Seagrasses: The ocean’s real superheroes

When we think superheroes, we think big, powerful and majestic, right? So, in the ocean, the superheroes have to be the all-powerful orcas, stealthy sharks or sentient whales? Wrong! These apex predators constitute an integral part of the marine food web and represent the top tier within the hierarchy, but the magic begins right at the bottom of the sea. I’m here to tell you why seagrasses are the ocean’s most humble yet mighty superheroes and explain each of their powers, as well as how they shape and alter the coastline, and even influence the pelagic seas. 

Seagrasses are the only marine angiosperms (flowering plants) and provide a plethora of ecosystem services benefitting both coastal human populations and the ocean. Their role within the marine environment is integral and has granted them the title of ecosystem engineers and foundation species. Below is an infographic representing each of their “superpowers” i.e. the ecosystem services they provide. Let’s take a look! 

Carbon sequestration and nutrient cycling

Seagrass anatomy

Seagrasses have a global distribution excluding the Antarctic and occupy only 0.1% of the ocean floor globally, however, are responsible for 10-18% of total oceanic carbon burial. Multiple studies have reported their carbon accumulation rates to stand at 48 to 112 Tg C yr-1 (Tg C=teragrams of carbon). Notably, seagrass meadows have the capacity to accumulate and sequester these large amounts of carbon with greater efficacy than any terrestrial ecosystem. They build this into their structure and when their different components and associated organisms die off, the carbon becomes buried in the sediment. Decomposition rates within seagrass meadows are slow due to anoxic conditions, meaning stocks persist for millennia. This unique carbon sink status affords seagrass a widely discussed role in climate change mitigation. On top of this ability to sequester carbon, seagrasses participate in nutrient cycling and the entrapment of sediment, ameliorating water quality and clarity. Through their root and rhizome systems, healthy seagrass meadows play a role in coastal protection through the stabilisation of sediment, hence preventing erosion during storms and severe weather events. 

Seagrass meadows as nurseries and protection grounds

Juvenile hammerhead shark seeking shelter within a seagrass meadow

The dense, three-dimensional leafy structure of seagrass meadows makes them the ideal shelter habitat for benthic invertebrates, while also providing safe nursery grounds for juvenile fish. Many studies have investigated the seagrass nursery function hypothesis and most state that the majority of seagrass meadows have a large abundance of fish and their prey owing to the availability of food and a low predation risk. In the UK, commercially valuable fish species such as Plaice, Pollock and Herring were found residing within seagrass meadows during their juvenile life stages. Common eelgrass (Zostera marina) beds also shelter rare UK marine species such as the Short Snouted Seahorse (Hippocampus hippocampus) and the Spiny/Long Snouted Seahorse (Hippocampus guttulatus). Elsewhere in the tropics, small-scale fisheries (SSFs) have been found to depend on healthy seagrass for supporting livelihoods. 

Source of food for marine vertebrates

Dugong grazing on a seagrass meadow

 In tropical regions, seagrass meadows act as a primary food source for large marine vertebrates such as dugongs and sea turtles which graze and crop these plants, sometimes to extinction. Thankfully, new studies have shown sharks to play a role in seagrass recovery by controlling this top-down pressure by predating on these seagrass-grazing vertebrates! This  resource function also makes seagrass meadows attractive SCUBA dive locations and has lead to the promotion of Eco-tourism in many seagrass areas. As well as being a direct food source, seagrasses become overgrown with macroalgae and epiphytes (sessile organisms that grow on plants) which are ingested by bottom-dwelling benthic communities. Furthermore, their resource role extends into deeper waters, with seagrass detritus supporting entire pelagic food-webs. 

Climate change mitigation

The aforementioned role in coastal protection, as well as their status as carbon sinks, affords seagrasses a role in climate change mitigation. The carbon stored within seagrass meadows has been coined “Blue Carbon” and these stocks are currently being assigned monetary values and incorporated into domestic climate policies. So-called Blue Carbon strategies have been emerging intending to access financing to compensate for the costs of protecting and restoring seagrass meadows. It has also been proposed that Blue Carbon strategies can contribute to Greenhouse gas (GHG) reduction targets, climate policies and the development of seaweed aquaculture as is being achieved in China.  

