The complex ecology of a submerged stone …

I was back at Smallhope Burn last week (see “Nitzschia and a friend…”), albeit a few kilometres further downstream from the site I discussed back in September.   Despite my visit being in mid-November, many of the stones I picked up here had tufts of young, healthy-looking Cladophora glomerata with, between them, apparently bare rock surface on which I could see the tiny, almost-black shells of Ancylidae snails.   These have rasping mouthparts and move across the stone surface grazing on the microscopic algae (probably mostly diatoms) that inhabit it.   Though the surface of the rock looked bare and felt rough to the touch, there is almost certainly a thin “sward” of diatoms and other algae here. Otherwise, the Ancylidae (the “cows” of my underwater pastures) would not be here.


A boulder from Smallhope Burn covered with tufts of Cladophora glomerata interspersed with Ancylidae snails with (right) a shell of an Ancylidae snail on my fingertip.   The shell is about two millimetres across and the boulder is about 30 centimetres across.

Under the microscope,I can see several different types of diatom, though most look as if they live on or around the Cladophora rather than in the patches between the tufts.   Cocconeis pediculus and Rhoicosphenia abbreviata are both common epiphytes (see “Cladophora and friends”) whilst Diatoma vulgare and Melosira varians form chains that are loosely-attached to the substrate and, if detached, will easily become entangled within the Cladophora tufts.   The Navicula species are both motile forms that can glide in and around the filaments in search of light.   I suspect that my standard method of sampling diatoms (brushing the tops of stones vigorously with a toothbrush) is a little too coarse to get a true indication of how the diatoms differ between the bare patches and the Cladophora tufts.


Diatoms from Smallhope Burn, Low Meadows, November 2014. a. Cocconeis pediculus; b.,c. d. Rhoicosphenia abbreviata; e., f. Diatoma vulgare; g. Ulnaria ulna; h. Melosira varians; i. Navicula tripunctata; j. Navicula gregaria.   Scale bar: 10 micrometres (1/100th of a millimetre).

Our thinking on the ecology of Cladophora has changed over the past few years. If you consult literature from the 1970s, you’ll see a general agreement that Cladophora prefers waters that are rich in nutrients. This is, indeed, my own observation but, at the same time, you can find a lot of Cladophora in rivers with quite low levels of nutrients and, sometimes, no or little Cladophora in rivers that are nutrient-rich. A recent paper from Ireland helps put these observations into perspective, by demonstrating that the quantity of Cladophora is strongly influenced by the density of grazers as well as the quantity of nutrients.   My suspicion is that grazing invertebrates can keep Cladopora under control until a “tipping point” is reached when either the density of grazers drops or production of Cladophora acceleratesers to an extent that the grazers can’t keep up with the rapid growth of the alga, at which point the bed becomes smothered.   It may even be an example of the “alternative stable states” theory developed by Brian Moss and colleagues for the Norfolk Broads, though that would take some more work to confirm.


Moss, B. (2010). Ecology of Freshwaters. A View for the Twenty-First Century. 4th Edition. Wiley-Blackwell, Oxford.

Sturt, M.M., Jansen, M.A.K. & Harrison, S.S.C. (2011). Invertebrate grazing and riparian shade as controllers of nuisance algae in a eutrophic river.   Freshwater Biology 56: 2580-2593.

Whitton, B.A. (1970). The biology of Cladophora in freshwaters. Water Research 4: 457-476.


The Really Rare Diatom Show

Having set out the limitations of my exercise to define nationally-scarce or rare diatoms, I have drastically reduced my list of candidates from 377 species down to eight.   I suspect that gathering some more data (see point 1 in my previous post) will mean that I can reinstate a few more species to the list, but that will have to wait for another day.

Six of the ten species on my list belong the genus Gomphonema or near relatives.   One of these is Didymosphenia geminata (see “A journey to the headwaters of the River Coquet”); of the others, the most intriguing is Gomphonema tergestinum, a species that occurred in 81 hectads but which seems to be particularly common in north-west England and south-west Scotland, for reasons that I do not fully understand. This needs further investigation but it could be another species that has a distinct biogeography that is not explained solely in terms of a particular chemical environment.   All six of these species qualify as “nationally scarce” rather than “rare” and do remember that my analyses are, at this stage, very preliminary.


