What are Seagrasses?
- Seagrasses are flowering plants that grow submerged in shallow marine waters like bays and lagoons.
- Evolution: Terrestrial plants evolved about 850 million years ago from a group of green algae. Seagrasses evolved from terrestrial plants that recolonised the ocean 70-100 million years ago. There are 60 species belonging to four families in the order Alismatales.
- Seagrasses occur all along the coastal areas of India. They are abundant in the Palk Strait and Gulf of Mannarin Tamil Nadu.
- Though seagrasses inhabit all types of substratas (layers) from mud to rock, the lush green seagrass beds are found extensively in muddy and sandy substratas.
- There are 21 islands in the Gulf of Mannar. Seagrasses abound in the waters around the islands of Kurusadi, Pumarichan, Pullivasal and Thalaiyari. All six genera and 11 species of seagrasses are found here.
- Some of the important seagrasses are
- Sea Cow Grass (Cymodocea serrulata)
- Thready Seagrass (Cymodocea rotundata)
- Needle Seagrass (Syringodium isoetifolium)
- Flat-tipped Seagrass (Halodule uninervis)
- Spoon Seagrass (Halophila ovalis)
- Ribbon Grass (Enhalus acoroides)
- These were once abundant in the Gulf of Mannar region but are now
Do they require photosynthesis?
- Like terrestrial plants, seagrass also photosynthesise and manufacture their own food and release oxygen.
- With tiny flowers and strap-like or oval leaves, they require sunlight for photosynthesis.
- Seagrasses reproduce through both sexual and asexual methods.
- The pollen from the flower of the male plant is transferred to the ovary of the female flower through the sexual reproduction method.
- This is known as submarine pollination. Most species undergo this process and complete their life cycle underwater.
- Seagrasses can also reproduce asexually by branching off at their rhizomes (modified subterranean plant stem that sends out roots and shoots from its nodes).
- Because of this character, they can recover after being cut by grazers like dugongsor disturbed by storms.
Threat to Seagrass
- Seagrass beds are facing declineall over the world at the rate of 2-5 per cent annually.
- Some 30,000 square kilometres of seagrass has been lost during recent decades at a global level.
- Seagrasses face natural disturbances like grazing, storms, ice-scouring and desiccation.
- Human disturbances like eutrophication, mechanical destruction of habitat, overfishing and release of nutrients play havoc on seagrasses.
- Siltation, trawling, coastal engineering construction, pollution, etc, are considered to be significant causes for the deterioration of seagrasses.
Why are they considered as ‘ecosystem engineers’?
Seagrasses are known for providing many ecosystem services. They are considered to be ‘Ecosystem Engineers’.
- Maintain water quality: Seagrasses help maintain water quality. They trap fine sediments and suspended particles in the water column and increase water clarity.
- In the absence of seagrass communities, the sediments are stirred by wind and waves, decreasing water quality.
- This reduced water clarity affects marine animal behaviour besides decreasing the recreational quality of coastal zones.
- Filter nutrients: They filter nutrients released from land-based industries before they reach sensitive habitats like coral reefs.
- Stabilises the sea bottom: Ocean bottoms without seagrasses are prone to intense wave action from currents and storms. The extensive vertical and horizontal root systems of seagrasses stabilise the sea bottomsimilar to land grasses that prevent soil erosion.
- Provide food and habitat: Seagrasses provide food as well as habitat for fishes, octopuses, shrimp, blue crabs, oysters, sponges, sea urchins, clams, etc. They are called ‘the lungs of the sea’ as they release oxygen into the water through photosynthesis.
- Protect small beings: Seagrass habitats protect juvenile and small adult fish from large predators and strong currents. Marine animals that live in soft sea bottom sediments also take shelter in seagrass meadows. Seagrass leaves support seaweeds by providing anchoring facilities. Seahorses and lizardfish are found living in seagrass meadows almost throughout the year.
- Releases nutrients after decomposition: When it is further decomposed, it releases nutrients like nitrogen and phosphorous. The dissolved nutrients in the water are absorbed by seagrasses and phytoplankton.
- Absorbs CO2: Even though seagrasses occupy only 0.1 per cent of the ocean floor; they sequester up to 11 per cent of the organic carbon buried in the ocean. Seagrasses absorb 83 million tonnes of carbon from the atmosphere annually.