Seagrasses are marine flowering plants, not seaweeds, that inhabit shallow coastal waters globally. These underwater meadows are more closely related to terrestrial flowering plants than to algae, possessing true roots, stems, and leaves, and producing flowers and seeds. Approximately 72 species of seagrass are found on every continent except Antarctica, forming some of the most productive ecosystems on the planet.
Seagrass Biology and Distribution
As true flowering plants, or angiosperms, seagrasses display a structure adapted to life underwater. They possess a network of roots and underground stems called rhizomes, which anchor the plant firmly into the seabed. The rhizomes extend horizontally, allowing a single plant to spread and form vast meadows, a form of asexual reproduction.
Seagrasses also reproduce sexually through underwater pollination. Their long, grass-like leaves are the primary sites for photosynthesis. To thrive, seagrasses require access to sunlight, which restricts their habitat to shallow, clear coastal waters. Most species grow in waters between 3 and 9 feet deep, but the deepest known seagrass has been found at 190 feet (58 meters).
The global distribution of seagrass is extensive, with meadows found in the salty and brackish waters of both tropical and temperate zones. While some coastlines may only host one or two species, the tropical waters of the Indian and Western Pacific oceans boast the highest diversity, with up to 14 species coexisting. The most widespread species is Ruppia maritima, which can be found in a variety of habitats in both temperate and tropical regions around the world.
The Ecological Importance of Seagrass
Seagrass meadows create complex habitats that support a high level of biodiversity. These underwater fields serve as nursery grounds for many commercially important fish, providing shelter for their juvenile stages among the dense leaves. A single hectare of seagrass can support as many as 80,000 fish and 100 million small invertebrates like crabs and shrimp. This structural complexity offers protection from predators.
Beyond providing shelter, seagrass is a direct food source for several large marine herbivores. Animals such as manatees, dugongs, and green sea turtles depend heavily on seagrass as a primary component of their diet. An adult dugong, for instance, can consume up to 88 pounds of seagrass daily. The breakdown of dead seagrass leaves also provides food for a host of smaller organisms, forming the base of a detritus-based food web.
Seagrass ecosystems also play a role in maintaining water quality. The dense network of leaves slows water currents, causing suspended sediments and particles to settle out of the water column. This trapping of sediment increases water clarity, which benefits the seagrass itself and other nearby habitats like coral reefs. Furthermore, seagrasses absorb excess nutrients from land-based runoff, helping to prevent the formation of harmful algal blooms.
Seagrass and Climate Change
The extensive root and rhizome systems of seagrasses bind and stabilize seafloor sediments. This stabilization, combined with the ability of the leafy canopy to dampen wave energy, helps to reduce coastal erosion and protects shorelines from the increasing intensity of storms. This natural defense is a valuable service for coastal communities facing threats to property and infrastructure.
These ecosystems are also “blue carbon” sinks, capturing and storing carbon from the atmosphere and ocean. Seagrasses sequester carbon through photosynthesis, storing it in their leaves, roots, and the surrounding sediment. Despite covering less than 0.2% of the ocean floor, seagrass meadows are responsible for storing an estimated 10-18% of the ocean’s total buried carbon. Per unit area, they can capture carbon up to 35 times more efficiently than tropical rainforests, locking it away in oxygen-poor sediments for thousands of years.
The degradation of these habitats poses a risk, as the destruction of a seagrass meadow can release this stored carbon back into the atmosphere, turning a sink into a source of greenhouse gases. Protecting and restoring these blue carbon ecosystems is therefore an important nature-based strategy for mitigating climate change.
Threats and Conservation Efforts
Seagrasses face a multitude of threats from both direct and indirect human activities. Direct physical damage is a major issue, caused by coastal development, dredging operations, and destructive fishing practices like trawling. Boat propellers and anchors can also create “scars” in the meadows, fragmenting the habitat. These physical disturbances directly remove plants and disrupt the ecosystem’s structure.
Indirect impacts are equally damaging, primarily through the degradation of water quality. Pollution from agricultural and urban runoff introduces excess nutrients into coastal waters, fueling the growth of algae. These algal blooms cloud the water, blocking the sunlight that seagrasses need to survive. Climate change exacerbates these problems through rising sea temperatures and sea-level rise, which can reduce light availability.
Recognizing their value, efforts are underway globally to protect and restore seagrass meadows. A key strategy is the establishment of Marine Protected Areas (MPAs) that restrict damaging activities. Improving coastal water quality by reducing nutrient pollution is another important approach, involving upgrades to wastewater treatment and better management of stormwater runoff. Active restoration projects, which involve transplanting seagrass shoots or planting seeds, are also being implemented to help recover lost meadows.