Seagrass-Herbivore Interactions: present, past and future

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The second day of the 12th International Seagrass Biology Workshop kicked off with a plenary lecture by Teresa Alcoverro, one of the worlds leading scientists in the field of seagrass-herbivore interactions.

Seagrasses are marine flowering plants, evolved from life on land about 100 million years ago. In coastal areas all over the world, seagrasses form extensive meadows, providing both food and shelter for all kinds of organisms. Seagrass and their associating fauna have a million year old co-evolutionary history. Some grazers like molluscs and isopods have specialized in eating the algae living on seagrass leaves; other organisms such as sea urchins, certain species of fish, turtles and dugongs eat the leaf tissue itself and therefore have a negative impact on the plants. The degree of impact will vary with the size, feeding preference and with the grouping behaviour and home range of the grazer. Schools of fish that exclusively eat all the young leaves for example will have a greater impact than small molluscs that forage on old leaves and additionally eat other benthic algae. The closer you look the more trophic layers you can discover in a seagrass ecosystem. Seagrass meadows are hotspots for herbivores and as a consequence these meadows attract predators. Predators show a preference for fragmented seagrass patches with a lot of edges. Alcoverro found that the fear generated by the presence of predators (e.g. sharks) in turn can reduce the amount of herbivory (e.g. turtle grazing), up to 50%. Cold spots of predators are richer in biodiversity of both prey species and other organisms, leading to the conclusion that herbivores, both direct and indirect shape these seagrass ecosystems.

Seagrasses in return have evolved to deal with grazing by using either avoidance or tolerance mechanisms. Seagrasses and their symbiotic epiphytes are known to contain several chemical components that could act as defenses, but their actual role in defense is probably low. Apart from self-defense, seagrasses were found to have certain escape mechanisms in order to avoid herbivore grazing: seagrasses can, for example, temporally increase their production when herbivore densities are low or have unpredictable or synchronized reproduction. Tolerance mechanisms focus on compensating for biomass loss due to herbivore grazing; for instance increased photosynthesis in existing leaf material or even increased growth rates. These mechanisms explain why grazing can actually have a positive effect on seagrass productivity – but only until a certain level. When turtle populations recover and their number rises, which is currently the case in certain marine protected areas, they can overgraze and deplete seagrass meadows, moving from one patch to another while the meadows they leave behind may not recover. Therefore, a fine balance between herbivores, seagrass standing biomass and highly resilient seagrass meadows are key to preserving both functional groups: we need to monitor the productivity, herbivory and predatory rates to truly understand the ecosystem and to be able to predict the future of seagrass meadows. Global change will for example lead to a rise in seawater temperature. How this temperature change will affect the aspects of seagrass-herbivore interactions is a one of the main questions for future seagrass research.

blue carbon

Securing a future for Seagrass Meadows

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I have known and worked with Dr Peter Macreadie for five years, so it was exciting to be present at his first plenary lecture at the 12th International Seagrass Biology Workshop in Wales last October. The topic of the plenary, seagrass carbon storage, centred on how seagrasses and their mud, or ‘blue carbon’, are important for dealing with our global climate change problem. Yes, we do work with mud, and blue carbon research has turned out to be quite a dirty and smelly job. I personally did not expect to enjoy playing in the mud well beyond my childhood (while getting paid for it!). But in his plenary, Macreadie laid out how much more there is to the story of blue carbon…

Macreadie began his talk with the facts about blue carbon biosequestration – a long word to describe how plants remove the atmospheric carbon dioxide (CO2) into their tissues or into their sediments. Terrestrial habitats are the most well-known ecosystems that biosequester carbon, but we are finding trees cannot take care of all the CO2 we have produced. A decade ago, clever scientists noticed that coastal or ‘blue’ carbon ecosystems can also biosequester carbon at a rate ~40 times higher than their ‘green’ carbon counterparts. These blue carbon habitats, including seagrasses, mangroves and tidal marshes, can also retain this carbon for hundreds to thousands of years without reaching full capacity. Seagrasses, in particular, not only have the power to help mitigate climate change, but their ecosystems are incredibly important for supporting half of the world’s fisheries and stabilising the coast against erosion.

Macreadie went on to describe the main questions in blue carbon research: 1) ‘Where is the blue carbon and how much is there?’ 2) ‘How do blue carbon stocks change under different conditions like habitat loss, sea level rise and increased temperatures?’ and 3) given that blue carbon ecosystems are a powerful tool against climate change, ‘How can blue carbon be incorporated into global carbon off-set initiatives?’

He highlighted a few areas of seagrass blue C research ranging from carbon loss after disturbance to microbes as the drivers of the carbon cycle to how to optimise carbon biosequestration through management. As a self-proclaimed ‘lab rat’ (with the occasional day out in the water), I enjoy researching these detailed dynamics of blue carbon science. And it is these nitty-gritty questions around seagrass biosequestration that we have answered over the last decade that are paving the way for blue carbon to shine on a cross-disciplinary, international platform.

Specifically, seagrass (and other blue C) habitats are gaining recognition for their monetary value. Valuating seagrasses helps translate their ecosystems services to policymakers, management agencies, and industry. While there are some that consider this valuation process to be a form of profiting off of nature, this new frontier is a priority for many blue carbon scientists, including Macreadie and his colleagues, because of the opportunity to see emphasis put on conserving these ecosystems we love and an opportunity for new funding to be put towards blue carbon restoration research. Many government bodies are coming out in support of both green and blue carbon biosequestration for its role in climate change mitigation, providing a great opportunity for blue carbon experts to be involved in blue carbon off-setting development.

At the end of the plenary, Macreadie circled back to answer the open question: Is seagrass blue carbon just mud? He answered with a yes with the hope that others can see the importance of this mud as a way to both mitigate climate change as well as an innovative pathway for seagrass restoration. As it turns out, we are never too old to play in the mud!