Notes on habit and environmental preferences
The ecological data that are included with the descriptions of each taxon are derived from both the authors’ personal experience and a variety of published sources. Anyone who is familiar with the literature on diatom ecology will know that comments on environmental preferences are often vague and there are frequent contradictions. Many such comments seem to be copied almost word-for-word from one Flora to the next. For this reason, we have summarised as much information as possible in the form of simple indices, concentrating on those most widely used in the UK and elsewhere in Europe for water quality monitoring. Even so, some apparent contradictions still occur, which might best be explained in terms of regional perceptions of different types of ‘pollution‘. A level of organic pollution that is regarded as ‘severe’ in a relatively pristine Alpine environment might, for example, be regarded only as ‘moderate’ in densely populated areas of northwest Europe.
Several of the indices are based on the weighted average equation of Zelinka and Marvan (1961: see also Birks et al., 1990):
where aj = abundance or proportion of valves of species j in sample, sj = pollution sensitivity (‘optimum’) of species j and vj = indicator value (‘tolerance’). Note that the principle behind this weighted average equation is identical to that behind the equation of Birks et al. (1990). If the relative abundance of a diatom taxon is plotted against an environmental variable, the data typically show a pattern that can be described by a unimodal response curve. The point of maximum abundance is the ‘optimum’ whilst the ‘tolerance’ describes the spread of values around the optimum, analogous to the standard deviation of a normal distribution. A normal distribution with a small standard deviation implies values tightly grouped around the optimum whilst one with a large standard deviation implies values spread widely around the optimum. From the point of view of biological monitoring, the former is clearly the more desirable situation and taxa that fulfil this criterion are given greater weight by treating ‘tolerance’ as the inverse of the weighted standard deviation and thus ‘weighting’ the equation in favour of ‘good’ indicators.
Habit information here concentrates on describing the relationship between the diatom and substrate (mode of attachment, if any), and sibling cells (type of association, if any) and on whether or not the species is motile. This latter point deserves a little explanation. Many diatoms — including some centrics and araphid diatoms: see Round et al., 1990) are capable of some movement, although it is the raphe-bearing pennate diatoms that are most often associated with motility. Many of these live attached to the substrate, but at certain stages in the life cycle (e.g. during sexual reproduction), it is advantageous for such species to move short distances in order to find a mate. A number of genera, however, have no means of attachment and exploit their motility more fully. An attached diatom requires the substrate to be relatively stable, in order to remain in a position favourable for photosynthesis. A motile diatom, however, can live on less-stable substrates, such as fine sediments, where there are ecological benefits to be gained from moving towards light whilst, at the same time, being able to exploit the relatively high nutrient concentrations often found in such habitats. The diatom assemblages that live on or in sediments are often described as ‘epipelon’; however, the same species are often found on larger substrates, particularly when the biofilm is thick or when the larger substrate is, itself, smothered with fine sediments. In reality, ‘epipelon’ and other types of diatom assemblage (‘epilithon’, ‘epiphyton’, ‘episammon’ are often just the extremes of continua. The term ‘motile’ here is used of those taxa that are capable of unimpaired movement for the greater part of their lifecycle. This roughly corresponds with the use of the term ‘epipelon’ in other sources (e.g. Round, 1981).
Additional information on habit and ecology is derived from the authors’ experiences.
Sensitivity to nutrients
Values from three trophic indices are used here:
- The Trophic Diatom Index (TDI) as used in the UK. This index was developed by Kelly & Whitton, 1995 and the version used here is the revised version, described in Kelly et al, 2001).
- The Trophic Diatom Index developed by Coring et al. (1999) for use in Germany.
- The Trophic Index developed by Rott et al. (1999) for use in Austria.
Note that all three indices were designed for use in rivers, and these values will not apply for standing waters.
The prevailing dogma is that phosphorus is the nutrient most likely to limit primary production in freshwaters and although there is good evidence to suggest that there are many situations where nitrogen may well be the limiting nutrient, in practice it is difficult to disentangle the effects of these nutrients. These trophic indices describe diatom distribution in relation to either ‘dissolved’ (≈ ‘orthophosphate’) or ‘total’ phosphorus but users should be aware that both are usually closely-correlated and also that phosphorus and nitrogen concentrations are often highly correlated. We recommend that these indices are treated as broad indicators of trophic status.
Sensitivity values (S) assigned as follows:
|Indicator value (S)
||Optimum occurs at or below 0.01 mg l-1 filtrable P|
||Optimum occurs between 0.01 and 0.035 mg l-1 filtrable P|
||Optimum occurs between 0.035 and 0.1 mg l-1 filtrable P|
||Optimum occurs between 0.1 and 0.35 mg l-1 filtrable P|
||Optimum occurs between 0.35 and 1 mg l-1 filtrable P|
In addition, a few taxa have sensitivity values of zero. These include a few taxa that are relatively rare in freshwaters and whose ecological preferences are not well defined, along with planktonic taxa, which are routinely excluded from calculations. Tolerance values (V) range from 1, for taxa with broad distributions to 3, for taxa that are restricted to a narrow range of nutrient conditions.
