Meetings with remarkable Gomphonema …

Having written about Gomphonema rhombicum in my previous post, I thought it would be worth staying with Gomphonema and showing some images of G. vibrio.   This is a diatom that I had rarely encountered previously but which cropped up in separate email conversations with Chris Carter and Geoff Phillips in the space of a couple of months.  Chris’ samples come from a small man-made pond at Yardley Chase, an SSSI in Northamptonshire (photographed above), whilst Geoff’s was from Phragmites stems in a Norfolk marsh dyke.  Both have hard water (Geoff’s location: pH: 7.6; alkalinity: 275 mg L-1 CaCO3; conductivity: 700 mS cm-1) and good water quality (TP: 60 mg L-1; TN: 1.5 mg L-1).   This set of conditions prompted me to dig out some samples from Croft Kettle, a location I wrote about a couple of years ago (see “The desert shall rejoice and blossom …”) where I had a vague memory of having seen something similar.

Valves of Gomphonema vibrio are relatively large (30 – 95 x 7 – 10 mm, according to Hofmann et al., 2017) and club-shaped with a slight swelling at the centre.  Overall, the valves are more slender than was the case for G. rhombicum (see illustrations in the previous post).   The striae are coarse (7 – 10 in 10 mm) and mostly radiate, but there is a distinct central area where there is a single stria on each side more distantly spaced from the adjacent striae than in the rest of the valve.  On one side, this stria is very short (sometimes it can be hard to see); on the other side, it is longer and ends with a distinct stigmoid (an isolated pore).    The central endings of the raphe are often turned to the same side.

Cleaned valves of Gomphonema vibrio from a pond at Yardley Chase, Northamptonshire.  Yardley Chase is shown in the image at the top of the post.   Images are in pairs, each at a slightly different focus plane.   All photos by Chris Carter.

Chris also sent me some photographs of the living cells, showing a clear stalk protruding from the narrower “foot” pole, as well as a beautifully-clear H-shaped chloroplast.  The presence of a stalk in this species just doubles my annoyance at not having checked for the same in G. rhombicum before cleaning the valves.

There are, it seems, remarkably few records of Gomphonema vibrio from the UK.  I can find no other records from rivers and Helen Bennion found just two other records of recent samples in the UCL database, both from Scotland: Loch Levan and Loch Davan.  Three of the five records are from ponds, which may be significant, and two of these were epiphytes, though there are not enough records here to make any firm pronouncements about habitat preferences.  However, the picture that is emerging is of a species that definitely has a preference for moderately hard to hard water with relatively low nutrients. If that is the case, then it could well be a species that used to be more common that it is now, as many habitats such as these will have deteriorated in recent decades due to agricultural enrichment.   It is certainly a very different habitat from the soft water, fast-flowing stream from which I recorded G. rhombicum in Bulgaria.

Live cells of Gomphonema vibrio from a pond at Yardley Chase, Northamptonshire.  Photos by Chris Carter. 

That makes a total of five records from the UK which, even allowing for the muddled taxonomy (which I’ll talk about in the next post) and the fact that the diatoms of small ponds are rarely studied, suggests that this may be a genuinely rare. It is listed as an “endangered species with persistent risk factors” on the German red list, with a forecast of further decline over the next ten years.   I’ve voiced my concerns about “rarity” and red lists before (see “A red list of endangered British diatoms?”) but will stick my neck out on this one and suggest that Gomphonema vibrio might be a candidate.

Reference

Lange-Bertalot, H., Hofmann, G., Werum, M. & Cantonati, M. (2017).   Freshwater Benthic Diatoms of Central Europe: Over 800 Common Species Used In Ecological Assessment (edited by M. Cantonati, M.G. Kelly & H. Lange-Bertalot).   Koeltz Botanical Books, Schmitten-Oberreifenberg.

