Hard science in hard water?

Having started to think about the ecology of small Fragilaroid diatoms in a recent post (see “When is a diatom like a London bus?”), I thought that it might pay to look in more detail at the habitats that these taxa do like, in the hope that this will help us to understand why they occur together so often.   I am just looking at two “species” in this post: “Staurosirella pinnata” (which we suspect to be a complex of several species) and “Staurosira construens” (which is also a complex, as the records in my database merge a number of varieties, most of which have subsequently been raised to the status of species in their own right).

One problem has to be confronted at the outset: these taxa also share a propensity to form chains which remain intact even after we’ve made slides.   This means that we often encounter aggregates of five or more cells, which violates the assumptions of random distributions of diatoms that underpin our statistical methods.   No-one, to my knowledge, has found a satisfactory means of dealing with this, but it should be borne in mind when considering the graphs which follow.

The first graph shows the distribution of records of these species in my database along an alkalinity gradient, and generally confirm the preference of both species for hard water.   I have highlighted two outliers on the chart for Staurosira construens. These samples are from the same location, the upper reaches of the River Wey (South) in Surrey, which receive a mixture of soft water, flowing off the Greensand, and harder water from the surrounding areas.   I have encountered anomalies between diatoms and water chemistry in this area before, which are probably the result of the complex hydrology of the area.


The distribution of Staurosirella pinnata (left) and Staurosira construens (right) along an alkalinity gradient. Records from the “DARES” dataset.   Two outliers from the River Wey (South) are highlighted.

The next two graphs show the distribution of records along phosphorus and nitrogen gradients and these show opposite responses: both seem to be most abundant when phosphorus is low and nitrogen is high. Again, we have the problem of the two outliers from soft water sites confusing the view for Staurosira construens but we can generalise and say that neither species is likely to be abundant (meaning > 10 per cent of all valves) except when these conditions are met.

The horizontal red lines on these graphs show the range of phosphorus and nitrogen measured in a single river, the River Wylye, during a study in 2011-2012. I have included these lines to give a rough idea of the precision that we should expect when defining the preferences of a diatom.   The River Wylye is a chalk stream, which tend to have relatively stable hydrology, so the range of nutrient concentrations measured in these streams is probably lower than is the case for many rivers.


The distribution of Staurosirella pinnata (left) and Staurosira construens (right) along an reactive phosphorus gradient. Records from the “DARES” dataset.   Vertical lines represent the approximate position of high (blue), good (green), moderate (orange) and poor (red) status boundaries.   The horizontal line shows the range of concentrations encountered in the River Wylye, Wiltshire in 2011-2012.


The distribution of Staurosirella pinnata (left) and Staurosira construens (right) along a nitrate-N gradient. Records from the “DARES” dataset.   Vertical lines represent the position of the (Irish) high (blue) and good (green) status boundaries.   The horizontal line shows the range of concentrations encountered in the River Wylye, Wiltshire in 2011-2012

Ecological assessment using diatoms is largely based on indices that calculate the relative position of a sample along a quality gradient based on a combination of the known ecology of the species and the representation of that species in the sample.   This means that the result is most strongly influenced by the most common species and anything that occurs below about five per cent has little influence. These charts suggest that Staurosirella pinnata and Staurosira construens will both be good indicators of a combination of low phosphorus and high nitrogen in hard water; however, there are a “tail” of records that extend into other types of water.   One valid question is whether the individuals responsible for these occurrences outside the “optimum” are the same species as those that are abundant at low P / high N / hard water.   Given what I wrote above about both of these taxa probably being complexes, this is a possibility.   However, the generally low numbers means that solving taxonomic riddles will be unlikely to lead to a great increase in precision in ecological assessments.

Personally, I lean towards the options I suggested in Baffled by the benthos (2) – that diversity within samples may be controlled by a wide range of factors unrelated to anthropogenic pressures and that interspecific diversity may give insights into ecological resilience. The problem is that this hypothesis is easier to propose than it is to test. It is not impossible to test; however, the hegemony of taxonomically-inclined diatomists over those with a genuine interest in functional ecology means that will probably remain no more than a theory for some time to come …

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