The German TDI is very similar to the UK TDI in principle, except that sensitivity values are not necessarily integers. The trophic classes defined by these values are as follows:
||Median total P (mg l-1)|
Austrian TI (Rott et al., 1999):
Rott et al. (1999) describe the trophic states indicated by the TI as follows:
||Total P annual mean (mg l-1)
||Total P extreme value (mg l-1)|
Sensitivity to organic pollution
Values from four indices are included:
- The Indice de Polluosensibilité (IPS: Coste, in CEMAGREF, 1982).
- The Generic Diatom Index (GDI: Rumeau & Coste, 1988)
- The Saprobienindex (Saprobic Index: SI: Rott et al, 1997)
- Saprobity values from van Dam et al. (1994)
Indice de Polluosensibilité and Generic Diatom Index
The Indice de Polluosensibilité is one of the widely used indices in Europe at present. It integrates the effects of all pollutants, including organic pollution, salinity, toxins etc. and, as such, it is a useful useful surrogate for an organic pollution index, if you are reasonably confident that organic pollution main “pressure” at the site that you are studying.
Sensitivity (S) values vary from 5 (very sensitive) to 1 (very tolerant) and indicator values vary from 1 to 3.
The Generic Diatom Index (GDI) is based on the same principles as the IPS but works at the level of genus rather than species.
Saprobic index (after Rott et al., 1997)
The Saprobien system, which evolved in the early years of the 20th century, recognises a series of stages in the oxidation of organic matter: typically ‘polysaprobic’, ‘α-mesosaprobic’, ‘β-mesosaprobic’ and ‘oligosaprobic’ along with various intermediate stages. Each stretch of a river can be assigned to a particular stage on the basis of the presence or absence of indicator species. Various indices based on the Saprobien system have been used in Continental Europe but not in the UK where the basis of the Saprobien system has been questioned. Briefly, the assumption that the response of the aquatic biota can be summarised solely in terms of the breakdown of organic matter has been rejected in favour of a more holistic understanding that recognises the impact that other constituents of organic effluents such as ammonia and suspended solid have on the biota. However, indices based on the Saprobien system are still useful for assessing the role of organic pollution in rivers. Of the many that have been developed, that of Rott et al. (1997) is one of the most useful and the sensitivity values can be interpreted as follows:
|Güteklasse I und besser|
(no or very little pollution)
moderate to strong
(moderate to heavily polluted)
(heavily to very heavily polluted)
(very heavily polluted)
Saprobity values from van Dam et al. (1994
Van Dam et al. (1994) present saprobity values, where known, for 776 species in 56 genera recorded from The Netherlands. The values are derived from Hamm (1969), Lange-Bertalot (1978, 1979) and Krammer and Lange-Bertalot (1986-1991), with the following categories:
||Water quality class
||Oxygen saturation (%)
||BOD520(mg l-1 O2)|
||α-meso / polysaprobous
Sensitivity to acidification
There is an enormous literature on the response of diatoms to acidification (see Battarbee et al., 1999). The approach adopted here is based on values reported in Van Dam et al. (1994) which are themselves derived from Hustedt (1938-39):
||Optimal occurrence at pH < 5.5|
||Mainly occurring at pH < 7|
||Mainly occurring at pH values about 7|
||Mainly occurring at pH > 7|
||Exclusively occurring at pH > 7|
||No apparent optimum.|
Salinity: from van Dam et al. (1994: after van derWerff & Huls, 1957-1974)
This is a key to freshwater diatoms; however, several of the taxa can survive in brackish water as well. Two sources of information on preferences for saline water are included: first, a simple descriptive term, ‘freshwater’ or ‘freshwater-brackish’, based on our own experience of the UK flora; and, second, the salinity classification of Van der Werff and Huls (1957-1974), as presented in van Dam et al., (1994). The latter places diatoms into one of four classes, which are related to chloride concentration and salinity as follows:
||Cl- (mg l-1)
We urge caution when using this classification, as the ‘fresh-brackish’ class includes many taxa that can thrive in hard water (i.e. where conductivity is derived from calcium and magnesium, rather than sodium and potassium salts).
Sensitivity to desiccation
This classification was derived from van Dam et al. (1994) based on an earlier classification by Denys (1991) and on their own experiences,
||Never, or only very rarely, occurring outside water bodies|
||Mainly occurring in water bodies, sometimes on wet places|
||Mainly occurring in water bodies, also rather regularly on wet and moist places|
||Mainly occurring on wet and moist or temporarily dry places|
||Nearly exclusively occurring outside water bodies.|