Last time I wrote, I mentioned that those diatom genera that did not have to be permanently submerged in order to thrive (so-called “aerophilous diatoms”) often appeared together in samples. Having seen some Luticola muticaearly in my analysis of the sample from Castle Eden Burn, it was no surprise to find Diadesmisand Simonsenialater in the same analysis. If anything, the biggest surprise was that I did not also find Hantzschia amphioxys, another habitué of the damp fringes of diatom society.
A quick analysis of my database puts these thoughts into context. There are 6500 samples in my database, so we can see, from the total number of records of each of the aerophilous genera that these are relatively scarce in the samples I encounter. That is largely because my sampling approaches are biased against the habitats where these thrive (more about this below). Aerophilous diatoms are more common than you might think; it is scientists with a yearning to learn more about them that is in short supply.
Hantzschiaand Simonseniaare both less frequent and less abundant than the other two genera, never occurring in numbers exceeding ten per cent of the total but, when they form more than one per cent of the total, there is a very high chance that you will also find other aerophilous taxa in the sample. Humidophilaand Luticolaare sometimes found in higher numbers, and when this is the case, then the proportion of other aerophilous taxa is also often high: 75 per cent of samples where Humidophilais abundant, for example, have at least one other aerophilous taxon present at one per cent or more.
Frequency of other aerophilous genera in samples with Hantzschia, Humidophila, Luticolaand Simonsenia. Each genus is represented by two rows: records where it formed 10 per cent or more of the total number of valves and records where it formed more than one per cent. Similarly, records for other aerophilous genera are also stratified into those where they comprise more than 10 per cent of the total and those where they comprise more than one per cent.
Genus | number of records | other aerophilous genera | ||
>10% | >1% | |||
Hantzschia | 147 | >10% | n/a | n/a |
>1% | 0.50 | 0.70 | ||
Humidophila | 248 | >10% | 0.25 | 0.75 |
>1% | 0.09 | 0.29 | ||
Luticola | 630 | >10% | 0.09 | 0.35 |
>1% | 0.05 | 0.16 | ||
Simonsenia | 61 | >10% | n/a | n/a |
>1% | 0.50 | 1.00 |
Over the years, I have come to use this information informally as a way of knowing whether the results of an analysis are likely to be giving me useful insights into ecological condition. Many of the samples I analyse are collected by other people and sent to me. These samplers should have been working to protocols that ensure that they check that the stones they choose were fully submerged for some time prior to their visit. However, the person collecting the sample may have to make a judgement about river and lake level fluctuations in the period before their visit. Finding lots of cells of aerophilous taxa in a sample is a good hint that something is awry.
The German method for ecological status assessment actually uses the proportion of aerophilous taxa as a check on the reliability of an assessment. I suspect that they are not the only ones, but They have a list of 46 species that they regard as aerophilous taxa, and use a threshold of five per cent in a sample as a threshold. The genera I’ve discussed all feature prominently, along with representatives of 19 other genera. Most of these are represented by only one or two species, although there are seven species of Nitzschia, five of Pinnulariaand six of Stauroneis. I suspect that some species on this list are more tolerant of desiccation than others. We do not know enough of the physiological mechanisms behind this tolerance but it would seem that a few genera (Hantzschia, Humidophila, Luticiola) have definitely got this hard-wired into their genotypes, whilst other genera have members which are mostly aquatic in their habit but with a few exceptions able to survive out of water for some time. I, personally, would trust the five per cent threshold if it was restricted to the hardcore aerophilous genera, with other taxa on the list providing supporting evidence. I would also add the proviso that there should be more than one aerophilous taxon contributing to that five per cent. I would be happier, too, if there were a few experimental studies behind these lists and thresholds but, as ever with the world of diatoms, taxonomists are several steps ahead of the physiologists and so we are heavily dependent on anecdotal information when interpreting results.
List of taxa regarded as aerophilous in the German system for assessing ecological status in rivers.
