Vanishing Seagrass Meadows

April 3rd, 2020

A talk presented by Barry Kaye (Lancashire MCS) March 2020. References and further reading are presented below the main talk.

Marine Plants

Line drawing of dabberlocks, a typical seaweed.

Most marine plants are microscopic ‘phytoplankton’. Of those that are large enough to see, the most common are ‘seaweeds’. These plants are adapted to live in shallow marine and intertidal areas where there is plently of light. Seawater provides the nutrients seaweeds need to live, so they just need to cling on to their preferred location, so they are not swept somewhere less hospitable: Where there is insufficient light or nutrients for them to compete with other species, or where they may be eaten faster than they can grow.

To do this they use a ‘holdfast’, which may appear root-like, but which works more in the manner of a hand, gripping a rock, rather than a root, which penetrates the soil.

Higher Plants

Daffodils in Williamson Park, Lancaster, Spring 2020.

‘Higher’ plants have largely solved the problems of living on land. There is lots of light above ground, but the entire plant is not bathed in nutrients – these must be extracted from ground water through roots, which are buried away from the sun. Light absorbtion requires specialised leaves or fronds, held some distance above the ground to get the most light. The division of labours requires a sophisitcated transport (‘vascular’) network to bring together the requirements for photosynthesis, and spread its benefits to all parts of the plant.

These plants are not, however, well adapted to life in the sea – underwater there is less light than they are used to. Further, marine sediments are typically anoxic, so any part of the plant penetrating them must be supplied with oxygen to survive. As a consequence the roots would depend entirely on the exposed leaves for all of life’s essentials.

While roots do not help sustain the plant in a marine ecosystem, they can anchor it in soft sediments. This is an ecosystem that seaweeds cannot easily colonise, as their holdfasts have very little to grip onto, so cannot hold their position in anything other than the slightest currents. Some seaweeds have developed ingenious methods for reducing the strain on their holdfasts, such as springy bodies that absorb wave currents, rather than transmitting these stresses to the holdfast, which might become dislodged (see the ‘rubberiest plant on the planet‘ elsewhere on this blog, which provides insight into the complex internal structures displayed by some seaweeds – though these structures are rarely associated simply with transport of nutrients).

As a consequence, areas of shallow seas with soft sediments – sands or muds – can be successfully colonised by higher plants, with little competition from the otherwise well established seaweeds. The plants that have succeeded in doing this are called ‘seagrasses’. There a number of superficially similar species around the world, their forms being dictated by the rigours of the environment they have colonised.

Sheltered community

Seagrass bed off the Isle of Gigha, with two spot gobies.

While seagrasses have been shaped by their environment, they also have an important role in shaping the environment in which they live. Seagrass shelters the water beneath its canopy, providing a refuge for juvenile fish, and stabilising the sediment for burrowers.

The blades of seagrass also exert a drag on the waves that pass over them. It has been calculated that substantial seagrass meadows can reduce wave height by as much as 50% on its trip from the open sea to the shore. (This is a very substantial reduction in wave energy, which is proportional to the square of the wave height).

In short, seagrass is as valuable as it is unlikely, supporting fisheries, and protecting coastal communities (and proterty). In the UK we only have one truly marine species of seagrass – Zostera maritima, which is shown in the photograph above. You can see some of the species that make it their home – two spot gobies, with a larger fish lurking in the background. The diversity of community is made clear when you compare the photo above to that below – taken of an adjacent patch of bare sand:

Photo adjacent to a seagrass bed, showing the paucity of life on the open sediment.

While seaweeds find it hard to establish on open sand, if there are no larger rocks to cling to, they have no problem colonising the seagrass itself. Often blades of seagrass are thickly tufted with fine filamentous algae – which are a serious problem, as they reduce the amount of light the seagrass gets. Fortunately, help is on hand from a range of small sea-snails, who are quite happy to eat the offending algae, providing a serendipitous cleaning service for the seagrass.

