Older … but not necessarily wiser?

Gubeikou_stream_April16

A stream flowed through Gubeikou, the village beside the Great Wall where we stayed.  It ran in an artificially-straightened channel, crossed by several small bridges, fords and stepping stones and, when I first saw it, a high percentage of its surface area was covered by floating mats of algae.   This, of course, piqued my interest.   However, when I returned the following morning equipped with rudimentary sampling equipment, the locals were busy clearing these flocs out of the stagnant areas.  The prospect of trying to communicate my interest in algae despite an almost total lack of Mandarin was too much and I skulked off, returning later to find a few of these flocs, along with some submerged leaves smothered in algae in a couple of sheltered backwaters.   Even so, the sight of a wuharin (foreigner) peering intently at the less-savoury aspects of their local stream attracted plenty of curious stares from passing locals.

Gubeikou_algal_floc_Apr16

A floc composed of leaves from an aquatic monocotyledon and associated algae in the stream at Gubeikou, Beijing Province, China, April 2016.

Local liquor is the travelling diatomist’s best friend, as it can be pressed into use as a preservative.  However, I did not add any baijiu to this sample, as this would have damaged the green algae that dominated the flocs.   Instead, I stuck the samples into a corner of my suitcase and hoped for the best.   This was rather optimistic on my part as, ten days later, when I finally had a chance to get the sample under my microscope, the green alga had disappeared completely and the sample was dominated by diatoms, particularly chain-forming araphid species, of which I could make out at least two species, even in this raw state.

Gubeikou_Staurosira_Apr16

A chain of diatoms, possibly Staurosira binodis associated with a green algal floc collected from Gubeikou stream, April 2016, along with (at the bottom of the picture), an out-of-focus chain of a smaller diatom.  Scale bar: 20 micrometres (= 1/50th of a millimetre). 

What I saw when I peered down my microscope at the samples, once they had been properly prepared and mounted, encapsulated many of the challenges faced by the modern diatomist.   Had I collected this sample thirty years ago, I would have confidently named most of what I could see.  The prevailing assumption was that diatom species were mostly cosmopolitan and I would have picked up a copy of Hustedt’s 1930 Flora from my bookshelf and matched the shapes that I saw in my Chinese sample with the illustrations.   However, looking at the sample through a mind conditioned by the developments over the past thirty years, I see subtle deviations from the outlines with which I am familiar, and I start to wonder …

It doesn’t help that the most abundant group in the sample from the algal floc were chain-forming araphids of the genera Staurosira and Pseudostaurosira, a group where there is much uncertainty over species and generic limits even within the geographical areas that I do know quite well.   The sample does, nonetheless, illustrate a principle that I discussed last year as, once again, we see several of these closely-related forms occurring together in the same habitat(see “When is a diatom like a London bus?”).   It suggests to me that, whatever the subtleties exist in species composition, the same general factors are ordering the community, whether in western Europe or China.   The sample from the submerged leaves had a quite different composition, dominated by Nitzschia species.  Some of the species looked familiar but at least two of those are known to be complexes that have still only been partially unravelled.

Gubeikou_araphids_Apr16

Chain-forming araphid diatoms associated with a green algal floc in the stream at Gubeikou, Beijing Province, China, April 2016.  a., b.: Staurosira cf. binodis; c.,d.: Pseudostaurosira cf. elliptica; e.,f.: Fragilaria capucina.   Scale bar: 10 micrometres (= 1/100th of a millimetre).

That there are patterns in the distribution of diatoms seems to be beyond dispute. There has been an enormous amount of research on this topic in recent years, much of it in reaction to a paper by Bland Findlay and colleagues who argued that biogeographic concepts were of limited applicability to microscopic organisms.   Yet we also know that some species are cosmopolitan, and the situation is further complicated because most diatomists base deductions about distribution on morphological criteria (what the [dead cell] looks like) and assume that this aligns with the biological species concept (ability of interbreeding pairs to produce fertile offspring) without further testing (the papers listed below are amongst the exceptions).   Finally, the limited geographical scope most studies, coupled with the prevailing belief that biogeographical variation exists, means that it is too easy to assume that a species has a restricted distribution.  It raises interesting questions about what we mean by a term such as “species” when considering diatoms, but that question will have to wait for another day.

My bigger concern is that the diatomist sees ecology in terms of nouns, whereas the dynamic systems that we study (and whose condition we are expected to advise upon) are perhaps better envisioned as a series of verbs.   Seen like this, the taxonomic complexity that diatomists love to unravel distils down into little more than a source of energy for the next trophic level.   Biogeographic differences only become important when they affect this flow of energy and, as we are often dealing with subtle variations in shape and size of cells, I doubt that all this taxonomic work will lead to radically different conclusions about the state of the environment.   But I may be wrong.  The problem is that this leads into a vicious circle: to answer questions about the extra information contained in all this diversity, we first have to unravel this diversity.  But this, in turn, takes up time that could be spent asking equally valid questions about ecosystem functioning.  Yet the unstudied diversity may, itself, be a confounding variable in studies on ecosystem functioning.   I’d like to think that diatomists get wiser as they get older; however, I am not fully convinced that this is always the case …

Gubeikou_Nitzschia

Nitzschia species associated with submerged monocotyledon leaves in Gubeikou stream, Beijing Province, China, April 2016.   a., b.: Nitzschia cf fonticola; c., d.: Nitzschia amphibia (girdle and valve views respectively); e.: Nitzschia palea sensu lato.  Scale bar: 10 micrometres (= 1/100th of a millimetre).

Reference

Findlay, B.J., Monaghan, E.B. & Maberley, S.C. (2002). Hypothesis: The Rate and Scale of Dispersal of Freshwater Diatom Species is a Function of their Global Abundance.  Protist 153: 261-273.

Rimet, F., Trobajo, R., Mann, D.G., Kermarrec, L., Franc, A., Domaizon, I. & Bouchez, A. (2014).  When is sampling complete? The effects of geographical range and marker choice on perceived diversity in Nitzschia palea (Bacillariophyta).   Protist 165: 245-59.

Trobajo, R., Mann, D.G., Chepurnov, V.A., Clavero, E. & Cox, E.J. (2006).  Taxonomy, life cycle and auxosporulation of Nitzschia fonticola (Bacillariophyta).  2: 1353-1372.

Trobajo, R., Clavero, E., Chepurnov, V.A., Sabbe, K., Mann, D.G., Ishihara, S. & Cox, E.J. (2009) Morphological, genetic and mating diversity within the widespread bioindicator Nitzschia palea (Bacillariophyceae). Phycologia 48: 443-459.

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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.

Staurosira_versus_alkalinity

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.

Staurosira_versus_PO4-P

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.

Staurosira_versus_nitrate

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 …