Ecology in the Hard Rock Café …

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.

A bigger splash …


This post continues my occasional series on insignificant black or dark brown spots on submerged stones (see “Both sides now …” for another recent episode).  I found these particular specimens on a cobble in Croasdale Beck in Cumbria, close to my regular haunts around the River Ehen and Ennerdale Water and thought that, with algae grabbing headlines for the wrong reasons yet again, I should write something positive about them.   What kind of weird world do we live in when people think it strange that algae thrive in a warm, well-lit water body, whilst simultaneously lauding other people who devote four years of their lives to practising jumping into a swimming pool?


Colonies of Chamaesiphon cf fuscus (mostly 2-3 mm in diameter) growing on a submerged cobble in Croasdale Beck, Cumbria, August 2016. 

There was something about the regularity of the outlines of the dark brown / black spots on some of the more stable stones in this flashy beck that attracted my attention.   I’ve scraped a lot of dark smears and smudges off rocks in the past and often been disappointed when all I find are inorganic iron or manganese deposits.  Over time, one gets an eye for what is and is not an algal growth (or, for that matter, a submerged lichen) and even, in some cases, for the type of alga that formed the growth.   In this case, I had a good idea, straight away, that I was looking at a member of the genus Chamaesiphon, a cyanobacterium (blue-green alga).

Members of this genus are unicellular and form dense mats of cells that can be difficult to photograph.   I could not get a really clear view of individuals within this particular colony so, instead, have included some of Chris Carter’s photographs of another member of the genus.   You can see the short, club-shaped cells, each in a sheath and many topped by small “exospores” which bud off from the mother cell to propagate the colony.   The sheath has a brown tinge, presumably to the “sun-screen” compounds that we have met before in cyanobacteria.   Most of the members of the genus live on submerged rocks, but a few live on other algae (see “More from the River Ehen”).   Most of the rock-dwelling species indicate at least good conditions in rivers, but one species, C. polymorphus, is tolerant of more enriched conditions, which complicates use of a straight genus-level identification for rapid assessments.


Chamaesiphon polonicus from Caldbeck, Cumbria, photographed by Chris Carter.  Top left: looking down on colony; other images: side views showing cells in their sheaths and, in a few instances, with exospores. 

Oddly (to me at least) press coverage of the Olympic diving pool story has only used the word “algae”, never telling us what sort of alga is responsible for the problem.   This is equivalent to the commentators saying that “animals” have just made a perfect leap off the 3 metre springboard, leaving the audience to work out whether the subsequent splash was made by a slug or a human.

But I should end on a positive note: better, perhaps to compare the algae not with the divers but with the judges who assign the final scores.   That’s because a few minutes mooching around a stream or beside a lake can usually reveal enough from the types of algae that live there to give some insights into the health of the stream.   My visits to Croasdale Beck over the past year or so have shown me enough to suggest that this little Cumbrian stream probably deserves the algal equivalent of an Olympic medal.  But I doubt that we’ll get much press coverage for saying that…