Friday morning arrived with blue skies, scattered cloud and sunshine over the Northumberland hills. More importantly, the river levels were much lower than on Thursday and my plans for fieldwork in the upper Coquet were back on course.
The River Coquet, just above the confluence with Rowhope Burn, May 2014.
I’m here in search of a remarkable diatom which grows luxuriantly on the rocks in this river and nearby rivers in Northumberland. The growths are visible with the naked eye in May; later in the year, they will be even larger. Each has the texture of damp cotton wool and, when you look at them through a low power microscope, you can see why: they are composed of dense masses of branched stalks, each topped by the distinctive, sarcophagus-shaped cells of Didymosphenia geminata. The left-hand image below shows the side (girdle) views of a group of cells which have recently divided; the right-hand image shows the front (valve) view. Each of the stalks was, in turn, smothered with many other diatoms.
A stone from the bed of the River Coquet, covered with colonies of Didymosphenia geminata, May 2014.
Didymosphenia has been the subject of a lot of interest over the last decade as mass growths (much larger than those I saw in the Coquet) have appeared in rivers in several parts of the world, notably New Zealand and Canada. It does not seem to have changed its distribution in Britain or Ireland markedly over this time. The paradox is that these huge biomasses seem to occur in rivers that are naturally low in nutrients.
Looking at Didymospenia down the microscope suggests a partial solution: the growths you can see smothering the stones are largely composed of the stalks which are made from carbohydrates which are just composed of carbon, hydrogen and oxygen and not phosphorus and nitrogen, the nutrients that normally limit growth in freshwaters. However, the Didymosphenia cells still need nutrients to survive and grow and some recent research has suggested an intriguing explanation for how low nutrients might actually be responsible for the high biomass that is often associated with Didymosphenia.
Didymosphenia geminata from the upper River Coquet, May 2014 (left hand image) and May 2006 (right-hand image). Scale bar: 20 micrometres (1/50th of a millimetre).
The first part of the story comes from a paper published by Brian Whitton and Neil Ellwood in 2007 which suggested that the stalk actually plays a role in helping the Didymosphenia cells scavenge phosphorus. Extracellular enzyme activity located in the upper part of the stalk helps the cells to liberate phosphorus that is bound into organic particles (from peat, for example). Even though routine measurements often indicate concentrations of phosphorus dissolved in the water are very low, there are occasional pulses of peaty water, associated with rainfall, that the Didymosphenia (and other algae) are ready to tap into.
The second part of the story follows on from this: Max Bothwell and Cathy Kilroy showed that low phosphorus actually stimulates growth of the stalk, presumably (my speculation here) to increase the potential to trap these organic phosphorus sources. They also lift the Didymosphenia colonies out of the narrow boundary layer close to the rock surfaces where it is exposed to any nutrients that might drift downstream.
The irony, as Bothwell and colleagues point out, is that most aquatic biologists associate high biomass of algae with high nutrients, whereas Didymosphenia actually seems to be associated with the opposite situation. Another irony, as I point out on my web pages is that, , when detached from the stream bed, these brownish masses floating downstream are often mistaken for raw sewage. So we have the rather unusual situation of an unsightly natural phenomenon (in the case of Northumberland, at least) being driven by the absence of pollution. So much for cleaner rivers!
Bothwell, M.L., Taylor, B.W. & Kilroy, C. (2014). The Didymo story: the role of low dissolved phosphorus in the formation of Didymosphenia geminata blooms. Diatom Research doi: 10.1080/0269249X.2014.889041.
Ellwood, N.T.W. & Whitton, B.A. (2007). Importance of organic phosphate hydrolysed in stalks of the lotic diatom Didymosphenia geminata and the possible impact of climate change. Hydrobiologia 592: 121-133.