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


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


How to be an ecologist #4

The British university system somehow manages to train ecologists despite two major impediments: most universities are in cites and all teach during the periods of the year that are least enticing for the budding field scientist.   Together, these factors work as two strong selective factors: getting thoroughly damp in a forest in Hampshire on a wet Sunday (because it is easier to timetable a whole day trip at the weekend), following a two hour coach trip, is a fine way of weeding out those with romantic notions of ecology gleaned from David Attenborough documentaries.

Then, of course, there is the residential field course, where students are immersed in an alien environment for an entire week (two, in some cases). These days, field trips seem to function as part of the marketing blurb in the prospectus, with departments vying with each other to offer the most exotic location. It makes as much sense to the customer (and that is what students are, let’s be honest) as a financial advisors offering a free pen to anyone who signs up to a pension plan.   Back then, we were taken, by boat, to Blakeney Point in Norfolk, and dumped outside the wooden hut that served as University College, London’s field station. We camped amongst the sand dunes and spent the days learning about the saltmarsh and sand dune plants under the tutelage of Dickie Clymo.

I still contend that you can learn more in a week about plant ecology on a British salt marsh or sand dune than in twice that time in a more exotic habitat.   Sand dunes and salt marshes have the additional benefit of having only a limited number of species, which means that we were able to focus on why they grew where they did rather than having to memorise long lists of names.   But then I do not have to recruit students for a degree course.


Left: saltmarsh vegetation at Blakeney Point, Norfolk, October 2011; right: close up of Suaeda maritima (Annual Sea Bite) from Blakeney Point.

I also learned a second very important lesson about ecology: that week among the sand dunes generated enough data to keep our ecology class busy through the winter months, as we analysed the data in all sorts of ways.   I learned multivariate statistics – ordination and classification – the slow way, crunching the numbers by hand.   The process worked, I think, because we could relate the graphs we produced to the communities that we had seen just a few months earlier.   I found Dickie Clymo’s way of teaching ecology sufficiently inspiring that I opted for a plant ecology-based project in my final year, and Dickie set me to work on his favourite type of habitat (the peat bog) and plant (Sphagnum moss).

I dug my dissertation out of the loft to see how it had stood the test of time as I was thinking about the post and memories came rushing back. My aim was to look at how the density of Sphagnum plants in a peat bog affected their properties.   I had read parts of John Harper’s book Population Biology of Plants during Dickie’s ecology course and was interested in how organisms competed with one another.   Bogs are seemingly prosaic habitats but, like the salt marshes where we had honed our ecological skills, they were excellent testing grounds for new ideas.


Sampling Sphagnum at Thursley Common, Surrey, early in 1983.

I made two visits to lowland bogs in southern England, Cranesmoor in Hampshire and Thursley Common in Surrey, pushed a metal quadrat with 3 cm sides into the top of the bog at various locations, and removed 1 dm2 squares of Sphagnum to take back to the laboratory. There, I counted the number of shoots and measured the length and mass of the green parts of the shoots. It was slow, painstaking work, which I slotted around my other lectures during my final year at Westfield, sitting in one corner of a research laboratory that Dickie’s PhD student shared with Brenda Thake’s students.   I also tried to look at the amount of chlorophyll in Sphagnum plants, and have a scar from pushing a pipette bulb onto a pipette with so much force that the pipette snapped and drove into my finger. This happened on a Sunday afternoon, when I was working in the laboratory on my own.   Back in those unenlightened days, I can’t even remember filling in an accident form.


The effect of density on shoot mass in different species of Sphagnum from bogs in southern England. Pearson correlation coefficient, r = 0. 75, P < 0.001. From my undergraduate dissertation.

The first graph that I have included makes a point that may be rather obvious to anyone who has grown vegetables – that the greater the density of shoots, the smaller each shoot tends to be.   They are competing with each other for resources and, the more plants that there are, the fewer resources each shoot can acquire.   Having made that point, you’ll see that species that tend to be found on the hummocks in peat bogs – such as S. capillifolium – tend to be smaller than those found in bog pools – such as S. subsecundum and S. cuspidatum.   The crowding may actually favour the former as they rely on capillary action to draw up the water that they need from the bog.

However, each shoot of Sphagnum is not really an independent “plant”, simply one of many genetically identical clones (called “ramets”) of a much larger organism.   The individual, in the sense that you and I are unique beings with a distinct genetic identity (a “genet”), is hard to distinguish from it’s neighbours; however, we can infer the response of the individual to density by looking at the frequency with which branches fork (leading to an increase in the size of the genet.   Once again, there was a linear relationship to density, with the greatest frequency of forks observed at low densities, suggesting that density was influencing the size of genets, as well as ramets. Of particular interest to me was that the slope of the relationship (ignoring the two pool species) was -1.42, which is close to -1.5, the theoretical relationship between density and the size of genets proposed in a 1963 paper by Japanese workers (“the -3/2 self thinning rule” which, I confess, I wrote in my lecture notes as the “three tooths law”, as if it were some arcane Japanese philosophical notion). It suggests that an element of “survival of the fittest” was working within Sphagnum populations, in ways that it was not easy to perceive.


The effect of density on the probability of a shoot of Sphagnum forking. The two points in the lower left hand corner (circled) are both pool species. Pearson correlation coefficient, r = 0.551; P < 0.05 (ignoring these two points). From my undergraduate dissertation.

Work on my dissertation took me perilously close to the start of my final examinations and I remember when Brenda Thake, phycology lecturer and unofficial mentor, took me aside and told me bluntly to finish writing and get stuck into some serious revision.   On reflection, I had reached the stage when data that has taken a long time to acquire (long enough, indeed, for you to question the validity of your original hypotheses) suddenly comes together. After all that time, there is a delirious sense of intoxication as you get stuck into the analyses and see patterns emerge.   It was my first taste of research as a vocation rather than as just another job, and it confirmed my desire to continue with my studies.

But that’s a story for another day.


Clymo, R.S. & Hayward, P.M. (1982).  The Ecology of Sphagnum.  In: Bryophyte Ecology (edited by A.J.E. Smith).  Chapman & Hall, London.

Harper, J.M. (1977). Population Biology of Plants. Academic Press, London.

Yoda, K., Kira, T., Ogawa, H. & Hozumi, K. (1963). Self thinning in overcrowded pure stands under cultivated and natural conditions. Journal of the Institute of Polytechnics, Osaka City University, Series D 14: 107-129.