Threats

As in any good superhero story, there has to be a villain. In the case of seagrass, it’s humans and anthropogenic influence that pose the biggest threats. Despite the discussed multitude of functions, two-thirds of seagrass meadows located in close proximity to populated areas have been destroyed. The main causes of seagrass decline have been eutrophication and mechanical damage due to mooring and anchorage. Climate change is predicted to have a synergistic effect with these stressors, further enhancing deterioration of seagrass habitats. Interestingly, increasing ocean acidification has been noted to have the potential to benefit seagrass carbon assimilation rates, a rare example of the positive effects of a changing climate. Preserving seagrass meadows and restoring those which have suffered damage or been lost is now a conservation priority, with a lot of work remaining to be done. Public awareness and stakeholder participation are crucial for this to be successful. 

Eutrophication

Eutrophication, as stated in many studies, has been one of the leading causes of seagrass disappearance. Growth in human coastal populations has led to a reduction in water quality and increased runoff. Further from the direct loss of meadows, eutrophication leads to several indirect stressors, including the promotion and subsequent dominance of phytoplankton and macroalgae. This consequently limits light availability through increased competition, even leading to reduced sequestration and enhanced GHG emission. The increase in nutrient and sediment loading is also having detrimental effects on seagrass-associated organic carbon deposits.

Seagrass wasting disease

Although not a primarily anthropogenic issue, increased runoff, nutrient overload and other environmental conditions may also alter the prevalence of infection in seagrass. During the last century, seagrass meadows underwent broad-scale damage due to the ‘wasting disease’. This was related to Labyrinthula zosterae and has sporadically been reducing Z.marina populations in the Atlantic since the 1930s. This is said to have coincided with the increased use of agricultural fertilisers and herbicides. One study tested for the association between nutrient over-enrichment and use of herbicides in making seagrass more susceptible to pathogens. Using an example herbicide Diuron, Z.marina was tested for resistance to infection by L.zosterae while another pathogenic slime-mould like protist from an Aplanochytrium species was cultured and also used in the experiments. The results confirmed a higher vulnerability of seagrass to pathogenic infection when exposed to elevated nitrate concentrations, Diuron, and the other tested pathogen. These results support the hypothesis that wasting disease outbreaks may be partially explained by an increase in runoff and fertiliser use. Despite these findings, studies call for more research to explore the role of abiotic and biotic factors on host-parasite interactions and highlight the need to investigate the Labyrinthula genus further. 

Moorings and anchorage

Diagram of a traditional swinging mooring consisting of a sinker block and chain

 Although not thought to be the greatest threat globally, conventional swinging moorings have had a significant contribution to seagrass decline. Standard moorings have been shown to cause scouring to meadows, contributing to the physical removal of rhizomes and shoots and creating gaps within a meadow. The loss of seagrass via physical removal has implications for the many mentioned ecosystem services seagrasses provide. Several studies have aimed to explore the effects of anchorage and mooring and found that dead matte areas lose the ability to be recolonised with recovery being hindered due to the slow clonal growth of seagrass, estimated at 1–7 cm yr−1. Consequently, the edges of the new bare area become unstable and more vulnerable to further disturbance. Anchoring scars are characterised by unprotected sediment which is subject to wave action, depressing the area and progressing the meadow toward fragmentation and regression from which it cannot recover. In the UK, it has been reported that at least 6 ha of seagrass meadow have been lost due to swinging mooring chains.

The future

For these marine superheroes to continue providing us and the ocean ecosystem with their services, we need to protect them. Reversing damage, mitigation strategies, and greater awareness are crucial. All their ‘powers’ render them a conservation priority, with some restoration programmes already attempted, yet a lot remaining to be done. In order for the complex marine food web to persist and our coastlines to be protected, seagrass meadows need to remain healthy and intact.


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