The Mysteries of The Mere

Shropshire Botanical Society Newsletter - Autumn 1999 - pages 7-10

Alex Lockton

 

The Mere at Ellesmere has long been one of the most popular botanical localities in Britain. With easy access to such a spectacular lake, botanists are practically invited to throw in a grapnel and see what comes out, and they have indeed been doing so for hundreds of years. Useful information about this site includes their herbarium sheets, as well as sketches and paintings, historical accounts and maps dating back into the mists of time. All this adds up to a fascinating history, with records from as long ago as 1632, when George Bowles and William Coote collected plants in the vicinity of Birch Hall, just south of The Mere.

In total there are hundreds of botanical records for The Mere, and more are being added to our database all the time. Specimens hidden away in museum collections and obscure literature sources help to shed light on this interesting and ever-changing site. But The Mere holds many surprises for ecologists, and in recent years we have discovered that it is worth exploring the facts carefully.

The first problem when researching The Mere is identifying the locations. A lot of old records simply state "Ellesmere," which would, these days, normally refer to the town. Unfortunately, the town is largely in one 10km square of the national grid (SJ33), while the mere is in another (SJ43). Because of the way botanical records are kept on computer nationally, that makes it quite difficult to trace them, and it often creates confusion. For example, in the 1994 book Scarce Plants in Britain, the map of Least Water-lily, Nuphar pumila, shows two dots in Shropshire: one for SJ33 and one for SJ43. Unsurprisingly, when the actual details are coaxed out of the Biological Records Centre, the SJ33 dot refers to a site listed as "Ellesmere" despite the absence of any suitable water body in that square; and it is probably misleading anyway: the original record was "a mere near Ellesmere" - not in fact The Mere itself, but more likely Blake Mere or Cole Mere. Many other records for The Mere have similarly migrated the few hundred metres west into the streets of the town; but they can usually be spotted because tarmac and cobblestones make very poor habitat for aquatic plants.

Another confusion is over a site called "The Moores." This is one of the places where Bowles and Coote botanised, but only a local would know that "The Moors" (as it is now called) is not a general reference to surrounding heathland, as it would have been anywhere else, but a specific reference to a stretch of common land that lies to the south of The Mere. The town of Ellesmere was quite unusual in not having any other unenclosed heathland or moorland nearby, and as a consequence this small piece of common land was of enormous significance to the townsfolk. The history books are full of stories of the people taking on the owners of the mighty Oteley estate to protect their precious Moors from enclosure or overgrazing. Again, the correct identification of the location of The Moors shifts the dots on the map from SJ33 to their rightful place in SJ43.

Once these old records are correctly assigned to The Mere and its associated Moors, it becomes possible to study the historical ecology of the area. This reveals some profound changes, and even sheds light on one of the hottest debates in current ecological thinking.

The conventional interpretation of the north Shropshire meres is that they are "naturally eutrophic" and therefore of little importance for aquatic macrophtyes. Many of the meres, with the exception of Brown Moss (which, for various reasons, is a special case), are rich in the dissolved salts of phosphorus and often have high levels of nitrogen and potassium, too - the nutrients used in fertilisers. It is these high levels of nutrients that contribute to the development of algal blooms (known as the "breaking" of the meres) in summer; which has been described since the late 19th century. On the basis of this, therefore, the meres are considered to be naturally eutrophic. But is this really a natural state, or is it a consequence of human activity? The answer may not be as simple as either of those. Bear with me for a moment, and I shall first explain a few other, related, mysteries of The Mere.

One strange feature is that The Mere contains drowned trees. A few metres below the shoreline there are ancient stumps of trees that must once have grown on dry land. As far as I am aware, nobody has yet worked out how old these trees are - which could be done by comparing their growth rings with those of other preserved tree remains that have been found elsewhere in Britain. But whatever their date, they must have grown there between about 10,000 years ago, after the last Ice Age ended, and maybe 1,000 years ago, when historical records for the area began. And at some time during that period, the level of The Mere must have risen and drowned them. As the north Shropshire meres generally have no inflow or outflow (they are filled by groundwater, and their surfaces are at the level of the water table), there is no known mechanism by which the level could have risen. None of the other meres in the area shows any fluctuations of similar magnitude.

