March is international seagrass month, and in recognition of this undersung and critically important marine plant we are focusing on seagrasses, and the integral part that sharks and rays can play together to combat climate change through regulating this critical marine habitat.
What is Seagrass?
Seagrasses are plants that live in shallow sheltered areas in temperate and tropical waters. These sensitive plants are different from seaweed (an algae) by forming flowers, having bright green blades, or leaves and a root (or rhizomal) networks that can form large, dense undersea meadows. Like coral reefs and rainforests, these underwater seabeds host complex communities of diverse species of invertebrates, plants, microbes and fish. Like rainforests and coral reefs, these productive ecosystems are also extremely threatened from human impacts. A 2009 study by an international team of scientists found that 29 percent of known seagrass meadows have disappeared since 1879. Over the entire 130-year period, seagrass was lost at a rate of 1.5 percent per year. Protecting what is left is critical.
Although seagrasses cover less than 0.2% of the seafloor globally, seagrasses absorb 10-18 % of the ocean’s carbon each year, making restoration an important tool in the fight against climate change. Seagrass captures carbon up to 35 times faster than tropical rainforests, mangroves and coastal wetlands. Seagrass meadows provide food and habitat for thousands of species such as larval fishes, shellfish, seahorses, marine mammals, and sea turtles. Over 30 times more animals live within seagrass compared to sandy habitat. Seagrass meadows support these diverse marine communities as well as human livelihoods.They provide vital nutrition for close to 3 billion people, and 50% of animal protein to 400 million people in developing nations.
Seagrass has declined globally, including a loss of 99% in the San Francisco Bay due to dredging, fill, sedimentation and habitat loss.
A major impact to seagrass beds globally are habitat loss, pollution, siltation, and dredging. The most emergent threat is climate change, causing bleaching and even killing beds in the warm shallow waters seagrasses inhabit. Exacerbated by greenhouse gas emissions, climate change creates rising ocean temperature events causing coral reef bleaching, and also impacting the shallow bays and coastal region where seagrasses flourish. Climate change is rapidly altering the marine environment globally. Species in all marine ecosystems are facing the threats of a changing environment, including sharks. Sharks are integral to the health of ocean food chains and many sharks and rays benefit from the ecosystem services searasses provide, and in turn can help those communities thrive. As apex predators, shark maintain the balance of trophic levels (feeding) in marine ecosystems and regulate the balance and health of prey species.
For a number of reasons, rising ocean temperatures and climate change also impact the range and livelihood of reef sharks and the coral reefs they depend on. Warm water events in the North Pacific have caused a decline in prey species and massive bleaching in the central Pacific, including the great Barrier Reef. Coral reefs host the highest biological diversity, including numerous species of sharks and rays. Coral provides habitat for small reef fish, such as tangs and damsels which serve as a food source for larger species, such as groupers, which in turn, provide food for sharks. Without sharks, the larger fish will consume the algae grazers and allow algae to overgrow the coral. Eventually the entire system will collapse with a decline in abundance and species diversity. This is one example of the consequences marine ecosystems face without sharks.
Rising temperatures are also leading to a change in the migration patterns for a number of shark species. In western Australia, Southern sharks and their relatives are likely to compete with and be ‘pushed out’ by tropical species, such as bull and tiger sharks, as they migrate southwards into increasingly warmer waters. Towards the equator, the geographical range of the reef shark have shifted and overlap causing hybridization. In the northwestern Atlantic a 2022 study off the coast of Florida reported tiger sharks moving further north than ever recorded. In turn, they are moving out of protected waters and into areas where they are being caught as bycatch by commercial fishermen. Seagrass, and especially coral cannot react as quickly as mobile species like sharks and rays, and are being left behind in warmer, less hospitable waters.
The Australia Institute of Marine Sciences conducted the first ever global survey of reef sharks, revealing a widespread decline in populations they studied. Their results published in 2020 showed reef sharks are functionally extinct on many coral reefs, primarily dueto overfishing. Studies show that the removal of sharks from reef ecosystems create a trophic cascade in coral reefs and seagrass ecosystems. In the latter, some smaller herbivorous fish, like the striped trumpeter, eat seagrasses, which are in turn eaten by dolphins, which switch where they forage when sharks are around.
Sharks help regulate trophic levels and maintain ecosystem stability. The results suggest that tiger sharks affect seagrass beds through both trophic pathways and seagrass beds. A study in Shark’s Bay, Western Australia, revealed that seagrass beds are less disturbed, or even denuded by dugongs and sea turtles, By distributing these grazers that forage on seagrass, the sharks in turn help increase eelgrass density and carbon fixation. Thus, these predators protect and enhance blue carbon by limiting grazing on seagrass meadows, particularly those affected by warming sea temperatures.
Protecting these ocean predators and restoring critical seagrass and reef habitat can nurture intact ecosystems, and may just help mitigate the impacts caused by climate change.
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Morgan, J.,et al. (2011). Detection of interspecies hybridisation in Chondrichthyes: hybrids and hybrid offspring between Australian (Carcharhinus tilstoni) and common (C. limbatus) blacktip shark found in an Australian fishery Conservation Genetics DOI: 10.1007/s10592-011-0298-6
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Journal: Nature DOI: 10.1038/s41586-020-2519-y
Learn more about seagrasses at https://www.projectseagrass.org/why-seagrass/