This article describes the habitat of the seagrass meadows. It gives an introduction to the characteristics, distribution, zonation, succession, biota, threats, functioning and adaptations of the organisms that live in seagrass meadows.
Seagrass communities are highly productive and dynamic ecosystems. Seagrasses are not true grasses but rooted vascular (flowering) plants of terrestrial origin that have successfully returned to the sea. This return needs several adaptations that allow them to live in submerged ocean regions. The sediments where they settle on can be muddy, rocky or sandy. Seagrass ecosystems are species-rich and include endangered species such as dugongs and seahorses. They are important for the geomorphology and ecology of coastal ecosystems through processes such as stabilizing sediments, recycling nutrients and providing the base of the oceanic detrital food webs. Although seagrasses cover only 0.15% of the oceans, they represent more than 1 percent of the total marine primary production, potentially acting as a sink for CO2 . Currently, they are facing many threats, due to human activities and natural causes 
Seagrasses generally inhabit the protected shallow waters of temperate and tropical coastal areas. Seagrass can be patchy, but more often it forms large swaths of vegetation, sometimes over 10,000 km2 in size. The most extensive areas are found in the tropics. A few species occur in colder regions. Worldwide 60 species exist, but only 4 closely related species are native of European waters. There are several distinct areas of seagrass meadows. These areas are the Indo-Pacific region, the seas around Japan and Australia, the central Western Atlantic region, the north East Atlantic region and the Baltic and Mediterranean seas.
Zostera marina is found from arctic waters along the northern Norwegian coast to the Mediterranean Sea. It is very abundant in the Baltic Sea, the North Sea and along the Atlantic coast to northern Spain. Zostera noltii ranges from the southern coast of Norway to the Mediterranean Sea, the Black Sea, the Canary Islands and has even been recorded on the southern coast of Mauretania. Cymodocea nodosa is found in the Mediterranean Sea, the Canary Islands and the North African coast. It is a warm water seagrass. Posidonia oceanica is restricted to the Mediterranean Sea. The distribution is restricted by the mixing zone of Mediterranean water and Atlantic water. Other species may successfully invade in European waters if seeds or fragments are accidentally introduced. Currently, the invasive seagrass Halophila stipulacea, native to the tropical and subtropical waters of the Red Sea, Persian Gulf and Indian Ocean, is settling in the eastern Mediterranean and has reached the southern coast of Italy. 
Thalassia is the dominant primary producer in tropical coastal seagrass communities, although other macrophytes (seagrasses and macroalgae), benthic and epiphytic diatoms and phytoplankton also contribute to the total community production. The species T. testudinum is found in the Western Atlantic and T. hemprichii in the Indo-Pacific. Thalassia-dominated meadows are considered to be amongst the most highly productive marine systems on Earth.
Requirements for development
Seagrasses develop under the following conditions:
- Salt or brackish water.
- Enough light for photosynthesis. They need an excess of 11% of the incident light in the surface water. They therefore grow in shallow regions along the coasts. The average depth is a few meters, but they are recorded down to 70 meters depth in clear water.
- Clear water. The plants die when the water becomes turbid by sediment suspension.
- A soft substrate such as mud or sand, but some species can also grow on rocky sediments and corals.
- A gently sloping coast, with little or no tidal currents or strong waves, is preferred.
Different species grow in different places. Zonation usually follows the ‘classical’ pattern of small and narrow leaved species in the intertidal zone, which in the shallow subtidal zone become replaced by the more broadleaved Thalassia.
In areas with consistent disturbance and unstable sediments, which are low in organic content, Syringodium filiforme may be the most abundant seagrass, where it is commonly found in a fringe at beaches.
Colonizing species of the high and mid intertidal zone are Halodule wrightii or Halophila decipiens (W. Atlantic) and Cymodocea rotundata, Halodule pinifolia and various Halophila spp (Indo-Pacific). They can be found just below the mangrove vegetation. Sometimes they are accompanied by sand-dwelling seaweeds such as Chondria, Hypnea and Caulerpa.
The mid and low intertidal zones are dominated by the climax vegetation Thalassia on stable substrates. Thalassia cannot tolerate prolonged exposure to high temperatures, nor long-term desiccation on intertidal flats. Thalassia-dominated seagrass meadows are generally multi-taxon conglomerates composed of various species of seagrasses, rooted calcareous and fleshy algae (rhizophytic algae), drifting fleshy or filamentous algae, and epiphytes. These smaller seagrass species or macroalgae (for example sand-dwelling Halimeda spp.) may replace Thalassia in deeper water.
