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.

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.

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