Banded snails on a strand of seagrass (BK, Gigha).
Above: Banded snails (Barleeia unifasciata) cleaning algae from a blade of seagrass.

Algae and snails are not the only organisms to attach themselves to seagrass blades, and for some the presence of seagrass is essential for their survival. The critter below is a marine oddity – a stalked jellyfish. It belongs to a broader group that includes anemones and jellyfish. Indeed, this staked jellyfish is one that has given up its free floating existance and become tied to the seagrass. As a group the stalked jellyfish are characterised by their ‘stay at home’ nature. Not only have they given up a life of constant voyage, but they do not travel even as juveniles.

Haliclystus octoradiatus
Haliclystus octoradiatus photographed off the Isle of Gigha, October 2019. Haliclystus seems to be associated with more overgrown blades, and I wonder if it is eating the seagrass cleaning service?

Disappearing meadows

As a home to stay-at-home species like the stalked jellyfish, you would think that seagrass meadows must be pretty stable places – stable enough that you don’t need to look for a new home very often at least! Indeed, seagrass meadows off the island of Ibiza in the Mediterranean have been estimated to be thousands of years old…

Sadly, the last century has shown that many seagrass meadows are in fact very fragile. It is estimated that approximately 90% of the area of seagrass meadows around the UK have been lost in the last century. The largest losses occurred in the 1930’s, but there has been limited or no recovery since.


Blades of seagrass showing bleaching and black spots characteristic of infection with Labarinthula in a meadow that is otherwise unaffected.

Zostera maritima has been lost from all North Atlantic coastal regions. The principle cause of the loss has been put down to disease. The slime mould Labyrinthula zosterae, thought to be the culprit, colonises modern meadows, but generally without ill-effect. It seems that since the mass deaths of the 1930’s Zostera and Labarinthula have come to an uneasy truce. If the Zostera is stressed in any way, however, then Labarinthula gains the upper hand, and the plant will quickly die…

Stressed seagrass

Pollution: When we think about marine pollution, catastrophic oil spills grab the headlines, and so dominate our perceptions. Most Zostera beds are, however, relatively resistant to oil spills; paradoxically the oil-dispersant mixtures used to break the slick up and get it off our beaches, can be more damaging. Problems due to elevated nutrient levels from sewage and agricultural run-off, are insiduous, and very much harder to quantify and mitigate…

One result of elevated nutrient levels in seawater is the growth of algae – in the water column, and attached to the Zostera blades. Both of these reduce the light reaching the plants. Human activity can also effect the amount of light reaching the meadow, by suspending fine sediments in the water column through dredging, or bottom trawling.

A final insult to seagrass beds from human activity are the moorings of leisure craft. Typically these moorings have a length of chain running along the bottom that provides play to allow the boat to rise and fall with the tide. As the boat does so, and moves in response to wind and currents, however, the chain is dragged accross the sea bed, leaving cleared circles in the seagrass beds that can be seen from space…

Not all of seagrasses stress is from humans – prior to 1930, seagrass beds were the primary food source for Brent geese Branta bernicla. When the seagrass died, so did the geese. In fact the species was nearly extinguished by the tragedy, and only escaped extinction by broadening its diet to take sealettuce (a seaweed). Modern brent geese have further diversified to forage on coastal grasslands , resulting in a resurgence for the species.

Unfortunately Brent geese still have a liking for seagrass; but the seagrass meadows have not recovered. As a consequence a large flock of geese can cause considerable damage to any meadows in their environ.

Plans for recovery

Globally, seagrass meadows still have massive economic significance. They are a nursery for many commercial and subsitance fisheries, and have an important role in coastal protection. Often, however, their loss is most keenly felt by the poorest; those whose means of subsistence has been lost, who cannot afford sea defences, or to move as the sea sweeps in…

It was heartening, therefore, to read of a UK innitiative to try and reverse this trend of loss. Reported in the Guardian on the 10th March 2020, Project Seagrass has a global outlook, but most interestingly for me, is looking at re-seeding areas of former seagrass beds at Dale Bay in Pembrokshire. I wish them luck in their enterprise (and you can donate to their efforts through the link in the references section below!)

