Before I was diverted by the delights of Bukhara and Samarkand, I was writing about the struggles that aquatic plants have to undergo in order to obtain the carbon that they need for photosynthesis (see “Concentrating on carbon …”). In this post, I want to show the scale of the effect of inorganic carbon supply on the diatoms that we find in freshwaters.
My earlier post pointed out that aquatic plants have two possible sources of carbon to use for photosynthesis: dissolved carbon dioxide or bicarbonate. The latter is derived, ultimately, from the rocks through which the water seeps before ending up in a stream or river. Calcium carbonate, in turn, reacts with hydrogen ions in the water to form the bicarbonate that plants can use for photosynthesis. A rock such as limestone, which is made of calcium carbonate, for example, provides a better supply than a hard siliceous rock such as granite.
Aquatic biologists use the term “alkalinity” to refer to the relative amounts of carbon dioxide, bicarbonate and carbonate in water. This can confuse people as, in this context, “alkalinity” has little to do with the pH of the water itself and, indeed, water that is alkaline (i.e. has pH > 7) does not have to have a high alkalinity. For now, just accept that low alkalinity water has little bicarbonate relative to dissolved carbon dioxide, whilst high alkalinity water has mostly bicarbonate and relatively little dissolved carbon dioxide. In practice, alkalinity is a good indicator of the geology underlying the catchment from which a sample was collected, with low values associated regions of hard rocks (such as the Ordovician granites in Ennerdale’s catchment) and high values particularly associated with limestone and chalk.
I’ve spent a quarter of a century trying to understand how diatoms react to pollution and one of the surprising by-products of those studies is the realisation that alkalinity is just as important as pollution in determining the diatoms that are found at a site. This is the case for most groups of freshwater organisms, but the scale of the effect on diatoms is particularly strong, as the graph below indicates.
Relationship between alkalinity and the average TDI at 430 UK river sites (r2 = 0.52). The blue line shows a regression line fitted to the 10th percentile using the “quantreg” package in R.
This graph shows a data from 430 UK sites where at least one TDI (Trophic Diatom Index) measurement was available, with alkalinity plotted on a logarithmic scale on the x axis and the TDI on the y axis. There is a clear relationship between the two variables with about half of all the variation in the TDI accounted for by alkalinity (i.e. geology) alone, and this is manifest, in particular, by alkalinity setting a “floor” below which the TDI is unlikely to fall at any given alkalinity value (indicated by the blue line). The red line, then, indicates the variation in TDI due to other factors, mostly human pressures such as eutrophication.
The blue line, in other words, indicates the best that the TDI is likely to be at any given alkalinity and if we were to look at samples which plot close to this line we will see quite marked differences in the diatoms as we moved from the low end towards the high. When alkalinity is low, we will find Tabellaria flocculosa, some Brachysira species (e.g. B. neoexilis) and maybe a few Eunotia species too. As alkalinity increases, so the diatom assemblage will be dominated by Achnanthidium minutissimum and relatives, but we will also see Hannaea arcus and Fragilaria gracilis, amongst other species. We will see some Achnanthidium and Fragilaria species at low alkalinity, too, but either different to those at moderate to high alkalinity or in lower numbers.
There are several possible explanations for this but Brian Moss, in a classic paper from 1972, suggested that the availability of dissolved carbon dioxide was a major factor. The “soft water” species, in other words, were better adapted to life without bicarbonate but were out-competed in moderate and hard water where the supply of bicarbonate was greater. Very roughly, this switch from domination by free carbon dioxide users to bicarbonate users occurs at no more than 20 mg L-1 CaCO3. There is more going on than just the supply of inorganic carbon: low alkalinity water is more likely to have low pH, which brings a separate set of challenges to aquatic organisms, and very high alkalinity water is often associated with productive agricultural areas. This means that effects at both ends of the scale may be hard to separate completely from human pressures. However, the broad story that emerges is that hard rock, in ecology as in music, is not to everyone’s taste.
Moss B. (1973). The influence of environmental factors on the distribution of freshwater algae: an experimental study. II. The role of pH and the carbon-dioxide-bicarbonate system. Journal of Ecology 61: 157-177.