Heidi, Marine Biologist, gives a great insight into the ecosystem services provided by seagrass and what we can do to protect them
Zostera noltii is "found in the intertidal zone, meaning that it can survive being completely exposed when the tide's fully out and fully submerged when the tide's in."
Seagrass meadows, often overlooked, play a crucial role in maintaining marine ecosystems and combating climate change. These underwater plants, which resemble the grass in your garden, are found in shallow waters and perform essential environmental functions, particularly in carbon sequestration. But how do they do this? And more importantly, what can we do to protect them? This article delves into the ongoing research by Heidi McIlvenny who featured on the Coastal Catch Ups Podcast.
Heidi's passion for the ocean began in her childhood, fostered by early swimming lessons from her father and summers spent bodyboarding at Benone on the coast of Northern Ireland. She started diving at 14 and has dived all over the world. Recently, she began diving in Northern Ireland, which deepened her appreciation for local marine environments. Her career focus shifted to coastal conservation, recognizing the significant human impact on these areas and their accessibility for public engagement.
After years in the NGO sector, Heidi has recently started a PhD at Queen's University Belfast, researching seagrass habitats and their carbon sequestration potential, aligning with current climate mitigation priorities.
I had to understand what types of seagrass are found around our coasts. Seagrass meadows consist of several species, two of which are prominent in Northern Ireland: Zostera noltii and Zostera marina. Zostera noltii is "found in the intertidal zone, meaning that it can survive being completely exposed when the tide's fully out and fully submerged when the tide's in." Heidi explained.
In contrast, Zostera marina, a subtidal species, remains underwater and has larger, thicker leaves that enhance its visual appeal. There are several established seagrass beds in Northern Ireland, including Waterfoot (Zostera marina) and Strangford Lough (Zostera noltii and Zostera marina).
A Home for the Fish
"They create food and home and a 3D structure for invertebrates and little anemones, and they act as nursery grounds for small fish species, even ones that will leave the nursery ground and go out and are important for our commercial fisheries."
Seagrass meadows are essential habitats supporting a diverse array of marine life. They provide food, shelter, and breeding grounds for numerous invertebrates, small fish, and other marine organisms. These meadows act as nursery grounds for juvenile fish species, some of which are vital for commercial fisheries. Heidi highlights their importance, stating, "They create food and home and a 3D structure for invertebrates and little anemones, and they act as nursery grounds for small fish species, even ones that will leave the nursery ground and go out and are important for our commercial fisheries."
Zostera noltii, on the other hand, provides an important source of food for wintering birds during low tide when the beds of grass are exposed. These birds include the light-bellied brent geese which travel from Canada to Ireland to escape the colder climate and rely on the grass to refuel.
Sucking in the carbon
"One of my questions is how much organic carbon is actually stored in the sediment below the habitat... It means that it hasn't re-entered the water column or potentially re-entered the atmosphere."
But it's time to answer the first question. The next important function provided by seagrass meadows is their ability to sequester carbon. Carbon sequestration refers to capturing and storing atmospheric carbon dioxide (CO2). Seagrass achieves this by trapping organic carbon in the sediment beneath it, which can be muddy or sandy and extend several meters deep. This sediment accumulation can store carbon for hundreds, thousands, or even millions of years, preventing it from re-entering the atmosphere or the water column.
As seagrass photosynthesises, they also contain carbon in their biomass. However, this biomass amount has been described as negligible compared to the sediment stores (Fourqurean et al, 2012).
All in all, seagrass is recognised as a significant carbon sink, with a study by Green et al, 2018 finding that 550 hectares of seagrass stored the equivalent of 10,512 individual UK people’s CO2 emissions per year. Although this is insignificant compared to the size of the UK emissions, it shows the power behind seagrass for such a small area they inhabit.
Research conducted on seagrass meadows in Northern Ireland aims to quantify the amount of organic carbon stored in these sediments. Understanding the extent of carbon storage is crucial for recognising the importance of preserving and restoring these habitats. Heidi explains, "One of my questions is how much organic carbon is actually stored in the sediment below the habitat... It means that it hasn't re-entered the water column or potentially re-entered the atmosphere."
