Seagrass in the Solent

I can still remember as a young teenager my amazement when told that there were flowering plants living completely submerged in the sea. Our sea grasses or eelgrasses, Zostera spp, form an important inshore plant community in the Solent and surrounding areas.

There are 3 eelgrass species in British waters and all are considered vulnerable and in need of protection and all live in the Solent. There are large eelgrass beds along the north coast of the Isle of Wight, Langstone, Portsmouth and Chichester Harbours and in Stanswood Bay, near Calshot, intertidal beds are easily seen. Leaves shoot from a creeping rhizome system that binds and stabilises the seabed sediment reducing coastal erosion. Leaves and rhizomes contain air spaces that aid buoyancy.

Eelgrass have separate male and female flowers on the same flower head. It usually flowers in late summer, dispersing threadlike pollen grains into the sea. Z. marina beds develop on firm sand, sometimes mixed sediments and usually grow below the low water spring tidal limit. Patches have been found in the Solent including to the west of Needs Ore, between Newtown and Gurnard Point, and to the east of the mouth of the Medina River on the north coast of the Isle of Wight (Tubbs, 1999).

The Hampshire and Isle of Wight Wildlife Trust have been running the Solent Seagrass Project to gather information on the extent of seagrass beds in the Solent area.

Eelgrass underwater. Image by Ronald C. Phillips PhD, creative commons.

Zostera beds are species-rich, particularly the subtidal beds of Z. marina. A large number of algal species occur as epiphytes on Zostera leaves (some species are found only in eelgrass beds). Other algae grow amongst the eelgrass or as mats on the sediment surface. Eelgrass offers an attractive and protective habitat for small animals including many crustaceans and fish.

For example, in Solent seagrass beds you can find deep-snouted pipefish, seahorse and fifteen-spined stickleback. There are also plentiful prawns and cuttlefish. When an area has healthy seagrass beds it is almost certain that it will hold plentiful marine life. Zostera spp. is also an important food for wildfowl including the dark‐bellied brent goose and wigeon which feed on intertidal beds.

Deep-snouted pipefish caught in Stanswood Bay Zostera bed. Image © P. A Henderson
Short-snouted seahorse captured in Southampton Water Image © P. A. Henderson
5-spined stickleback caught at Calshot. © P. A. Henderson

Although seagrass beds are critically important habitats that support human well-being the extent of the beds throughout the world are declining at a rapid rate. Within British waters the decline in extent and well-being of seagrasses is linked to pollution from industrial effluents and sewage, mechanical disturbance, land reclamation etc. Zostera marina is susceptible to a wasting disease caused by a slime mould. In the 1930s populations were decimated by this disease and some have never fully recovered. Zostera angustifolia and Zostera marina are both affected by nutrient enrichment from nitrates, oil pollution and anti-fouling paints used on boats.

Recent reductions in pollutant discharges have aided seagrass recovery, but we are still introducing large amounts of nitrogen and phosphates into the sea which encourage algal blooms and metabolic imbalance in eelgrasses. Eelgrass beds are not physically robust, and the plants are easily killed or damaged by trampling, digging, dredging, bivalve harvesting or other forms of physical disturbance.

Unfortunately, our direct impacts on the beds during our leisure activities have intensified. The damaging mechanical effects on Zostera marina (Common Eelgrass) seagrass beds in UK waters from recreational boating activities, anchoring and traditional swing mooring scour, have been of continuing concern. There is a clear need to implement good practices to limit these impacts while allowing people to enjoy their boating activities. Eco-moorings, a design that reduces the abrasion pressure of anchoring and mooring on the seabed have been developed and are being tested. However, there has been a limited uptake of eco-moorings to date. Eelgrass beds are a natural feature which we all need to protect and cherish if we are to maintain the rich marine life of the Solent.

Dr. Peter Henderson – SPS