Seagrass – More than just carbon sinks

Abstract
Seagrasses are marine flowering plants that form meadows along the coastlines of every continent except Antarctica, and inhabit mostly sandy and muddy substrates. Globally, it is estimated that 7% of seagrass meadows decline annually. Scientists attribute this to poor water quality caused by pollutants, especially nutrients and sediments. Seagrasses are important nursery grounds for a rich diversity of marine species such as certain finfish, shellfish, sea cucumbers, penaeid prawns, dugongs and sea turtles. Covering a mere 0.1% of the ocean surface, seagrasses provide more than 24 ecosystem services that benefit humans. Seagrasses are often considered to be the “lungs of the ocean” due to their capacity to trap carbon, thus making them one of the Earth’s greatest carbon sinks. Research has shown that 10 seagrass species are at an elevated risk of extinction (14% of all seagrass species), with 3 species qualifying as endangered. On 23 May 2022, the United Nations General Assembly adopted a resolution to commemorate 1 March annually as World Seagrass Day. The conservation and management of seagrass ecosystems is paramount, demonstrating the critical role of seagrass meadows in achieving 16 of the 17 Sustainable Development Goals. This paper examines an open dialogue with representatives from Pacific Island countries on best practices, lessons learned, and developing joint strategies on seagrass conservation and management.

Background
Seagrasses are believed to be the third most valuable ecosystem in the world, preceded only by wetlands and estuaries (McKenzie 2008). Additionally, seagrass meadows contribute to vital ecological functions in the marine environment, many of which provide up to 24 different ecosystem services for humans, including coastal protection, nursery habitats, and sediment accretion and stabilisation (Nordlund et al 2017; Singh et al. 2022). They also provide important fishing grounds for local communities for sustenance and income. Around the Pacific, seagrass meadows are critical habitats for marine species such as dugongs and sea turtles. Notably, these coastal ecosystems are often interconnected by means of migrating animals, nutrient fluxes and organic carbon. They are also effective in purging pathogens that threaten humans and coral reefs. Globally, there are 72 known seagrass species, which occupy less than 0.2% of the ocean floor. The highest biodiversity is found in the Indo-Pacific region (Waycott et al. 2009).
The distribution of seagrasses around the world, as shown in Figure 1, indicates areas where seagrass ecosystems are declining (shown in red) due to human activities and areas where seagrasses are increasing (indicated in green). While global data have shown that there are no changes in some areas, this does not translate to seagrass meadows showing no actual change; instead, this may be due to limited seagrass research. Because most of the available data are from developed countries, which have a larger variety of seagrass species, these data could not be used for a comparative analysis in the Pacific Islands region. This is because differences in species composition and environmental conditions play a key role in estimating carbon stocks, which are valuable data for the conservation and monitoring of seagrass ecosystems. Seagrass meadows consist of different species that vary in size, form, growth and turnover rates. As a result, carbon sequestration in seagrass meadows vary in different regions and within meadow landscapes themselves. Recognition of seagrass species and seagrass meadow carbon data cannot rely on a “universal approach” based on carbon storage data from developed countries where they have been estimated from a greater variety of seagrass species (Singh et al. 2022). Hence, current regional and global estimates of carbon stocks and accumulation rates are based on limited datasets. Pacific Island countries need more studies and research on seagrass to further assess variability in carbon accumulation in seagrasses and sediments. By localising carbon estimates, it is possible to map the carbon accumulation in various seagrass species and the type of sediments they grow in coastal areas of the Pacific Islands region. Such carbon estimates will be useful for planners and policy-makers in blue carbon management plans.

Seagrass as a potential carbon storage
Seagrass is one of the largest coastal carbon sinks on the planet (Duarte et al. 2013). An important part of the oceanic carbon sink is that it can capture and store up to 55% of atmospheric carbon known as “blue carbon” (Singh 2019). This carbon is reserved and stored in the form of sediments in mangroves, salt marshes and seagrass meadows. Thus, seagrass meadows play a key role in global carbon cycling and are responsible for storing up to 10– 18% of the total ocean carbon mass each year. Singh et al. 2022). Research has also established that seagrass carbon storage rates are up to 35 times greater than that of tropical rainforests (Fourqurean et al. 2012; Singh et al. 2022). Blue carbon in coastal systems is more efficient than green carbon on land due to slower decomposition and greater storage in sediments. Carbon storage in seagrass ecosystems is divided into three carbon pools: 1) upper seagrass biomass, including sheaths, leaf blades and attached epiphytic biota; 2) lower seagrass biomass, including rhizomes and seagrass roots; and 3) sediments, originating both inside the ecosystem (autochthonous) and outside the ecosystem (allochthonous) (Fourqurean et al. 2014). The carbon content at the bottom of the substrate is higher than at the upper biomass because carbon accumulates in the sediment. The complex and dense root system of seagrass meadows results in trapped carbon in the sediments, and continues to increase as the seagrass beds expand. Therefore, lower seagrass biomass is up to 90% of the total plant biomass which, if left undisturbed, can be stored in seagrass ecosystems for a very long time (millennia) (Howard et al. 2014).