Finally, there is one other mystery about The Mere that needs to be solved. At the southern tip of the lake there is a marsh, which lies within The Moors, the area of common land mentioned previously. This marsh contains a very odd range of plants, notably Common Cotton-grass, Eriophorum angustifolium, Narrow Buckler-fern, Dryopteris carthusiana, and Bog Pondweed, Potamogeton polygonifolius. These plants typically occur in very acidic places, such as moorland flushes on the top of the Stiperstones. They are not lowland plants at all, except maybe on raised bogs, and some of them can be found at Whixall Moss. The water of The Mere, however, is very base-rich (with a pH of between 8 and 9) and entirely unsuitable for such plants. Last year during a course at the Field Studies Council we took Britain's leading expert on pondweeds, Chris Preston, to see The Moors, and although we insisted that Bog Pondweed grew there he clearly didn't believe a word of it until he saw it himself. It is not that these plants are rare; it is that they are growing in the wrong place. In the Flora of Shropshire, Charles Sinker et al. explain this incongruity by suggesting that "perhaps the marsh is losing contact with the base-rich groundwater of the mere and becoming ombrogenous" (p. 92). However, as we shall see, their hypothesis may have been wrong.

So, without spinning out the mystery more than is necessary, what did happen at The Mere? Why did the water level rise; why are there upland plants in a lowland site; and what is the significance of all the old records that have been misplaced? Well, we don't know all the answers, but we can be pretty sure that they involve global warming, dairy farming, a canal, and the North American state of Minnesota.

 

Bowles and Coote recorded, among other things, several peat bog species at "the Moores of Ellesmere." In fact, it is thought that they made the first British record of Great Sundew, Drosera anglica, there. There is no doubt, if such plants were present, that there must have been a lowland raised bog there, somewhere between Birch Hall and the edge of the water. The only such place where such a bog could have been is in a great basin that lies south of the Mere, and which to this day is mostly lined with peat. Historical accounts show another interesting thing: there used to be a brook, significantly called the Blackwater, that flowed out of The Mere at just this point. It used to run along where the canal now lies, and during the winter it would flood, blocking the main road along the side of The Mere, forcing people to use another road that ran around behind the castle.

The fact that The Mere had an outflow is unusual and significant. And even more significant is the knowledge that this outflow ran either through, or very close to, a raised bog. Remember, the water in the lake is too base rich to allow a peat bog to develop, according to conventional wisdom. But the Blackwater was there, and it would surely have been called "black" because of the tannin extracted from the peat.

Since Bowles & Coote there are many other records by botanists who collected plants from the Mere itself. A lot of these are listed in Leighton's Flora of 1841. These records pre-date Fig 1: the Mere as it may have been in 1970the earliest mention of the breaking of the meres by nearly 100 years, and they do not give the impression of a nutrient-rich water-body. Among the plants listed are Lesser Water-plantain, Baldellia ranunculoides, Floating Water-plantain, Luronium natans, and Alternate Water-milfoil, Myriophyllum alterniflorum. These are species you might confidently expect to find in a crystal-clear upland lake in Wales or Scotland. These days you will probably not find any of them, and certainly not all three, in any of the eutrophic meres.

 

Fig 1: The Mere as it may have been in 1790:

 

This is clear evidence that 200 years ago The Mere was not eutrophic at all. The water may have been somewhat base-rich (i.e. containing calcium salts), but it would have had very little of the agricultural fertilisers N, P & K - and especially very little nitrogen. A similar situation occurs at Malham Tarn in Yorkshire, where a base-rich but nutrient-poor lake lies adjacent to an acidic peat bog. Many of the same plants occur there as used to occur at The Mere. Our hypothesis is that this is reminiscent of what The Mere used to be like.