References – further reading

The drawings and photographs illustrating this piece are my own – I am not able to display some of the slides I used in the talk here for copyright reasons.

Estimates of the reduction in wave energy due to seagrass beds can be found in: Effect of a seagrass (Posidonia oceanica) meadow on wave propagation by E. Infantes, A. Orfila, G. Simarro, J. Terrados, M. Luhar and H. Nepf. in Mar Ecol Prog Ser 456: 63– 72, 2012.

A study on the diversity and stability of seagrass meadows can be found in Long-term persistence of structured habitats: seagrass meadows as enduring hotspots of biodiversity and faunal stability by A. Challen Hyman, Thomas K. Frazer, Charles A. Jacoby, Jessica R. Frost and Michał Kowalewski. Proc Roy Soc B Published:02 October 2019

Stalked jellyfish are not a common find underwater, you can find out more about their lifestyle at STAUROMEDUSAE / STAUROZOA. The identity of species photographed was established with the assistance of of the Stauromedusae UK website, I had previously mis-identified it as H. auricula, which is found in guide books, though apparently a rather rare…

The seagrass meadows off Ibiza are composed of Posidonia oceanica, and lay a claim to be home to the oldest plants on the planet. Cloned individuals have been estimated to be 100,000 years old. Sadly they are under threat – for more information see Ibiza and Formentera Preservation (Posidonia).

The loss of Seagrass meadows around the British Isles is catalogued by the Botanical Society of Britain and Ireland. You can see maps showing the declining distribution at BSBI Maps.

The factors causing problems for British seagrass beds have been documented in ZOSTERA BIOTOPES An overview of dynamics and sensitivity characteristics for conservation management of marine SACs by D.M. Davison and D.J. Hughes. Scottish Association for Marine Science, which is available in online.

You can see the circles cut in the seagrass in the satellite view of Studland Bay on Google maps.

Efforts to re-seed former meadows were reported in the Guardian on the 10th March 2020. The scheme is an initiative of the Project Seagrass charity.

Posted in MCS talks, Science

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February 16th, 2020

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Posted in Conservation, MCS talks

Lancashire’s Living Seas

December 13th, 2019
Sea slug in a sponge garden at Roa Island, 2015, by Barry.
Sea slug in a sponge garden at Roa Island, 2015, by Barry Kaye.

The North West has a diverse range of coastal habitats, from the tidal mudflats of Morecambe Bay, to seacliffs at St Bees in Cumbria, and dune systems of the Fylde coastline. These provide homes for a wide range of life, which is fed by the nutrient rich waters of the Irish Sea.

Recently a number of new MCZs have been approved in the North West to help protect the organisms that live here – including the estuaries of the Ribble and Wyre and Lune, which have been designated in part to protect the smelt (or cucumber fish – so called because it smells of cucumber!).

It will be important in the future to actively manage these new conservation zones, which are under pressure from a range of factors, including:

  • Plastic (and other) pollution (see the amounts of plastic being collected at Silverdale, for example)
  • Overfishing, where by-catch and practices such as bottom trawling are immensely damaging to communities that dwell in and on the soft sediments of the Irish Sea.
  • Development, the installation of offshore wind turbines (for example) causes sound pollution that has an effect on marine wildlife that is not restricted to cetaceans. (Members of the audience pointed out that once installed, the turbines do provide a managed and effective haven for fish stocks, as sea traffic around the turbine bases is restricted).
  • Climate change, with increased temperature moving the preferred range for species Northwards, and acidification reducing reducing the fitness of species that rely on calcareous structures, e.g. shells or exoskeletons.

