Seagrasses are a group of around 72 flowering plant species that resemble the grasses found on land. One notable feature that differentiates them from land plants is that they do not have thick stems. The buoyancy of water provides support for the plant. They also have structures called rhizomes which act as holdfasts attaching them to the sediment below.
Seagrasses have adapted to be able to survive in saltwater, which would kill most land plants. They release water from their leaves and store salt, in order to make sure that they have more salt in their leaves, than is present in the surrounding water. Without this, all water would leave the plant and kill it. They are known as ecosystem engineers because they form a whole ecosystem which is vital for a range of different species. They act as a food source, habitat and spawning area to many species that cannot survive anywhere else. Different species of seagrass live together in meadows providing a wide variety of habitats and functions within the ocean, including; stabilising ocean sediment and reducing water currents, keeping water clear. Seagrasses are found in shallow coastal waters no more than two metres deep, due to the fact that they require lots of light in order to photosynthesise; providing them with enough energy to grow, reproduce and survive in a hostile environment. Seagrasses also play a role in the formation of a wider ecosystem, in combination with coral reefs and mangrove forests. Seagrasses filter pollutants from river run-off and prevent a harmful build up of sedimentation from reaching coral reefs.
Seagrass meadows are thought to be the most threatened ecosystem on Earth. An international team of scientists concluded that 58% of the world’s seagrass meadows are currently declining at a rate of approximately 110km2 per year. This decline has been linked directly and indirectly to human causes through pollution and habitat destruction. One of the major causes of the decline is the decreased water quality due to eutrophication; an excess of nutrients in the water from pollution sources on land including run-off from fertilisers and phosphates from agriculture. This causes blooms of phytoplankton to increase, which use up valuable nutrients in the water, depriving other organisms of these essential nutrients and poisoning marine animals. The phytoplankton floats on the surface blocking out the light to marine plants including seagrasses, preventing them from photosynthesising effectively, and can therefore die. Boats are also a problem as they anchor within seagrass, creating bare patches due to erosion of the bed. Dredging by fisherman for shellfish and by creating shipping lanes, is also a huge problem, as meadows can be completely removed, leaving a bare sea floor. In addition, shallow regions along the coast are often prime areas for coastal development, but this increases sediment build-up, which can bury small seagrass shoots, blocking out sunlight and ultimately killing them.
The importance of seagrass and why we should care
Seagrasses are incredibly important to the wider marine environment because they slow down water currents and their roots help stabilise the soil, trapping and consolidating it, allowing other plants to inhabit the area. If seagrasses are removed, then water currents will speed up, causing increased coastal erosion. This means, they are not only important to the marine species, but also provide a huge benefit to man in a variety of ways; these benefits are known as ecosystem services. Some species are specialist shade loving grasses and if conditions are not suitable, this will increase the risk of local or even global extinction. Scientists have shown that seagrasses are a major source of photosynthetic primary production, allowing them to convert energy from sunlight into plant matter, used by a variety of marine animals as a source of food. There are currently around 115 species that rely upon seagrasses (many almost entirely) and 27% of these are already vulnerable including the dugong, manatee and some species of sea turtles. As further seagrass is lost, this figure is likely to rise as more and more species are going to be affected by the loss of these habitats. Conservation International, a non profit organisation whose aim is to ensure the health of humanity by protecting the Earth’s ecosystems and biodiversity, states that seagrasses help transport nutrients throughout the ecosystem and the value of this is estimated at $1.9 trillion annually. The process of photosynthesis allows them to produce organic matter which forms the building blocks of the plant. This organic matter is eaten by marine organisms like fish, lobsters and crabs; all of which are commercially important food sources.
Seagrass meadows also form a vital spawning ground and nursery for many species of fish, shrimp and crab. For example, in the UK the spiny and short-snouted seahorses only breed within seagrasses, so if this habitat is lost, they will be vulnerable to extinction. Furthermore, a lot of the species associated with these habitats are of high economic value; providing local people with their livelihood. Local people have also used seagrass meadows as a source of medicine for many years, making it important to them and potentially to the rest of the world, as there may be cures waiting to be discovered within these grassy meadows. Another ecosystem service the meadows deliver is the filtering of water, increasing water quality. If they are destroyed the decrease in water quality will dramatically affect local people as well as lots of marine species. Scientists have calculated that approximately 15% of the carbon dioxide held within the oceans is locked within seagrasses, and if destroyed, there will be a higher concentration of carbon dioxide in the atmosphere; a further significant contribution to climate change.
The solution: what we can do
Protecting the remaining seagrasses that we have is a much more viable solution than trying to plant more, as seagrass meadows can re-establish themselves and spread as long as they are protected from further damage. This is an easier option than trying to establish meadows in areas where conditions might not meet their requirements. Education of local people is also needed to highlight the importance of the seagrasses and what they can do to help, through improved fishing practices that can replace dredging and other harmful methods. There also needs to be a reduction in coastal development which damage the seagrass beds, or at least setting aside protected areas where seagrasses are most vulnerable . Waste disposal also needs to be improved so that point source and non point source pollution is reduced, which causes algal blooms that block out the light. Point pollution is where there is a single source of pollution that can be traced back to the source, whereas non point pollution comes from everyday sources, such as driving a car. These pollutants can build up and the cumulative effect can be harmful to wildlife.
Furthermore, the monitoring of seagrass populations is essential. There are current volunteer monitoring programs including seagrass watch and the global seagrass monitoring network. However, the most effective way that seagrasses can be protected is through the formation of marine reserves, places where the marine environment is conserved and fishing and anchoring of boats is prohibited or regulated. These marine reserves, also known as Marine Protected Areas (MPAs), help protect a range of marine environments, not just seagrass meadows. To ensure the future of seagrass meadows, you need to also conserve coral reefs and mangrove forests and you can help by signing a petition to create more marine reserves here.