Pleasures in my own backyard

Crowtrees_pond_July16

One of the delights of my part of County Durham is the range of natural history that is available without the need to travel great distances.  That, indeed, has been the theme of this blog right from the start (see “Cassop”) and today’s post continues the theme of nature on my doorstep, with a visit to a local nature reserve within walking distance of my house.  Like Cassop Pond, it is at the foot of the Magnesian Limestone escarpment and, at this time of year, the grassland is rich with Northern marsh and Common spotted orchids.   It is, of course, the ponds that draw my attention: they are rich in aquatic plants including, once I start to look closely, beds of the alga Chara, which I’ve written about before (most recently in “Everything is connected …”).  And then, once my eyes are adjusted to looking at natural history at this more intimate scale, I can see that the stones on the bottom of the pond are covered with tiny snails (probably Hydrobiidae) with shells coiled in the shape of an ice-cream cornet.  Freshwater snails crawl across submerged surfaces rasping off attached algae with their tough radula so I started to wonder what snails in this particular pond might be feeding upon.

Crowtrees_stones_July16

Submerged stone from the pond at Crowtrees Nature Reserve, County Durham, covered in Hydrobiidae snails (left: the stone is about 10 cm across) and (right) a stone removed from the bottom of the pond showing the marl-covered part that was exposed and the marl-free part that was buried in the sediment. 

Viewed from just above the water, the surface of the stone looked as if it could be an algal film but, when I picked it up, the stone did not have the yielding texture that I associate with such films, but was a hard, mineral-rich marl.  More intriguingly, it was only present on the exposed surfaces, possibly, I suspect, due to the subtle interactions between chemistry and biology that I wrote about in “Everything is connected …”.

The calcite crystals make it hard to get a good view of the material under the microscope, but I managed to see a number of diatoms, mostly Gomphonema pumilum, or a relative, but also a good number of tiny, slightly asymmetric cells of a species of Encyonopsis, a genus that was, until recently, included in Cymbella, and which is usually a good indication that the water is about as untainted by human influences as it is possible to get.   It is, however, hard to get a really clear view of these under the microscope as they were scooting around.   With valves that are barely more than a hundredth of a millimetre long, I really needed to use an oil immersion objective to see them clearly, but the calcite crystals on the slide made it almost impossible to get a clear view of the live cells.  Not surprisingly, most of what we know comes from studies of carefully-cleaned preparations of the empty frustules.   Encyonopsis shares with Tyrannosaurus Rex the distinction of being an organism better known dead than alive.   It is rather ironic, given that healthy populations are living so close to my house, but that’s very often the case with diatoms.

There was one other abundant alga living amidst the rock (and, indeed, probably the major food source of the snails), but I am having some problems giving it a name, so a full account of that one will have to wait until another day.

Crowtress_diatoms_July16

Diatoms at Crowtrees Nature Reserve, July 2016: a.-d.: Gomphonema (possibly G. pumilum) in girdle and valve views; e.-g.: Encyonopsis sp.   Scale bar: 10 micrometres (= 1/100th of a millimetre).

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Everything is connected …

I’ve written about a curious group of algae called stoneworts (or charophytes) on a couple of occasions (see “The desert shall rejoice and bloom” and “Croft Kettle through the magnifying glass”. The significance of the name “stonewort” becomes obvious when you pick up a Chara plant, expecting it to be soft and pliable, and are struck by the rough texture of the axes, caused by the deposition of lime.

Stonewort-Chara-hisp-macro_

Chara hispida, photographed by Chris Carter.   Note the main axis and branches, from which whorls of branchlets arise at intervals.