The Wildlife Trusts organise a range of coastal activities that you can get involved with, from shore searches (grown up rock pooling) to christmas tree planting on the Fylde, to protect and enhance the dune system. For more information, or to get involved see:

Living Seas North West (Lancashire, Manchester North Merseyside)

Lancashire’s Living Seas, was a talk by Eleanor Falch to Lancashire MCS on Wednesday 11th December 2019

Posted in Conservation, MCS talks

Colour Underwater

September 2nd, 2019

The current storms indicate that summer is passing into autumn, and at Lancashire MCS we are starting to think about our winter lectures; which we hope will bring some interest into the darker months for you! Each year those of us on the committee dive deep into our store of knowledge to bring some element of the underwater world to life…

A number of years ago the subject of underwater colour was brought up, and I had thought this would be an interesting subject (though I did not know enough about it to present a talk;-) Over the years since then I have been gathering some relevant publications, and thought that perhaps this year I would try to bring them together.

The subject rather quickly expanded, as considerations of physics (transmission of light underwater), incidental colour (plants cannot help but be green – though seaweeds often are not!) and behaviour (how animals manipulate colour for communication and camouflage) all have an important part to play. When we look at how organisms produce colour, we get a glimpse into deep-time; the genes for green fluorescent protein (or their analogues) are present in all metazoans, suggesting colour may have been important to the Ediacaran biota 540 MYA.

Is red a colour?

Our eyes have adapted to life above water, but reds and oranges are strongly absorbed in seawater, leaving a monochromatic green-blue world. A lot of sea life is red, however, and some deep sea fish generate red light. We might, therefore, suggest that the colour red is significant even if it is only visible up close: At distance red light is absorbed, so anything red appears grey or black. If you only want to advertise locally, and don’t want to attract the attention of the big fish lurking in the gloom, red might be the most important colour!

Two photographs of brittlestars on kelp. the main image is taken from 5m in natural light, and they appear monochrome. The inset is taken at a distance of 30cm with a white strobe.
Two photographs of brittlestars on kelp. the main image is taken at a distance of 5m, and they appear monochrome. The inset below is taken at a distance of 30cm. To the best of my knowledge the colours displayed do not have any importance for the brittlestars concerned.

A comparison with other land animals suggests that colour perception in different species is likely to be very different to our own. Indeed, when age, visual defects, ill-health and genetics are taken into account, I might argue that colour is a personal experience, with even crude descriptions of ‘blue’ or ‘yellow’ meaning quite different things to each of us. (Web design in my day job, and it is quite important to ensure that text/background colour combinations are likely to be legible to readers!)

When we look at marine life, we see species with true colour perception ‘superpowers’. The most studied of these is that of the mantis shrimp – with twenty visual receptor types – twelve for colour (we have three), six for polarisation (we cannot detect this at all) and two for luminance (we have one). This suggests that the oceans are far from monochromatic, and there is hope for my talk…

See Unconventional colour vision by Justin Marshall and Kentaro Arikawa in Current Biology 24.24 (2014), R1150-R1154, for a primer in colour vision, and animal superpowers!

Barry Kaye

We are currently finalising our winter programme of lectures, and hope to have some external speakers this year alongside the ‘old guard’. please join us if you can – our Newsletter will keep you up-to-date.

Posted in Marine science update, MCS talks, Science


March 1st, 2019

Photographs of Orkney, sea slug and diving the High Seas Fleet.
Above top: An Orkney seascape (with defensive positions) by Lewis. Below the seaslug Coryphella by Gordon, and a diver on one of the wrecks of the German High Seas Fleet by Lewis.

Two talks about two very different types of destroyer – Lewis Bambury will talk about Orkney, including a look at how events from 100 years ago gave the islands some of the best diving in the world. Gordon Fletcher will look at the colourful world of sea slugs, giving you the chance to hear about the feeding habits of these predatory carnivores, their unusual sex lives, and the extraordinary defence mechanisms they utilise to avoid being eaten by larger predators!

