What is seagrass

What is seagrass?

Since the time of the dinosaurs, four groups of flowering plants (angiosperms) colonised the oceans. Known as ‘seagrass’, they are the only flowering plants that can live underwater. More closely related to lilies and gingers than to true grasses, they grow in sediment on the sea floor with erect, elongate leaves and a buried root-like structure (rhizome).

Seagrass live in near-shore waters of most of the worlds’ continents. They are the main diet of dugongs and green turtles and provide a habitat for many, smaller marine animals, some of which, like prawns and fish, are commercially important. They also absorb nutrients from coastal run-off and stabilise sediment, helping to keep the water clear.

  • Seagrass meadow, Lakshadweep Islands (India)
  • Photographer: Umeed Mistry

Seagrass

quick facts

60

There are approximately 60 seagrass species (possibly 72) globally that belong to four major groups.

40

40 Times more animals occur in a seagrass meadow than in bare sand

40ºC

Seawater temperatures above 40ºC will stress tropical seagrass. Death occurs at temperatures above 43ºC

Reproduction

Seagrass is unique among flowering plants, in that all but one genus can live entirely immersed in seawater. Enhalus plants are the exception, as they must emerge to the surface to reproduce; all others can flower and be pollinated under water. Adaptation to a marine environment imposes major constraints on morphology and structure. The restriction of seagrasses to seawater has obviously influenced their geographic distribution and speciation.

  • Enhalus female flower with small male flowers (white)
  • Photographer: Ria Tan (Wild Singapore)

What is seagrass?

Seagrasses are flowering plants (angiosperms) which have flowers and pollination systems that are well adapted for pollination via water. Seagrass grow in sediment on the sea floor with erect, elongate leaves and a buried root-like structure (rhizome).

Marine Plant

Seagrasses are unique flowering plants that have evolved to live completely submerged in sea water. There are 60 described species of seagrasses worldwide, with the greatest diversity found in the tropical waters of the Indo and western Pacific.

Photosynthesis

Seagrasses rely on light to convert carbon dioxide and water into oxygen and sugars. Oxygen transported to the roots creates an oxic environment in the sediment around the roots, facilitating nutrient uptake.

Habitat

Seagrasses occupy a variety of coastal habitats. Seagrass meadows typically occur in most shallow, sheltered soft-bottomed marine coastlines and estuaries. These meadows may be monospecific or may consist of multispecies communities.

Reproduction

Seagrass is unique among flowering plants, in that all but one genus can live entirely immersed in seawater. Enhalus plants are the exception, as they must emerge to the surface to reproduce; all others can flower and be pollinated under water. Adaptation to a marine environment imposes major constraints on morphology and structure. The restriction of seagrasses to seawater has obviously influenced their geographic distribution and speciation.

  • Enhalus female flower with small male flowers (white)
  • Photographer: Ria Tan (Wild Singapore)

Where does seagrass grow?

A number of environmental parameters are critical to whether seagrass will grow and persist. These include physical parameters that regulate the physiological activity of seagrasses (temperature, salinity, waves, currents, depth, substrate and day length), natural phenomena that limit the photosynthetic activity of the plants (light, nutrients, epiphytes and diseases), and anthropogenic inputs that inhibit access to available light for growth (nutrient and sediment loading). Various combinations of these parameters will permit, encourage or eliminate seagrass from a specific location.

Seagrasses occupy a variety of coastal habitats. Seagrass meadows typically occur in most shallow, sheltered soft-bottomed marine coastlines and estuaries. These meadows may be monospecific or may consist of multispecies communities, sometimes with up to 12 species present within one location.

The depth range of seagrass is usually controlled at its deepest edge by the availability of light for photosynthesis. Exposure at low tide, wave action and associated turbidity and low salinity from fresh water inflow determine seagrass species survival at the shallow edge. Seagrasses survive in the intertidal zone especially in sites sheltered from wave action or where there is entrapment of water at low tide, (e.g., reef platforms and tide pools), protecting the seagrasses from exposure (to heat, drying) at low tide.

  • Posidonia oceanica
  • Photographer: Dimitris Poursanidis

What is seagrass?

There are 60 described species of seagrasses worldwide, with the greatest diversity found in the tropical waters of the Indo and western Pacific.

Angiosperms

Seagrasses are flowering plants (angiosperms) which have flowers and pollination systems that are well adapted for pollination via water. Seagrass form tiny flowers, fruits and seeds.