Nationally scarce Gomphonema species? a. Gomphonema clavatum; b. Gomphonema insigne; c. Gomphonema ventricosum; d. Gomphosphenia (Gomphonema) grovei; e. Gomphonema turgestinum. Scale bars: 10 micrometres (1/100th of a millimetre). All images from or

One species that may qualify as genuinely rare is Tetracyclus emarginatus, for which we have just two records.   The genus itself is rare, and known mostly from the fossil record but is also sufficiently distinctive that it would not be misidentified or overlooked by analysts.   Another representative of the genus, T. rupestris, has been recorded from Britain but does not feature at all in our database.   When it has been recorded, it is from rock surfaces and damp mosses rather than submerged in streams, so it could have been overlooked in my preliminary analysis. A third representative of the genus, T. lacustris is, as the name suggests, likely to be under-represented in a database composed of samples from rivers, so I will reserve judgement on the rarity of this species though I suspect that it is another candidate for the red list.


Tetracyclus emarginatus. Scale bar: 10 micrometres (1/100th of a millimetre). Image from (photographer: Bernie ní Chatháin).

Another candidate is Cymbellonitzschia diluviana. Though I have tried not to comment on the distribution of species found in lakes, I will make an exception for this species because the habitat is quite well understood, thanks to the work of David Jewson and colleagues at the University of Ulster.   They found it to be most abundant on sand grains exposed to wave action in the littoral zone of Lough Neagh and a few other loughs and lochs with hard water and high pH.   As this combination of characteristics is relatively rare in the UK, it is reasonable to assume that C. diluviana will also be very limited in its distribution.


Cymbellonitzschia diluviana (left: valve view; right: girdle view of two recently-divided cells. Scale bar: 10 micrometres (1/100th of a millimetre). Image from (photographer: David Mann).

Finally, Chris Carter has suggested Entomoneis ornata as a candidate for the diatom “red list”, pointing out that it has not only rare in this country, but is also already on the Red List of Plants of Germany and is also considered to be rare in The Netherlands. Cells of Entomoneis are characteristically twisted around the apical axis, which tests even Chris’ photographic skills, and the genus is more common in brackish and marine waters than in freshwaters. However, it is certainly a species that should be on our preliminary list, and deserves further investigation.

In the next post I’ll consider the pros and cons of a “red list” of British diatoms.


Entomoneis ornata from the Oxford Canal, England, photographed by Chris Carter. Scale bars: 10 micrometres (1/100th of a millimetre)


Carter, C.F. & Belcher, H. (2010). A UK record of Entomoneis ornata (J.W. Bailey) Reimer in Patrick & Reimer 1975. Diatom Research 25: 217-222.

Jewson, D.H. & Lowry, S. (1993). Cymbellonitzschia diluviana Hustedt (Bacillariophyeae): habitat and auxosporulation.   Hydrobiologia 269/270: 87-96.

Ludwig G., Schnittler M. (1996) Rote Liste Gefahrdeter Pflanzen Deutschlands. (12 volumes but available as the list only from

The meaning of … nothing

I had to dig out some old papers today as background reading for a report I am writing.  In the process, I came across one by Horst Lange-Bertalot written in 1979 which implied that, with a knowledge of the ecological requirements of 50 – 100 species of diatom, the condition of almost any river in Europe could be assessed.   About a decade later, Frank Round made a similar assertion, going on to suggest that, for a group of organisms to be useful as environmental indicators “… the species should be easily identifiable (modern floras must be available), quantifiable (preferably without time consuming labour and preferably by workers who can be trained to perform the analyses without the need for detailed knowledge of the biology of the organisms”.

How times change.  Both Lange-Bertalot and Round played a major role in the paradigm shift that has overwhelmed diatom taxonomy over the past three decades.   My own view is that Round’s statement is broadly correct, as I have tried to illustrate in earlier posts (“Lago di Maggiore under the microscope”, “Subsidiarity in action”, “’Speed dating’ with diatoms”).  Many of my colleagues around Europe would contest this, and a veritable flood of books and papers describing new species has pushed the process of accurate identification of diatom species out of the reach of the generalist biologists Round was envisaging, to highly-specialised individuals with very expensive microscopes.

However, here is a problem: assume that the community of European diatom analysts is a finite resource, and that effort is disproportionately directed towards taxonomy.  Something else has to sacrificed, doesn’t it?   To test this idea, I scanned the abstract booklet for the most recent International Diatom Symposium and made a rough classification of the subject matter for the oral presentations.  49 papers dealt with freshwater diatoms.  Of these, 20 (41%) were concerned with taxonomy and a further 26% dealt with the spatial or temporal distribution of diatoms with no reference to other groups of organisms.   Only two papers dealt with physiology and none at all with functional ecology.   Lots of people are interested in the microscopic structure of the diatom cell wall yet almost no-one seems to care about the role that these actually play in freshwater ecosystems.

