Seagrass

Seagrasses are flowering plants (angiosperms) which grow in marine environments. There are 60 species of fully marine seagrasses which belong to four families (Posidoniaceae, Zosteraceae, Hydrocharitaceae and Cymodoceaceae), all in the order Alismatales (in the class of monocotyledons). Seagrasses evolved from terrestrial plants which migrated back into the ocean about 75 to 100 million years ago.Species subtotal:  Species subtotal:  Species subtotal: 2 to 9  Species subtotal:   Seagrasses are flowering plants (angiosperms) which grow in marine environments. There are 60 species of fully marine seagrasses which belong to four families (Posidoniaceae, Zosteraceae, Hydrocharitaceae and Cymodoceaceae), all in the order Alismatales (in the class of monocotyledons). Seagrasses evolved from terrestrial plants which migrated back into the ocean about 75 to 100 million years ago. The name seagrass stems from the many species whose leaves are long and narrow, who grow by rhizome extension and often spread across large 'meadows', which resemble grassland: many species superficially resemble terrestrial grasses of the family Poaceae. Like all autotrophic plants, seagrasses photosynthesize, in the submerged photic zone, and most occur in shallow and sheltered coastal waters anchored in sand or mud bottoms. Most species undergo submarine pollination and complete their life cycle underwater. Seagrasses beds/meadows can be either monospecific (made up of a single species) or in mixed beds. In temperate areas, usually one or a few species dominate (like the eelgrass Zostera marina in the North Atlantic), whereas tropical beds usually are more diverse, with up to thirteen species recorded in the Philippines. Seagrass beds are diverse and productive ecosystems, and can harbor hundreds of associated species from all phyla, for example juvenile and adult fish, epiphytic and free-living macroalgae and microalgae, mollusks, bristle worms, and nematodes. Few species were originally considered to feed directly on seagrass leaves (partly because of their low nutritional content), but scientific reviews and improved working methods have shown that seagrass herbivory is an important link in the food chain, feeding hundreds of species, including green turtles, dugongs, manatees, fish, geese, swans, sea urchins and crabs. Some fish species that visit/feed on seagrasses raise their young in adjacent mangroves or coral reefs. Seagrasses trap sediment and slow down water movement, causing suspended sediment to settle out. Trapping sediment benefits coral by reducing sediment loads, improving photosynthesis for both coral and seagrass. Although often overlooked, seagrasses provide coastal zones with a number of ecosystem goods and services. Seagrasses are considered ecosystem engineers. This means that the plants alter the ecosystem around them. This adjusting occurs in both physical and chemical forms. Many seagrass species produce an extensive underground network of roots and rhizome which stabilizes sediment and reduces coastal erosion. This system also assists in oxygenating the sediment, providing a hospitable environment for sediment-dwelling organisms. Seagrasses also enhance water quality by stabilizing heavy metals, pollutants, and excess nutrients The long blades of seagrasses slow the movement of water which reduces wave energy and offers further protection against coastal erosion and storm surge. Furthermore, because seagrasses are underwater plants, they produce significant amounts of oxygen which oxygenate the water column. These meadows account for more than 10% of the ocean’s total carbon storage. Per hectare, it holds twice as much carbon dioxide as rain forests and can sequester about 27.4 million tons of CO2 annually. The storage of carbon is an essential ecosystem service as we move into a period of elevated atmospheric carbon levels. However, some climate change models suggest that some seagrasses will go extinct – Posidonia oceanica is expected to go extinct, or nearly so, by 2050. Seagrass meadows also provide physical habitat in areas which would otherwise be bare of any vegetation. Due to this three dimensional structure in the water column, many species occupy seagrass habitats for shelter and foraging. It is estimated that 17 species of coral reef fish spend their entire juvenile life stage solely on seagrass flats. These habitats also act as a nursery grounds for commercially and recreationally valued fishery species, including the gag grouper (Mycteroperca microlepis), red drum, common snook, and many others. Some fish species utilize seagrass meadows and variaus stages of life cycle. In a recent publication, Dr. Ross Boucek and colleagues discovered that two highly sought after flats fish, the common snook and spotted sea trout provide essential foraging habitat during reproduction. Sexual reproduction is extremely energetically expensive to be completed with stored energy, therefore, they require seagrass meadows in close proximity to complete reproduction. Furthermore, many commercially important invertebrates also reside in seagrass habitats including bay scallops (Argopecten irradians), horseshoe crabs, and shrimp. Charismatic fauna can also be seen visiting the seagrass habitats. These species include West Indian manatee, green sea turtles, and various species of sharks. The high diversity of marine organisms which can be found on seagrass habitats promotes them as tourism attraction and a significant source of income for many coastal economies in along the Gulf of Mexico and in the Caribbean. Historically, seagrasses were collected as fertilizer for sandy soil. This was an important use in the Aveiro Lagoon, Portugal, where the plants collected were known as moliço.