What is it?
Seagrasses are unique, being the only flowering plants (angiosperms) able to live and flower in saline water. Seagrass beds require soft sediments in which to root and high light levels, as such they only develop in intertidal and shallow subtidal areas on sands and muds. Seagrasses only grow in sheltered conditions, protected from significant wave action, and can be found in estuaries, harbours, bays and lagoons.
Four species of seagrass occur in the UK, two species of tasselweeds (Ruppia sp.) and two species of Zostera., commonly called eelgrass. Only the Zostera species form BAP Priority Habitats and both are considered to be scarce (present in 16-100 ten km squares). Dwarf eelgrass Zostera noltii is found highest on the shore, often adjacent to lower saltmarsh communities. Common eelgrass, Zostera marina, is the larger of the species and is predominatly found in the shallow sublittoral. A variant of Zostera marina, narrow leaved eelgrass Zostera marina var angsutifolia, previously considered a separate species can sometimes be found on the mid to lower shore amongst the dwarf eelgrass.
The plants stabilise the substratum, preventing erosion, are an important source of organic matter, and provide shelter and a surface for attachment by other species. Eelgrass is an important source of food for wildfowl, particularly brent geese and widgeon which feed on intertidal beds. Where this habitat is well developed the leaves of eelgrass plants may be colonised by diatoms and algae such as Enteromorpha spp, Cladophora rectangularis, Rhodophysema georgii, Ceramium rubrum, stalked jellyfish and anemones. The soft sediment infauna may include amphipods, polychaete worms, bivalves and echinoderms. The shelter provided by seagrass beds makes them important nursery areas for bass, flatfish and, in some areas, for cephalopods. Adult fish frequently seen in Zostera beds include pollack, two-spotted goby and various wrasse. Two species of pipefish, Entelurus aequoraeus and Syngnathus typhie, are almost totally restricted to seagrass beds, while the red algae Polysiphonia harveyi, which has only recently been recorded from the British Isles, is often associated with eelgrass beds. Both species of UK seahorse, Hippocampus hippocampus and Hippocampus guttulatus, have been recorded in seagrass beds and can be considered an associated species.
Five different community types have been identified for seagrass beds from the southern North Sea and the Channel and 16 microhabitats including the seagrass itself, sessile epifauna, infauna and free swimming animals not confined to a special part of the community. The diversity of species will depend on environmental factors such as salinity and tidal exposure and the density of microhabitats, but it is potentially highest in the perennial fully marine subtidal communities and may be lowest in intertidal, estuarine, annual beds.
The situation in the South East
As with elsewhere in the UK, populations in the SE need further survey to provide accurate data on location and bed extent, however, general trends have been identified. Seagrass was once prolific in the estuaries and harbours of the Solent and Kent, notably Lymington River, Southampton Water, Portsmouth, Langstone and Chichester Harbours, North coast iof the Isle of Wight and the Swale, Medway and Thames Estuaries. These populations were almost wiped out during the 1930’s wasting disease outbreak and have never fully recovered.
In Kent, where little is known about current seagrass populations, it is considered to be present in sparse beds in the Cleve Marshes, within The Swale Special Protection Area (SPA)/Ramsar site, which contains Z. angustifolia and Z. noltii; and Hoo Marina, within Medway Estuary and Marshes SPA/Ramsar site, containing Z. noltii. The citation for Thames Estuary and Marshes SPA/Ramsar also lists Z. angustifolia and Z. noltii. It is mentioned in the Kent Biodiversity Action Plan.
In the Solent area, Portsmouth Harbour hosts sparse beds of Z. noltii and Z. marina var. angustifolia. Langstone and Chichester Harbours still host some dense beds of Z. noltiiand Chichester also contains some dense beds of Z. marina. Elsewhere on the North coast of the Solent, Z. marinacan be found in isolated beds of the Beulieu River and Calshot Spit. The North Coast of the Isle of Wight hosts the most extensive beds of seagrass found in the SE with notable sites for Z. marina including Yarmouth, Cowes and Osbourne Bay and Z. noltiiincluding Ryde.
Disease. A wasting disease was responsible for dieback of large areas of seagrass in the UK in the 1930s. It is thought this was caused by a fungus and further outbreaks have been noted in the South East in recent years, notably in Langstone Harbour.
Natural cycles. The extent of seagrass beds may change as a result of natural factors such as severe storms, exposure to air and freshwater pulses. Grazing by wildfowl can have a dramatic seasonal effect with more than 60% reduction in leaf cover reported from some sites. Warm sea temperatures coupled with low level of sunlight may cause significant stress and dieback of seagrass.
Physical disturbance. For example, by trampling, dredging and use of mobile bottom fishing gear, land claim and adjacent coastal development through the construction of sea defences and potential for changes in the hydrological regime.
Alien species. Introduction of, and competition from, alien species such as Spartina anglica and Sargassum muticum
Increased turbidity reducing photosynthesis. Nutrient enrichment, at low levels, may increase production in Zostera while high nitrate concentrations have been implicated in the decline of mature Z. marina Phytoplankton blooms, resulting from nutrient enrichment, have been shown to reduce biomass and depth penetration of eelgrass. Eutrophication can also result in a shift to phytoplankton epiphyte or macroalgal dominance.
Marine pollution. Eelgrass is known to accumulate Tributyl, tin and possibly other metals and organic pollutants. Several heavy metals and organic substances have been shown to reduce nitrogen fixation that may affect the viability of the plant, particularly in nutrient poor conditions. Herbicides in antifouling paints and agricultural run-off can reduce photosynthesis and growth. Accumulated pollutants may become concentrated through food chains.
Vision for seagrass beds
- No further loss of existing habitat
- Improved knowledge of seagrass beds; location, composition, extent and health
- Long term monitoring strategy in place to identify changes to populations over time and identify threats
- Good management of sites, including management of boating and fishing activity, and monitoring of water quality
- No damage to site integrity from activities arising outside the sites, e.g. poorly considered coastal development affecting hydrodynamics and sediment processes
- Greater public awareness of seagrass beds and their specialist wildlife, including greater awareness of the impacts of human pressures, such as boating.
How we can deliver this vision
- Set up SE seagrass working group to steer and guide seagrass research and conservation
- Increase surveying effort to provide accurate data on seagrass attributes
- Set up coordinated long term monitoring strategies to identify changes over time
- Work with EA to monitor water quality in regard to seagrass health
- Work with Natural England to highlight current threats in SAC’s, SPA’s and SSSI’s and work towards reducing them
- Work with boating fraternity to raise awareness of seagrass and seek to implement management options to reduce impacts
- Provision of ecological and management advice to Local Authorities, developers and other stakeholders
- Project funding through landfill tax credits or Statutory Agency Grants etc