The role of seagrass in climate change mitigation
Seagrass habitats also increase coastal protection by trapping and stabilising sediments and dissipating wave energy, thereby allowing suspended material to settle on the bottom and increase water clarity. They are also considered to be “ecosystem engineers”, playing key roles in ecosystem organisation. Seagrasses provide conditions and resources essential for species to complete their life cycles, and help to maintain niche diversity by supporting complex habitat structures on which thousands of other species depend. Seagrasses are nutrient sinks, buffering or filtering excess chemicals, and act as nutrient pumps by releasing important compounds into nutrient-poor regions. This service is estimated to be worth over USD 29,000 (equal to FJD 65,000) per hectare per year (Singh 2019). Additionally, seagrasses could potentially have wastewater treatment properties. They may be able to remove various disease pathogens from seawater, such as Enterococcus, which affects humans, fishes and invertebrates, and reduces coral reef diseases by 50% in relative abundance of bacteria (Lamb et al. 2017).

Conservation efforts around the Pacific
Scientists have found 16 varieties of seagrass in the Pacific, with one subspecies found only in Fiji, Tonga and Samoa. A recent study from selected coastal Fijian sites have acknowledged and confirmed that seagrass habitats in these areas can be carbon sinks. The soil carbon contents ranged from 0.50% to 0.95%, which is lower than the reported global average of 2.0% (Singh et al. 2022). This study, which used the Intergovernmental Panel on Climate Change (IPCC) methods, investigated the carbon storage potential of small seagrass plants in Fijian coastal communities. The study confirmed that more carbon is stored in seagrass meadows than adjacent unvegetated areas, and carbon contents depend on seagrass species, geomorphic context and local environment influences. Carbon storage in plants and seagrasses help combat the increase in atmospheric carbon. Therefore, it is critical to preserve and sustain seagrass areas.

In Solomon Islands, dugongs and seagrass meadows are closely linked to customs through myths such as that of Lau Lagoon on the north coast of Malaita Island (Worldfish 2018), and many people in Lau Lagoon will not hunt or eat dugongs. The lagoon harbours the largest seagrass meadow in Solomon Islands. In the past, conservation efforts have been coordinated by the Wildlife Conservation Society, WorldFish, WWF and The Nature Conservancy (TNC). TNC for instance is currently including nesting beaches for endangered hawksbill turtles and a focus on nature-based solutions.

Papua New Guinea has the highest number of seagrass species in the Pacific, yet these species remain underestimated and threatened. Work on the conservation of seagrass ecosystems and protection have been largely done by the Locally Managed Marine Area Network in partnership with local tribes. Due to a lack of monitoring and limited survey data, local threats are not fully understood. While the recently passed Protected Areas Policy6 allows for seagrass protection, it does not include seagrass species. Currently, TNC in collaboration with the PNG Climate Development Authority is working on developing a Blue Carbon Ecosystems Policy Framework to provide a structure and policy pathway for its blue carbon ecosystems. PNG is also home to tidal marshes, which makes it unique to other Pacific Island countries.

The Vanuatu Fisheries Department (VFD) and the Pacific Community (SPC) have developed a monitoring system that has been adopted from Seagrass Watch. To date, 13 species of seagrasses have been confirmed in Vanuatu’s waters, with one likely error as it is not supported by an herbarium specimen (McKenzie and Yoshida 2017). Marine protected areas, locally known as tabu erias, are set up by traditional landowners in coastal communities that cover the reef, seagrass meadows and mangrove ecosystems as part of the larger Community-Based Fisheries Management Program that VFD implements. Through adaptive management, VFD collaborates with local communities to conduct various habitat and resource monitoring surveys (creel and underwater), presenting the results back to communities to drive management changes. Non-governmental organisations (NGOs), such as the Vanuatu Environmental Science Society, also contribute to the monitoring of seagrass areas in prominent communities around Vanuatu.

