Seagrasses are angiosperms that are restricted to life in the sea. Seagrasses colonized the sea, from terrestrial angiosperm ancestors, about 100 million years ago, which indicates a relatively early appearance of seagrasses in angiosperm evolution. With a rather low number of species (about 50-60), seagrass comprise < 0.02% of the angiosperm flora. Seagrasses are assigned to two families, Potamogetonaceae and Hydrocharitaceae, encompassing 12 genera of angiosperms containing about 50 species (Table 1). Three of the genera, Halophila, Zostera and Posidonia, which may have evolved from lineages that appeared relatively early in seagrass evolution, comprise most (55%) of the species, while Enhalus, the most recent seagrass genus, is represented by a single species (Enhalus acoroides, Table 1). Most seagrass meadows are monospecific, but may develop multispecies, with up to 12 species, meadows in subtropical and tropical waters.
Adaptations to Colonize the Sea
The colonization of the sea required a number of key adaptations including (1) blade or subulate leaves with sheaths, fitted for high-energy environments; (2) hydrophilous pollination, allowing submarine pollination (except for the genus Enhalus) and subsequent propagule dispersal; and (3) extensive lacunar systems allowing the internal gas flow needed to maintain the oxygen supply required by their below-ground structures in anoxic sediments. Seagrass species are all clonal, rhizomatous plants, a necessary adaptation for angiosperm growth in the high-energy marine environment. The rhizome is responsible for the extension of the clone in space, as well as for connecting neighboring ramets, thereby maintaining integration within the clone. The growth rates of seagrass rhizomes vary from a few centimeters per year in the larger, slow growing species, to more than 5 m yr-1 in the smallest species. These horizontal extension rates result in estimated times to develop seagrass meadows ranging from less than 1 year, for fast-growing species (Halophila, Syringodium and Cymodocea species), to centuries for the slowest growing ones (e.g. Posidonia oceanica, Photo 1).
|Table 1. List of seagrass species and their membership to the different seagrass floras. After Hemming and Duarte (2000).|
|Amphibolis antarctica||S. Australian flora|
|Amphibolis griffithii||S. Australian flora|
|Cymodocea angustata||Indo-Pacific flora|
|Cymodocea nodosa||Mediterranean flora|
|Cymodocea rotundata||Indo-Pacific flora|
|Cymodocea serrulata||Indo-Pacific flora|
|Enhalus acoroides||Indo-Pacific flora|
|Halodule pinifolia||Indo-Pacific flora|
|Halodule uninervis||Indo-Pacific flora|
|Halodule wrightii||Caribbean flora|
|Halophila baillonis||Caribbean flora|
|Halophila beccarii||Indo-Pacific flora|
|Halophila capricornii||Indo-Pacific flora|
|Halophila decipiens||Caribbean and Indo-Pacific floras|
|Halophila engelmannii||Caribbean flora|
|Halophila ovata||Indo-Pacific flora|
|Halophila ovalis||Indo-Pacific flora|
|Halophila spinulosa||Indo-Pacific flora|
|Halophila stipulacea||Indo-Pacific flora|
|Heterozostera tasmanica||S. Australian flora|
|Phyllospadix iwatensis||Temperate W. Pacific flora|
|Phyllospadix japonicus||Temperate W. Pacific flora|
|Phyllospadix scouleri||Temperate E. Pacific flora|
|Phyllospadix serrulatus||Temperate E. Pacific flora|
|Phyllospadix torreyi||Temperate E. Pacific flora|
|Posidonia australis||S. Australian flora|
|Posidonia oceanica||Mediterranean flora|
|Posidonia ostenfeldii||S. Australian flora|
|Posidonia sinuosa||S. Australian flora|
|Posidonia angustifolia||S. Australian flora|
|Posidonia coriacea||S. Australian flora|
|Posidonia denhartogii||S. Australian flora|
|Posidonia kirkmanii||S. Australian flora|
|Posidonia robertsoniae||S. Australian flora|
|Syringodium filiforme||Caribbean flora|
|Syringodium isoetifolium||Indo-Pacific flora|
|Thalassia hemprichii||Indo-Pacific flora|
|Thalassia testudinum||Caribbean flora|
|Thalassodendron ciliatum||Indo-Pacific flora|
|Thalassodendron pachyrhizum||S. Australian flora|
|Zostera asiatica||Temperate W. Pacific flora|
|Zostera capensis||S. Atlantic flora|
|Zostera capricorni||S. Australian flora|
|Zostera caulescens||Temperate W. Pacific flora|
|Zostera japonica||Temperate W. Pacific flora|
|Zostera marina||N. Atlantic, Mediterranean, W. and E. Pacific floras|
|Zostera mucronata||S. Australian flora|
|Zostera mulleri||S. Australian flora|
|Zostera noltii||N. Atlantic and Mediterranean floras|
|Zostera novazelandica||New Zealand flora|
Seagrass Distribution and Habitat