These talks will be followed by the local group AGM.

Wednesday, 13 March: 19:30 – 21:00 at the Gregson Centre, 33 – 35 Moor Gate, Lancaster LA1 3PY.
Admission £3.00, everybody welcome!

Posted in MCS talks, Science

Invisible World: marine primary production

February 5th, 2019

Man and animals are in reality vehicles and conduits of food, tombs of animals, hostels of Death, coverings that consume, deriving life by the death of others. Leonardo da Vinci

Plants are rather different – quietly converting sunlight into the food we need to survive; the shepherd with his grazing flock is the subject of a painting, the meadow, a beaucolic backdrop. In the worlds oceans, however, the plants that form the meadow are microscopic – completely invisible to the naked eye. Indeed, for most of the 20th century, the main players remained elusive even to the best optical microscopes!

Over the last decade or so satellite imagery, coupled to unmanned submersibles, have begun to reveal the true extent of marine ‘plant life’. We find a complex, dynamic pattern of blooms, and rapid disappearances keyed to the seasons, currents and climate. Alongside this, genetics has begun to unravel the complexities of the interrelationships between the different groups of marine plants – and animals…

Join us on Wednesday 13th February between⋅19:30 and 21:00 at the Gregson Centre, 33 – 35 Moor Gate, Lancaster LA1 3PY for a personal look at some of the recent research in this area.
Admission £3.00, everybody welcome!

Posted in Events, Marine science update, MCS talks, Science

Dark and dull? Underwater wildlife photography in British waters

November 27th, 2018

Tunicates and worms by Barry Kaye

At our next meeting, members of the group will take personal views on the subject of photography around UK coastline. This will include a look at the photographic equipment they use, and the challenges they face in getting a ‘good’ photograph. Main speakers: Gordon Fletcher (film), Lewis Bambury (digital), Jo Kaye (macro).

Dark and dull? Underwater wildlife photography in British waters (PDF 117kB)
Wednesday, 12th December, 19:30 – 21:00 at the Gregson Centre, 33 – 35 Moor Gate, Lancaster LA1 3PY.
Admission £3.00, everybody welcome!

Please note, we do not have a meeting in January 2019, our first meeting of the New Year wil be in February, the remainder of our winter lecture programme is available in the PDF linked below:

MCS winter lectures 2018 (PDF format 82kB)

Posted in Marine science update, MCS talks

Life in Liverpool docks

April 7th, 2018

Review of ‘What’s up Dock?’, a talk presented by Wendy Northway (MCS North West England Seasearch Coordinator) on 14th March 2018.

While Liverpool’s pre-eminence as a maritime centre in the North West is undisputed today, it was only in 1751, with the construction of the world’s first commercial wet dock that she cemented her call to that title. Previously the most important port in the area was Chester, a position the city had held since Roman times.

The first wet docks allowed ships to be unloaded in a day and a half, significantly undercutting the older practice of unloading cargo to smaller boats at sea for transfer to shore – a process that could take two weeks on good weather. This gave the port a massive commercial advantage, and soon the docks were over-subscribed, leading to the rapid construction of new, larger facilities, which were also required as the expansion of trade with the industrial revolution lead to ever larger vessels…

Liverpool's Albert Dock with the Liver Building in the background. Photo B Kaye
Above: Liverpool’s Albert Dock (with the Liver Building in the background) has been cleaned up and is now a major tourist destination, housing Tate Liverpool and a number of museums. Diving in the docks is now strictly controlled. Photo B. Kaye

As each new dock was built, unseen and rather unheralded, we also built a new marine habitat, and it is these that were the subject of the talk What’s up Dock? by Wendy Northway (MCS North West England Seasearch Coordinator) to the group on 14th March 2018. The docks are no longer used commercially (having closed in 1972), but are actively managed leisure spaces. Water from the river Mersey is not used, as it is too polluted, instead the docks are topped up from the Irish sea on high water springs, giving a salinity in the range 24‰ to 28‰. The exchange of water with the Irish Sea allows for migration of marine wildlife between the Irish Sea and the docks, which have as a consequence become colonised by a fairly select group of organisms.