The stoneworts are asssociated with hard water, so this deposition should not be a great surprise (the process by which kettles are coated with lime scale in hard water areas is very similar) however, most of the other plants in these habitats don’t share this property, so what is so special about Chara?   The answer is that, in hard waters, the carbon dioxide that plants need for photosynthesis is in short supply, but much more carbon is available as the bicarbonate ion.   Some aquatic plants can absorb the bicarbonate and then use an enzyme, carbonic anhydrase, to convert this bicarbonate to carbon dioxide. Chara, however, has a different strategy, actively pumping out hydrogen from inside the cells which, in turn, react with the bicarbonate and release carbon dioxide, which can then be absorbed by the plant.   However, as the water is also rich in calcium, a further series of reactions produces insoluble calcium carbonate, generating some additional carbon dioxide in the process As this series of reactions occurs very close to the cells from which the hydrogen ions are leaking, the precipitates end up on the plant surface, creating the rough texture.   The chemistry is way beyond this blog (meaning “… this blogger”) but you can follow it up in the references below.

Stonewort-Chara-intermedia-

Marcroscopic view of Chara intermedia showing an internode with a whorl of branchlets, along with spine cells and cortex cells (photograph: Chris Carter).

Another ion that is not very soluble is phosphate and this often gets caught up with the precipitating lime to form calcium phosphate.   This can be beneficial, as this phosphorus might otherwise fuel growth of phytoplankton which, in turn, would shade the Chara.   This means that Chara meadows should be resilient to artificial enrichment of nutrients to a limited extent at least.   However, there is some evidence that this capacity might be much less than was previously thought.   Hawes Water, a small tarn in Lancashire (not to be confused with Haweswater in Cumbria), for example, used to have rich and diverse communities of Chara spp, even in the deepest parts, but now the Chara and other submerged aquatic plants are confined to the shallow margins of the lake.   There is also good evidence of artificial enrichment in this catchment. The surprise is that concentrations of phosphorus in the water are still relatively low, yet the Chara meadows are much reduced compared with their condition fifty years ago.   The team that did this work also looked at another small marl lake, Cunswick Tarn, near Kendal in Cumbria, and found very similar changes.

It suggests a sensitivity to eutrophication that, perhaps, has previously been under-estimated, but it also points to the importance of balancing mechanisms in nature. On the one hand, Chara has some inbuilt capacity to counter-act increased nutrient concentrations. But others have shown that the ability of Chara to precipitate calcium carbonate is, itself, based on the photosynthesis rate.   The Chara meadows will reach a point when their natural capacity to absorb this extra phosphorus will be exhausted and then, as the phytoplankton take advantage of this, the water will get more turbid, reducing the amount of light reaching the Chara.   Less light means less photosynthesis and that will reduce the need for bicarbonate and, in turn, mean less carbonate deposition and less phosphorus removed. The evidence from Hawes Water is that this change happens very quickly: an ecological “domino effect”, if you like. As ever, everything is connected; sometimes in surprising ways.

Chara-virgata-Skye-fruit

Chara virgata (with oospores) from the Isle of Skye, photographed by Chris Carter.

Reference

McConnaughey, T. (1991). Calcification in Chara corallina: CO2 hydroxylation generates protons for bicarbonate assimilation. Limnology and Oceanography 619-628.

Pentecost, A. (1984). The growth of Chara globularis and its relationship to calcium carbonate deposition in Malham Tarn. Field Studies 6: 53-58.

Walker, N.A., Smith, F.A. & Cathers, I.R. (1980). Bicarbonate assimilation by freshwater charophytes and higher plants: I. Membrane transport of bicarbonate ions is not proven. Journal of Membrane Biology 57: 51-58.

Wiik, E., Bennion, H., Sayer, C.D., Davidson, T.A., McGowan, S., Patmore, I.R. & Clarke, S.J. (2015). Ecological sensitivity of marl lakes to nutrient enrichment: Evidence from Hawes Water, UK   Freshwater Biology 60: 2226-2247.

Wiik, E., Bennion, H., Sayer, C.D., Davidson, T.A., Clarke, S.J., McGowen, S., Prentice, S., Simpson, G.L. & Stone, L. (2015). The coming and going of a marl lake: multi-indicator palaeolimnology reveals abrupte cological change and alternative views of reference conditions.  Frontiers in Ecology and Evolution 3:82. doi: 10.3389/fevo.2015.00082.