Seagrass

Seagrasses are submerged angiosperms that evolved from land plants and independently returned to the sea. Their leaves contain air spaces that transport oxygen to buried roots and rhizomes, while their creeping rhizomes anchor plants and allow clonal expansion. Most species tolerate salt water, though some occupy brackish lagoons and estuaries. Seagrass meadows occur on shallow, generally well-lit seabeds on every continent except Antarctica, with especially extensive beds in tropical and subtropical coasts and in protected temperate bays.¹

Pollination is an unusual underwater process: many species release pollen directly into the water, where currents carry it between flowers. Reproduction may also occur through seeds, but vegetative growth from rhizomes can dominate meadow expansion. Taxonomically, seagrasses belong to several flowering-plant lineages rather than forming one close evolutionary family; examples include Zostera, Posidonia, Thalassia, and Halophila.²

Seagrass meadows are foundation habitats that alter the physical and biological conditions around them. Dense leaves slow currents and waves, while roots and rhizomes bind sediment, reducing coastal erosion and improving water clarity. The three-dimensional canopy offers shelter and feeding areas for juvenile fish, crustaceans, mollusks, and epiphytic organisms. Species such as dugongs and green turtles graze directly on seagrass, and many commercially important fish use meadows during early life stages.³

These meadows also participate in global carbon cycling. Organic material from plants can become buried in oxygen-poor sediments, creating “blue carbon” stores that persist for centuries or longer. Seagrasses cover a small fraction of the seafloor but can make a disproportionately large contribution to coastal carbon burial. Their sediments also retain nutrients and pollutants, although disturbance can reverse that service by exposing stored carbon and reducing water quality.⁴

Seagrass supports human societies through fisheries, shoreline protection, nutrient removal, and cultural values. Meadow-associated habitats contribute to catches of fish and invertebrates, while stable sediments can protect infrastructure and beaches from wave energy. In some regions, seagrass leaves have also been used traditionally as roofing or packing material. The benefits are often indirect: a meadow may support a fishery by providing nursery habitat rather than by being harvested itself.³

Decline is driven chiefly by reduced water clarity from nutrient pollution, sediment runoff, dredging, coastal construction, anchoring, boat scarring, destructive fishing, and physical disturbance. Climate change adds marine heatwaves, stronger storms, sea-level rise, and changing sediment conditions. Excess nutrients can stimulate algal growth that blocks light, while warming and disease can cause rapid losses in some species. Global assessments report substantial historical decline, but rates vary by region and species, and some meadows recover when stressors are removed.⁵

Seagrass restoration is not simply a matter of planting shoots: water quality and seabed conditions must first support continued growth. Projects may transplant adult plants, sow seeds, protect natural recolonization, or combine these approaches. Success depends on species, depth, season, hydrodynamics, sediment, and the availability of genetically diverse source material. Monitoring often uses shoot density, meadow area, leaf traits, flowering, and below-ground biomass rather than surface appearance alone.⁶

Some of the largest organisms on Earth are clonal seagrasses: a meadow can consist of genetically connected shoots produced by a single long-lived plant, although estimates of age depend on growth rates and genetic sampling. Seagrass habitats also have a less visible microbial dimension; oxygen released around roots supports chemical transformations that influence nitrogen and sulfur cycling. Conservation therefore includes watershed management, vessel controls, protected areas, restoration, and long-term observation—not only planting campaigns.²⁶

Angiosperm

A flowering plant that produces seeds enclosed within an ovary, often developing into a fruit.

Blue carbon

Carbon captured and stored in coastal and marine ecosystems such as seagrass meadows, mangroves, and tidal marshes.

Rhizome

A horizontal stem, usually underground or buried in sediment, that produces roots and new shoots.

Epiphyte

An organism that grows on another plant without necessarily parasitizing it; seagrass leaves commonly host algae and small animals.

Meadow

A contiguous area dominated by seagrass plants, often containing several species and associated organisms.