In brief
Seagrasses are not seaweeds. They are true flowering plants with roots, stems, leaves and flowers that have returned to life in the sea. Meadows feed turtles and dugongs, shelter young fish, steady the sea floor and store carbon, but they are declining where water becomes cloudy or the sea floor is disturbed.
What it is
About 70 species of seagrass are known, found on every continent except Antarctica. They grow in shallow water where light reaches the bottom, usually in sand or mud. Common groups include eelgrass (Zostera) in cooler waters, turtle grass (Thalassia) in the tropical Atlantic and Caribbean, and Neptune grass (Posidonia) in the Mediterranean and Australian waters.
A seagrass plant has a creeping underground stem, called a rhizome, that anchors it and spreads sideways. Upright shoots with long ribbon leaves rise from the rhizome. Hollow air channels in the leaves and roots carry oxygen down into the mud. Seagrasses flower and even release pollen into the water, which carries it to other plants.
| Feature | Seagrass | Seaweed (macroalgae) |
|---|---|---|
| What it is | Flowering plant | Alga; not a flowering plant |
| Roots | True roots and rhizomes in sediment | A holdfast that grips; no true roots |
| Where it grows | Mostly on sand or mud | Mostly on rock and shell |
| Reproduction | Flowers, pollen and seeds, or spreading rhizomes | Spores or other algal reproduction |
| Examples | Eelgrass, turtle grass | Kelp, rockweed, sea lettuce |
Why meadows matter
- Food. Green turtles graze on some seagrass, and dugongs and manatees depend on it. Many small animals eat the algae and tiny animals that live on the leaves.
- Shelter and nurseries. Young fish, shrimp and crabs hide among the leaves from larger predators.
- Steadying the sea floor. Leaves slow water so that fine sediment settles, and roots and rhizomes bind it. A meadow can make water clearer and reduce erosion.
- Carbon. Meadows trap particles and bury plant matter in sediment where there is little oxygen to break it down. A global analysis (Fourqurean and others, 2012) found that seagrass soils are an important carbon store.
What a meadow needs
Seagrasses need more light than many seaweeds. Most need roughly a tenth to a fifth of the surface light to reach the leaves. That is why they live in shallow, clear water and why cloudiness is such a threat. They also need sheltered water, because heavy waves uproot them, and sediment that is not too coarse or too soft.
Growth is slow to repair. A damaged patch can take years to regrow, and in some species a scar from a boat propeller can last a long time.
Declines and recovery
A global review (Waycott and others, 2009) found that seagrass area has been declining for decades and that the rate of loss has accelerated. It estimated that about 29 per cent of the known area has been lost since seagrass was first recorded in 1879. A 2011 assessment of 72 species (Short and others) found most to be not threatened but a number at elevated risk of extinction.
- Cloudy water. Runoff of soil and fertiliser clouds the water and fuels algae that smother leaves and block light.
- Dredging, anchors and propellers tear plants and rhizomes and resuspend sediment.
- Heat and storms. Heat waves can kill leaves, and storms can bury or uproot meadows.
- Disease. In the early 1930s a “wasting disease” swept through eelgrass on both sides of the North Atlantic and killed most of it. It was linked to a slime-mould-like organism, and meadows recovered over decades in many places.
Where the cause of decline has been removed, for example when runoff was reduced and water cleared, seagrass has recovered, and in some places it has been replanted from shoots and seeds. Cleaning the water is usually the most effective step.
Field exercise
At low tide on a sheltered flat, or from a clear-water jetty, look closely at a seagrass leaf and a seaweed. Sketch where the roots or holdfast are. Count how many different small animals you can find on a handful of leaves in a tray of seawater, then return everything to where you found it.
Check yourself
Read the question, think of your answer, then open it with a tap or the Enter key.
Why are seagrasses not seaweeds?
They are flowering plants with true roots, stems and leaves; seaweeds are algae.
What is a rhizome?
A creeping underground stem that anchors a plant and spreads sideways.
Why do seagrasses live only in shallow water?
They need a good share of surface light to reach their leaves, often a tenth to a fifth.
Name two reasons cloudy water harms seagrass.
It blocks light and often comes with fertiliser that fuels algae growth on the leaves.
Which large animals graze on seagrass?
Green turtles (some species of seagrass), dugongs and manatees.
What did the 1930s wasting disease affect?
Eelgrass on both sides of the North Atlantic.
Basis of the facts
Where these facts come from. The count of about 70 species and the extinction-risk summary are from Short and others (2011); the decline figures (about 29 per cent of the known area lost since 1879, and an accelerating rate) are from Waycott and others (2009). The carbon-store finding is from Fourqurean and others (2012). The light requirement is given as a rounded range. The 1930s wasting disease is described in general terms in the standard marine biology textbooks. See the sources list.