Importance of Seagrass in Coastal Island Ecosystems
Seagrass ecosystems represent a fraction of marine vegetation yet have an immense sequestration ability that is vital in tackling the climate crisis. Seagrasses are a crucial part of the marine ecosystem due to their productivity level, biodiversity, and sensitivity to water quality changes. Due to this, they play a significant role as an indicator of the overall health of a coastal ecosystem. Despite their significance, their numbers are dwindling globally, and conservation efforts are essential to ensure these crucial ecosystems are protected.
Seagrass as a blue carbon ecosystem
Seagrasses are located along the shore of every continent except Antarctica. They are submerged flowering plants with deep roots. Seagrass ecosystems can sequester significant amounts of carbon and store it as organic carbon in sediment for long periods, making them one of the most significant natural carbon sinks globally. Blue carbon refers to the removal of carbon dioxide from the atmosphere by the world’s ocean ecosystems. As carbon accumulates over time in seagrasses, it is stored primarily in soils, with 50-99% of coastal blue carbon stored in the soils below ground. These accumulates have been measured up to four meters deep and remain for extensive periods (up to millennia).
Seagrasses account for a minimal proportion of the world’s oceans at less than 0.2%; however, they sequester approximately 10% of the carbon buried in ocean sediment annually, sequestering 27.4 Tg of carbon per year. In comparison to terrestrial forests, seagrasses can store up to twice as much carbon per hectare. Current estimates propose that the global seagrass ecosystem organic carbon pool could be up to 19.9 billion metric tons.
Seagrasses’ extensive root system which extends vertically and horizontally helps stabilise the seafloor. Therefore, areas of the ocean floor that are devoid of seagrass are more vulnerable to intense wave action. Coastal ecosystems without seagrasses to reduce the force of current along the seafloor can be subject to increased damage from storms – this can be seen on Florida’s beaches. Seagrasses also provide food and shelter for many flagship species, alongside essential nursery areas for fish species and numerous invertebrates living in seagrass communities such as lizardfish and seahorses.
Green sea turtles and Florida manatees feed directly on seagrass leaves, while other organisms use seagrasses indirectly to provide nutrients. For example, Bottlenose dolphins feed on organisms that inhabit seagrass ecosystems. Filter feeders, crabs, sea cucumbers, and other marine species feed on detritus from the bacterial decomposition of dead seagrass plants. Phytoplankton and seagrasses benefit from the further decomposition of seagrass which dissolves in water enabling them to re-absorb it.
Juvenile fish and invertebrates can easily conceal themselves from predators in seagrass meadows. Seagrass leaves also provide an ideal surface for the attachment of eggs and larvae. Alongside small adult fish and juveniles, infaunal organisms also inhabit seagrass meadows, including worms, clams, and many echinoderms. Seagrasses’ extensive root system deters predators from digging through substratum on the hunt for infaunal prey organisms.
Seagrasses increase water clarity by helping to trap fine particles and sediments suspended in the water column. A lack of seagrass in an area causes sediments to be more frequently stirred by wave action and wind, decreasing the water clarity. In turn, this affects marine animal behaviour and decreases the recreational quality of the coastal area. Seagrasses also filter nutrients from stormwater runoff or land-based industrial discharge before they are washed out further to sea or other sensitive marine habitats such as coral reefs.
Contribution of blue carbon ecosystems
As seagrasses are degraded, converted to other land uses, or lost, the blue carbon stored in the soil is exposed and released into the atmosphere and/or ocean as CO2. Loss of seagrass ecosystems could result in the annual release of 0.15–1.02 billion tons of CO2. Seagrasses combined with mangroves and tidal marshes have a global area that equates to only 2–6% of the total area of tropical forest, however, degradation of these ecosystems accounts for 3-19% of carbon emissions from global deforestation. A recent study compared the emissions resulting from the annual loss of these blue carbon ecosystems to the annual fossil fuel CO2 emissions of the United Kingdom. Currently, the emissions of the United Kingdom are ranked 9th globally.
Why do seagrass ecosystems need restoring?
However, despite their importance, approximately 29% of Earth’s seagrass ecosystems have been lost globally. With annual global losses of around 1.5%, they are among the world’s most threatened ecosystems; this rate has been increasing in recent decades. Such losses are due to major threats, including deforestation and dredging, which degrade water quality due to poor land use. Other drivers of conversion and degradation include aquaculture, urban and industrial coastal development, and marine and terrestrial sources of pollution. While the cause may vary, they are all driven by human activities. As climate change continues to be a threat, these impacts are expected to intensify.
Between 1997 and 2004, almost half of Bermuda’s offshore seagrass beds declined. Another culprit has contributed to further declines in recent years: the green turtle. A lack of predators in the ecosystem due to the overfishing of sharks has resulted in an imbalance, resulting in the green turtle overgrazing on seagrass beds. In a healthy ecosystem, turtles and seagrass co-exist as sharks control the turtle population and limit the time the turtles spend grazing on the meadows. Alongside a lack of sharks, numerous successful conservation efforts around the Caribbean Sea and Florida have increased the population.
The Bermuda Seagrass Project
Climate Wise launched the Bermuda Seagrass Project to restore the seagrass habitat and micro-ecosystem surrounding Bermuda. By caging and fencing patches of seagrass, they are protected from grazing by green turtles and allowed to grow. The turtles can access part of the seagrass, but not enough to hinder its growth. This method allows the seagrass to flower and produce seeds which are eventually distributed to increase the seagrass coverage. When the marine ecosystem is fully restored, this includes the return of a healthy shark population, the seagrass can grow out from under the cages and colonise other areas.
Improving the seagrass meadows improves the marine environment. The primary aim of the project is to restore balance in the ecosystem and allow the vibrant system to regrow. Regenerating the seagrass meadows has numerous benefits for various other species that inhabit seagrass, including rays, seahorses, and parrotfish. Additionally, species outside the meadows also benefit from the increase in species.
Another key benefit of the project is the carbon sequestration ability of the resorted seagrass. As mentioned, seagrasses are a vital carbon sink and are considerably better at removing carbon from circulation than many land-based solutions. The seagrass meadows will therefore contribute to reducing the volume of carbon in the atmosphere, and therefore, reduce the global carbon footprint. The carbon impact of the project is being monitored, which can then be used to inform the establishment of similar projects in other locations.
While benefitting the environment, the community, tourism, and local businesses are also supported. A total of 2,384 square metres of seagrass habitat were restored by the end of 2021, with the aim of bringing the total area of seagrass covered and protected in Bermuda to 19,000 metres squared. There is still a long way to go, but the impact of the project and restoring the ecosystem has unparalleled effects in tackling climate change.
The implications of losing such important aids in fighting climate change will have devasting consequences. We need to protect and restore these crucial carbon sinks and biodiversity hotspots to reduce the impact of climate change on coastal ecosystems.