Featured Creature: Seagrass

Photo by Peter McGowan, USFWS

What creature is an ecosystem engineer, a hero of carbon capture, and a standout among its relatives for its unique environment? 


Photo by Peter McGowan, USFWS

Sorting out our relationships 

Recently, I’ve been marveling at the strange science of taxonomy, and all of the examples of organisms that have defied classification or created challenging puzzles of how to name, sort, and associate them within the web of life (I’m thinking of slime molds, lichens, Portuguese man-o-wars, and the whole fungal kingdom, just to name a few). As I look back over planetary history, I marvel at the long time spans it took to form the planet we find familiar today, and how the life that came before us created these conditions in which we can now live so comfortably as complex, multicellular, perhaps even intelligent organisms. 

In particular, I was intrigued by the movement of life from the oceans to land, and the types of photosynthesizing life forms that were able to harness the energy of the sun to survive in such different environments. When I think of ocean-dwelling “plants,” my mind jumps to seaweed or kelp, imagining long stalks of algae “rooted” on the ocean floor, swaying in the currents. But as Tania explained in a recent Featured Creature, such algae are actually not plants at all, but protists. The majority of photosynthesizing life in the oceans are in fact algae rather than plants, while plants tend to be situated on the continents, including in freshwater systems and tidal basins.

However, there is one type of plant that truly does thrive in the sea – seagrass! Complete with the physiological features of true roots, rhizomes (or thick submerged stems) from which the bodies of seagrasses sprout long leaves, and flowers for sexual reproduction, these organisms are rooted firmly in the plant kingdom, and the ocean floor. 

From Smithsonian Ocean 

Form and function

Though algae and seagrasses are understood to be in different kingdoms with a different evolutionary history, their similarities allow them to serve similar functions physically and ecologically. At the most basic level, both organisms are sea dwelling creatures that derive their energy from photosynthesis and dwell in coastal marine environments, where they stay stationary throughout the tides and currents by attaching to the sea floor. 

Algae accomplish this with a root-like structure called a holdfast, that comes in different shapes across species and contexts, and performs the function of attaching the organism to rocks. Seagrass, in contrast, has a more sophisticated root system that not only anchors the plant in place, but also absorbs nutrients that help the creature grow. In both cases, swaths of anchored algae or seagrass can withstand the dramatic changes of the tides and the more intense waves that come in storm surges. They help slow down that movement and buffer the immense amounts of energy being unleashed, acting as a dynamic and living sea wall. 

As they grow (up to heights of 35 ft, or 7 m, depending on species), seagrasses filter the water around them and collect sediment including nutrient rich silt that settles to the floor. In this way, they help improve water quality and clarity and reduce erosion along coastlines. They also participate in nutrient cycling, using their roots to absorb certain nutrients that have come off the land and releasing them through their leaves, which helps support life in nutrient poor regions. Finally, their photosynthetic process releases vast amounts of oxygen — about 10 L a day from a single square meter.

Seagrass comes in many different varieties, with about 72 different species across the four main families, and a geographic range spread across the Earth’s oceans. They form meadows along the seafloor that provide shelter to many other organisms, who may spawn, prey, or hide in the shelter of the grasses. Both larger, free-swimming creatures (like sea turtles, sharks, rays, and many more) and smaller immobile organisms are found in seagrass meadows. Small invertebrates attach to the leaves themselves, while larger organisms pass through in their search for food, or for a safe place to hide from their own predators, lay eggs, or raise their young. It has been found that one acre of seagrass meadow can harbor over 40,000 fish and 50 million small invertebrates! Talk about a complex food web. 

Seagrasses are found across the world, from the tropics to the Arctic. Shades of green indicate the number of species reported for a given area. The darker shades of green indicate more species are present. (Short, F. et al. 2007; Image and caption from Smithsonian Institute)

A climate change champion (with its own challenges)

Seagrasses are not only ecosystem engineers that provide a foundation for coastal life of many forms, but also prolific contributors to the carbon cycle that shapes our global climate. Carbon in the ocean forms an elemental building block of the structures within seagrasses. In the same way that land-based vegetation takes in carbon through photosynthesis and converts it to sugars used for energy and physical structures, seagrasses use the carbon they consume to form their leaves and roots. When parts of these organisms die, they decay on the ocean floor where they are buried under sediment, settling the carbon there long-term. 

Though they only occupy about 0.1% of the ocean floor, seagrasses are estimated to sequester up to 11% of the carbon stored in the ocean. This function, in which seagrasses clearly punch above their weight, leads many people to call them ‘carbon sinks’ or ‘climate change allies’. 

While seagrasses certainly deserve such credit and praise, like the most dedicated among us working for healthy ecosystems and a livable planet, they can’t manage alone. These special creatures face their own vulnerabilities to intensifying climate challenges, as physical disturbances from great waves or from certain animals uproot the rhizomes and root systems anchoring seagrasses in place. More than anything else, the greatest threat to seagrasses is disturbance from human activity, including direct removal of seagrass beds, pollution caused by runoff of chemicals used in industrial agriculture and land development, consequent algal blooms that block sunlight from reaching the natural seagrass meadows, dredging at industrial scales, and fragmentation of habitat from anchors and propellers that then causes erosion and leaves seagrass beds susceptible to further damage and die-off. 

People have begun to recognize the importance of seagrasses for the health of marine creatures and ecosystems and our planet as a whole. There are many restoration efforts aimed at monitoring, bolstering, and reestablishing seagrass beds, led by organizations such as Seagrass Watch and Seagrass Net. Projects from the Chesapeake Bay to the coasts of Australia have made strides in regeneration of these remarkable meadows, and we can continue to build off this momentum as we value living shorelines and the integration of nature and its beautiful complexity into our understanding of planetary health and climate resilience. 

Seagrass meadows offer us a glimpse of what abundance looks like, and as we map our pathways into the future, let us remember what we stand to gain by participating with respect and reciprocity in the living Earth. 

Take a look, and imagine our coastlines flourishing this way all over the globe: 

“Sea” you later!