Our project to restore seagrass in Plymouth Sound, by replacing damaging boat moorings, has shown incredible results. We’ve found that this plant really is an ocean superhero, creating a home for marine life such as cuttlefish, catsharks and conger eels among its fronds

By Anna Turns

First a dog whelk glides slowly into view. Next, a fairly well camouflaged spider crab walks through the tall, green leafy blades. Then, a common cuttlefish swims along. Its retractable tentacles suddenly shoot out to catch the small fish, a sprat. (Captured in the video above.) Marine life is thriving here in the seagrass beds within Plymouth Sound, by the South Devon coast. 

Frith Dunkley, Marine Protected Areas (MPA) Researcher at the Marine Conservation Society, has been using baited remote underwater video systems (BRUVs) to record biodiversity beneath the waves and assess the importance of seagrass as a habitat for fish, crustaceans and molluscs. “Essentially, a BRUV is a camera in a waterproof casing mounted onto a metal frame with an arm that sticks out – at the end there’s a bait box containing 100g mackerel,” she explains. That bait attracts the attention of scavenging fish and other predators. 

Once Frith and the research team have lowered each BRUV down on a rope and left it to settle on the seabed, the video records for at least 45 minutes before the kit is lifted back up to the research vessel, a 12-metre catamaran. Back on land, she counts the maximum number of each individual species present within every minute of footage.

During this four-year Plymfish project, in collaboration with the University of Plymouth, Frith has analysed more than 40 videos and logged a total of 35 species, from catsharks and Couch’s bream to conger eels and thornback rays.

Frith has been comparing a nearby sandy seabed to areas with established beds of seagrass at four sites within Plymouth’s designated Marine Protected Area, an area of ocean earmarked for recovery and protected from human activities such as trawling. Frith’s study reveals that seagrass meadows support biodiversity; showing a 70% increase in individual animals, and a 7% increase in different species when compared to the nearby sandy seabed.

Seagrass is the only flowering plant to live in the sea and pollinate while submerged. The complexity it adds to the seabed enables more animals, including native seahorses, to shelter, breed and feed here. “It creates a three-dimensional habitat with lots of different nooks and crannies for things to live in, on and under.

“Just as wildlife differs between the tree canopy and the forest floor, seagrass provides refuge for so many different species. Seagrass beds make great fish nurseries because they provide food and cover, so they’re important economically for fish stocks too,” explains Frith. Herring lay their eggs in seagrass, while juvenile cod and haddock shelter among the fronds for protection from predators. 

The plant’s deep root system adds stability to the seabed, helping to dissipate the power of storm waves and reduce coastal erosion. Seagrass captures carbon dioxide from the water column during photosynthesis. Over time, the seagrass dies and decomposes, and along with other sediment suspended in the water column, this organic ‘blue carbon’ gets trapped in the seagrasses’ deep root systems, locking it down in the process. Together with mangroves, kelp forests and tidal marshes, seagrass meadows are critical carbon sinks that help mitigate climate impacts.