Possible sources of error in diatom community analysis

When implementing monitoring programs based on assessments of diatom community composition, earlier studies in Europe would aid in identifying the likely sources of error. Several sources of error, at all stages of the analysis, have been highlighted by Prygiel et al., (2002) in an inter-laboratory comparison exercise. For example, when the sampling protocol is not strictly followed then variability due to sampling can be very high. Errors include sampling from exposed substrata — from areas subjected to water level change and from areas of low-velocity flow as compared to other parts of the river — and sampling from stones covered by abundant filamentous algae. Laboratory and counting errors may include the use of high temperatures when drying slides (leading to clumping of diatom valves) and the settling out of large taxa during the preparation of consecutive slides from a single sample. The main source of variability is, however, in the identification ability of individuals (Prygiel et al., (2002). That is why biological quality controls focus mainly on counts and misidentification (Kelly, 1999). Diatoms are suitable for such controls and proposals relating to quality control have been made by Kelly (1999). Prygiel et al., (2002) made recommendations which should be seen as the way forward for India in terms of quality control and the validation of diatom analysis data. Most of the variability due to sampling and slide preparation can be avoided by organizing comparisons between different studies. Such comparisons are very useful because, with field and laboratory approaches, they make operators aware of the consequences of not following protocols. They are also useful because they highlight some taxonomic problems. Diatoms are good subjects for photomicrography and therefore many diatomists make use of the diatom archives such as (http://keisou.hp.infoseek.co.jp/mokuji.html#m1, http://craticula.ncl.ac.uk/Eddi/, http://www.math.ualberta.ca/~bowman/diatom/) to check problematic identifications. This approach should be further encouraged by formalizing expert-practitioner exchanges, by creating iconographic databases, and by organizing regular workshops to allow updating of knowledge. The archiving of permanent slides also facilitates the creation of reference collections, which are particularly useful for the identification of difficult species.