Threats and challenges
There are various threats to seagrass ecosystems, including climate change, coastal development, sedimentation (landuse changes), storm surges, pollution, tropical cyclones and overfishing. These are further exacerbated by a lack of regulations to monitor seagrass ecosystems. Furthermore, Pacific communities’ lack of awareness and recognition of seagrass ecosystems often hinders conservation efforts, even though most coastal communities rely on its marine resources. Research gaps exist on seagrass ecosystems’ adaptive capacity to climate change, including ocean acidification and investigation of overpredation, which calls for long-term monitoring and data collection. Monitoring should include assessing seagrass distribution and health over time. The potential for blue carbon markets is still unknown. Therefore, knowledge gaps need to be addressed by further research to assess variability in carbon accumulation in seagrasses and sediments around the Pacific for site-specific data.

Opportunities
More research and outreach awareness are needed for the promotion and recognition of seagrass ecosystems in the Pacific. It is important, however, that this awareness provides linkages between seagrass ecosystems and with other marine life and ecosystems. For instance, the link between seagrass as a source of food for dugongs and sea turtles. Additionally, it can be incorporated into other ecological conservation awareness and community-based fisheries management initiatives that already exist within government, academia and NGOs. The importance of NGOs such as the Locally Managed Marine Area Network, and local communities in protecting and managing marine ecosystems through engagement and community involvement is crucial. Emphasis should be placed on hands-on learning for citizen science initiatives.

To create a better understanding of the loss and general state of seagrass ecosystems in the Pacific Islands,7 MACBLUE is currently mapping the extent of seagrass ecosystems in Fiji, PNG, Solomon Islands and Vanuatu. This will help better understand drivers of degradation and identify the most important threats. The project will additionally test appropriate management strategies. The resulting data will allow inventories of associated natural capital and will support government partners in their efforts to strategically develop and implement conservation, management and rehabilitation efforts.

This initiative, as well as many others by civil society, governments, and academia, should finally help keep seagrass meadows in the public mind and effectively contribute to their conservation. Only then, can we maintain these unsung heroes of ocean life so that they can continue to contribute to our well-being.

Conclusions
The Pacific Islands region has been identified as having limited information on carbon storage in tropical seagrass species and meadows. The available literature on seagrasses in the Pacific is very limited and for a lot of the available data, ground-truthing is needed. Where possible, more research is needed to drive actions to lobby for a specific or standalone seagrass policy. A healthy seagrass meadow supports fisheries that in turn support people and income-generating activities; therefore, more attention is needed in seagrass conservation and management. While there is slow shift towards a greater recognition of seagrass ecosystems in the Pacific, this is mostly done as part of community-based fisheries management in collaboration with conservation NGOs. Due to their importance in climate change mitigation, biodiversity conservation, and the local economy, it is vital that we protect this ecosystem together using a coordinated and unified approach.

Acknowledgements and disclaimer
The MACBLUE project is jointly implemented by the Secretariat of the Pacific Regional Environment Programme, the Pacific Community and the Deutsche Gesellschaft für internationale Zusammenarbeit GmbH (GIZ Pacific) in close cooperation with their four partner governments Fiji, Papua New Guinea, Vanuatu and Solomon Islands. The MACBLUE Project implements its activities with financial support from the German Federal Ministry for Environment, Nature Conservation, Nuclear Safety and Consumer Protection through its International Climate Initiative. The views expressed in this article are the authors’ and do not necessarily reflect the view of their organisations.

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Authors
Falma Aiviji, Vanuatu Department of Fisheries.
Henry Kaniki, WWF Solomon Islands.
Joeli Bili, Germany Agency for International Cooperation (GIZ).
Mazzella Maniwavie, The Nature Conservancy PNG.
Shalini Singh, Fiji National University.

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Aymeric Desurmont, Team Leader - Fisheries Information and Knowledge, Fisheries, Aquaculture, and Marine Ecosystems | [email protected] 
Tracey Holley, Science Networks and Knowledge Management Officer, Fisheries, Aquaculture, and Marine Ecosystems | [email protected]

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