http://faculty.umb.edu/anamarija.frankic/eeos476/Class%20Materials/Habitats/burkholder2007.pdf

Seagrasses and eutrophication

https://drs.nio.org/drs/bitstream/handle/2264/3872/Biol_Conserv_144_1961a.pdf?sequence=1&isAllowed=y

Extinction risk assessment of the world’s seagrass species

Effects of irradiance, temperature, and nutrients on growth dynamics of seagrasses: A review

The recent focus on carbon trading has intensified interest in ‘Blue Carbon’–carbon sequestered by coastal vegetated ecosystems, particularly seagrasses. Most information on seagrass carbon storage is derived from studies of a single species, Posidonia oceanica, from the Mediterranean Sea. We surveyed 17 Australian seagrass habitats to assess the variability in their sedimentary organic carbon (Corg) stocks. The habitats encompassed 10 species, in mono-specific or mixed meadows, depositional to exposed habitats and temperate to tropical habitats. There was an 18-fold difference in the Corg stock (1.09–20.14 mg Corg cm−3 for a temperate Posidonia sinuosa and a temperate, estuarine P. australis meadow, respectively). Integrated over the top 25 cm of sediment, this equated to an areal stock of 262–4833 g Corg m−2. For some species, there was an effect of water depth on the Corg stocks, with greater stocks in deeper sites; no differences were found among sub-tidal and inter-tidal habitats. The estimated carbon storage in Australian seagrass ecosystems, taking into account inter-habitat variability, was 155 Mt. At a 2014–15 fixed carbon price of A$25.40 t−1 and an estimated market price of $35 t−1 in 2020, the Corg stock in the top 25 cm of seagrass habitats has a potential value of $AUD 3.9–5.4 bill. The estimates of annual Corg accumulation by Australian seagrasses ranged from 0.093 to 6.15 Mt, with a most probable estimate of 0.93 Mt y−1 (10.1 t. km−2 y−1). These estimates, while large, were one-third of those that would be calculated if inter-habitat variability in carbon stocks were not taken into account. We conclude that there is an urgent need for more information on the variability in seagrass carbon stock and accumulation rates, and the factors driving this variability, in order to improve global estimates of seagrass Blue Carbon storage.

/pdf/variability-in-the-carbon-storage-of-seagrass-habitats-and-4ww0xnpbij.pdf

Variability in the carbon storage of seagrass habitats and its implications for global estimates of blue carbon ecosystem service.

Seagrass meadows are considered to be among the most important marine ecosystems, with regard to both ecology and biodiversity and for the services they provide. Seven species occur in the Mediterranean Sea: Posidonia oceanica (the most common in the open sea), Cymodocea nodosa (particularly common in the eastern basin), Ruppia cirrhosa, R. maritima, Zostera marina and Zostera noltii (mainly in estuaries and brackish lagoons), and Halophila stipulacea (introduced from the Red Sea). Seagrass regression may be due to natural processes and/or natural or anthropogenic disturbances and stress. It can also be due to long-term climate trends, e.g., the post-Last Glacial Maximum rise in sea-level, the Little Ice Age (LIA) cooling and the post-LIA warming, resulting in possible misinterpretation. Human-induced losses of P. oceanica have been mainly related to coastal development, pollution, trawling, fish farming, moorings, dredging, dumping and introduced species. All other seagrasses have also undergone more or less dramatic regression events. In fact, accurate data are generally of very local value and they are lacking for most of the Mediterranean Sea. In the absence of a reliable baseline, some widely cited cases of regression are questionable. Relatively healthy P. oceanica meadows, whose limits have changed little since the 1950s, may thrive in highly anthropized areas. In addition, the decline of one species can benefit another, so that the overall seagrass balance may remain unchanged (e.g., Cymodocea replacing Posidonia). However, to conclude that everything is for the best would be erroneous. First, the lack of data supporting the general regression hypothesis does not invalidate the hypothesis. Indisputably dramatic seagrass losses have been documented (e.g., P. oceanica and Z. marina). Second, the Posidonia regression is irreversible at human scales, while other seagrasses can rapidly recover, and the expansion of some seagrasses (e.g., Cymodocea) cannot counterbalance, in terms of ecosystem services, the decline of the P. oceanica meadows. Third, human pressure (demography, tourism, etc.) on Mediterranean seagrass ecosystems is destined to strongly increase in the coming decades. Finally, the rise in sea-level due to global climate change will automatically induce a withdrawal of the lower limit of seagrass meadows whenever the limit is beyond the compensation depth. So the regression trend observed in Mediterranean seagrasses, even if it proves to be currently weaker than postulated, will significantly increase and become a major concern in the future. There is therefore an urgent need for the adoption of a set of efficient indicators and the setting up of a robust comparative baseline in order to draw up an accurate assessment of the losses and, for seagrasses other than Posidonia, possible gains at Mediterranean scale. In addition, seagrasses and seagrass habitats should be granted legal protection and, where such protection already exists, it should be implemented.

