Seagrass: Essential underwater meadows

Mar 23, 2021

An article by Nina de Villiers

Nina is part of Dr Louw Claassens’ team researching the status of the Estuarine Pipefish in the Boesmans and Kariega Estuaries. She has written a fascinating article on Seagrass: essential underwater meadows. and the importance of these meadows to the health of estuaries.

Seagrass: essential underwater meadows

By: Nina de Villiers

Have you ever looked out on the estuary on a day where you can see right to the bottom and noticed swaying grass? Or perhaps you’ve been sitting watching the water and long green strips have drifted passed you? It’s likely you’ve been looking at seagrass. Seagrasses are marine flowering plants that have, like their terrestrial grass counterparts, roots or rhizomes. Therefore, they gain their nutrients from sediment as opposed to through the water like algae. They are found on every continent except Antarctica and reside at the margin between land and sea. And while you look at these unassuming plants maybe you haven’t realised how lucky you are to see such glorious meadows. Threatened globally, seagrasses play an essential role for many marine and estuarine animals.

Not only do fish and other marine critters use this habitat for foraging but many use it as a nursery. Thousands of baby fish spend their first couple of months growing within the swaying strands and some spend their entire lives within seagrass beds. Without these lush habitats the fish you find in your supermarket or favourite restaurant would probably be missing. Seagrass meadows supply 50% of the world’s fisheries as 32% of commercial fish species caught use these habitats during some part of their life cycle. Seagrasses are extremely important for biodiversity and far more species are found within these beds compared to adjacent sandy habitats. The benefits of seagrass don’t just stop at the role it plays for animals. They also act as filter systems, sediment stabilisers, and can capture and store carbon 35 times faster than a rainforest. Although, they only occupy 0.1% of the seafloor they are responsible for 11% of the organic carbon buried in the ocean and create more oxygen than forests and grasslands combined. Without seagrasses there is an increased risk of property damage from storms as they buffer the intensity of waves. Even when seagrass blades have died and become free floating they will eventually sink to the bottom and provide nutrient rich detritus which is fed on by many animals. Tiny animals also remain on these strands and are an important food source for many fish.

In South Africa the dominate seagrass is eelgrass (Zostera capensis) and it can be found in estuaries all along the coast from Namibia to Kenya (Fig. 1):

Figure 1: Beds of eelgrass (Zostera capensis) found in 62 of the 300 estuaries from Olifants Estuary to Kosi Bay along the South African coastline

Some of the estuaries with the largest areas of eelgrass in South Africa are Knysna, Langebaan, Keurbooms, and Bushmans. These eelgrass beds are a vital habitat and helps support the high biodiversity of many estuaries (Fig 2):

Figure 2: Seagrasses are home to a number of animals. (Starting at top left moving clockwise) Spiny starfish (Marthasterias africana), Clingfish (Apletodon sp.), Carpet flatworm (Thysanizoon sp.), Four-tone nudibranch (Godiva quadricolor), Anemone (Actiniaria), Long snouted pipefish (Syngnathidae temminckii), Hydroid (Eudendium sp.), Blue speckled dorid (Dendrodoris caesia)

When alive these meadows create homes from the tiniest sea slug to sneaky octopus (Fig 3):

Figure 3: A tiny sea slug (Oxynoe sp.) does acrobatics on an eelgrass blade (left). Blending in perfectly an octopus (Octopus vulgaris) hides away (right).

A very special animal in the Bushmans and Kariega estuary that relies on eelgrass beds is the Critically Endangered Estuarine Pipefish (Syngnathus watermeyeri) which hides within these beds (Fig. 4):

Figure 4. The Critically Endangered Estuarine Pipefish (Syngnathus watermeyeri) relies on dense beds of eelgrass for refuge.

Pipefish are part of the Syngnathidae family along with seahorses and seadragons, which all have a straw-like fused jaw. This family of fishes has a very special trait: the male carries and gives birth to the young. The Estuarine Pipefish has only been recorded in the Bushmans, Kariega, Kasouga, and Kleinemonde estuaries. Historical surveys have found very few individuals and numbers fluctuated dramatically. According to the latest IUCN Red List Assessment the population in each estuary is estimated to be fewer than 50 individuals. At an average length of only 10 cm, these guys are particularly difficult to find especially within the dense beds of vegetation. The extensive study carried out from October 2019 to July 2020 found a total of 31 pipefish in Bushmans and 28 in the Kariega estuary. These recent surveys found no pipefish in any of the other estuaries within the range, and at this stage, the Estuarine pipefish is only present in the Kariega and Bushmans estuaries. This animal depends on the presence of suitable habitat, such as seagrass and macroalgae. Any decrease in the extent of suitable habitat would have a significant negative impact on this species. Protection of these habitats would aid in the conservation of this species as well as the many other animals that rely on it.

Around the world seagrass, like many habitats in close proximity to land, is threatened by human impacts and it is estimated that globally every hour 1 ha of seagrass is lost. Threats include displacement from agricultural and wastewater runoff, direct damage by boating, removal of patches of seagrass especially owing to development, illegal bait digging (i.e. using spades) and climate change. Zostera capensis is listed as Vulnerable on the IUCN Red List of Threatened Species and has a limited ability to disperse and therefore once lost from a system it is unlikely to return. Due to this, eelgrass has a high risk of local extinction. Major threats to eelgrass within the Bushmans and Kariega estuary are the direct removal through propeller damage from boating, illegal bait digging, and the removal by residents. To combat this destruction, baiting and boating restrictions need to be more strictly enforced. To reduce your impact, you can lift your boat engine or travel at a slower speed when going through patches of eelgrass. Even better confine your boating time to high tide (Find out more about tide times here: http://www.sanho.co.za/). Other threats include increased eutrophication, this is when water is overly enriched with nutrients due to waste water or agricultural run-off, and introduction of invasive species. You can also learn more about seagrasses and how to help protect them on the Project Seagrass website (https://www.projectseagrass.org/why-seagrass/).

Eelgrass is more than what it appears beneath the surface lies a three-dimensional habitat. It plays an integral role in the lives of animals and humans through its direct benefits and its indirect services. More and more research focusing on our eelgrass beds should be carried out with active conservation actions on the ground. Estimates of the area of eelgrass are required to be used as a baseline with which future changes can be compared. Around the world these habitats are diminishing every day and we are incredibly privileged to have such a large amount on our doorsteps in Kenton on Sea. However, eelgrass requires our protection in order to thrive into the future. It is our responsibility to aid in its survival through improving water quality, education, monitoring changes to the extent, and being conscious of our boating impacts. Protecting this habitat will have significant knock down effects for other threatened species such as the Estuarine Pipefish

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