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The green mantle of the standing pond* …

One of the highlights of a wet and windy weekend at Malham Tarn Field Centre for the annual British Diatomist Meeting was a talk by Carl Sayer on the ecology of a small pond in Norfolk.  The work was not new to me, as I had been the external examiner for Dave Emson’s PhD thesis on which the work was based.  I remember, at the time, making a mental note to write a post once the work was fully in the public domain, and Carl’s talk has finally jogged me into action.

Carl’s starting point was the observation that small ponds are often covered with dense growths of floating aquatic plants such as duckweed (Lemna minor).  Repeated visits to ponds in north Norfolk, close to where he grew up, had shown that this cover of duckweed often lasted for a few years before disappearing, only to reappear some years later.   As this duckweed blocks out sunlight, periods of dominance are likely to have unfortunate consequences for other aquatic plants in the pond and, as these pump oxygen into the water as a by-product of photosynthesis, life for other pond-dwelling organisms – such as the Crucian carp (Carassius carassius) that Carl likes to catch from the pond – will also get tougher.

There’s a lot of questions that could be asked about what’s going on here, and not all can be answered in a single study, but establishing whether these periodic episodes of duckweed dominance were one-offs or if they were regular events is a good place.  Here Carl and Dave  were able to use a well-known association between a diatom – Lemnicola hungarica – and duckweed to track changes in Lemna over time.   Lemnicola hungarica grows attached to the roots of duckweeds and similar species and seems to be unusually fussy about its habitat compared to many diatoms, which means that when Lemnicola is found in the sediments of a pond, that is a fairly good indication that Lemna was abundant when those sediments were being laid down.   In the process, they also discovered another diatom, Sellaphora saugerresii, also seemed to be strongly associated with Lemna, at least in this habitat (it is also common in many rivers were Lemna is sparse or absent).

The relative abundance of a) Lemnicola hungarica and b) Sellaphora saugerresii in surface sediments of north Norfolk ponds with and without Lemna dominance.   The two species are illustrated on the right hand side (S. saugerresii is typically about 10 micrometres  (= 1/100th of a millimetre) in length).

Armed with this information, Dave and Carl went back to one of Carl’s local ponds and extracted a core of the sediments from the middle in order to see how numbers of Lemnicola hungarica and Sellaphora saugerresii changed through the length of the core.   Because they were also able to date the core, they were able to show that the period when there are documentary records of duckweed dominance coincides with both of these indicators being abundant in the pond sediments.  Below these levels (i.e. further back in time), the relative abundance of these two species waxes and wanes several times, suggesting that the duckweed cover, too, had come and gone over the years.

Left: Dave Emson and the core from Bodham Rail Pit; right: changes in the relative abundance of Lemnicola hungarica and Sellaphora saugerresii at different levels of the core.    The grey rectangle indicates the period during which Lemna is known to have been dominant in the pond (all photos in this post: Carl Sayer).

Quite why this is so is not clear.   There are several species of floating aquatic plant (water hyacinth and Salvinia, the floating fern are two good examples) that are able to cover large areas of standing water bodies in a short period of time and they often do this by vegetative growth rather than by seed.   This means that the plants are mostly clones of a very small number of plants that first colonised the water body.   And this, in turn, may mean that a virus that infects one frond will be able to infect every other frond as well as there is a very narrow range of genotypes within the population.  That’s one possibility but there may be others.

But back to the story: knowing that Lemna abundance fluctuates is not quite the same as being able to describe the consequences of this for the rest of the organisms that inhabit these ponds.   The Crucian carp was the species that attracted Carl to the pond in the first place so it would be good to know whether this species can survive the dark, oxygen-poor years when the surface is covered with duckweed.   They did find scales of Crucian carp in the cores right through the pond’s dark ages suggesting that this tough little fish had managed to hang on.  In 2008, a few years after the most recent duckweed episode, they found just a single carp when they cast their nets out into the pond but there were three by the following spring and, in 2011 there were over 200 juveniles.  So it looks like the carp populations definitely retrench during the duckweed episodes but that they do, eventually, recover.   And, maybe, another generation of north Norfolk natural historians will become enthralled by the aquatic world as a result?