This is important as data for particular areas dominate the global literature at the moment such as a particularly effective seagrass species found throughout the Mediterranean (Lavery et al, 2013). A more diverse and local-scale understanding is needed. The sediments below seagrass meadows represent a long-term carbon sink, and their protection ensures that this stored carbon remains locked away, contributing to global efforts to mitigate climate change.
How can we protect seagrass?
"Our first priority has to be protecting what we have, and if you can alleviate physical disturbances, then we should be doing that pretty quickly."
So given the substantial carbon sequestration potential of seagrass meadows, there is a growing emphasis on their conservation and restoration. So how can we do this? Many seagrass habitats have been lost due to human activities and environmental changes. This is the bit that particularly interests me. The UK has lost at least 44% of its seagrass coverage since 1936, with losses potentially as high as 92% over longer periods (Green et al, 2021).
Heidi mentions concerns about changing climatic conditions and coastal squeeze, where habitats can't migrate due to hard sea defences, highlighting that "there's actually nowhere for them to move back to." I suppose this is a concern for the intertidal species Zostera noltii. Shorelines will typically migrate landward as the mean sea level rises, and therefore the habitats you find on the shoreline will move landward too.
However, when these habitats come up against a hard structure, for example, a sea wall, they have nowhere to migrate to and are therefore lost. Now some solutions do exist but this is a huge challenge (both currently and in the future) for balancing human development and environmental protection and conservation.
However, Heidi highlights, "Our first priority has to be protecting what we have, and if you can alleviate physical disturbances, then we should be doing that pretty quickly." She emphasizes the role of eco-friendly moorings, noting, "Eco moorings are a proven technology... It's great now that Northern Ireland has a trial." This is important as usually shallow sheltered bays that seagrass favour, is also used for anchorages.
Anchor chains normally scour the seabed, creating large circular areas of barren seabed. Heidi explained that removing disturbances can lead to sea grass recovery, describing it as passive restoration, where "as soon as you lift that chain off the ground, it's likely that it will grow back and fill in those patches."
Another mitigation accompanies the eco-mooring - engagement - no doubt it will take a while for boat owners to purchase and install eco-moorings. So in the meantime, the other important mitigation is engagement with water users. Educating boat owners about the location of seagrass beds and implementing voluntary no anchor zones (demarcated by buoys) to protect seagrass is a good start.
SavvyNavvy, who provides a navigation app for water users, have placed seagrass beds on their charts so users can make informed decisions on anchoring. There is no reason why this couldn't be added to other areas around the coast in Ireland.
From a planning and development point of view, a method of protecting seagrass comes in the form of Marine Conservation Zones which restrict particular human activities by law along with management and monitoring.
For example, at Waterfoot in Northern Ireland, commercial demersal and static fishing have been banned. Any equipment such as fishing gear which would scour the seabed, destroying seagrass and disturbing sediment, should be avoided.
Heidi also discussed the need for active restoration, where "you are planting either seagrass seeds or taking little plugs of the plant and putting it in a new place." This is more complex than it sounds, requiring proper site selection that can be populated with seagrass, finding donor sites that can supply the seagrass seed, and summoning enough people to help plant the seedlings.
What does the future hold for Seagrass?
There is potential to triple the estimated blue carbon sequestration rate of Northern Ireland’s inshore marine protected area network by protecting and enhancing blue carbon habitats like seagrass meadows (Strong et al, 2021).
Heidi points out the political nature of environmental decisions, asserting that "the environment and nature have not been top of the priority list politically." However, she is hopeful about Northern Ireland's new Climate Change Act, which requires consideration of nature-based solutions, suggesting that it could lead to better decision-making that supports Nature's recovery.
Seagrass meadows are indispensable components of marine ecosystems, offering a myriad of benefits, from supporting biodiversity to mitigating climate change through carbon sequestration as we discussed. There is potential to triple the estimated blue carbon sequestration rate of Northern Ireland’s inshore marine protected area network by protecting and enhancing blue carbon habitats like seagrass meadows (Strong et al, 2021).
Protecting and restoring these habitats is essential for maintaining their ecological functions and enhancing their role in carbon storage. As research continues to uncover the full extent of their benefits, it becomes increasingly clear that seagrass meadows are a crucial ally in our efforts to combat climate change and preserve marine biodiversity.
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