Name | Authority |
Achnanthes coarctata | (Brébisson) Grunow in Cleve & Grunow 1880 |
Chamaepinnularia parsura | (Hustedt) C.E.Wetzel & Ector in Wetzel et al. 2013 |
Cosmioneis incognita | (Krasske) Lange-Bertalot in Werum & Lange-Bertalot 2004 |
Denticula creticola | (Østrup) Lange-Bertalot & Krammer 1993 |
Diploneis minuta | Petersen 1928 |
Eolimna subadnata | (Hustedt) G. Moser, Lange-Bertalot & Metzeltin 1998 |
Fallacia egregia | (Hustedt) D.G. Mann 1990 |
Fallacia insociabilis | (Krasske) D.G. Mann 1990 |
Fistulifera pelliculosa | (Brébisson ex Kützing) Lange-Bertalot 1997 |
Halamphora montana | (Krasske) Levkov 2009 |
Halamphora normanii | (Rabenhorst) Levkov 2009 |
Hantzschia abundans | Lange-Bertalot 1993 |
Hantzschia amphioxys | (Ehrenberg) Grunow 1880 |
Hantzschia elongata | (Hantzsch) Grunow 1877 |
Hantzschia graciosa | Lange-Bertalot 1993 |
Hantzschia subrupestris | Lange-Bertalot 1993 |
Hantzschia vivacior | Lange-Bertalot 1993 |
Humidophila aerophila | (Krasske) Lowe, Kociolek, Johansen, Van de Vijver, Lange-Bertalot & Kopalová, 2014 |
Humidophila brekkaensis | (J.B.Petersen) D. Lowe, Kociolek, Johansen, Van de Vijver, Lange-Bertalot & Kopalová, 2014 |
Humidophila contenta | (Grunow) Lowe, Kociolek, Johansen, Van de Vijver, Lange-Bertalot & Kopalová, 2014 |
Humidophila perpusilla | (Grunow) Lowe, Kociolek, Johansen, Van de Vijver, Lange-Bertalot & Kopalová, 2014 |
Luticola cohnii | (Hilse) D.G. Mann 1990 |
Luticola dismutica | (Hustedt) D.G.Mann1990 |
Luticola mutica | (Kützing) D.G. Mann 1990 |
Luticola nivalis | (Ehrenberg) D.G. Mann 1990 |
Luticola nivaloides | (W.Bock) J.Y.Li & Y.Z.Qi 2018 |
Luticola paramutica | (W. Bock) D.G. Mann 1990 |
Luticola pseudonivalis | (W.Bock) Levkov, Metzeltin & A.Pavlov 2013 |
Luticola saxophila | (W.Bock ex Hustedt) D.G.Mann 1990 |
Mayamaea nolensoides | (W. Bock) Lange-Bertalot 2001 |
Melosira dickiei | (Thwaites) Kützing 1849 |
Muelleria gibbula | (Cleve) Spaulding & Stoermer 1997 |
Neidium minutissimum | Krasske 1932 |
Nitzschia aerophila | Hustedt 1942 |
Nitzschia bacillarieformis | Hustedt 1922 |
Nitzschia disputata | J.R. Carater 1971 |
Nitzschia harderi | Husedt 1949 |
Nitzschia modesta | Hustedt 1950 |
Nitzschia terrestris | (J.B. Petersen) Hustedt 1934 |
Nitzschia valdestriata | Aleem & Hustedt 1951 |
Orthoseira dendroteres | (Ehrenberg) Genkal & Kulikovskiy in Kulikovskiy et al. 2010 |
Orthoseira roseana | (Rabenhorst) Pfitzer 1871 |
Pinnularia borealis | Ehrenberg 1843 |
Pinnularia frauenbergiana | E. Reichardt 1985 |
Pinnularia krookii | (Grunow) Hustedt 1942 |
Pinnularia largerstedtii | (Cleve) Cleve-Euler 1934 |
Pinnularia obscura | Krasske 1932 |
Simonsenia delognei | (Grunow) Lange-Bertalot 1979 |
Stauroneis agrestis | J.B. Petersen 1915 |
Stauroneis borrichii | (J.B.Petersen) J.W.G.Lund 1946 |
Stauroneis gracillima | Hustedt 1943 |
Stauroneis lundii | Hustedt 1959 |
Stauroneis muriella | J.W.G. Lund 1946 |
Stauroneis obtusa | Lagerstedt 1873 |
Surrirella terricola | Lange-Bertalot & Alles 1996 |
Tryblionella debilis | Arnott ex O’Meara 1873 |
Reference
Schaumburg, J., Schranz, C., Steizer, D., Hofmann, G., Gutowski, A. & Forester, J. (2006). Instruction protocol for the ecological assessment of running waters for implementation of the EC Water Framework Directive: macrophytes and phytobenthos. Bavarian Environment Agency