What tickles your toes when you walk into the water at the beach? Most people refer to it as seaweed, but it looks similar to the grass that may grow outside your house. Seagrasses are a group flowering plants that live in shallow waters all over the world. Here in the Puget Sound area of the Salish Sea, we commonly see a species called Zostera marina, referred to as ‘eelgrass’. Most of what you can see of the eelgrass is the long ribbon-like blades which flow with the current. The hidden roots are just as important, though. Eelgrass beds are an essential ecosystem and provide many services for both ocean and land-dwelling life.

The importance of eelgrass

Eelgrass beds serve as a nursery for juvenile fish and animals. The long blades of grass, up to three feet tall, make hiding from predators easy. In the Pacific Northwest, herring and salmon spawn in eelgrass beds. The nursery supports ecosystems around the globe, protecting the vulnerable life stages of animals important in the ocean ecosystem, food web, and to human fisheries.

Eelgrass beds improve water quality by trapping sediment with their blades. Epibiotic algae living on the blades help accomplish this. Eelgrass has a unique underground network of thick horizontal roots called rhizomes. The rhizome root network can connect a single eelgrass bed over a mile long, making eelgrass one of the largest living organisms in the world. The root network helps to stabilize soil and prevent coastal erosion.

Another amazing function of eelgrass beds is the ability to sequester carbon. This means they can remove carbon dioxide (CO2) from the atmosphere and turn it into organic carbon, which is recycled back into the environment. Climate change is the result of increased atmospheric CO2. Carbon sequestration by so-called “carbon sinks”—a term scientists use for areas that are natural reservoirs which store organic carbon—is essential to the health of the ocean and Earth. By itself, carbon is an important element that all life needs to grow. It is harmful when large amounts are converted to CO2 through burning fossil fuels.

Seagrass is responsible for over 20% of marine carbon sequestration, even though seagrass beds only occupy about 1% of the entire ocean (Duarte et al. 2011). This ends up accounting for 40 to 100 thousand tons of carbon per year! Seagrass carbon sinks are even greater than terrestrial carbon sinks, like rainforests!

Eelgrass is able to trap carbon in two ways: one way is by using photosynthesis, converting CO2 to oxygen like land plants. The other way is trapping sediments with their long blades and accumulating it around their roots. Although eelgrass produces oxygen by photosynthesis, the mud in which they live is anoxic, or without oxygen. And, without other essential nutrients like nitrogen or phosphorus, this prevents animals or other bacteria from coming to eat the organic carbon, which would cycle it back to CO2 through respiration. So, carbon is suspended then trapped in the mud for years.

All carbon sinks are important in the carbon cycle; whether they are in the ocean or on land. Because of the effectiveness and importance of seagrasses and other algal species in sequestering carbon, scientists are working to farm and promote the growth of more seagrass. It is important to keep this in mind when considering human impact on seagrass communities.

Threats to eelgrass

Nearshore and underwater construction or dredging can be detrimental to eelgrass beds. Seagrasses grow in shallow water to be able to absorb sunlight for photosynthesis. Runoff from construction and dredging to make way for ports are some of the biggest threats to eelgrass. Removal of eelgrass beds destroys the soil where eelgrass roots have worked to trap carbon for so long. Runoff from construction can cause murky water that prevents eelgrass from being able to photosynthesize. Chemical runoff can cause algal blooms which also lead to the decreased ability for eelgrass to absorb sunlight and perform photosynthesis. Furthermore, the loss of eelgrass communities threatens animals that depend on eelgrass beds for nurseries, ultimately disrupting entire ecosystems.

With increasing ocean temperature, disease is becoming more prevalent among species, including eelgrass. Eelgrass wasting disease by a slime mold pathogen Labyrinthula poses serious threat the health of eelgrass beds (Bockelmann et al. 2013). This pathogen can be spread rapidly through beds and appears as black markings on leaves, not to be confused with helpful brown ebibionts that also live on eelgrass blades.

It is of utmost importance that we protect our eelgrass ecosystems. If you are a diver, you can take place in restoration projects for seagrass. Use your voice to speak up against the construction of shorelines. Dispose of your waste properly, and don’t use fertilizer if possible, to prevent harmful runoff.

We must protect these essential ecosystems for the future. Next time a little piece of seaweed tickles your toe, remember how amazing it is for our world!

Abbey is a PADI Divemaster and aspiring marine biologist. Read Abbey’s Bio.