 

Why did the water level rise?

If that is an apt description of The Mere 200 years ago, it suggests a possible mechanism for the raising of the water level. The Mere's outflow - the Blackwater - would have, uniquely among the meres, kept the level in check. If the Blackwater were blocked, somehow, the water level would have risen. The most obvious way for the Blackwater to be blocked would have been the formation of the dome of the lowland raised bog that we have shown occurred in its path. A lowland bog has the ability to raise itself above the water table and keep on growing - they can grow up to 10m above the groundwater level. Unfortunately, until a few years ago, there was no known mechanism by which a bog could even exist in the path of a base-rich stream, let alone grow so big as to block it. But in 1997 scientists in Minnesota demonstrated that rivers and moving groundwater could boost the water supply to peat bogs in areas where there was not enough rainwater otherwise to sustain them: in other words, a peat bog can, against all previous assumptions, grow using base-rich water from below. As long as the acrotelm, or living surface of the bog, is fed only by pure rainwater, the bog should survive happily. This revelation has led to a spate of papers in ecological journals, all endorsing this theory and revolutionising our understanding of peat bog hydrology.

This, therefore, gives us our mechanism explaining how The Mere could have risen, and it also explains all the wonderful acidic, upland plants that used to (and still do) occur there. The Mere probably once had a huge bog, as much as 10ha in extent, at its southern end. All this, unfortunately, was destroyed abruptly in the year 1800. In that year the Ellesmere & Chester Canal was carved through The Moors. Routing canals through peat bogs was common practice, partly because the land was flat, and partly because the peat itself holds water, which means the canal doesn't have to be lined with clay. When they built the canal, they dug another outflow from the north-west side of The Mere to stabilise the water levels and prevent The Moors from flooding. The level of the Mere, which had risen maybe 5m since the Ice Age, was suddenly dropped by a metre or so and stabilised permanently. This caused the Blackwater to dry up, leaving the peat bog high and dry, and the farmers moved in to reclaim the land. Nowadays all that is left of the Blackwater is a small ditch, flowing the other way, draining the tiny remnant of peatland known as the marsh. But the Bog Pondweed remains, as does a small selection of other peat bog plants, with little other purpose than to confuse the unwary ecologist.

The canal, of course, was built as an extension of the industrial revolution into the Welsh borderlands. One of the main incentives for the canal company was to bring dairy products Fig 2: The Mere as it is todayquickly from rural regions into the towns. By the 19th century dairy farming had become the most profitable system of agriculture in this part of the country, replacing sheep herding as the climate warmed after the end of a 300 year cold period known as the "Little Ice Age."

 

Fig 2: The Mere as it is today.

 

The agricultural changes brought about by climate change, industrialisation and political stability led to the ploughing up of permanent pasture and the felling of woodlands (Oteley Park had been one of the largest Deer Parks in England - all lime woodland). This in turn would have led to the release of nutrients into the groundwater and the eutrophication of the meres even before artificial fertilisers were available to farmers. For the whole of the 19th and 20th centuries, the meres have been accumulating excess agricultural nutrients and warming up (even until the middle of this century The Mere used to freeze over regularly), a process that has led to the loss of most of the former aquatic flora. That process continues apace today, with even the rain bearing a load of nitrogen from the dust and spray of agricultural use.

That brings to an end one explanation of the history of The Mere. Once upon a time, it was just like an upland lake: cold, clear, and with a wealth of rare plants. Even now the evidence for that is visible: computer analysis of the vegetation at The Moors links it clearly to the community S27 Carex rostrata-Potentilla palustris tall-herb fen, despite the fact that the Bottle Sedge, C. rostrata, disappeared 100 years ago. S27 fen is typical of upland Welsh lakes; but it is virtually unknown in the Midlands and the south of England.

