The problem with the case I set out for a “switch” from a winter / early spring biofilm community to a summer / autumn assemblage is the sample that I was writing about contained elements of both. This, I think, is another aspect of an issue that I touched upon in “The River Wear in January”: that the scale that we work at is much greater than the scales at which the forces which shape biofilms operate. There is no intrinsic driver for this switch beyond the physical forces in the river but each stone will have a slightly different history. A smaller cobble will be more likely to be rolled than a boulder, as will one that is not sheltered from the main current, or not well bedded into the substratum. The sample I collect is a composite from the upper surface of five separate cobbles so will blend these different histories. The more stable stone might have more Navicula lanceolata and Gomphonema olivaceum whilst the recently rolled might be dominated by early colonisers such as Achnanthidium minutissimum.
The same processes can even work on a single stone. Arlette Cazaubon, a French diatomist, now retired, wrote several papers on this topic (see references at the end of this post). She highlighted how the diatom assemblages differed across the surface of a boulder, depending on the exposure to the current. However, that is only part of the story. The picture at the top of the post was taken in January, when I was collecting my first samples of the year. You can see the streak where I ran my finger through the biofilm and some other marks, perhaps where the heel of my wader had scuffed the stone (I’m trying to keep my balance in the middle of a northern English river in January whilst holding a waterproof camera underwater, remember). But such damage could have arisen just as easily from twigs or stones that were being washed downstream. Taken together with Arlette’s work, it shows how a mature Navicula lanceolata / Gomphonema olivaceum assemblage can live alongside a pioneer Achnanthidium minutissimum assemblage.
A schematic view of the biofilm in the River Wear at Wolsingham, March 2018. a. Navicula lanceolata; b. Gomphonema olivaceum complex; c. Fragilaria gracilis; d. Achnanthidium minutissimum. Scale bar: 10 micrometres (= 1/100th of a millimetre).
I’ve tried to depict that in the schematic diagram above. On the left-hand side there is a mature biofilm, with long-stalked Gomphonema species creating a matrix within which motile diatoms such as Navicula lanceolata live whilst, on the right, there is a pioneer community dominated by Achnanthidium minutissimum. However, whilst this patchiness is a natural phenomenon, it can contribute to the variability we see in ecological data and, indirectly, to an impression that ecological data are not precise. If I were to divide the diagram above into two halves, the left-hand side would return a higher TDI than the right. This is because the diatoms on that side have broader ecological tolerances than those on the other (the sample size, by the way, is far too small to do this seriously but I just want to make a point). In practice, however, the entire diagram represents little more than the width of a single bristle of the toothbrush that I use to collect samples so a sample is, inevitably, an amalgam of many different microhabitats on a stone. Our assessment of the condition of the river represents the average of all the patches across the five stones that form a typical sample on that day.
The importance of patchiness in determining the structure and composition of stream communities has been known for some time (see review by Alan Hildrew and Paul Giller in the reference list). What we have to remember when trying to understand phytobenthos is that patchiness is, to some extent, embedded in the samples we collect, rather than being something that our present sampling strategies might reveal.
* “… for we know our patchwork heritage is a strength not a weakness ..” Barack Obama: inaugural address, 2009
A useful review on patchiness in stream ecosytems (several other papers in this volume also discuss patchiness in freshwater and marine environments):
Hildrew, A.G. & Giller, P.S. (1994). Patchiness, species interactions and disturbance in the stream benthos. pp. 21-62. In: Aquatic Ecology: Scale, Pattern and Process (edited by P.S. Giller, A.G. Hildrew & D.G. Rafaelli). Blackwell Scientific Publications, Oxford.
Some of Arlette Cazaubon’s papers on variability in diatom assemblages across the surfaces of single stones:
Rolland, T., Fayolle, S., Cazaubon, A. & Pagnetti, S. (1997). Methodological approach to distribution of epilithic and drifting algae communities in a French subalpine river: inferences on water quality assessment. Aquatic Science 59: 57-73.
Cazaubon, A. & Loudiki, M. (1986). Microrépartition des algues épilithiques sur les cailloux d’un torrent Corse, le Rizzanese. Annals de Limnologie 22: 3-16.
Cazaubon, A. (1986). Role du courant sur la microdistribution des diatomées epilithiques dans une Riviere Méditerranéenne, L’Argens (Var, Provence). pp. 93-107. Proceedings of the 9th Diatom Symposium. Bristol.
Cazaubon, A. (1988). The significance of a sample in a natural lotic ecosystem: microdistribution of diatoms in the karstic Argens Spring, south-east France. pp. 513-519. In: Proceedings of the 10th Diatom Symposium, Joensuu, Finland.