My explorations of the microscopic world of the River Wear have now gone one step further with the transformation of the schematic representation that I presented in The River Wear in January into a three-dimensional diorama. This shows the “biofilm” on the top of submerged stones, with a layer of Navicula lanceolata at the top (the chocolate brown layer in the photograph from the earlier post) intermingled with small Gomphonema cells on long stalks and some cyanobacterial filaments. A large part of the biofilm, however, is inorganic particles and aggregations of organic matter.
I’m curious about why this biofilm is thickest in the winter, not just in the River Wear but in many other rivers too. Part of the reason is that the organisms that form this film can outpace the bugs that want to eat them at this time of year but this is not the whole story. As the image shows, the biofilm is about far more than just algae, so we need to know a little more about all that organic matter that takes up so much of the space in the picture. Where does it come from and why does it accumulate on stone surfaces?
The story starts with the polysaccharides that algae and other microorganisms (fungi and bacteria) secrete as they grow. These polysaccharides play several roles – they provide the stalks for diatoms such as Gomphonema, they help motile diatoms such as Navicula move and they also ensure that any enzymes that the organisms secrete stay in the proximity of the cell while they perform their functions. However, as well as servicing the organisms that produce them, they also alter the chemical and physical environment on the stone surface. Organic and inorganic particles, for example, can be trapped amongst the stalks of diatoms such as Gomphonema, but there are also chemical interactions. River water contains dissolved organic matter, the end-result of the breakdown of organic matter such as leaves further upstream. This can flocculate to form small particles which can be physically trapped, or it may be adsorbed onto the various polysaccharides in the biofilm.
If you think of a snowball rolling down a hill and growing in size as more and more snow gets stuck on the outside, you have a very rough idea of how a biofilm grows. Simply being a biofilm is enough to help it become a bigger biofilm, as the wide range of biological, chemical and physical interactions that take place will increase the quantity of living and dead organic material, along with inorganic particles. The supply of organic material varies through the year, and is greatest in autumn, following leaf fall (see “A very dilute compost heap …”). The biofilm, unlike the snowball, is largely static; it is the water around it which is moving, bearing with it the raw materials to help it grow. However, the biofilm also bears the seeds of its own destruction: all that organic matter – whether produced by algae in situ or imported from upstream – makes it a nutritious food source for the small invertebrates that inhabit the stream bed. I often see midge larvae eating their way through both living and dead matter when I am examining samples under my microscope. They are there throughout the year, but are busier in the warmer months when, as a consequence, the biofilms are thinner.
Curiously, despite having collected this sample from a stretch of the Wear where I could feel the strength of the current pushing against my legs, flow has relatively little effect on biofilms. There is a thin layer just above the bed of the river where there is almost no current, due to frictional drag and the biofilms exist in this zone. Only when the discharge becomes so strong that the stones themselves are overturned do we see major losses to the biofilm itself. I have seen a medium-sized summer spate in the Wear lead to the opposite effect: a rapid increase in biofilm thickness, presumably because the invertebrates were more vulnerable than the smaller algae.
I will return to the same location on the River Wear in March to see how things have changed.
Blenkinsopp, S.A., & Lock, M.A. (1994). The impact of storm-flow on river biofilm architecture. Journal of Phycology 30: 807-818.
Liu, W., Xu, X., McGoff, N.M., Eaton J.M., Leahy, P., Foley, N. & Kiely, G. (2014). Spatial and seasonal variation of dissolved organic carbon (DOC) concentrations in Irish streams: importance of soil and topography characteristics. Environmental Management 53: 959-967.
Lock, M.A., Wallace, R.R., Costerton, J.W., Ventullo, R.M. & Charlton, S.E. (1984). River epilithon: toward a structural-functional model. Oikos 42: 10-22.
Stevenson, R.J. (1990). Benthic algal community dynamics in a stream during and after a spate. Journal of the North American Benthological Society 9: 277-288.