Seagrasses comprise a functional group of about 60 species of underwater marine flowering plants worldwide. Seagrasses belong to the families Hydrocharitaceae and Potamogetonaceae. There are 13 genera of which six (Amphibolis, Heterozostera, Phyllospadix, Posidonia, Ruppia and Zostera) are mostly restricted to temperate seas and seven (Cymodocea, Enhalus, Halodule, Halophila, Syringodium, Thalassia and Thalassodendron) are distributed in tropical seas. The Indo-Pacific / Australian region is the most seagrass-diverse region in the world, with seagrass meadows consisting of 7 (or more) species (Les et al., 1997; Den Hartog and Kuo, 200x; Short et al., 2007).
Seagrasses grow primarily on soft substrates, from the intertidal down to maximum depths of around 70 m. In shallow (coastal) waters, they can form dense meadows which constitute valuable and often overlooked habitats providing important ecological and economic functions and services. Seagrass beds provide nursery grounds and adult habitats for fish and shellfish, including many species of economic importance. The latter makes seagrasses important components of coastal fisheries productivity. Seagrasses have important physical functions, such as filtering coastal waters, dissipating wave energy, sediment stabilisation and biogenic sediment production, thus maintaining coastal water quality and clarity and keeping up with relative sea-level rise. Seagrass meadows play a vital role in supporting coastal and marine communities and in maintaining diverse flora and fauna. They are an important food source for megaherbivores such as gees, green sea turtles, dugongs, and manatees, and provide a critical habitat for many associated organisms, including several charismatic and/or endangered animal species of importance to nature conservation. They often occur in proximity to coral reefs, mangroves, salt marshes, bivalve reefs and other marine species, with which there can be significant ecological interactions.
Seafloor stabilization, erosion prevention, sediment trapping / production and other ecological and socio-economic benefits have been described for seagrass meadows around the world, the effect of which is highly variable and both positive and negative impacts have been reported. The ability of a seagrass meadow to perform ecosystem services depends on local abiotic and biotic conditions such as tidal regime, currents and waves, latitude, seasonal dynamics, prevailing climate, timing of weather conditions in relation to seagrass life cycles, seagrass-specific characteristics, biomass (above and below the ground), ecosystem interaction, bio-physical feedback mechanisms, etc. (e.g. Koch, 2001; Gacia et al., 2003; Bouma et al., 2005; Boer, 2007; Koch et al., 2009; Barbier et al., 2011; Van der Heide et al., 2012).
Seagrass areas are globally declining at a rate of 110 km2 per year since 1980. Decline rates have accelerated from 1% per year before 1940 to 7% per year since 1990 (Waycott et al., 2009). Human impacts are to a large extent responsible for this decline and in many cases, dredging operations have directly or indirectly contributed to it (Erftemeijer and Lewis, 2006). The 10 most important threats to the remaining seagrasses in order of impact appear to be (Grech et al., 2012):
- Urban/industrial runoff
- Urban/port infrastructure development
- Agricultural runoff
- Boat damage (commercial)
- Boat damage (recreational)
- Shipping accidents
- Changes in sea surface temperature