The ecology of cold days …

It is counter-intuitive but algal communities in rivers are often at their most diverse and abundant during the coldest months of the year.   Three months ago, the upper surfaces of stones at a site I visited last week were rough to the touch but, today, they are covered with a thick, chocolaty- brown film.   The explanation may lie in the absence of the small snails that were so abundant on my previous visit (see “The Complex Ecology of a Submerged Stone”) though this is probably only part of the story.   Were you to lift up the bonnet and poke around in the engines of these algae, you might find some clever adaptations to the cold that the bugs that graze them lack, though there are only a few tantalising hints in the scientific literature. More about this in a future post …


Thick biofilms from Smallhope Burn, February 2015.   Left hand image: a submerged brick removed from the water at Knitsley Bridge; right hand image: a cobble photographed in situ at Low Meadows.  

The dark-brown layer is usually very thin and underlain by a thicker, lighter layer (think of the stone as a slice of toast, topped with butter and marmite).   Under the microscope, this dark-brown layer resolves into a mass of motile diatoms that have congregated at the top of a mixture of organic and inorganic particles and other microorganisms, many of which will be fungi and bacteria involved in the breakdown of organic matter.   The most common diatom in these films, in my experience is Navicula lanceolata, but you rarely find pure growths, and other Navicula species, particularly N. gregaria and N. tripunctata are often intermixed and sometimes dominate.

Navicula lanceolata has very characteristic kayak-shaped cells which contain two parallel chloroplasts.   Navicula gregaria is generally smaller but has a similar shape, except that the ends are drawn out to a short “beak”.   Once again, there are two chloroplasts but these are slightly offset from one another.   Navicula tripunctata is has parallel sides, reminiscent of a Canadian canoe and, again, two parallel chloroplasts.   All three move around constantly under the microscope slide, making it hard it measure them accurately.


Navicula lanceolata from Smallhope Burn, County Durham, February 2015.   Scale bar: 10 micrometres (1/100th of a millimetre).


Navicula gregaria from Smallhope Burn, County Durham, February 2015.   Scale bar: 10 micrometres (1/100th of a millimetre).


Navicula tripunctata from Smallhope Burn, County Durham, February 2015.   Scale bar: 10 micrometres (1/100th of a millimetre).

All three of these species are both taxonomically well-defined and very widely distributed. Many Floras refer to them as having preferences for enriched water but my data contradicts this, as they are common across the pollution gradient. I have also found them in numbers at many sites almost free from human influence.   They seem to grow well on the top surface of stones in almost any type of water so long as it is well-buffered and close to neutral pH,. The seasonal preference is easier to demonstrate: the graph below shows how much more likely it is to encounter Navicula lanceolata in abundance in late Winter and Spring compared to other months.   N. gregaria and N. tripunctata show similar (though not identical) trends and I suspect that the ecology of all three species is defined more by physical than chemical conditions: give them cool, well-lit conditions and they will thrive. Indeed, for a river-dwelling organism, “cool” and “well-lit” often go hand-in-hand as there is less marginal vegetation at this time of year, compared to in the summer.

At this point, hard evidence to support my comments dries up. We know a lot about how the distribution of diatoms varies in relation to chemical variables that are fairly straightforward to sample and/or measure in the field – pH, conductivity, nutrient concentrations etc – but far less about the detailed interactions of these organisms with other organisms and, indeed, with less straightforward parameters.   To paraphrase Donald Rumsfeld, there are far more “known unknowns” than there are “known knowns”, and I have no idea (obviously) about the “unknown unknowns”. Except that I suspect that there are some very interesting stories yet to be revealed.


Distribution of records of Navicula lanceolata by month. The line represents sampling effort (percent ofamples collected in a given month) and vertical bars represent samples where N. gregaria forms >16% of all diatoms (90th percentile of all samples where N. lanceolata is present, ranked by relative abundance).

Note: my comments about these three species being taxonomically well-defined are partly based on extensive analyses of the RbcL genes of these species in a study that I have written about previously (see “When a picture is worth a thousand base pairs …”) though which is still unpublished.   There are some nuances in the case of Navicula gregaria, as there are at least three distinct forms, though one is largely brackish and the other mostly found in more oligotrophic (low nutrient) habitats. Our study has probably focussed mostly on the third type (“Navicula gregaria B” in Cox, 1987).  More about Navicula gregaria in “On the trail of Arthur Scott Donkin”.


Cox, E.J. (1987).   Studies on the diatom genus Navicula Bory. VI. The identity, structure and ecology of some freshwater species. Diatom Research 2: 159-160.

Kelly, M.G., Juggins, S., Guthrie, R., Pritchard, S., Jamieson, J., Rippey, B, Hirst, H. and Yallop, M (2008). Assessment of ecological status in U.K. rivers using diatoms. Freshwater Biology 53: 403-422.

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.