Seagrass in Plymouth
This extract is taken from: Holt, P. 2023. Pollution in Plymouth Sound and the Tamar Waterway – A Historical Review. The SHIPS Project CIC, Plymouth, ISBN 978 1 7397053 4 3. For references, please see the original document.
One of the most noticeable changes to the ecology of Plymouth Sound was the loss of the seagrass (Zostera marina) beds.
Seagrass has been a focus of the rewilding movement as there have been claims of its ability to capture carbon and its beneficial role as a habitat for seahorses, juvenile fish, and invertebrates. Seagrass, or eel grass as it is often known, is mentioned in historic papers and newspaper articles by many names, including ‘sea wrack’, ‘grass wrack’, ‘eel-wrack’, and ‘widgeon grass’. However, ‘sea wrack’ is also used for seaweed so context is important when reading an article. Seagrass is mentioned in 19th-century accounts as a resource for several manufacturing industries. It is also mentioned as supporting scenery in documentary accounts describing some other marine life that lives within the seagrass. Accounts from this time suggest that seagrass was abundant on the UK coastline as ‘it abounds all along the sea coast’ (WT, 1862), ‘Zostera marina or sea wrack is a very abundant and hence probably a cheap material’ (CT, 1861) and ‘It grows in great numbers, or rather in great fields, affording pasture to innumerable living beings’ (ST, 1863). Seagrass was so readily available that it was used as a packing material, for upholstering and sound insulation (Butcher, 1933), it was collected from beaches and used as manure for fields, along with the seaweeds Laminaria, Fucus and Ulva (NDJ, 1920). It was so abundant in Jersey in 1863 that the large quantities that washed up on the strandline were taken away to be used as fuel and manure (DCC, 1863). The American Civil War between 1861 and 1865 created a shortage of cotton for the mills of Lancashire and a serious proposal was made in 1862 to use seagrass as a substitute (WDM, 1862), but it was later proved to be unsuitable. Where seagrass is mentioned in historical accounts about Plymouth it is only ever in its supporting role, and no one felt the need to record where it could be found.
‘(Seagrass) grows in great numbers, or rather in great fields, affording pasture to innumerable living beings’
But we can gain a few clues from scientific reports about where the seagrass grew as the locations are sometimes mentioned, and particularly interesting are the locations mentioned where seagrass now no longer grows (Fig. 35).
- Today there is still a small patch of seagrass in Cawsand Bay on the west side of Plymouth Sound but in 1890 Zostera was found ‘between North Point and Pier Cove, in the 2-4 fathoms limit‘. Note the 4 fathom (8m) maximum depth range. North Point is shown on the 1939 Imray chart as being off Kingsand Beach and Pier Cove is also known as Pier Cellars (Johnson, 1890).
- Johnson goes on to say, ‘in this part and towards Penlee Point Zostera beds are plentiful, affording anchorage for different algae’ (Johnson, 1890). Zostera is now only found in a small area of Cawsand Bay which could not be described as plentiful, and it no longer extends southwards to Penlee Point.
- Wembury Point is described as having excellent Zostera beds on the shore between the Point and the Mewstone (Johnson, 1890) but there is no seagrass to be found there now.
- Holt (1898) refers to Zostera in Plymouth Sound at Jennycliff Bay and northeast of Drake’s Island as well as Cawsand Bay. Seagrass can still be found in these places, but it is very sparse and patchy in Jennycliff. Patches of sand covered with seagrass were reported in Marra Pool (Allen, 1904) or ‘south of Batten Castle’ (MBA, 1904, p209) at the north end of Jennycliff Bay in 1904. which are now no longer present. Holt also mentions the Cattewater but this I believe is when he refers to ‘sandy and muddy ground’ in the same paragraph.
- The document Plymouth Marine Invertebrate Fauna mentions a seagrass bed to the north of Drake’s Island (p162), the mouth of the Yealm (p166), and along the southern shore (p167), the ‘eastern shore of Yealm mouth’, Drake’s Island, Jennycliff Bay, Cawsand Bay, Whitsand Bay, Yealm Estuary (MBA, 1904, p244 & p270). Unusual here is the mention of seagrass in Whitsand Bay to the west of Plymouth.
- Lebour reported taking samples ‘outside the Breakwater’, and ‘specimens from both within and without the Breakwater’ (Lebour, 1918 p443, p456, p457). It is not clear where this refers to as there are no known seagrass beds outside of the Breakwater, perhaps it is referring to the area between Pier Cellars and Penlee Point mentioned above. Were this referring to the Yealm beds which are the only known seagrass beds ‘outside’ of the Breakwater then it is surprising that the author was not more specific about the location.
The document Plymouth Marine Fauna also mentions a seagrass bed at Queen’s Ground at the north end of Cawsand Bay (MBA, 1957, p87) which is no longer present and nor can it be found near the New Ground buoy further to the northeast which is also mentioned (p125). The length of the seagrass fronds is mentioned in one report by Wilson (1949) where he says that Plants from the estuary of the river Yealm near Plymouth certainly reached a length of 5ft or more prior to 1931 and a width of at least 8mm’.