Pollination

Seagrasses reproduce by pollination while submerged and complete their entire life cycle underwater. Pollination in seagrasses is hydrophilic (aided by water). Enhalus plants are the exception, as they must emerge to the surface to reproduce.

Roots & rhizomes

The roots and rhizomes of seagrass are often buried in sand or mud. They anchor the plant, store carbohydrates and absorb nutrients. Seagrasses depend upon the clonal growth of rhizomes to increase the area they occupy.

SAVING SEAGRASS

An exploration of the priceless environment of Roebuck Bay, Western Australia – the traditional lands of the Yawuru people – now under threat on many fronts. The immense bay is home to dugong and turtles, myriads of migrating birds and countless shellfish. It’s also been the food bowl for generations and generations of the Yawuru people.

Where does seagrass grow?

A number of environmental parameters are critical to whether seagrass will grow and persist. These include physical parameters that regulate the physiological activity of seagrasses (temperature, salinity, waves, currents, depth, substrate and day length), natural phenomena that limit the photosynthetic activity of the plants (light, nutrients, epiphytes and diseases), and anthropogenic inputs that inhibit access to available light for growth (nutrient and sediment loading). Various combinations of these parameters will permit, encourage or eliminate seagrass from a specific location.

Seagrasses occupy a variety of coastal habitats. Seagrass meadows typically occur in most shallow, sheltered soft-bottomed marine coastlines and estuaries. These meadows may be monospecific or may consist of multispecies communities, sometimes with up to 12 species present within one location.

The depth range of seagrass is usually controlled at its deepest edge by the availability of light for photosynthesis. Exposure at low tide, wave action and associated turbidity and low salinity from fresh water inflow determine seagrass species survival at the shallow edge. Seagrasses survive in the intertidal zone especially in sites sheltered from wave action or where there is entrapment of water at low tide, (e.g., reef platforms and tide pools), protecting the seagrasses from exposure (to heat, drying) at low tide.

  • Posidonia oceanica
  • Photographer: Dimitris Poursanidis

saving seagrass

The habitat complexity within seagrass meadows enhances the diversity and abundance of animals. Seagrasses on reef flats and near estuaries are also nutrient sinks, buffering or filtering nutrient and chemical inputs to the marine environment. They also stabilise coastal sediments.

They also provide food and shelter for many organisms, and are a nursery ground for commercially important prawn and fish species. The high primary production rates of seagrasses are closely linked to the high production rates of associated fisheries. These plants support numerous herbivore- and detritivore-based food chains, and are considered very productive pastures of the sea. The associated economic values of seagrass meadows are very large, although not always easy to quantify.

Seagrass/algae meadows are rated the 3rd most valuable ecosystem globally (on a per hectare basis), only preceded by estuaries and wetlands. The average global value of seagrasses for their nutrient cycling services and the raw product they provide has been estimated at 1994US$ 19,004 ha-1 yr-11.. This value would be significantly greater if the habitat/refugia and food production services of seagrasses were included.

nutrient sinks

Seagrasses on reef flats and near estuaries are nutrient sinks, buffering or filtering nutrient and chemical inputs to the marine environment. They stabilise coastal sediments, preventing coastal erosion.

food and shelter

Seagrasses provide food and shelter for many organisms, they are a nursery ground for commercially important prawn and fish species. Seagrasses a major food source for a number of grazing animals, like dugongs and green turtles.

Carbon Sequester

Globally, seagrasses are as important as forests in storing carbon (on an areal basis) and can store carbon 35 times faster than rainforests. Coastal seagrass meadows hold up to 83,000 metric tons of carbon km-1

Algae (seaweed):

Often confused with seagrass

Algae are plants that also colonised the sea and are often confused with seagrasses, however, they are more primitive than seagrasses. In contrast to seagrasses, algae do not have a true root system (they have holdfasts) and do not have veins that carry molecules around the plant.

Algae have spores and do not flower or produce fruit, while seagrasses have seeds and fruit.

  • Photographers: Ria Tan (Wild Singapore)

Image : Project Seagrass

Algae

  • Simple holdfast to anchor to hard substrate such as rocks or shells
  • Photosynthesis undertaken by all cells
  • Uptake of minerals and nutrients from water column via diffusion
  • Reproduction via spores

Seagrass

  • Complex root structure to anchor plant in the sediment, and extract nutrients and minerals
  • Photosynthesis restricted to cells in leaves
  • Transport minerals and nutrients in aerenchyma and the lacunae (veins)
  • Reproduction via flowers, fruits and seeds