West Cork Palaeoecology - Blog
The Environment of West Cork through Time
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A long blog.
Lakes are not usually on top of hills, principally because they form in depressions where water drains into them from higher ground. But in this case, things are different.
A view of Glandart Lake looking west, over the edge.
Glandart Lake lies at a height of between 330 m and 340 m above sea level, with a total surface area of about 0.4 hectares, and a circumference of 230 metres. It is largely circular. The most significant characteristics about this lake are the facts that it lies at the top of a steep slope, with only a very small amount of land to the east at a higher level. Surrounding high ground to the north and south are separated from the land the lake lies on by valleys that extend deeper than the lake level and that drain to the west. In addition, the water of the lake is retained from flowing down the slope to the west by a narrow bund of land that extends just 2.5 m above the lake level and is only some 5 or 6 m wide. Glandart Lake is thus perched right on the edge of a steep slope that descends 100 metres within 435 metres, a slope gradient averaging a 1m drop for every 4.35m – 1 in 4. A stream emerges from the slope below Glandart Lough at a height of 260 m, that is 80 metres below the lake level.
A 3D (rotatable and zoomable - use a mouse) virtual model can be seen here. This was constructed using QGIS mapping software, with the 3DJS plugin, and data purchased from BlueSky surveys. The data was not freely avaiulable because of the fine definition - based on a grid of 25 cm, 100 times more detailed than satellite imagery. In the case of this topography, it was necessary - satellite images at a resolution of 30m miss the essential nooks and crannys of this landscape, the ones that effectively isolate the lake.
Glandart Lake lies within a band of the Castlehaven Formation which with the older Gun Point formation outcropping further to the east north east, lies within the core of a faulted anticline, flanked by the younger Toe Head and then the Old Head Formations. These Late Devonian sediments are all essentially a very similar series of sedimentary rocks, principally sandstone, siltstone and mudstone. They are generally not massive, but very heavily cleaved by folding and faulting, and relatively easily shattered and split. Although the siliceous sediments, with a hard siliceous cement, can create an impermeable surface, the degree of faulting and cleavage means that the bedrock is very often free draining.
The bank holding the water back from rushing down the slope into the valley is only 5 or 6 metres thick, and yet clearly impervious. The formation of this depression, so close to the edge of the hillslope, but separate, is as yet unknown. Possibly a kettle hole, from the last ice age, formed when a buried block of ice melts, leaving a 'hole' in the ground. Or possibly the ground here is till, impervious boulder clay left on the hillside by the retreating ice at the end of the ice age. However, probing the land around the lake, including the bank, suggests bedrock lies not far beneath the surface.
The westcorkpalaeo.com webpage for Glandart is here.
The water in the lake would appear to be principally ground water, topped up with whatever rain might fall. As a consequence, the lake having no surface drainage flowing in, has an unusual and seemingly fairly limited biota. In addition the vegetation around the lake and in the immediate locality is also very limited, and does not provide a lot of organic matter into the lake. It is not known how deep the lake is, nor how deep any lakebed sediment might be. Scooping from the edge of the lake has yielded only thin organic sediment with a very high silt content and seemingly a low oxygen (anoxic) content. No living algae or diatoms have been found in the bottom sediment, only some live testate amoebae, but many tests of testate amoebae as well as cast off Cladocera (water flea) carapaces, pollen grains, and spores.
An extremely interesting find from the bottom sediment was a Suctorian, attached to a Cladocera carapace. These predatory, sessile, non-ciliated forms of a group of ciliates, are not widely studied, nor widely known. This one has not been named, yet, but unlike others has a lovely regular shape as a seven pointed star. Take note of the 'tentacle' like structures coming from the star points - these are for sensing, and catching, and ingesting - by 'sucking' - the prey, any passing protist that touches a tentacle. No other suctorians have yet been found.
A suctorian from Glandart Lake (mag x600). It is about 30 microns across (3 hundredths of a millimetre).