In 1931 the seagrass in Plymouth Sound suddenly died off leaving only a few small patches surviving. The problem was widespread; in 1933, Cotton reported that the loss of Zostera on the Atlantic coast of the US had started in 1930, the seagrass died off in France in 1931, then along the coast to Holland and by 1933 the loss had reached Denmark and Sweden (Cotton, 1933). Several localities in Devon were noted in 1932 to have bare seagrass beds with dead roots, with a few stray plants surviving, and the swans that had lived on the river Yealm for years deserted that estuary for Plymouth in search of food. Over 90% of the Zostera populations disappeared (Muehlstein, 1989) and the speed of loss was rapid; the abundant and extensive beds of seagrass at Cape Ann, Massachusetts, started to disappear in 1931-32 and were practically gone by 1933 (Dexter, 1944). The problem may have started earlier and was only noticed in the 1930s. Cottam reported that ‘In England, two definite periods of decrease in Zostera can be distinguished; one immediately after the War, that is between 8 and 12 years ago or perhaps even longer, and another at the same time as the epidemic in France and America, namely in 1931 and 1932’ (Cottam, 1935). The die-off was sudden and widespread so many attempts were made to determine the cause which had affected all the Zostera on two continents in the space of just a few years. In 1933, some experts thought the cause was oil pollution, but the French thought it was disease (TC, 1933; MS, 1934). Lack of sunshine in 1931-32 was proposed as a reason but this theory was proved incorrect by Atkins (1947). The die-off was not caused by variations in seawater, or salinity and no causal link could be established with climactic factors (Giesen, 1990). By 1949 the situation had not improved, with one newspaper reporting that the ‘Eel grass in the River Yealm today is also nothing like the length it used to be. There has been very little in Cawsand Bay for years’ (WMN, 1949).
We still do not know what caused the loss of 90% of the seagrass in America and Europe a but historical review of pollution in Plymouth rules out some possible causes. Changes to the ecosystem brought about by the Plymouth Breakwater were not the cause as this structure had been in existence for more than 100 years, so the seagrass had by then established a new equilibrium with the changes to the marine habitat caused by extra silt deposition in Plymouth Sound. Before the 1930s, Plymouth Sound was still the repository for the town sewage, pumped directly into the sea through several outfalls and into the Tamar and Plym estuaries, yet the seagrass appeared to thrive. Mines were still active in the Tamar Valley so pollution from mine runoff would still be occurring, and agriculture had not changed its practices for decades, so the seagrass was already tolerating any contemporary pollution runoff from the mines and fields. All these pollutants were local in character so unlikely to be the culprit for what became a problem that occurred on both sides of the Atlantic Ocean. Oil pollution in some form is a candidate for the cause of the die-off as it was being used as fuel in both the US and Europe. The use of oil as fuel was relatively new in 1930 but cars had been in Plymouth for 30 years and oil-powered ships for more than 20 years. Another potential candidate is the changes to the environment caused by the addition of Tetraethyl lead to petrol that started in 1923, a change that happened in both the US and Europe and one that could affect the environment in both locations. Modern pollutants such as plastics, DDT and TBT can be ruled out as causes as they had yet to be invented, but they may still be adversely affecting seagrass now in other ways.
‘Eel grass in the River Yealm today is also nothing like the length it used to be.’
The seagrass in Plymouth Sound has not recovered since the die-off in the 1930s and historic accounts tell us that seagrass could be found in areas of Plymouth Sound where it cannot be found today. Johnson’s comment about the maximum depth where seagrass can be found in the Sound to be 4 fathoms or 7.3 metres, is a deeper range than today where the seagrass can usually be found in a depth of 5m or less (Johnson, 1890). It is thought that the depth range of seagrass is limited by available light, which is a function of turbidity, hinting that underwater visibility was perhaps better in 1890 than today which allowed more sunlight to reach the seabed. In 1890, the waters of Plymouth Sound were a repository for all of Plymouth’s sewage so it is hard to understand how the water could be clearer then than it is now. Perhaps the presence of large oyster beds and their ability to filter seawater had a larger beneficial effect on the ecosystem than is currently appreciated.
It is also important to note potential changes to the seabed sediment caused by the loss of seagrass in the 1930s. In Salcombe, the Zostera was holding sandbanks together with its extensive network of roots and when the seagrass died off the sand washed away leaving just stones and seaweed, with no substrate remaining for the seagrass to recolonise (WMN, 1949). This may also have happened in Plymouth and other locations, so some areas where seagrass once grew cannot now be re-established because all the substrate suitable for growing seagrass has washed away. Unfortunately, no detailed records exist of what the seabed in Plymouth Sound was like before 1930 so it is impossible to know.
Last updated 18 October 2023