* King Lear Act III scene IV

References

Buczkó, K. (2007).  The occurrence of the epiphytic diatom Lemnicola hungarica on different European Lemnaceae species.  Fottea, Olomouc 7: 77-84.

Emson, D., Sayer, C.D., Bennion, H., Patmore, I.R. & Rioual, P. (2017).  Mission possible: diatoms can be used to infer past duckweed (lemnoid Araceae) dominance in ponds.  Journal of Palaeolimnology https://doi.org/10.1007/s10933-017-0008-6.

More pleasures in my own backyard

Back in early July I wrote about a visit to a pond in a local nature reserve (see “Pleasures in my own backyard”) and ended with the hint that there was one other abundant alga there that I was unable to name at the time.  I was reticent about naming it, as it seemed to be a rare alga and the habitat where I had found it did not match the locations where it had been found to live.

I’ve now shown it to Brian Whitton and he has joined me on another excursion to the same pond, and I can confirm that it is, in fact, Chroothece richteriana, a freshwater red alga.   We’ve met (and even eaten) red algae several times over the lifetime of this blog (see “More from the Lemanea cookbook …”), but Chroothece is different in that it does not form filaments or thalli, but lives in mucilaginous masses.   The individual cells, each of which are ovoid, with a single star-shaped chloroplast, live embedded within this mass.

Chroothece_Crowtrees_July16

A colony of Chroothece richteriana growing on marl-encrusted rocks from Crowtrees Pond, County Durham, July 2016.  Scale bar: 10 micrometres (= 1/100th of a millimetre).

This is a species that was, until recently, known only from two very old records. However, searches over the past few years have found it growing at a number of different locations.  There are now half a dozen locations in the UK, plus one in the Isle of Man.   Interestingly, the population at Crowtrees matches these other records in respect to the underlying geology – limestone – which yields very hard water, but differs in being permanently submerged.  The other records are from seepages and other semi-aerial habitats.   The population at Crowtrees formed a thin film that was firm to the touch due to the deposition of calcite crystals within the matrix.   There were also some cyanobacterial filaments mixed in amongst the Chroothece, as well as the diatoms that I mentioned in the previous post.  I suspect that the snails that I observed on my earlier visit were scraping up a mixture of all these species from the thin surface layer that had not yet had time to become hardened by calcite crystals.

One theory for the success of Chroothece here is that habitats such as this are naturally low in phosphorus, an essential nutrient that is naturally scarce but which is relatively insoluble and consequently is precipitated out of the water along with the calcite.   Studies in Spain (in a river, rather than a seepage or pond) showed that Chroothece shares the characteristic of several other algae from this type of habitat, of producing enzymes that can scavenge phosphorus from tiny particles that are suspended in the water.  The enzymes are thought to be concentrated in the mass of mucilage (which is actually formed from the organism’s stalks)

Ironically, our excursions to Crowtrees Nature Reserve have become more frequent over the past year or so as our usual running and walking beats in the countryside around Bowburn have been changed as a local quarry expands its activity (seen in the gouge in the skyline in the picture below).  The pond, itself, looks natural, but local drainage is strongly influenced by mining and quarrying.  The area around here, especially associated with the Permian limestone, abounds in nature.   But whether or not this nature is natural is a topic for another day …

view_from_Crowtrees_July16

The view from Crowtrees Nature Reserve towards the Tarmac quarry, July 2016.

References

Aboal, M., García-Fernández, M.E., Roldán, M. & Whitton, B.A. (2014).  Ecology, morphology and physiology of Chroothece richteriana (Rhodophyta, Stylonematophyceae) in the highly calcareous Río Chícamo, south-east Spain.  European Journal of Phycology 49: 83-96.

Pentecost, A., Whitton, B.A. & Carter, C.F. (2013).  Ecology and morphology of the freshwater red alga Chroothece in the British Isles.  Algological Studies 143: 51-63.