind.
Sphagnum’s unseen major natural use is one of the carbon storage. When the plant dies its remains do not decay, and thus store greater quantities of carbon than forests. The dead vegetation simply turns into peat thus storing carbon for thousands of years. It is an extremely absorbent plant, soaking between 16 to 26 times its own weight in water. Peat bogs act as giant sponges, slowly releasing water over time, keeping a steady flow and preventing surges of rainwater running into streams and thereby creating downstream flooding. These bogs help to filter and clean water which is used in flavouring Scottish and Irish whiskies!
Anaerobic, acidic bogs decay very slowly and thus preserve human remains for millennia as has been found with the discovery of the Tollund Man in Denmark and the Lindow Man in the UK. Sphagnum moss was used for field dressings for wounds in the Crimean War and later in World War I for its absorbent and acidic nature prevents the growth of bacteria. It is also used as an alternative to chlorine in swimming pool sanitation processes. In the Far East it is used as a potting material for orchid growth and in the Arctic as a dried, roof insulating material.
In Europe and the USA, peat is widely used as a compost material. The ‘cutting ’of peat from bogs, to be dried for heating purposes, has been in widespread use worldwide particularly in thermal electricity generating stations thus releasing the trapped carbon dioxide into the atmosphere.
The Fenlands of East Anglia, UK, were drained in the 18th century for highly intensive agriculture but as the peat has dried out it is picked up by strong winds and whisked away causing massive topsoil losses. In 2030, DEFRA (the UK’s Department for the Environment, Food, and Rural Affairs) will totally ban the sale of peat for amateur and professional growers.
All around our world, governments are awakening to the valuable resources that sphagnum peat bogs provide in carbon capture and flood control. Plans are afoot to plant sphagnum moss in large quantities in highland areas to not only capture carbon but as importantly to alleviate downstream flooding in lowland areas.
Whilst our efforts to reduce carbon emissions are commendable, more frequent and longer spells of drought together with the drainage of peat lands can release more stored carbon into the atmosphere. Climate change is with us and will determine the short-term future of both seagrass meadows and sphagnum moss bogs.