Regression of Mediterranean seagrasses caused by natural processes and anthropogenic disturbances and stress: a critical review

Seagrasses are aquatic angiosperms, which are confined to the marine environment. The term seagrass (with several linguistic variants in the Germanic language group) refers undoubtedly to the grasslike habit of most of its representatives. The term has been long used by fisherman, hunters, farmers, and other inhabitants of the coastal areas of several European countries, i.e. areas where only species occur with long linear leaves. Ascherson (1871) probably was the first researcher to introduce the term into the scientific literature. The seagrasses form an ecological group, and not a taxonomic group. This implies that the various seagrass families do not necessarily have to be closely related. The taxa regarded as seagrasses belong to a very limited number of plant families, all classified within the superorder Alismatiflorae (Monocotyledonae) (Dahlgren et al., 1985), also generally known as the Helobiae (Tomlinson, 1982). The subclass Alismatanae (Kubitzki, 1998) is with respect to its contents identical with Alismatiflorae. Three out of four families consist exclusively of seagrasses, viz. the Zosteraceae, the Cymodoceaceae, and the Posidoniaceae. In the past these families generally have been classified as subfamilies of the Potamogetonaceae (Ascherson and Graebner, 1907; den Hartog, 1970). Further studies have shown that the latter family appeared to be too heterogeneous (Tomlinson, 1982; Dahlgren et al., 1985), and had to

https://www.springer.com/cda/content/document/cda_downloaddocument/9781402029424-c1.pdf?SGWID=0-0-45-167836-p46606360

Taxonomy and Biogeography of Seagrasses

This chapter describes how the seagrass species can be identified. Seagrasses are defined as flowering plants growing in shallow marine environment. They form an ecological group and not a taxonomical one. . They are represented by twelve genera, which have been classified in diverse taxonomic groups. These are the entire families: Zosteraceae (three genera), Posidoniaceae (one genus), Cymodoceaceae (five genera), and three of the seventeen genera in the family Hydrocharitaceae. Seagrasses are aquatic plants, which have fewer morphological and anatomical features for species identification than terrestrial flowering plants. Some species have a vast geographic distribution and occupy different niches; this may result in considerable morphological variation, and some of these variations, when studied more closely, may turn out to be separate taxa. All chromosomal studies on seagrasses indicate that chromosome numbers may be different between genera and families, but that the basic numbers are similar among the different species within the same genus. Classification of the various seagrasses in the taxonomic system, and even more their phylogeny, will continue to be debated for many years to come. The definition of seagrasses at the species level is far from satisfactory in certain genera, and can cause difficulties in arriving at a correct identification. Thus, researchers should be careful in citing and interpreting the published literature, which may contain incorrect species names.

C. Den Hartog

Papers

Taxonomy and Biogeography of Seagrasses

Seagrass taxonomy and identification key

Seagrass Morphology, Anatomy, and Ultrastructure