The subtidal fringe is an area with Syringodium isoetifolium (Indo-Pacific). This seagrass has tough, cylindrical leaves. Enhalus acoroides can also be present. This seagrass also has very large and tough leaves. In the subtidal zone, the species Halophila stipulacea and Halodule uninervis occur when the substrate is not yet stabilized, whereas Thalassodendron ciliatum and Enhalus acoroides grow on stabilized substrates (Indo-Pacific). Other seagrasses below the depth range of T. hemprichii are Cymodocea serrulata, and at places Enhalus acoroides. Fine carpets of Halophila spp. (principally H. decipiens, H. engelmannii in the W-Atlantic, and H. decipiens, H. ovalis and H. spinulosa in the Indo-Pacific) can be found extending to 30 – 40 m depth. Deeper down the slopes of the W. Atlantic coasts, Thalassia testudinum is replaced by Syringodium filiforme or Halodule wrightii that tolerate lower light levels.
European genera are Zostera, Posidonia and Cymodocea. Zostera or eelgrass is a small genus of widely distributed seagrass, with Z. marina and Z. noltii occurring in western Europe. It is found on sandy substrates and in estuaries where it is submerged or partially floating. The meadows are important for sediment deposition, substrate stabilization, as substrate for epiphytic algae and micro-invertebrates and as nursery grounds. Z. marina is a subtidal species and may grow down to 10 -15 meters depth. Z. noltii forms dense beds in the muddy sand of intertidal areas. Z. marina has a lower tolerance to desiccation and therefore less adapted to intertidal areas. Z. noltii can also occur in subtidal areas, but is usually outcompeted by other seagrasses. Posidonia oceanica is another European species. It is endemic to the Mediterranean Sea. It occurs in dense meadows or in bands along channels. Balls of fibrous material, known as egagropili, from the foliage of the plant can be found on adjacent beaches. P. oceanica grows from shallow subtidal waters to depth of 50 - 60 meters in areas with very clear water. Cymodocea nodosa is another seagrass that occurs in the Mediterranean Sea. It grows on sandy sediments in waters down to 20 meters deep. 
The sequence of succession in the seagrass beds generally involves early species that stabilize the sediments and increase the sediment nutrient content, and by doing so allow establishment of later species. In the final state of succession, populations of plants or animals remain stable and exist in balance with each other and their environment. This so-called climax community remains relatively unchanged until destroyed by strong disturbance (e.g. invasive species introduction) or human interference (fishing, dredging, etc.).
In the tropical Western Atlantic the following step in the succession sequence is the colonization by rhizophytic green algae, mainly species of Caulerpa, Halimeda, Penicillus, Rhipocephalus and Udotea, that use a net of rhizoids to anchor in unconsolidated sediments. These algae supply some amounts of organic matter and nutrients to the sediment, but have limited sediment binding capability.
After a while, colonizing seagrass species start to germinate and grow. These species are especially opportunists that grow fast and have long internodes, providing a fast formation of a network of stolons. They survive well in unstable or depositional environments, thereby further stabilizing the sediment surface. Species are, Halodule wrightii (western Atlantic) and Halophila stipulacea, Halophila ovalis and Halodule uninervis (Indo-Pacific). In the intertidal zone, pools are formed by grazing activities of turtles. The margins of these pools are overgrown by seagrass species. Cymodocea species arrive afterwards and take over from the first colonizers. They have shorter internodes, more fleshy roots and can better fixate the sediments. In some sequences of succession, Syringodium filiforme appears instead of H. wrightii, or S. filiforme colonizes after the latter species, in which case the two species grow intermixed. S. filiforme is the least constant member in the sequence of succession and is frequently absent.
With time and increasing development of the community, Thalassia testudinum colonizes the Caribbean Sea and the Gulf of Mexico, and as far north as Cape Canaveral in Florida region. Its dense leaf canopy and rhizome and root system efficiently trap and retain particles, increasing the organic matter of the sediment and fueling the sedimentary microbial cycles. In the Indo-Pacific, Thalassia hemprichii and Enhalus acoroides are the last to colonize. They have very short internodes, densely branched stolons and numerous fleshy roots. This results in very dense vegetation.