Further evidence would be very useful to corroborate (or refute) this account. It would be interesting to attempt to date the tree stumps still submerged in The Mere. It would also be useful to analyse the peat in The Moors to see what is there. Most of the recent deposits may have been lost, but the older, deeper peat may yet retain fragments of such rarities as Rannoch-rush, Scheuchzeria palustris, which is known to occur in peat deposits at Whixall Moss and elsewhere. It is difficult to think of a direct proof that the Blackwater was dammed by a peat bog, but research might help to clarify the exact route of the brook and the extent of the bog. Finally, is there any chance of re-creating any of these lost habitats and restoring some of the lost plants? Sadly, a full restoration is out of the question, but recent work by County Council rangers is aimed at protecting as much as possible of what remains of The Moors (previously their management plan aimed to convert it to woodland) and the charity Plantlife are investigating the possibility of reintroducing the national rarity Starfruit, Damasonium alisma, which is one plant that can probably tolerate the nutrient-rich waters of the present-day Mere.

Lost Plants

Species which have gone missing from The Mere or The Moors since 1632:

Equisetum palustre Marsh Horsetail

Nymphaea alba White Water-lily

Ceratophyllum demersum Rigid Hornwort

Ranunculus circinatus Fan-leaved Water- crowfoot

Montia fontana Blinks

Stellaria palustris Marsh Stitchwort

Elatine hexandra Six-stamened Waterwort

Hypericum pulchrum Slender St. John's-wort

Rorippa palustris Marsh Yellow-cress

Myriophyllum spicatum Spiked Water-milfoil

Myriophyllum alterniflorum Alternate Water-milfoil

Lythrum salicaria Purple-loosestrife

Lythrum portula Water Purslane

Oenanthe crocata Hemlock Water- dropwort

Apium inundatum Lesser Marshwort

Cicuta virosa Cowbane

Myosotis secunda Creeping Forget-me-not

Littorella uniflora Shoreweed

Veronica scutellata Marsh Speedwell

Petasites hybridus Butterbur

Baldellia ranunculoides Lesser Water-plantain

Luronium natans Floating Water-plantain

Damasonium alisma Starfruit

Hydrocharis morsus-ranae Frogbit

Scheuchzeria palustris Rannoch-rush

Potamogeton coloratus Fen Pondweed

Potamogeton x zizii Long-leaved Pondweed

Potamogeton perfoliatus Perfoliate Pondweed

Potamogeton obtusifolius Blunt-leaved Pondweed

Spirodela polyrhiza Greater Duckweed

Lemna trisulca Ivy-leaved Duckweed

Juncus subnodulosus Blunt-flowered Rush

Eriophorum vaginatum Hare's-tail Cottongrass

Schoenoplectus lacustris Common Club-rush

Carex diandra Lesser Tussock-sedge

Carex ovalis Oval Sedge

Carex elongata Elongated Sedge

Carex lasiocarpa Slender Sedge

Carex rostrata Bottle Sedge

Carex viridula ssp. oedocarpa Common Yellow Sedge

Carex elata Tufted Sedge

Carex pulicaris Flea Sedge

Deschampsia setacea Bog Hair-grass

References

English Nature. 1998. A Strategy for the conservation of the Meres & Mosses. EN, Attingham Park, Shrewsbury.

Glaser, P.H., Siegel, D.I., Romanowicz, E.A. & Yi, P.S. 1997. Regional linkages between raised bogs and the climate, groundwater, and landscape of north-western Minnesota. Journal of Ecology 85: 3-16.

Lockton, A.J. & Whild, S.J. 1998-99. An Ecological Review of The Moors, Ellesmere. Whild Associates report to Shropshire County Council.

Sinker, C.A. 1962. The North Shropshire Meres and Mosses - a background for ecologists. Field Studies Vol. 1 No. 4. Reprinted Headley Brothers Ltd., London.

Wheeler, B. 1998. The results from research into the location of historical Damasonium alisma sites. Plantlife Back from the Brink Project report no. 90.

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