Please keep off the (sea)grass! 

By Tiaan Engelbrecht, in association with the von der Heyden lab, Stellenbosch University 

I am often asked about my research as a postgraduate student, and when I tell people I work on seagrass, I am met with confusion and questions such as: “Is it kelp, or seaweed?” As a passionate scientist and above all, a nature enthusiast, I love these dialogues, which allow me to share my knowledge about my favourite marine plant and at the same time create awareness regarding the beautiful environment around us. 

Seagrass is not kelp or seaweed, but rather a flowering plant growing in the shallow waters of sheltered estuaries, where brackish water meets the open ocean. Seagrass, much like the grass in your backyard, is delicate, with long, slender leaves that can easily break. Nestled within estuaries, bays, and lagoons, seagrass thrives in sheltered environments, protected from the powerful waves that crash along the shore. 

Seagrass plays a vital role in our ecosystem – absorbing carbon dioxide, releasing oxygen, and utilising nutrients to grow. Unlike terrestrial grass, however, seagrass performs these essential functions while partially or fully submerged in shallow waters. 

In South Africa, the Cape Dwarf eelgrass (scientific name Zostera capensis) is the most abundant seagrass species, which inhabits sheltered estuarine banks and lagoons. Stretching from the Olifants Estuary near Papendorp on the West Coast to Kosi Bay near the Mozambique border, Zostera capensis spans an impressive range. 

Focusing on the West Coast region, this seagrass species is found in just three key sites: Langebaan Lagoon and the Berg and Olifants River Estuaries. Of these, Langebaan Lagoon represents the second-largest seagrass coverage in South Africa, highlighting its ecological significance. This site marks an important breeding site for numerous fish and bird species and supports a wide range of biodiversity. 

Langebaan Lagoon holds dual significance as a Ramsar site – an internationally recognized wetland of importance – and one of the few Marine Protected Areas (MPAs) along the West Coast. Additionally, it is one of only two sites in the country where seagrass falls under formal protection. Berg Estuary, another Ramsar site, is one of the most important coastal wetlands in South Africa, hosting a large number of rare and endangered plant species. 

The Olifants Estuary represents the upper limit for seagrass on the West Coast, with no seagrass occurring north beyond Papendorp. The local community in this town relies heavily on fishing for their livelihood and similar to the two sites mentioned above, seagrass plays a crucial role as nursing grounds for fish and provides habitat to a myriad of bird species. During one of my visits to this estuary, I witnessed a family of Cape clawless otters running over the floodplains. This is truly a remarkable ecosystem that is worth protecting at all costs.

Estuaries are unevenly distributed along our coastline and those hosting seagrass are extra vulnerable to threats. It’s a bit like putting all your eggs in one basket – if the basket falls, everything inside is lost. Sadly, in recent years we have lost 742 hectares of seagrass coverage, which from a relatable South African perspective is roughly 1060 rugby fields. It is no surprise that the threat status of seagrass has been upgraded from ‘threatened’ and is now considered an endangered species by both the International Union for Conservation of Nature (IUCN) and the South African Red List of Species. 

Seagrasses are facing threats from all angles, with rivers among the most polluted systems in the country. Run-off from wastewater treatment facilities and agricultural practices introduces nutrients and pollutants into these waterways, which eventually accumulate and flow into the ocean through estuaries. 

Before escaping to the open sea, these nutrients, chemicals and pollutants settle in seagrass meadows and poison them. The presence of excess nutrients in the system causes algal blooms, with thick algae mats covering seagrass meadows that smother them. Among other industries, mining poses a critical threat to seagrass on the West Coast. Estuary mouth alterations, upstream run-off, pollution and land use changes are some of the disturbances that have led to the loss of seagrass cover – and these are often associated with mining. 

However, studies that assess the direct influence mining has on seagrass on the West Coast are lacking, highlighting a knowledge gap for future research. In other areas such as Langebaan Lagoon, nearly 40% of seagrass has been lost due to human-related activities such as bait digging using prawn pumps, kite surfing activity or trampling by beachgoers. 

It’s not all doom and gloom though, seagrass is an unsung hero. When they are strong and healthy, they make life better for all of us in the form of ‘ecosystem services’. Ecosystem services are the benefits that humans receive from healthy environments. Robust seagrass meadows are vitally important for improving water quality, because they absorb nutrients and capture sediments in the water column, acting as a natural water filter. 

Additionally, many different species depend on seagrass for food or shelter. If you enjoy fish and chips now and then, ever wonder where marine fish babies grow up to become big and strong adults? Estuaries with abundant seagrass are perfect nursing grounds for fish – they provide plenty of shelter and food to support them. 

If you’ve ever visited Langebaan Lagoon, or the Berg or Olifants Estuaries, and you have a keen eye for birdwatching, photography or simply enjoy observing nature, you may have noticed the beautiful pink flamingos, pelicans or black oystercatchers roaming the seagrass meadows in search of food. Their diet consists of invertebrates like snails, crabs, or prawns: creatures often found in seagrass meadows. 

Overall, a healthy estuary equals a healthy living environment, so many bird pairs can breed or raise their young here. Down below, in the shallow waters covering seagrass meadows, many beautiful shark and ray species can also be found feeding on the small critters mentioned above. An example of a fauna that is completely dependent on seagrass for its survival is the false eelgrass limpet, an endangered species endemic to South Africa. 

In addition to its ecological value, seagrass can help fight climate change. I told you seagrass is an unsung hero. Through photosynthesis, plants naturally remove carbon dioxide (CO₂) from the atmosphere – the very CO₂ emitted by vehicles, aeroplanes and other forms of fossil fuel combustion – and in return, provide us with the oxygen we need to survive. 

What happens with this CO₂ they capture? Seagrasses store it in their sediment and plant material. When left undisturbed, this stored CO₂ can remain locked away for thousands of years, making seagrass vitally important in combating climate change. This stored carbon is commonly known as blue carbon. Seagrass is internationally recognized as an important carbon sink due to its ability to remove large amounts of atmospheric CO₂ and store it on our behalf. 

This CO₂  storage is what my research mainly focuses on. By taking a sediment or plant material sample, we can calculate how much carbon is stored in a seagrass meadow. In my research group, the von der Heyden lab, we have quantified the carbon stocks for seagrass in Langebaan Lagoon and the Berg and Olifants Estuaries, along with several others on the eastern coast of South Africa. 

To sum up, seagrass forms the basis of several vitally important food webs in South African estuaries, providing a home and food to many species big and small. They help keep our water clean and remove the CO₂ that we emit into the atmosphere, and they do all this free of charge, with no T’s and C’s. 

I hope that this information has sparked an interest in you to look out for seagrass on your next visit to one of these destinations where seagrass meadows peacefully exist, and to be aware of why it is so important to protect them from human intrusion, especially potentially destructive industries such as mining.

And if you are lucky to see these meadows, please keep off the (sea)grass!