‘Ugly’ seagrass meadows vital for atoll survival

Often referred to as the ‘ugly duckling’ of marine ecosystems, these vital habitats are frequently removed for aesthetic reasons - a trend that is increasingly prevalent across the Maldives. 

Beyond aesthetic removal, seagrass meadows face a myriad of threats, including coastal pollution and construction. Superimposed upon these are the impacts of climate change: increased storm frequency and severity, rising sea temperatures, elevated sea levels, and ocean acidification. These ecosystems are succumbing to the “coastal squeeze” between climate-driven impacts and coastal development.

Seagrass meadows play multifaceted roles, particularly crucial for atoll nations. One such overlooked contribution is their capacity to produce substantial volumes of sediments suitable for reef island building. For atoll nations such as the Maldives, where islands are primarily composed of coral sediments, the decline in coral health underscores the importance of alternative sediment sources. Seagrass ecosystems emerge as key contributors, particularly in producing calcareous sand. This type of sand, rich in calcium carbonate, is common in tropical and subtropical regions and forms the basis for many coral reefs and atoll islands.

A study conducted in Huvadhoo Atoll quantified sediment production by organisms on seagrass surfaces, revealing an annual production of 763,000 – 943,000 kg of calcium carbonate over 1.1 square kilometre. Of this, 518,000 – 564,000 kg were sand-sized, suitable for reef island building. Extrapolating these findings nationally, Maldivian seagrass meadows produce 62,762 – 99,604 tonnes of calcium carbonate annually, with 63% being sand-sized material. When sediments remain trapped by seagrass, they contribute to the shallowing of the seabed, protecting the reef islands from coastal erosion by reducing the force of waves hitting shorelines.

To provide a highly simplified understanding of the potential impact of this, consider an island with an area of one square kilometre. Raising this island by just one centimetre (0.01 meters) would require about 10,000 cubic meters of sediment. If we assume the bulk density of calcium carbonate is 1,500 kilograms per cubic meter (a commonly used average), then 15,000 tonnes of calcium carbonate would be needed. Using the annual sediment production of Maldivian seagrass meadows, which produces about 62,750 tonnes (63% of 99,604 tonnes) of sand-sized calcium carbonate, this material could theoretically elevate the island by approximately four centimetres annually.

Of course this is not as simple in reality. Island elevation is a gradual process that typically occurs over centuries, not annually. Additionally, natural processes like currents and waves redistribute sediments, and human activities such as coastal development can disrupt sediment deposition. These factors, combined with the slow pace of natural sediment compaction and accumulation, mean the actual impact is far more intricate.

With the world facing its fourth global coral bleaching event, the second in a decade, coral reefs across the globe are in a precarious state. While seagrass meadows do not directly address coral bleaching, they play a crucial role in mitigating the adverse effects of ocean acidification on coral reef health. Ocean acidification results from increased carbon dioxide and other carbon emissions due to human activities, making seawater more acidic. This acidity reduces the calcification rate, the pace at which coral polyps secrete calcium carbonate to build their skeletons, forming the foundation of coral reefs.

Seagrass ecosystems can help offset ocean acidification if they absorb more carbon dioxide than they release. This condition, known as "net autotrophy," occurs when seagrass takes in greater amounts of carbon dioxide through photosynthesis, the process by which plants convert sunlight into energy, than it releases during respiration. According to a study, Indo-Pacific seagrass meadows appear to be net autotrophic over an annual cycle. Hence, they display the capacity to locally offset the impacts of ocean acidification and promote coral calcification.

Often characterised by their seemingly uniform green hues, seagrass meadows are also vital habitats that support diverse biodiversity. A recent review published in Nature Reviews Earth & Environment highlights the abundant and diverse microbial communities harboured by seagrass ecosystems, including viruses, bacteria, and fungi. Additionally, these meadows provide essential habitats for 121 megafaunal species - large marine animals exceeding 45 kilograms - such as sea turtles, dolphins, sharks, rays, and seabirds. Many of these species rely on seagrass for sustenance, migration pathways, or breeding grounds.

Furthermore, according to a UNEP report, these ecosystems act as nurseries for 20% of the fish caught worldwide for food or trade, making seagrass ecosystems incredibly important for supporting fisheries. Without these foundational habitats, fisheries would face significant disruptions, threatening food security, particularly for island nations like ours.

Seagrass ecosystems play a crucial role in alleviating climate change through their blue carbon storage capacity, ranking among the most efficient carbon sinks in the world. It is estimated that seagrass globally stores 140 megagrams (Mg) of organic carbon per hectare in the top meter of soils. Seagrass meadows are up to 40 times more efficient at capturing organic carbon than land forest soils.

They capture and store carbon through photosynthesis and by slowing the decomposition of plant matter, allowing them to store carbon for extended periods, often spanning hundreds or thousands of years. Hence, they hold significant potential for carbon offsetting, especially for small island states. A recent study in the Bahamas highlights the remarkable capacity of its seagrass ecosystems to store and absorb substantial amounts of carbon annually. The findings suggest that these ecosystems could, theoretically, offset up to 68 times the country's carbon emissions in the year 2018, potentially enabling the Bahamas to achieve carbon neutrality. Given the Maldives' aim to achieve carbon neutrality by 2030, preserving our seagrass meadows could be instrumental in realising this goal. Furthermore, it is important to note that any damage or disturbances to these ecosystems have the potential to release trapped carbon dioxide back into the atmosphere, exacerbating climate change.

It is imperative that we recognise the critical role seagrass ecosystems play in our atoll nation and beyond. These habitats are vital not only for mitigating climate change - a pressing issue for island nations like ours - but also for supporting biodiversity, sustaining fisheries and protecting coastlines.  

Given that our economy is heavily dependent on tourism, it is also worth emphasising that conservation efforts of seagrass ecosystems inherently benefit our tourism sector, by protecting coral reefs, contributing to marine diversity, and mitigating island erosion. For years, we have removed these ecosystems to enhance the aesthetic appeal of our islands, yet we are now at a point where efforts must be made to revive these so-called ‘ugly ducklings’ of our marine ecosystems. Doing so is essential not only to preserve the aesthetic charm of our nation but also to safeguard the very existence of it.

Taking steps forward, we can look at strategies such as:

The coordinated efforts of the government, resorts, and island communities are essential to implementing strategies such as those mentioned above to safeguard our seagrass ecosystems for future generations. It is through our shared commitment that we can secure the survival and prosperity of our islands, ensuring a sustainable future for all.