So we have two problems: the first is that the use of diatoms for ecological assessment has got much more complicated than when Lange-Bertalot and Round were writing their pioneer papers.   This has pushed the work into the realm of “experts” who take longer (and cost more) whilst, at the same time, producing outputs that are harder for lay people to digest.  The second problem is that the work on which this is based is barely, now, connected to the ecosystem functioning that we claim to want to preserve.   Diatomists, it seems, may end up knowing the shape of everything yet the meaning of nothing.


Lange-Bertalot, H. (1979).  Pollution tolerance of diatoms as a criterion of water quality estimation.  Nova Hedwigia 64: 285-304.

Round, F.E. (1991).  Use of diatoms for monitoring rivers.  pp. 25-32.  In: Whitton, B.A., Rott, E. & Friedrich, G. (editors) Use of Algae for Monitoring Rivers.  E. Rott, Institut für Botanik, Universität Innsbruck, Austria.


Ecosystem services … again

Sorry to bang the same drum repeatedly, but I want to return to the theme I explored on 17 November, when I suggested that not all situations where man interacts with water will necessary benefit from “good ecological status”.   Last time, I used the example of angling, suggesting that, as fish yields were a consequence of productivity, there would be situations where anglers, an important stakeholder community, would prefer enriched ecosystem to the pristine ecosystems that us Fundamentalist Ecologists yearn for.

I also mentioned in my post on 9 November that other recreational users of water, rowers, for example, might not appreciate the removal of weirs, even if some conservationists regarded this as desirable.  Having written these words, I started to wonder if there were any situations where conservationists themselves might not regard good status to be a desirable outcome?

I think we can take as a general rule-of-thumb that protecting natural habitats and restoring degraded habitats to their “pristine” state is a general goal for conservation.   But maybe there are exceptions that go against this general dogma?   Perhaps, too, conservationists sometimes overstate the link between high quality habitat and naturalness?  One example that springs to mind is the recent spread of the otter.  For a long time, we regarded the spread of the otter as a sign of the gradually increasing health of our freshwaters.  Yet it is now so widely distributed, often in rivers that are not pristine, that we need to re-examine this assumption.   Evidence for the ink between otters and toxic pollutants that biomagnify along the food chain is quite good.  However, some other types of pollution, such as a moderate amount of enrichment by organic and inorganic nutrients might not be problematic and, indeed, by boosting overall productivity, might raise the carrying capacity of the habitat.  I have never seen this idea explored in detail but it would be worth a look.

Another example of an organism that might actually thrive from enrichment is an unprepossessing but rather rare aquatic plant called Najas marina which is found in only six locations in the UK.   One of these is Upton Great Broad, a habitat that is far from pristine.   Yet it appears that Najas marina is a relatively recent arrival to this lake, only being recorded after the onset of enrichment.   This, of course, creates a conundrum as restoring Upton Great Broad back to more “natural” conditions might bring other conservation benefits, but what would happen to the population of Najas marina if we did this?

I recall a situation that arose in the early 1990s when Northumbrian Water were required, by EU law, to build a sewage works at the mouth of the Tees, rather than discharge raw sewage as they had been doing.  The problem was that the sewage provided an excellent food supply for worms on the tidal mudflats which, in turn, sustained an internationally-important wading bird sanctuary.   This location was, indeed, protected by a different piece of EU legislation, the Birds Directive.   Work on the wading birds of the Tees Estuary was led by Professor Peter Evans of Durham University whilst I was still working there.  I have, however, not seen any of this published in a peer-reviewed journal, which is a shame as it would provide a thought-provoking case study of the trade-offs that many involved in applied ecology have to face.   Here, as in the other examples I’ve mentioned, it might well be the case that an ecosystem at less than good status is actually of greater conservation value than one that has been restored back to good status.

At this point I had better duck my head below the parapet and wait to see what kind of responses this generates.


A diagram illustrating the relationship between conservation and ecological status.  The EU’s Water Framework Directive expresses the quality of an ecosystem in terms of five classes, from “high” to “bad”, with good status being the theoretical target that all water bodies should achieve.


Ayres, K.R., Sayer, C.D., Skeate, E.R. & Perrow, M.R. (2008). Palaeolimnology as a tool to inform shallow lake management: an example from Upton Great Broad, Norfolk, UK.  Biodiversity and Conservation 17: 2153-2168

Mason, C.F. & MacDonald, S.M. (1990).  Impact of organochlorine pesticide residues and PCBs on otters (Lutra lutra) in eastern England.  Science of the Total Environment 138: 147-160.