Around the edge in shallower water the principal algae found is Paludicola turfosa, a dense algae formed in whorls around a central 'stem'. Despite being a lovely deep green colour, it is in fact classified as one of the red algae (Rhodophyta). This algae produces large amounts of mucilagenous strings, presumably to deter browsers - it's sticky, slimy and tangling. These can just be seen in the photo below as fine transparent strings emanating out from the main greenery (click on the image and zoom right in). Like the suctorian mentioned above, Paludicola has a complex and interesting life cycle.
Paludicola turfosa - note the transparent strands of slime secreted by this algae, just visible in this photo (mag x200).
The Paludicola provides both anchorage and shelter for a selection of diatoms, a separate family of algae which are distinctive for their silica skeletons, or frameworks. Although still single celled organisms, diatom species can be recognised by the intricate and distinctive shapes and patterns of these silica frameworks. These only become visible once the surrounding cytoplasm is removed, and the lakebed is littered with these skeletons of diatoms past. Surirella and Pinnularia, two large (relatively) and motile ('raphid' - bearing raphes by which they exude a substance enabling a gliding motion across a surface) diatoms, living on the sediment surface (epipelic). I haven't yet found these species alive, just their remains in the bottom sediment. They probably browse the deeper edges of the lake, but the lake edge overhangs the water, and the one thing nearly all diatoms neeed, is light. They photosynthesise, like green plants. But they are motile and can move to places where conditions are better; lighter, cooler, warmer...
So to add to the strange behaviours of predatory non- ciliate ciliates, Paludicola life cycle, and single celled amoeba making a shell to live in, we have photosynthesising algae... that moves.
Surirella (left, x400) and Pinnularia (right, x200) from the lakebed sediment at Glandart.
But on, and in, the Paludicola we can see other, smaller diatoms, hiding, or making use of the protecting lines of sugary slime. Some of these motile diatoms dwell within their own tubes of jelly, or slime, for protection possibly. And they commune within these tubes.
Dangling off the ends of the Paludicola 'fronds' are the curvy Eunotia exigua, in their thousands.
Eunotia exigua on Paludicola, and (inset) Eunotia, from the lakebed sediment at Glandart.
The Paludicola is the main surface to which these diatoms can attach, the lake edge descends abruptly to a depth at which light is poor, not good for photosynthesising organisms; and the actual lake edge is overhung by vegetation and peat outgrowth, largely mosses. This is a common situation in many lakes, so there is no gradual deepening of the water with a slowly descending shoreline, but instead an abrupt depth overshadowed by peat and moss and vegetative detritus.
However, some diatoms are freely floating - planktonic - amongst the Paludicola - round ('centric') Cyclotella, strings of colonies of attached Tabellaria flocculosa, and the larger boat shaped Frustulia saxonica. Less common here are Eunotia tetraodon, with the crescent shape bearing four (tetra-) humps (-odon) along the dorsal side.
Cyclotella sp.- the circular diatom centre right (x1000), Tabellaria flocculosa - individual at centre left (x1000), attached colony at bottom right (x1000), Frustulia saxonica individual (x200) bottom left, four communing in a mucus tube top right (x1000), and Eunotia tetraodon bottom centre (x1000), from the lake at Glandart. Anything else is Eunotia exigua (x1000)
Recent research on lake bodies and the behavior of microbes in them has thrown up some interesting issues relevant to the changing climate. One study showed that the amount of CO2 released by microbes in a lake is greater both at higher temperatures, and also more the closer to the shore they are. Another study suggests that the ability of microbes to either photosynthesise, or act as predators on other microbes, might change as temperature within the lake water rises. This latter study found this was more the case in high nutrient waters.
The first finding might well be simply linked to the amount of light penetrating the shallower water and allowing a greater concentration of microbes - both CO2 using photosynthesisers, and their CO2 producing predators. How the dynamics might change as temperature rises, as in the second finding, is uncertain. Even more so in lakes such as Glandart, which appear to have a low biodiversity, and a low nutrient status.
References
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