Mussels and algae colonising a discarded bicycle. Photo (c) BrokenDiver
Above: An old bicycle thrown into the dock has been colonised by mussels and algae. Photo © BrokenDiver.

In 1988 the docks were colonised by up to 1000 mussel spat per square meter. The absence of common starfish in the docks have allowed very large population densities of mussels to be established, and larger individuals have grown too big to have any natural predators. The mussels have been calculated to filter the entire of the docks water every four days, and are probably vital to maintaining water quality. Other filter feeders include bryozoans, sponges and tunicates, while cockles have colonised the soft, muddy bottom of the docks.

Black goby amidst mussels, bryozoans and sea squirts in Liverpool docks. Photo (c) Catherine Gras
Black goby amidst mussels, colonial hydroids and sea squirts (Ciona and Ascidiella) in Liverpool docks. © Catherine Gras

The modern docks are home to a thriving marine community, including a number of fish and crustaceans. I was, however, particularly interested to hear that the seasearch divers had confirmed the presence of an introduction from the southern hemisphere, the worm Ficopomatus enigmaticus in the Collingwood Dock. This species may have been introduced on the bottoms of ships or as larvae in bilge water. Unfortunately we do not know if this is a relatively modern introduction on pleasure craft, or a relict from the Liverpool port’s trading days.

Ficopomatus enigmaticus, an Austrolian introduction. Photo Wendy Northway (c) PhoebeSparke
Above: The characteristic pagoda shape of the tubes created by the worm Ficopomatus enigmaticus, an Australian introduction. Photo Wendy Northway © PhoebeSparke

NOTES: Ficopomatus has become a nuicance in many areas it has colonised, producing dense aggregations that can interfere with dock gates and other marine/estuarine structures, though at high densities in docks may help clear the water of particulates, and improve bottom biodiversity (JNCC report linked below). In some places it forms large reef structures in shallow brackish water. Needing a temperature above 18°C to breed, it may not be as invasive in our local open waters, but has been recorded in the docks at Barrow, so we should be looking out for it in the Bay! More information on Ficopomatus enigmaticus at ‘The Exotics Guide‘ and JNCC.

Posted in Marine science update, MCS talks

Sea Level Rise – the hidden coastal process?

January 3rd, 2018

Above: Simulation of sea level rise on the Fylde coastline.

Our next talk, by Trevor Lund of Blackpool and Fylde College, will examine the evidence from around the Bay for sea level change in the past, look at the processes involved and consider how this will affect us all in the future. The process of sea level rise is one of the most important in shaping our coasts and shallow seas, but what exactly is it and can we see any evidence for it in the environment around us?

Wed. 10th January 2018 at 19:30 ‘Sea Level Rise – the hidden coastal process?’ by Trevor Lund (Blackpool and Fylde College)
Meeting in the cinema upstairs at the Gregson Community Centre, Lancaster, LA1 3PY. Admission £2 – all are welcome!

Sea Level Rise (PDF 294kB) Poster with more information.

Posted in MCS talks

Marine Jellies

December 3rd, 2017

Photograph of the jellyfish Aequorea vitrina, by Gordon Fletcher

Contributing to the festive season, we have an illustrated talk on ‘Marine Jellies’ by Gordon Fletcher on the 13th December. Gordon is a good story teller, and an excellent marine life photographer. I cannot think of a more able person to bring some of the strangest and most beautiful creatures in our seas to life for us!

Wed. 13th December 1t 19:30 ‘Marine Jellies’ by Gordon Fletcher (Lancashire MCS)
Meeting in the cinema upstairs at the Gregson Community Centre, Lancaster, LA1 3PY. Admission £2 – all are welcome!

Posted in Events, MCS talks, Science