Although the structure of T. testudinum and T. hemprichii communities may vary considerably, mainly because T. hemprichii meadows contain more seagrass species, general patterns in structural changes and processes during succession are similar. With the progression toward a climax community, there is an increase in the belowground biomass of the community as well as the leaf portion exposed in the water column, and more nutrients are sequestered by the seagrasses. The increase in leaf area provides an increase in surface area for colonization by epiphytic algae and fauna, with the surface area of the climax community being many times that of either the pioneer seagrasses or the initial algal colonizers. In addition to providing a substrate, the larger leaf area also increases sediment-trapping effects. The climax species T. testudinum and T. hemprichii (with also E. acoroides in the Indo-Pacific), have the highest leaf area, the highest total biomass, and by far the greatest amount of material in the sediments of any species dominant in the earlier stages of succession.
Functioning and adaptation
Seagrasses are basically land plants that returned to sea and secondarily colonized marine habitats. The seagrasses have adapted to the marine environment in several ways:
- Salinity. They are halophytes, with different adaptations to seawater. They can regulate osmotic pressure by sequestering salt ions within the cell wall or vacuoles. When a cell swells due to external osmotic pressure, membrane channels open and allow efflux of osmolytes (carbohydrates and amino acids) which carry water with them, restoring normal cell volume.
- Submergence. They are hydrophytes able to grow under submerged conditions. For this reason, the tissues of the seagrasses consist of aerenchyma. Through this aerenchyma, air can be provided to the submerged parts of the plants. Another adaptation to these conditions is that the seagrasses have an epidermis with chloroplasts. This is exceptional because the epidermal cells of other higher plants lack chloroplasts. Above this epidermis, a very thin cuticle is present, enabling the uptake of nutrients through the whole leaf surface. Stomata’s are absent and the vascular bundles are strongly reduced.
- Desiccation. Sulfation facilitates water and ion retention in the cell wall to cope with desiccation and osmotic stress at low tide. Sulfated polysaccharides are also important both in terms of resistance to mechanical stresses and as protection from predators.
- Erosion. Seagrasses are resistant to erosion by waves and tidal currents, due to well-developed rhizomes and numerous, fleshy roots that anchor into the substrate. The supple leaves are better resistant to water movement than stiff leaves.
- Pollination. Pollen is released from the flowers in gelatinous clumps that are carried by water currents to the pistils (female reproductive organ).
The major stems of seagrasses are called rhizomes. They grow horizontally, mostly just below the surface of the substrate. Together with the roots, they help stabilizing the substrate. The leaves and the roots function as sediment traps. The roots are thicker and more fleshy than the fibrous roots of terrestrial grasses. They thus provide protection against coastal erosion. Nutrient uptake from the seawater is very efficient. This is called nutrient-stripping. As filtering system, seagrasses fulfil an important role in the quality of the coastal waters. They absorb [math]CO_2[/math] and are important primary producers, together with epiphytic algae. They have a shelter function for many organisms and they provide important nursery grounds for commercial fish species. 
In the dense seagrass meadows, a wide variety of organisms are fouling such as hydroids, sponges, bryozoa and seaweeds. These organisms mainly attach on older leaves. Many invertebrate species are dependent on seagrasses and would become extinct or greatly reduced in abundance if the seagrasses disappear. An example of the importance of the seagrasses for the survival and development is the Atlantic bay scallop Argopecten irradians. This scallop needs the seagrass habitat for the development of its larval stage and for the protection of juveniles from predators. The dense meadows can discourage predators from entering it. Most seagrasses are not directly consumed by herbivores, because they are too tough. Species that do graze on the seagrasses are sea urchins, sea turtles, dugongs, manatees, some fishes and waterfowl. Filter feeders are abundant in the sediment. Some examples are scallops, jackknife, clams and sea cucumbers. 
Decline of the population of seagrasses is a worldwide trend. This can be caused several natural and human disturbances. Over the last 2 decades, the estimated loss from direct and indirect human impacts amounts to 18% of the documented seagrass area. But this is a small fraction of the actual numbers, because many losses remain unreported. In turn, seagrass loss leads to a loss of the associated functions and services in the coastal zone. 
The causes of this declining trend are:
- Coastal pollution
- Natural disturbances such as grazing, storms and desiccation
- Dredging for harbors, ports and shipping lanes
- Beam trawl fishing and overfishing
- Aquaculture: shading, eutrophication and sediment deterioration
- Eutrophication: excessive input of nutrients is toxic to seagrasses
- Clearance for beaches
- Shipping: anchoring, pollution
- Introduction of exotic species; accidentally, ballast water, aquaculture, hull fouling
- Tourist facilities
- Global warming, sea level rise, [math]CO_2[/math], UV increase
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