Seagrasses form dense meadows in shallow seas worldwide
Living in cities, often far from the ocean, means that many of us are not always able to appreciate how important the oceans are for supporting humanity. The oceans and their coastal margins, where land meets the sea, are some of the most productive environments in the world, and support not only biodiversity, but also human well-being.
Of the most overlooked, yet important parts of ocean diversity are seagrasses. Many people may not be aware that seagrasses exist, although once you know what to look for, they are easy to spot. Imagine an underwater lawn of long grass, with blades moving in the current. There will be lots of animals hiding amongst the blades, from crabs and shrimps to large grazing fishes.
Seagrasses form dense meadows in shallow seas worldwide, in both tropical and cold water oceans. They differ to kelp forests, which grow much deeper and larger and belong to a very different evolutionary lineage.
Unfortunately, much of the world’s seagrass is highly threatened through human actions such as coastal degradation, as well as impacts of climate change. Many seagrass meadows globally are shrinking with an estimated 7% loss per year since 1990 and there have been repeated calls to enhance seagrass conservation and management worldwide.
The most common seagrass in the coastal waters of southern Africa is called Zostera capensis. It is unique in many ways, one of which is that it is capable of straddling a wide diversity of environments and temperatures.
My work, and that of many collaborators in South Africa, focuses on increasing our knowledge around seagrasses in the region, which can then be used for supporting conservation and management decisions. Examples of our work includes analysing the genomes of seagrasses from different areas along the South African coastline to see if any genomic variations might make some populations better able to withstand changing climates.
Add to that work on pollutants, the interactions of seagrasses with other marine and estuarine species, as well as laboratory experiments on how seagrasses respond to changes in temperature and herbicides and the stage is set for a cutting edge research that places South African seagrass science on the global map.
The overall aim of our collaboration is to halt the loss of seagrass and to provide mechanisms for its restoration where it has gone extinct.
Seagrasses in South Africa
Zostera capensis is found only in about 62 estuaries along the southern Africa coastline extending from the Olifants River estuary on the cool-temperate west coast to Kosi Bay, almost on the border with Mozambique and a much warmer environment. Some isolated populations are found as far north of SA as southern Kenya. This distribution means that populations are isolated from each other making them more vulnerable to local pressures.
For example, bait collecting, trampling, pollution and coastal development all contribute to seagrass declines in South Africa. This is problematic, because seagrasses grow slowly and do not easily recover from disturbances. This means that once meadows are lost, it is almost impossible for seagrasses to re-establish from other populations, without human intervention.
But the threats to seagrass extend beyond the actual plants. Seagrasses provide shelter for many other species, including those that support human livelihoods through fisheries.
Seagrasses support a wide array of biodiversity, from fishes and birds to many different kinds of invertebrates.
Seagrasses also provide many other crucial ecosystem services. For example, the three-dimensional structure of seagrass meadows capture floating materials which is then trapped in the sediment around the plants. This makes seagrasses an excellent store of blue carbon (which is carbon stored in marine sediments, usually long-term if not disturbed), helping fight the rising levels of atmospheric carbon dioxide and associated effects of climate change.
Seagrasses also provide coastal protection through buffering strong wave action, as well as contribute to improving water quality.
Solutions to the seagrass crisis
It is difficult to estimate the global monetary value of the combined services from seagrass. However, research on seagrass off the coast of Sweden suggests the meadows there have a value up to USD 20,700 per hectare per year.
This makes conserving seagrasses a priority, so that their ecosystem services can continue to contribute towards supporting human well-being, even into an uncertain climatic future.
Seagrass restoration involves taking plants from a donor population and using them to replant areas where they have gone extinct. In this example from Langebaan Lagoon, the initial seagrass transplant is spreading outwards, forming a denser meadow ten months later
Restoring seagrasses, as well as other aquatic vegetation such as mangroves and saltmarshes, is seen as critical for ensuring healthy coastal ecosystems. There are many initiatives to support coastal restoration worldwide, including in Kenya and Mozambique, but much of the focus has been on the northern Hemisphere.
In South Africa, the search for an optimal restoration protocol is only just beginning, with numerous research gaps to be addressed before the full potential of the seagrass meadows off the African coast can be realised. Much of the work will also include improving the health of estuaries to ensure that seagrass restoration and the future of seagrasses is provided the best possible chance.
Sophie von der Heyden receives funding from National Research Foundation. Sophie is also part of Project SeaStore https://www.vonderheydenlab.com/project-seastore-science-for-seagrass-restoration.html