Eelgrass (Zostera marina) is a flowering plant that grows entirely underwater in shallow coastal waters. Unlike seaweed, which is algae, eelgrass is a true vascular plant with roots, leaves, veins, and flowers. It forms dense underwater meadows that serve as nursery habitat for dozens of fish and invertebrate species, making it one of the most ecologically important plants in temperate oceans.
A Plant, Not a Seaweed
One of the most common points of confusion is the difference between eelgrass and seaweed. They look superficially similar, but they’re fundamentally different organisms. Seaweeds are algae. They attach to the seafloor with a simple holdfast and absorb nutrients directly through their body surfaces by diffusion. They have no flowers, no veins, and no true roots.
Eelgrass, by contrast, belongs to the monocotyledons, the same broad plant group that includes grasses, lilies, and palms. It has an internal vascular system that transports water and nutrients through veins, just like a land plant. Its roots actively absorb nutrients from the sediment. It produces flowers, gets pollinated underwater, and disperses seeds. Small air pockets called lacunae run through the leaves, keeping them buoyant and helping the plant exchange oxygen and carbon dioxide with the surrounding water.
What Eelgrass Looks Like
Eelgrass has long, ribbon-like leaves with rounded tips that grow from a creeping underground stem called a rhizome. The leaves can reach up to 1.2 meters (about 4 feet) long and are typically 2 to 12 millimeters wide, which is why it’s sometimes called tapegrass. The rhizome sits just below the sediment surface, stretching 2 to 5 centimeters between nodes, with roots anchoring each node into the sand or mud. When you see a dense meadow of eelgrass, what looks like thousands of individual plants is often a network of connected rhizomes sending up new shoots at each node.
Where It Grows
Eelgrass thrives in shallow, protected coastal waters across the temperate Northern Hemisphere, from the Atlantic coasts of North America and Europe to the Pacific. It needs a lot of light, which limits how deep it can grow. Transplant experiments in moderately turbid water found that eelgrass survived and branched at depths of 3 meters or less but died off completely at 5 to 8 meters within months. The key factor is how much sunlight penetrates to the bottom. Seagrasses generally need at least 4 to 36 percent of surface light to survive, so in clearer water, eelgrass can grow deeper, while in murky bays it’s restricted to the shallowest zones.
Salinity ranges are fairly broad. Eelgrass grows well in water ranging from about 26 to 31 parts per thousand, which covers most coastal and estuarine environments that aren’t extremely brackish or freshwater.
How It Reproduces
Eelgrass reproduces two ways. The simpler method is clonal: the rhizome elongates underground and sends up new shoots from its nodes, steadily expanding the meadow outward. This is how most meadow growth happens day to day.
Sexual reproduction is more complex and happens entirely underwater. Reproductive shoots develop branches containing both male and female flower parts enclosed in sheaths called spathes. Pollination occurs underwater, and after successful fertilization, it takes roughly four weeks for seeds to mature. The reproductive shoots eventually detach and drift with currents, releasing ripe seeds over that four-week window. Seeds that settle on suitable sediment germinate and establish new meadows, sometimes far from the parent bed. This drifting seed dispersal is how eelgrass colonizes new areas and how restoration scientists have developed creative planting methods, collecting reproductive shoots in mesh bags suspended from buoys and letting seeds drop naturally to the seafloor below.
Why Eelgrass Meadows Matter
Eelgrass meadows are among the most productive ecosystems in coastal waters. Their tall, dense leaf canopy creates structurally complex habitat that shelters juvenile fish and invertebrates from predators while providing abundant food. Surveys in Chesapeake Bay seagrass beds collected over 21,000 fish across 31 species, dominated by silver perch, spot, weakfish, spotted seatrout, and Atlantic croaker, many of which are commercially or recreationally important. Species like pinfish and Atlantic croaker actively choose seagrass habitat based on dissolved oxygen levels, food availability, and predation risk.
Beyond fish habitat, eelgrass meadows stabilize sediment with their root systems, filter nutrients from the water, and store carbon in both their living tissue and the organic matter that accumulates in the sediment below. This carbon storage capacity, often called “blue carbon,” makes seagrass beds significant players in coastal carbon cycling.
Threats to Eelgrass
The biggest ongoing threat is eutrophication, the overloading of coastal waters with nitrogen from agricultural runoff, sewage, and stormwater. Excess nitrogen doesn’t directly poison eelgrass, but it fuels explosive growth of algae that shade out the plants beneath. In eutrophied areas of eastern Canada, one type of epiphytic algae increased 40-fold, and a mat-forming green algae called sea lettuce increased 670-fold, eventually covering 61 percent of the bottom. Under this blanket of algae, eelgrass shoot density drops, both aboveground and belowground biomass decline, and the meadow’s carbon storage capacity shrinks along with it.
Disease is the other major threat. Seagrass wasting disease, caused by a microorganism called Labyrinthula zosterae, produces distinctive black or brown lesions on leaves, often with a pale center. The pathogen destroys chloroplasts, stripping the plant of its ability to photosynthesize. In the 1930s, a wasting disease outbreak along the Atlantic coasts of Europe and North America wiped out up to 90 percent of eelgrass, disrupting coastal food chains and sediment processes across entire regions. Smaller outbreaks have continued to appear around the world since then, and recent research has confirmed that the disease can spread through waterborne transmission, not just direct leaf-to-leaf contact.
Global Decline and Recovery
Seagrass meadows have declined globally over the past century due to the combined pressures of coastal development, pollution, eutrophication, and climate change. In the temperate North Atlantic East (the European coastline), net losses have reached roughly 69 percent. The picture is more encouraging in parts of North America: the temperate North Pacific and North Atlantic West bioregions show no net loss of seagrass extent, likely because strict management measures in many countries have slowed or even reversed declining trends.
Restoration efforts use a combination of transplanting adult shoots and deploying seeds. One approach suspends collected reproductive shoots from buoys over target areas, letting seeds release naturally over several weeks and germinate on the seafloor below, forming new arc-shaped meadows. These techniques are labor-intensive but have proven effective in areas where water quality has improved enough to support regrowth. The limiting factor is almost always light: if the water is too murky from nutrient pollution or sediment runoff, no amount of planting will sustain a meadow.