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.