What has become known as Doonfoot Lagoon in Ayrshire has provided a great deal of excitement for birders and photographers. This year, as well as in 2017, a Spotted Crake (Porzana porzana), a scarce summer visitor to Britain from its wintering grounds in Africa was not only in residence but could easily be seen.
What most people visiting the locality do not realise is that the ‘lagoon’ has formed relatively recently in an area once inundated by the sea at high tide. Water flows into and out of the lagoon from the River Doon which enters the Firth of Clyde at this point on the coast. To illustrate how recent it is I have downloaded present and historic satellite views from Google Earth and brought them roughly to the same scale (judged by the length of the nearby car park). The photographs were of course taken at different states of the tide in the various years but at the start of the series (1999) the sea reached the sand dune in front of the car park. There was no lagoon at all. On the latest photograph available (2018) not only can the lagoon be seen virtually in its present state but also the presence of a now large and vegetated sand dune on its seaward side.
In the intermediate years, a shallow area filled with water, often containing seaweed or with seaweed round the edges (which rotted with that characteristic aroma) formed on occasion but was then sometimes washed away by the sea. That is what appears to be present in the 2005, 2007 and 2011 photographs. It was sometime between 2011 and 2016 that the lagoon in its present state formed, as the dune on the seaward size increased in extent.
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| Satellite Images from Google Earth |
Flow into and out of the lagoon is now confined to passage over a shallow bar that forms a sandy bank of the River Doon. At high water, the level of water in the lagoon appears to be the same as that in the river. As the tide ebbs, the level of water in the lagoon falls.
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| At high tide, the water level in the 'lagoon' (left foreground) equals that in the River Doon (flowing right to left) |
Having seen the various changes at Doonfoot over the past 42 years and the recent formation of the lagoon, I had always assumed the water in it would be brackish. Over the past couple of years though I began to think that, with the changing vegetation and the fact that terrestrial birds like starlings can be seen bathing in it, the salinity may be low, not qualifying even for the term ‘brackish’.
I should say at this point that I do have some proprietary interest in marine birds and their existence on salt water, having written a monograph on salt glands in the 1970s and having worked for ten years on how those glands work to remove salt, ingested as part of their food or by drinking salt water, from the blood. Because salt glands need a lot of energy I often look at birds in estuaries and look for clues as to whether the salt glands are working at the time, drips off the end of the beak, swelling over the salt glands in the head, for example.
Having been fooled before about the salinity of a pool at the top of a sandy beach in Hong Kong which contained tadpoles (it was fresh water), I decided to measure the salinity of the lagoon. Having had, for 40 years, access to laboratory equipment that would have provided the answer I rather grudgingly had to buy the cheapest but best tool to do the job—a refractometer. Amazon soon had one at my door which on checking the calibration proved to have been set spot on at the factory somewhere in China.
To cut to the chase, the salinity of the lagoon was indistinguishable from Ayrshire tap water; no sign of any degree of saltiness. I also took sample from the wash over the entrance to the lagoon and from the river to the seaward side of the lagoon. Again, they were all fresh water. By contrast, at the point where on the day waves were starting to break on the shore, the salinity was 2%* (Atlantic sea water is around 3.5%). These samples were taken at high tide on 10 September. At low tide on 12 November, the lagoon and its outflow was fresh. I then walked to the furthest point of the beach I could reach seaward to the south of the lagoon to take a water sample there. It was full-strength (3.5%) sea water.
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| Salinity measurements mapped to GPS co-ordinates on Google Earth |
What seems to be happening is that the lagoon fills from and empties into the river at a point which at high tide is above the mixing zone of fresh water flowing down the Doon and sea water in the Firth. With the incoming tide the level of the river rises not as a result of a movement of salt water but of a backup of river water. Therefore, the lagoon is partially flushed with fresh water twice a day, which would account for the fact there is no residual salt. I took the samples at times when rainfall and therefore flow down the Doon was and had been high. I am interested to see what happens on a high storm-driven tide at a time when flow down the river is relatively low. Will the mixing point be further upstream and will the lagoon turn brackish as a result? If it does, then my guess is that the salt water would soon be washed out when things returned to what appears to be the present normal state.
The appearance of bulrushes or reedmace (Typha latifolia) also gives some indication of salinity. Although it will tolerate slightly brackish water, there is in the north American populations tested a major reduction in the length of the leaves when the salinity is raised from 0 to 0.4%.
The mixing of water from rivers entering the sea is complicated. Satellite photographs show that the river water still forms a discrete moving mass well into the Firth (and birds like swans are often lined up along the flow, feeding on whatever is being brought down). Depending on the temperature difference there may be a layering of less dense fresh water over the salt. I must at this point thank Robin Turner in a discussion on Ayrshire Birding for pointing out a useful website on what happens when rivers meets the sea.
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| This image from Google Earth shows the channel into the Firth of Clyde made by the River Doon |
The formation of the ‘lagoon’ raises several questions:
- Will the ‘lagoon’ remain or will it and the sand dune which protects it be washed out in the future with the coast returning to the state it was in 1999?
- Has a freshwater ‘lagoon’ formed previously and then been washed out?
- Is its presence part of the phenomenon of rebound from the ice ages (with falling sea levels) in northern Britain?
- Is it increasing in length?
- Are changes at the mouth of the Doon connected with the state of the numerous weirs and/or flow from the dammed Loch Doon?
I find it difficult to know what to call the Doonfoot ‘lagoon’. By some definitions, since it is not sea water, it cannot be called a lagoon. Perhaps ‘inlet’ would be more appropriate.
Finally, a biological question: how do birds like the Spotted Crake—which fly by night—find a suitable habitat in which to land and live? Do they use the sense of smell to detect plants or rotting fresh water vegetation, for example? Whatever the mechanism, there is no doubt that Water Rails (Rallus aquaticus), as well as the occasional Spotted Crake, soon found this entirely new location and moved in.
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*I am using percentage (%) to express salinity. Geographers and industry usually uses parts per thousand (0/00) while physiologists work in molarity or molality. Percentage is most easily understood by all.
Peaker M, Linzell JL. 1975. Salt Glands in Birds and Reptiles. Cambridge: Cambridge University Press
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| The bed of bulrushes at the western (distal) end |
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