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Green barriers in the blue

Seagrass meadows as coastal defenses

By Blue Forests Project

September 20^th, 2017

Seagrass meadows, vital but threatened

Seagrasses are marine plants found across the world. They form underwater meadows and are vitally important through the many benefits they provide, including water filtration, serving as a nursery and home for many organisms, including commercially important fish and shellfish species. Seagrass meadows are important for marine biodiversity, helping to support coastal and island communities through revenue from coastal and marine tourism.

Seagrass ecosystems are under intense pressure globally, with 65% of them worldwide are thought to be degraded. The rate of this loss is equivalent to those occurring in tropical rainforests and coral reefs, yet they receive a fraction of the attention. To add to this loss, rising sea levels are now threatening these important ecosystems and this could be the straw that breaks the camel’s back.

“Seagrass meadows are critical to the functioning of healthy and productive coasts around the world, but unfortunately they are often overlooked and unappreciated. In addition to protecting coastlines against erosion and providing food security through fisheries, seagrasses sequester remarkable amounts of carbon in their soils and are thus important in the fight against climate change. We need to do more to protect these incredibly valuable ecosystems” says Gabriel Grimsditch, Programme Officer with UN Environment.

Seagrass meadows bind sediments together and protect coasts from erosion

On every coastline, the waves and currents create change, sometimes bringing sediments to the coast, but other times causing erosion and loss of land. In order to understand the role of seagrasses in sediment dynamics and patterns, researchers have been monitoring the effect of seagrass on change in sediment elevation locations in Scotland, Kenya, Tanzania and Saudi Arabia. In research published this week, they have found that “the presence of seagrass had a highly significant, positive impact on sediment deposition at all sites”, stated Maria Potouroglou, co-ordinator of SeagrasSeDy (Seagrasses and Sediment Dynamics), a network of scientists based across the globe.

“Combined data from the current work and the literature showed an average difference of 31 mm per year in elevation rates between areas with seagrass and areas without”, says Maria and adds that this emphasizes the important contribution of seagrass in facilitating sediment deposition and reducing erosion.

“This study provides quantitative evidence of the value of seagrass conservation to coastal resiliency. This paper will be of interest to and used by not only seagrass scientists, but also coastal managers and policy makers” says Steven Lutz, Blue Carbon Programme Leader at GRID-Arendal.

The researchers found that where seagrasses occur, they enhance sedimentation or reduce erosion. The seagrass canopy reduces wave energy and slows the flow of water over the sediment surface, reducing the water’s capacity to dislodge sediments. At the same time the slower water flows can allow already suspended sediments to settle out from the water, resulting in increased deposition of sediment. Also, the seagrass roots help to armour the sediment from erosive forces and to trap and bind sediment particles together.

Seagrasses and sea level rise

Sea- level rise is seen as a rapidly increasing threat to coastal and island communities across the globe, threatening property and potentially increasing the impacts of climate change and extreme weather events, such as hurricanes. Many communities, such as Miami Beach, Florida, are investing hundreds of millions of dollars into climate resilient infrastructure including the elevation of property and new drainage pumps.

Seagrasses, along with the other coastal habitats, are known to be effective in the buffering of shorelines from storm surges, but quantitative information on this benefit is sparse. Recognition of this important natural service by engineers, city planners and government agencies could help provide cost effective resilience to the impacts of climate change.

“Our study is an important first step in assessing the role of seagrasses in controlling sediment surface elevation, and illustrates that seagrasses may be able to grow and keep up with sea-level rise ”, says Maria. She also calls for the development of an extensive scientific network which will monitor this race between the seagrass floor and sea level to see who can stay on top, identify the areas of high risk and make sure seagrass meadows still line the coast in the future.