Seagrasses occur in all coastal areas of the world, except along Antarctic shores. The four most obvious habitat requirements of seagrasses are a marine environment, adequate rooting substrate, sufficient immersion in seawater and illumination to maintain growth. Seagrasses are found in waters with salinity greater than 10‰ in estuaries to salinities of about 45‰, in hypersaline coastal environments. Seagrass grow from the intertidal, where they are exposed to full sunlight during the emersion periods to depths receiving, on average, 11% of the irradiance incident just below the water surface, allowing seagrasses to grow deeper than 40 m in the clearest ocean waters. Most seagrass species are confined to sandy to muddy sediments, although some species can grow over rock. High sediment mobility by currents and waves, causing successive burial and erosion, may cause seagrass mortality. Consequently, highly mobile, but otherwise suitable, sandy sediments, may be bare of seagrass cover. High inputs of organic matter, which stimulate bacterial activity, are conducive to seagrass mortality due to the accumulation of phytotoxic compounds, such as sulphide. The organic matter concentrations of sediments supporting seagrass growth is generally less than 6% of the dry weight, with redox potentials spanning from highly oxidized to moderately reduced (> - 100 mV). Seagrass encounter suitable conditions along a global area estimated at about 0.6106 km2, equivalent to 10% of the coastal ocean, an estimate of seagrass cover that involves considerable uncertainty.
Seagrass form extensive meadows (Photos 1 and 2), which are highly productive and often support high biomass, with a global average biomass of about 180 g C m-2 an average net production of about 400 g C m-2 yr-1, ranking amongst the most productive ecosystems in the biosphere. These estimates represent, when scaled to the estimated global cover of seagrasses, a contribution to marine primary production of 0.61015 g C yr-1, or about 1.13% of the total marine primary production. Because herbivory rates are low in most seagrass meadows, most of their primary production is either stored in the sediments or exported to neighboring ecosystems. Seagrass bury about 27 Tg C year-1, or about 12% of the total carbon storage in marine ecosystems. Hence, seagrasses are important components of the marine carbon cycle, being responsible for a significant fraction of the net CO2 uptake by marine biota.
Seagrass meadows enhance the biodiversity of coastal waters. They harbor, virtually without exception, more animals and more species than nearby unvegetated areas. The fish fauna of seagrass meadows can be of considerable diversity, typically reaching more than 100 species in any one region, often dominated by juvenile specimens, as seagrass meadows often play a nursery role. The largest animals that are associated with the seagrass habitat are the green turtle, Chelonia mydas, and species of the order Sirenia (sea cows), notably the dugong Dugong dugon, and the West Indian manatee Trichechus manatus. These animals are the largest marine herbivores, and forage over seagrass meadows. A second manatee species, T. senegalensis (the West African manatee) may also consume seagrass, but data on this animal are scanty.
Seagrass meadows have other important ecological functions. They improve water quality by reducing the particle loads in the water and absorbing dissolved nutrients. Seagrass stabilize sediments, diminishing sediment resuspension while promoting sedimentation. Seagrass meadows dissipate wave energy and protect coastlines. In addition, a significant fraction of seagrass production accumulated in the beach, as beach-cast detritus, where they deliver carbonate materials that nourish the beach and contribute to dune formation.
Seagrass meadows are believed to be experiencing a world-wide decline, with global loss rates estimated at 2-5% year-1, compared to 0.5% year-1 for tropical forests. The causes for seagrass loss are multiple and include disease, extreme events, such as hurricanes and typhoons, burial by shifting sand, excess nutrient inputs to coastal waters and a reduction of water and sediment quality associated to eutrophication, and overgrowth by opportunistic algae, leading to seagrass loss, excess organic supply from aquaculture and effluents, water quality deterioration by excess sediment inputs, mechanical damage from fishing activities, coastal engineering and boat activities; climatic extremes, such as heat waves and associated hypoxic events; displacement by invasive species, and excessive herbivory. Whereas actions are being taken to curb these trends, including legislation to protect seagrass meadows, transplanting efforts, and monitoring efforts to detect change, there is, as yet, no evidence that the associated recoveries compensate for the losses.
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